Glacial sequence and environmental history in Sierra Nevada del Cocuy - van-der-Hammen et al. 1981

Palaeogeography, Elsevier Scientific

Palaeoclimatology, Palaeoecology, Publishing Company, Amsterdam

-

32 (1980/1981): 247-340 Printed in The Netherlands

GLACIAL SEQUENCE AND ENVIRONMENTAL SIERRA NEVADA DEL COCUY (COLOMBIA)

T. VAN

DER

HAMMEN,

J. BARELDS,

H. DE JONG

HISTORY

February

15, 1980;

revised

version

IN

THE

and A. A. DE VEER

Hugo de Vries-Laboratorium, Department of Palynology and Paleoecology, Amsterdam, Amsterdam (The Netherlands) Instituut voor Aardwetenschappen, Department of Biostratigraphy and Free University, Amsterdam (The Netherlands) Gelderse leergangen, Nijmegen (The Netherlands) Soil Survey Institute, Wageningen (The Netherlands) (Received

247

accepted

July

University

of

Paleoecology,

14, 1980)

ABSTRACT Van der Hammen, T., Barelds, and environmental history Palaeoclimatol., Palaeoecol.,

J., De Jong, H. and De Veer, A. A., 1981. Glacial sequence in the Sierra Nevada de1 Cocuy (Colombia).

Palaeogeogr., 32: 247-340.

The glacial sequence in the area of the Sierra Nevada de1 Cocuy (Cordillera Oriental, Colombia) was studied in the field and by means of aerial photographs in relation to the environmental history, which was studied by means of pollen analysis and radiocarbon dating. At least five (possibly six) glacial drift bodies could be recognised, and on the basis of the (groups of) bordering moraines about six main glacial stades were defined and named. Drifts 2, 3, 4 and 5 are presumably all of Fuquenian (Last Glacial) age, whereas Drift 6 is of Holocene (Neoglacial) age. In the pollen diagrams the Saravita, SusacL and Guantiva interstadials are clearly reflected as periods of a somewhat higher forest limit, a more abundant growth of Polylepis and an interesting pioneer vegetation (often with Dodonaea).

The greatest extension of the glaciers took place before ca. 25,000 B.P., probably in the period between 45,000 B.P. and 25,000 B.P. The climate was relatively wet at that time and the forest limit something like 800-1000 m below the present one, and during the early period of major extension of the ice, glaciers and forest may have locally been in contact (at elevations between 2200 and 2700 m), and the paramo belt was relatively narrow and wet, with Polylepis abundant in the lower parts.

Between 21,000 and 14,000 B.P., the ice extension was much less, the forest limit lower, and the climate drier, which resulted in a relatively broad and dry paramo belt.

The Late Glacial had a relatively wet climate again, but the annual temperature increased; there are two clear “glacial stades”.

There are very clear signs of a late medieval neoglaciation; glaciers started to retire from the outermost Neoglacial bordering moraines probably after 1850 A.D., and are still decreasing.

RESUMEN Se investigd la secuencia glacial en el area de la Sierra Nevada de1 Cocuy (Cordillera Oriental, Colombia) en el campo y por medio de fotografias abreas, en relation con la historia medioambiental, estudiado por medio de analisis de polen y de radiocarbono.

Se pudieron reconocer 5 (y posiblemente 6) “drifts” glaciales y en base de 10s (grupos de) 0031-0182/81/0000-0000/$02.50

o 1981

Elsevier

Scientific

Publishing

Company

248 morrenas terminales y laterales se definieron unos 6 estadios glaciales. “Drift” 2,3,4, y 5 deben ser de edad Fuqueniense (Ultimo Glacial), “Drift” 6 es de edad Holocene (“Neoglacial”). En 10s diagramas de polen 10s interestadiales de Saravita, Susaca y Guantiva estan claramente reflejados coma periodos con un limite de bosque algo mas alto, una abundancia mayor de Polylepis y una vegetation pionera interesante (frequentemente con Dodonaea). La mayor extension de 10s glaciares fue anterior a 25.000 B.P., probablemente entre 45.000 y 25.000 B.P. El clima en este period0 era humedo y el limite de1 bosque se hallaba unos 800-1000 m debajo de1 actual, y durante el period0 temprano de mayor extension de1 hielo, glaciares y bosque pueden haber estado localmente en contact0 (en elevaciones entre 2200 y 2700 m), y la zona de paramo era relativamente angosta y hfimeda, con abundante Polylepis en la parte baja.

Entre 21.000 y 14.000 B.P., la extension de 10s glaciares era mucho menor, el limite de1 bosque se hallaba a una elevation menor y el clima era m&s seco, resultando en una zona de p&ram0 relativamente ancha y seca. El Tardiglacial tiene nuevamente un clima mls htimedo, pero las temperaturas anuales estan subiendo; hay dos estadios bien definidos. Hay indicios muy claros de una “neoglaciacion”; 10s glaciares principiaron a retirarse de las morrenas terminales neoglaciales exteriores, probablemente despues de 1850 A.D., y continuan su receso en la actualidad.

INTRODUCTION

The Sierra Nevada de1 Cocuy constitutes the highest part of the Colombian Cordillera Oriental between lat. 6” 20’N and 6” 35’N (see the index map in Fig.18). It is the only part of this Cordillera that reaches altitudes above the snow-line (over a length of 33 km), and the highest peak (the Ritacuba) has an elevation of 5493 m. Geologically the area consists of a series of folded and faulted sandstones, shales and occasional lenses of limestone, the folds having a north-south strike. The higher crests and ridges are formed by sandstones dipping westward; the main bodies of ice are present on these dip-slopes. Being the only part of the Eastern Cordillera with relatively extensive present-day glaciation, and reaching high altitudes in a relatively extensive area, it is the most important area for the study of high-altitude vegetation belts (and of their relation to the local climate), and also for the study of older and more recent glaciations in this part of the Andes.

In January 1959 the first author carried out field studies in the area, together with E. Gonzalez and R. F. Flint. These studies of vegetation, lake sediments and glacial deposits, were mainly carried out in the area of Valle de Lagunillas with additional observations in the area of Laguna de la Plaza (all in A4 on the map of Fig.18). A brief account of this expedition was given in Kraus and Van der Hammen (1960), and the scientific results (including pollen-analytical studies) were reported in Gonzalez et al. (1965).

In July, 1967, Roberto Jaramillo and the first author carried out a study of the forests in the area of the Rio Casanare, which forms the southern limit of the Sierra de1 Cocuy (AB7 in Fig.lS), and in the adjoining Piramo de Chita. In the same year F. Lucas collected lake sediments in the Sierra de1 Cocuy, and made observations on moraines. The most important of these lake sequences were studied palynologically by the second author,

249

while the last two authors prepared a glacio-morphological map of the area, based on the interpretation of aerial photographs and on earlier field studies

by Gonzalez et al. All these studies were carried out as part of a research project on the Quaternary of Colombia, sponsored by the Netherlands Foundation for Tropical Research, WOTRO. In 1972 and 1973 extensive studies of the vegetation of the highest part of the Sierra were carried out by Antoine M. Cleef (equally sponsored by WOTRO). These studies (Cleef, 1978, 1979; Cleef, in press) now provide a much sounder basis for the interpretation of pollen diagrams.

When these pollen-analytical studies were finished, the vegetational studies completed, and the finished glacio-morphological map elaborated, the first author made another field study in the area in March 1977 (together with A. M. Cleef) to verify in the field some critical parts of the glaciomorphological map, partly in relation with the dating of glacial stages, and to gain a better understanding of the various vegetation types (in relation to local climatic conditions) and the local vegetational history. The main results of the above-mentioned field and laboratory studies are reported here; first the vegetational and environmental history will be treated, subsequently the glacial history, and finally the results of both parts will be compared, discussed and integrated.

In order to make this paper more readable, we did not mention the authors’ names of the plant species, nor did we indicate the families to which the various genera and species belong. The relevant documentation is provided by the enumeration at the end of this article.

PRESENT

VEGETATION,

ENVIRONMENT

AND POLLEN

RAIN

The present-day altitudinal zonation of the p&ram0 vegetation was studied

by Cleef (1979) and the altitudinal zonation of forest vegetation by Van der Hammen and Jaramillo (in prep.). Only a concise survey will be given here of these partly unpublished data, as far as they are of importance for the present study (above ca. 2000 m). The names of plant communities mentioned here are provisional ones; they will be described more definitely in the near future. There are considerable differences between the eastern and the western side of the Sierra, and for that reason they will be discussed separately.

The eastern side is very wet; the annual precipitation and atmospheric humidity are high, and there is only one drier season annually.

From slightly above 2100 m to ca. 2550 m a.s.1. Weinmannia pinnata (Cunoniaceae) is one of the commonest trees in this forest. There are always many cyatheaceous tree ferns (Cyathea, Akophila) in this forest, and Alchorneu and Acalypha are usually represented (the last genus only below ca.

2400 m). Below 2200 m Cecropia and Heliocarpus occur. Other common arboreal genera or families in this “Weinmannietum pinnatae” are Clusiu,

250

Stylogyne, Brunellia, Rapanea, Eugenia, Hedyosmum, Guarea, Bilia, Piper, Saurauia, Fresiera, Psychotria, noma (a species with pinnate leaves), Melastomataceae,

Myrsinaceae, and with a percentage of 30-50%.

The and 14.5”C. From ca. 2550

Ocotea, Ternstroemia, Sapium, the palm Geo-

Lauraceae, Rubiaceae, Araliaceae. This forest may reach a height of ca. 40 m, of herb-cover of 15-40% and a percentage of moss-cover average annual temperature in this zone is between 17°C

m upward to at most 3300 m (sometimes to 3100 m only) is the dominant tree, with W. pinnata still occurring up to ca. 2850 m (or possibly even 3300 m). Up to 2950 m Cyatheaceae (tree ferns) are common, above that elevation they are replaced by tree ferns of the genus Blechnum (section Lomaria).

Up to ca. 3050 m the forest is characterized by the presence of the palm Geonoma weberbaueri, and the bamboo Chusquea is usually present. The following genera or families are commonly represented: Miconia, Clusia, Clethra, Brunellia, Rapanea, Drimys, Weinmannia

rollottii

Ternstroemia, Fresiera, Ocotea, Geissanthus, Hedyosmum, Piper, Monnina, Ericaceae, Myrsinaceae, Melastomataceae,

Hieronima,

Araliaceae, Rubiaceae. This “Weinmannietum rollottii” cloud forest is 20 m to 35 m high, has a percentage of herb-cover of 5-35% and a percentage of mosscover of 89-90%.

The average annual temperature in this zone is between 14.5” and 9°C. Above the “Weinmannietum rollottii” zone, between ca. 3100-3300 m and ca. 3500-3600 m, lies a zone of low forest and high scrub where Ageratina tinifolium is in most cases the dominant species. The percentage of individuals belonging to the Compositae is very high, usually between ca.

60% and 100%. Hypericum is mostly present with a few percent coverage only, but occasionally the coverage may be as high as 20%. Other common taxa are: Baccharis, Senecio vaccinioides, Gynoxys, Diplostephium and Espeletia cf. curialensis, Cestrum, Ribes, Monnina, Hesperomeles, Tibouchina, Miconia, Bucquetia, while Aragoa lycopodioides, Rapanea dependens and cf. Oreopanax may occur. This “Ageratinetum tinifoliae” is 5-7 m high, has a percentage of herb-cover of 15-50% and a percentage of mosscover of normally 90-100%.

The average annual temperature is between ca.5”C and 9°C. Senecio vaccinioides may become very common to form pure stands especially in the higher parts. Going upward in this zone, open spaces between the low forest and shrub formation increase in number and in size. This zone, between 3100-3300 m and ca. 3500 m altitude, corresponds with Cleef’s subparamo (shrub piramo and bamboo dwarf shrub paramo) of the humid eastern slopes. Bamboos of the genera Chusquea and Swallenochloa occur, the first in the forest, whereas the latter abounds mainly in the open areas.

The zone between 3500 and ca. 3800 m and between ca. 3800 and ca.

4250 m is called by Cleef “lower bamboo paramo” and “upper bamboo bunch grass paramo”, and is respectively dominated

by Swallenochloa cf. tesselata and Swallenochloa-Calamagrostis (average annual temperature

251

between ca. 6°C and 3°C); many characteristic paramo herbs and Swallenochloa-Sphagnum bogs occur.

At the border between this belt and the next higher superparamo, a narrow belt of low bushes of Loricaria complanata may occur locally.

The superparamo belt occurs between ca. 4200-4300 m altitude and the limit of vegetation (at ca. 4800 m: nival belt). The average annual temperature is between 3°C and nearly 0°C. The vegetation cover is discontinuous here, owing to movements of the uppermost soil layer caused by the daily rhythmic process of freezing and thawing. In the lower part of the superparamo the following genera or species are common: Draba, Montia, Valeriana plantaginea, Arenaria and Cerastium, several Compositae (e.g. Senecio niveoaureus) and the fern Hymenophyllum trichophyllum.

On the slopes of the atmospherically drier western side much of the forest has disappeared.

Below 2800-2900 m, down to the very deep valley of the Chicamocha, proper stands of forest are scarce, whilst the vegetational aspect tends to become more and more xerophytic, so that it has become difficult to establish where the erstwhile lower forest limit lay. Between ca.28002900 m and ca. 3500 m remnants of the original montane forest still occur.

The dominant tree in this forest is Weinmannia fagaroides.

In one of the remnants of this “Weinmannietum fagaroides”, we found Ilex cf. hun thiana, Styrax and Prunus sp. to be very common, other genera or families represented being Clusia, Clethra, Rapanea, Rhamnus, Viburnum, Vallea, Psychotria, Cestrum, Xylosma, Araliaceae, Lauraceae and Melastomataceae.

The forest was 15 m high, the percentage herb-cover was 10% and the percentage of moss-cover 40%.

Between 3500 m and ca. 3850 m altitude lies the subpkamo, subdivided

by Cleef into a lower shrub paramo (till 3700 m) and a higher dwarf shrub piramo.

In this zone, and up to ca. 3750 m locally a low forest may be present in which Hesperomeles lanuginosa is the dominant species. This “Hesperomeletum lanuginosae” (sensu Cuatrecasas) attains a height of 6-8 m. Another type of low forest is the “Polylepietum quadrijugae” that may occur locally, and as high as 4000 m or even higher. Polylepis quadrijuga is the dominant species. Locally Polylepis forest may be found as a continuous zone near the altitudinal forest limit proper around 3500 m, and may contain, e.g., Weinmannia fagaroides.

It seems as if at such high altitudes the same may hold for the Hesperomeles forest.

In the shrub paramo, bushes of Ericaceae, Compositae and Melastomatacae are common, and locally dense thickets of Myrica paruifolia. These thickets may occur up to almost 3750 m. In the dwarf shrub piramo the vegetation is mainly dominated

by Arcytophyllum nitidum (Rubiaceae), the grass Calamagrostis effusa also being common (Cleef, 1979).

The grass paramo (the “proper paramo”), between ca. 3850 and ca.

4300 m altitude, is dominated

by Calamagrostis effusa and Espeletiopsis colombiana, while many typical p&am0 herbs abound (Bartsia, Jamesonia

252

Castilleja, Acaena cylindristachya, etc.) (Cleef, 1979).

In the lower superparamo there is again a narrow belt of the shrub Loricuria complanata, and thickets of other Compositae (Senecio, Diplostephium) may occur. Otherwise, the floristic composition of the superparamo of both sides is very much the same (Cleef, 1979).

It should be mentioned here, that the position of the limit between grass paramo and superparamo (often lying at ca. 4200-4300 m altitude) may be largely determined by soil conditions (presence of very young moraines).

Where this young moraine material is absent (e.g., in nunatak-like areas), grass piramo may be found locally up to at least 4500 m. We shall discuss this problem in greatfr detail in the next section. bogotensis,

The observed vegetational differences are doubtlessly caused by the differences in local climate. The main cause of the change in vegetation with altitude is, naturally, the temperature (i.e., the mean annual temperature: seasonal temperature fluctuations are small), but it is not so clear yet precisely which factors determine the differences observed between the eastern and western slopes of the Sierra. Doubtlessly the differences in humidity and in the amount of precipitation constitute one of the factors.

Although no exact figures are known, a comparison with similar situations elsewhere permits the conclusion that the annual rainfall at ca. 3000 m altitude is ca. 1000 mm on the west side (see Fig.l), and more than 2000 mm on the east side. There is a marked seasonality on the west side, with one well-pronounced and one indistinct drier season; on the east side there is only one, and not very distinct drier season. The average relative atmospheric humidity on the east side may be between 70 and 100% during most of the day throughout most of the year, whereas on the west side it may be considerably less during many days of the year. The eastern slopes are mostly clouded and misty, the western slopes have more clear days.

On the clouded side the temperatures will remain relatively low but even and with little diurnal extremes. On the less clouded side the temperatures tend to fluctuate appreciably, with more frequent and greater diurnal extremes, especially during the dry season. This could possibly lead to the incidence of night frosts at the west side at lower altitudes than at the east side. A probably important aspect is the influence of warm ascending aircurrents from the precipitous and deep Chicamocha Valley to the west of the Sierra whose bottom reaches the lower Subandean and upper tropical belt, and contains much open terrain with xerophytic vegetation. This might cause higher mean annual temperatures on the west side, and keep the night temperatures at a higher level than would correspond with the effect of clear nights.

In many places of the northern Andes the altitudinal forest limit (and partly the vegetation belts) lie somewhat higher on the exposed, wetter outer slopes than on the drier, inner slopes (see, e.g., Van der Hammen, 1960, 1968 and 1974; Lauer, 1979). This seems to be best explained by

253

EL COCUY La! r:t;’ :,’ ; t ,

- Boy& p;

mrr 150

,“‘/.I’

,*<I “,

CH ITA

- Boya&

![I+ Or I,,’ ..,! ? ( 1

! II’.]

‘-

iCi.G’

10

mrr 130,

130

I

I

90 80 70.

‘Fig.1. Average monthly m) and Chita (alt. 3005

‘~

precipitation and yearly mean at the villages El Cocuy m), on the western side of the Sierra Nevada de1 Cocuy.

(alt.

2749

the above-mentioned effect of almost unremitting cloudiness and mist, which prevents extreme temperature fluctuations.

In the Sierra Nevada de1 Cocuy, however, the reverse seems to be the case: there is a higher altitudinal forest limit and the boundaries of vegetation belts ascend to greater altitudes in the dry west than they do in the wet east (Cleef, 1979).

Conceivably this apparent exception is attributable to the effect of ascending warm air currents from the Chicamocha Valley. A possible additional circumstance is that forest regeneration in an extremely humid area, with dense bamboo growth and, in open sites perhaps rapidly extending Sphagnum bogs, is seriously hampered or altogether delayed. ‘These problems can certainly not be more definitely resolved until intensive meteorological and microclimatological studies have been completed.

Such studies will form a part of the recently started long-term Ecoandes project.

Some plant taxa identified and probably of some special tion of the vegetation belts.

in connection with palynological inquiries importance were omitted from the descrip-

254

One of these is the shrubDodonaea uiscosa (Sapindaceae), of very common occurrence in xerophytic vegetation and on eroded soils. It is frequent between 2000 and 3000 m altitude and was recorded as relatively frequent at altitudes up to 3400 m on the western slopes of the Sierra Nevada de1 Cocuy, in somewhat degraded open vegetation, and here associated with, e.g., Espeletiopsis jimenez-quesadae and Buddleja lindenii. Pollen of Dodonaea apparently easily becomes air-borne and is found frequently in certain significant intervals of the pollen diagrams (see below). A relatively high percentage (up to more than 35% of the pollen total, but with a very low pollen density) was found in the very recent sediment of a small lake in the superparamo of the Sierra at 4220 m altitude (Gonzalez et al., 1965). The species is very common on the slopes OT the Chicamocha Valley (between 2000 and 3000 m), and the high pollen values found in the superparamo seem to substantiate the importance of ascending air-currents (see above).

Urticaceae are common in the herbaceous undergrowth of the montane forest, especially in the more humid type, but representatives of the genus Pilea have been recorded at altitudes up to 3650 m and Urtica ballotaefolia has been found in the open paramo, between rocks, up to 3950 m. The occurrence of the latter species seems to be related to animal dunging.

Another species found recently in the paramo is Purietaria debilis (in rock shelters).

Alnus jorullensis occurs often along streams and in wet peaty soil, and was observed in the Cocuy up to ca. 3550 m.

Quercus was not encountered in the Cocuy area; it is, however, common in the more western part of the Eastern Cordillera, at the other side of the Chicamocha Valley. Hedyosmum is a common tree, particularly in the more humid montane forests; it seems to have pioneer qualities, and to increase in number when more light is available.

Plantago rigida occurs between 3000 and 4300 m altitude, but between 3400 and 4250 m it is found in cushion communities and bogs (mainly in the grass piramo). Such bogs are found in all grass paramos, but an optimum of development is only attained in the most humid piramos (Cleef, 1978).

Distichiu muscoides is another cushion plant participating in bog formation, mainly in the lower superparamo (Cleef, 1978).

Isoetes is very frequent in lakes and bogs of the p&.ramo. In lakes it may be the dominant plant in aquatic vegetation.

Grabandt (1980) studied the relation between forest vegetation and pollen rain in the Eastern Cordillera. She calculated the tree/pollen relation for the most common tree-genera. These t/p values may contribute towards a more correct “translation” of the pollen diagrams into terms of stands of vegetation. In a transect from Piramo de Chita through the Cusiana Valley to the Llanos Orientales, she found a reasonable correlation between the local stands of vegetation and the pollen rain. Such Subandean anemophilous species as Alchornea, Acalypha and Cecropia disappear at the limit

255

of the Subandean and Andean forest belts. Small quantities of their pollen are, however, found higher up the slopes, and the quantities tend to rise again in open paramo vegetation. There is a very close relation between the occurrence of stands of Weinmannia and the presence of its pollen; Weinmannia is insect-pollinated, and, generally speaking, under-represented in the pollen rain. Quercus and Alnus are anemophilous and higher percentages of their pollen in the pollen deposits correspond neatly with the presence of stands of trees, but they are represented by small numbers (as a “background” fallout) almost everywhere.

High percentages of pollen of Compositae were mainly found in the paramo, especially in the subphramo, where shrubby representatives of this family abound.

VEGETATIONAL

AND

ENVIRONMENTAL

HISTORY

The seven pollen diagrams from the Sierra Nevada de1 Cocuy published and described in Gonzalez et al. (1965) together cover the last 12,500 years. Nothing was known up to now of the earlier vegetational history of the area. In 1967 F. Lucas, then a member of our project-team, collected lake sediments in the central-western part of the Sierra, the most important being several sections taken with the Dachnowski sampler in a filled-up lake at ca. 3500 m altitude, on the drier western side. This lake, called Laguna Ciega, is situated 4 km NE of the village of Guican (B3 on the map, Fig.18) in the area of Drift 3 (see below). All previously published pollen diagrams were from lakes in the area of the younger Drifts 45 and 6. One of the sections was taken in the NE central part of the former lake-area (Laguna Ciega I, 10.75 m deep; Fig.2) and the other more in the SW part (Laguna Ciega III, 8.75 m deep; Fig.3).

One sample from section I and two from section III were dated in the Groningen 14C Laboratory (see Figs.2 and 3).

Sample

No.

Col. 200a Cal. 201 Cal. 202

Laboratory GrN 7 065 GrN 7066 Gt-N 6907

No.

Section

and depth

L. Ciega I, 56*570 cm L. Ciega III, 367.5-378 cm L. Ciega III, 652-674 cm

I4 C Age 14,140i 120 12,830*80 20,840+ 140

B.P. B.P. B.P.

These dates make it clear already that the infill of Laguna Ciega represents a considerable time interval, presumably the last 25,000 years. Therefore, first the two pollen diagrams from Laguna Ciega will be discussed and subsequently an overview of the already published data, augmented by some new evidence, will be given.

The samples for palynological analyses were taken at an average vertical distance of 12.5 cm. In the pollen total both the pollen of trees and shrubs (including Compositae and Polylepis) and that of grasses (Gramineae) is included. As far as possible a total of about 300 pollen grains was counted (the sum is indicated in the diagram per sample). The general diagram is of

256

the Iversen-type, the width representing the total of pollen-total-elements, cumulatively from left to right: trees and shrubs, Compositae, Gramineae, and Polylepis. Some of the separate curves of trees are indicated within the corresponding area of the diagram, not cumulatively but all plotted from the left axis to the right. At the right of the general diagram the separate pollen curves are drawn, first of the elements included in the pollen total, subsequently those not included in that total but calculated in ratio to it.

At the left of the general diagram the lithological column, the depth scale, and the local zone numbers used here for the description of the diagram are indicated.

The zones are briefly described here, by referring to the two diagrams (Figs.2 and 3; the zone boundaries are indicated, the zone numbers are at the extreme left). Zone numbers are preceded by a C to indicate that they are local zones from Laguna Ciega only.

Not far from the site there are remnants of forest, extending even up to 3600 m altitude, so that originally the altitudinal forest limit must have been present close to the site. We may consider its position at the boundary of Andean forest and subparamo. The local vegetation type of the Laguna Ciega site is marshy and open, locally with stands of Espeletiu. A rivulet traverses the marsh.

Zone Cl The base of the two sections is formed by coarse material of presumably glacial origin. Lake sedimentation in the deepest part (Ciega I) must have started immediately after the glaciers of Drift 3 had begun to retire (see below). Somewhat later the lake level rose, the open water covering also the area where Ciega III was taken. Grass (Gramineae) pollen is completely dominating in the diagrams, while the presence of Portulacaceae (Montiu), Cruciferae (probably Druba) and Malvaceae (Acaulimuluu sp.) indicate that elements of the upper grass p&ram0 or the lower part of the superparamo prevailed. In Ciega I, a slight rise of forest elements (Alnus, Quercus) occurred in the lower part of zone Cl, possibly as a reflection of a distant minor upward shift of the forest limit. This phenomenon is not present in Ciega III where sedimentation apparently started after this fluctuation.

In the shallower part of the lake, Isoetes apparently formed close stands on the bottom. At the top of the zone, these were gradually replaced by the next zone of the hygroseries dominated by Cyperaceae (possibly Eleocharis mucrostuchya or Curex ucutatu: A. M. Cleef, pers. comm.).

Extrapolation of l4 C dates yields a possible age of ca. 27,000 years B.P. for the base of zone Cl; however, sedimentation probably proceeded much faster in the lower part of the section and we have to consider this date to represent a maximum value, and the real age might be one or a few millenia younger. We will discuss this problem later on.

Zone Cl probably corresponds with zone W-III (and possibly W-II, inclu-

O

tO

N

E

c

O

--

t-

r~

JE:

-5

o_

_9 °

n

L. C I E G A r" (3.

AIt,

3510

m

I

pp. 2 5 7 - - 2 6 4 .

absl.

8 W

E

P

D u~ Cw

E~

~ Do

<u9

~

:E Z ~ ~)>-

=~-z OO~

u-~ o~:

u ueu_~-

c 20

30

40

50

60

70

8 0 90 lOOM°

.~J

<

v

-¢ L)-~ ~.1 =

Fw

.,{ uj , L)

-

=~ 10

w

,

~ w<

~

-;

Z~'O

~o~ u-O-L)

L)

w

-

uw

L.w w ~{ o

~J>. w

>. ,

~

>-

~,,, , w~:E:E

>~DD

z'~>FO-J

~:=u OL)

~

=-;:~

,

ululo

ro ~ Q : Z

L) >~L D-J :E ~ C= LAB#ATAE 0 . 3 ,

CORIARIA 0,3

5O

100

150.

C8 200 •

250

300 ANACAR DIACEAETO 3

iC7 350.

PORTULACACEAE

PANOPSIS

/C6

0.3

0.3, S A X I F R A G A C E A E 0 3 , RUBIACEAE 0.3

PORTULACACEAE

0,3

450-

[5

~ !

500-

AMARANTHACEAE

0.3

AMARANTHACEAE

0.3

550-

~2;g~ ~

COL

GAIADENDRON

600-

0.3

ANACARD~ACEAET0.3

I 650-

C3 700-

ANACARDIACEAE

T 0.6

750

500-

C2 850 ANACARDIACEAE T 0.3

900.

A N A C A R D I A C E A E T 0.3 ANACAR DIACEAE TO,3

950

A N A C A R D I A C E A E TO.3

C1 I000 TILIACEAE

1050

0 3

ANACARDIACEAE7[O.3, PORTULACACEAE 0 3

)O

90 \ 8 0

TREES AND

70

60

50 ~40 . "

30

20~ 10

0%

COMPOGRAMINEAEACAENA/ SITAE FOLYLEPIS

I,i

.

.

.

.

.

.

.

.

' I ' Included

' I I I in t h e s u m

A N A C A R D I A C E A E T0.3 0

0i 10 20 30L4140 150 ~ 70 8Oi 9Oi 1010O/o SCALE FOR A L L CURVES UNLESS OTHERWISE INDICATED

SHRUBS

PEAT CLAY SAND

i[3 •

ALNUS PODOCARPUS

x SUM OF OTHER FOREST E L E M E N T S

Fig.2. Pollen diagram o f section Laguna Ciega I (Sierra Nevada del C o c u y ) . Alt. ca. 3 5 1 0 m.

20

40

60

80

100 "Is

0

100

200

300

400

500

600 %

pp. 265--272.

CIEGA

L. ALE 1

ca. 3 5 1 0 m

r~J

~ 0

-~

.~

r~lD

-

G-

abs{.

>, .~

P

'T

E

0-

O~

n -

]]]

D u3

u~ fl

,:,.', _~

~) =~

~ 0

10

20

30 40

~

GO 70 ~

I-:l,LI

o

o

go~oo°6 r,

WW 0

"J "J Z OOw ZZ

oo~ I[ IE :~

wo

, u< S ~

W~O0

~88

<.o< z,~ 01~1o ~ i.- LIJ ~WL.5

.~_J~

50

SYM PLOCOS

0.6

CLUSIA 0.3 ~00-

C8 M ALVACEAE 0.3

150

VALLEA 0.3, RUBIACEAE 1.0 BORRERIA

1,0

200

JUSSIAEA 0.3 250-

C6

POLYGALACEAE 0.3

300

i

CS GFN-7065 12830±8{

350-

I COL 201

POLYGALACEAE 0.3

400-

C4 MYRIOPHYLLUM

0.7

450-

500A N A C A R D I A C E A E TO.3

1~0.3 .................

DRYMIS

0.5

CROTON 0 . 3 RUBIACEAE

0.3

ROSACEAE 0.3

ZX3/"9 0 80 70 GO 50 40 30 20 )0 Oi

TREES AND

COMPOGRAM INEAE ACAENA/ SITAE ~LYLEPIS

" INCLUDED IN

THE SUM

0

40 50

10

20 30

60

0 70

80

90 IO0°A

SCALE FOR ALL CURVES UNLESS OTHERWISE INDICATED

•

ONE POLLENGRAIN

FOUND

SHRUBS SPHAGNUM PEAT

D

ALNUS

CLAY

•

QUERCUS

GRAVEL

x

SUM OF OTflER FOREST ELEMENTS

Fig.3. Pollen diagram of section Laguna Ciega III (Sierra Nevada del Cocuy). Alt. ca. 3510 m.

100

2~

~0

~

~0

~0

N O MeG

100

2~

3~

400

~

°1,

273 ding the top of W-I) of the section Ffiquene (Van Geel and Van der Hammen, 1973); the base of zone W-II (top of zone W-I) was calculated, by extrapolation of ~4C dates, to have a possible age of 24,500 years B.P.

Zone C2 This zone shows a remarkable decrease of Gramineae and an increase of Compositae pollen. At the same time there is an increase of the pollen of certain trees and shrubs: Alnus~ Quercus, Myrica, Rapanea, Melastomataceae, Dodonaea (and some Acalypha), to a total m a x i m u m of representation of more than 30% (in Ciega III). The pollen of the same group of genera is f o u n d to increase also in later interstadials of Laguna Ciega, but also of the P~ramo de Guantiva (Van der Hammen and Gonzalez, 1965) and FOquene (Van Geel and Van der Hammen, 1973). They are apparently the first to appear on the virgin p~ramo soils left after a stadial. Dodonaea, a pioneer on eroded soils, is an especially interesting case. There seems to be no d o u b t that this percentage of arboreal pollen does n o t indicate the presence of Andean forest in situ but simply indicates an upward shift of the forest limit; even pollen of the Subandean anemophilous Acalypha increases slightly. It is probable, however, t h a t the zoophilous Melastomataceae grew in close proximity of the lake. Together with the increase of Compositae and a decrease of grass pollen this indicates correspondence with the subp~ramo belt. The manifest m a x i m u m of Polylepsis indicates that this tree was growing near the Ciega III site; this also points in the same direction.

The pollen of a number of entomophilous p ~ a m o plants (Gentianaceae, Caryophyllaceae, some Plantago and Umbelliferae) increases. They may have grown on the dry ground around the lake, or in the hygrosere. High maxima of cyperaceous pollen, short-lived maxima of the alga Botryococcus and the local increase of spores of Mougeotia (Zygnemataceae; see Van Geel and Van der Hammen, 1978), all point to the fact that the water was shallower than it was before, and that the stands of Cyperaceae vegetation extended to the site of Ciega III. The upper boundary of zone C2 was directly 14C dated in section Ciega III as 20,840 + 1 4 0 B.P. (see above and also Fig.3). This means that this boundary corresponds with that between zones W-IV and W-V of Laguna de Ffiquene, directly 14C dated as 20,575 + 190 B.P. (Van Geel and Van der Hammen, 1973), and that the zone C2 must coincide with the Saravita interval.

Zone C3 In this zone the arboreal pollen percentage is very low again. Upon the whole grass pollen dominates; some intervals show rather high Compositae values, but this is principally the case in Ciega III, where it may have been caused by the local presence of a marshy Compositae scrub. Gentianaceae,

274 Cruciferae and some other p~ramo herbs are relatively frequent and there can be but little d o u b t that the area lay within the upper grass p~ramo belt (or in the lowermost part of the superp~ramo).

In Ciega III the succession from open water with Isoetes to Cyperaceae marsh seems to have continued, and probably culminated in a marshy shrub vegetation of Diplostephium revolutum (Compositae), locally with Hydrocotyle. Botryococcus is poorly represented, and Mougeotia occurs. In the upper part of zone C3, towards the boundary with C4, the Compositae decrease, Botryococcus increases again, and the Cyperaceae show a maximum.

This suggests that primarily the local scrub disappeared and subsequently there was open water again, and some extension of the Cyperaceae marsh.

At the same time the deposition of peaty clay was replaced by that of pure clay. In Ciega I, where the lake was deeper, this development is also reflected in the pollen diagram but, as grey clay was deposited during nearly the whole interval C3, it m a y be that there was standing water most of the time. However, the very prominent peak of Lycopodiurn spores, and the maxima of Mougeotia suggest that some vegetation may have been present on the spot, during some of the intervals and that the lake may temporarily have fallen dry.

Zone C3 is very well dated by two 14C dates. One is at the base and was already mentioned above (20,840 + 140); the other is at the top: 14,140 + 120 B.P. (see above and Fig.2). Zone C3 would, therefore, correspond with the Ffiquene stadial (zone W-V of Laguna de Fdquene; Van Geel and Van der Hammen, 1973).

Zone C4 There is again an increase of the pollen of some anemophilous trees (Alnus, Quercus) and of the shrub Dodonaea viscosa, indicating a temporary amelioration of the climate (see above). At the same time the pollen ratios of Compositae increase considerably at the cost of t h a t of grasses (Gramineae).

There is an increase of Gentianaceae pollen, and a slight increase of pollen of Caryophyllaceae, Plantago, and (at the top) of the Chenopodiaceae-Amaranthaceae type. Apparently the site once more lay within the subp~ramo zone. Botryococcus reappeared (it is abundant in the lower part, where also a m a x i m u m of Cyperaceae is noticeable). As this might indicate a rise of the lake level, this was apparently counteracted by the more definite filling-up of the lake, that t o o k place at the sites of both sections during the second half of this interval. From then on, the possible incidence of pronounced local influences must be taken into account.

Zone C4 is well dated, on the one hand by the above-mentioned date from the top part of zone C3 (14,140 B.P.) and on the other hand by a 14C date in the basal part of zone 5 : 1 2 , 8 4 0 + 80 years B.P. (see above and also Fig.2).

Hence, zone C4 may safely be dated as between ca. 14,000 B.P. and 13,000 B.P., and coincides with the Susac~ interstadial (Van der Hammen and Gonzalez, 1965; see below).

275

Zone C5 The pollen of Compositae dominates the picture in this zone, in both diagrams, and the percentage of arboreal pollen becomes very low again.

There is a slight rise or minor m a x i m u m of grass pollen. The site still seems to have lain in the subp~ramo, but at a higher level than during zone C4; it is, however, difficult to establish whether Compositae (including Espeletia lopezii) growing in situ in the marsh contributed towards the pollen rain; if so, the site may also have lain within the grass p~ramo belt.

Zone C5 corresponds with the cold phase between the Susadt and Guantiva interstadials (Van der Hammen and Gonzalez, 1965). This cold phase was dated in the Pfiramo de Guantiva as 12,770 + 130 B.P. (ibid). The lower part of zone C5 was dated as 12,830 + 80 B.P. (see above and Fig.3). If datings from other localities are also taken into account, zone C5 may tentatively be dated as between 13,000 B.P. and 12,400 B.P. (ibid.; Van der Hammen and Vogel, 1966).

Zone C6 This zone shows a very clear rise and two maxima of the total arboreal pollen, up to 40--50%. Here again, the maxima are of Alnus, Quercus, Melastomataceae, Acalypha and Polylepis, but in this zone also of Podocarpus.

Gentianaceae and Urticaceae pollen becomes more frequent, and the Compositae pollen somewhat scarcer. This might be associated with wetter conditions in the bog, so that any conceivably present Compositae shrub became replaced by stands of Cyperaceae, cyperaceous pollen becoming more abundant, especially in Ciega III. The reappearance of Mougeotia also suggests temporarily higher water levels.

The site lay within the subpfiramo belt, but certainly nearer to the forest limit than ever before during the lacustrine deposition, possibly in the zone of Polylepis that may have been present immediately above the Andean forest proper. However, it is clear that no Weinmannia forest was present around the lake.

Zone 6 corresponds without reasonable d o u b t with the Guantiva interstadial. The diagram from the type site (Cienaga del Visitador, P~ramo de Guantiva; Van der H a m m e n and Gonzalez, 1965) is very similar to that of zones C6 to C8. It was dated there by interpolation between 14C dates (the above-mentioned 12,770 -+ 130 B.P., below and 9830 + 140 B.P. above) as ca. 12,400 B.P. to ca. 10,900 B.P. The Guantiva interstadial could also be recognised in the diagram Laguna de Ffiquene, where the upper limit could be directly 14C dated as 10,820 -+ 60 B.P. (Van Geel and Van der Hammen, 1973). The climatic amelioration apparently ushering in the Guantiva interstadial could also be dated in Valle de Lagunillas in the same Sierra Nevada del Cocuy, as very shortly before 12,320 + 100 B.P. (Gonzalez et al., 1965).

276

Zone C7 and C8 Immediately after zone C6, there is a marked rise of Gramineae and Compositae pollen, and a strong decrease of arboreal pollen, concomitant with a minor but not very marked rise of Caryophyllaceae and Cruciferae.

The site probably again lay within the lower grass p£ramo belt. A little later the first and manifest increase of Plantago (possibly the cushion plant P. rigida, that may form a very peculiar type of " b o g " ) t o o k place. At this rise we place the boundary between the zones C7 and C8 which is otherwise obscure.

The Gramineae values remain high at the beginning of zone C8 to become lower only in the higher part of this zone, at least in Ciega I. In the same diagram the arboreal pol]~en increases gradually at the beginning of zone C8 (especially t h a t of Alnus, which attains values of ca. 30%). The marked rise of Hymenophyllum spores and Isoetes spores in the beginning of zone C8 is striking.

Zone C7 presumably corresponds with the E1 Abra stadial, and zone C8 with most of the Holocene. The E1 Abra stadial is a very manifest colder interval in pollen diagrams of, e.g., Ffiquene (Van Geel and Van der Hammen, 1973) and the Sabana de Bogot~ (Schreve-Brinkman, 1978). It was reasonably well dated as beginning at ca. 10,900 B.P. and, as calculated by interpolation, terminating somewhere between 10,000 and 9500 B.P. The diagram of Cienaga del Visitador mentioned above is very similar to the upper part of the Ciega diagrams, and exhibits but little differences between the equivalent of the E1 Abra stadial and the Holocene. This was explained (Van der Hammen and Gonzalez, 1965) by an increasingly drier climate during the Holocene as compared to the Late Glacial. A 14C dating from immediately below the rise of Plantago in that diagram yielded 9830 + 140 B.P. The very characteristic course of the Plantago curve can also be noticed in other diagrams of the Cocuy area (e.g., in VL-III and VL-V in Gonzalez et al., 1965); the rise of that curve was directly ~4C dated in the section VL-III as having taken place at 10,030 + 90 B.P. Fig.19 shows a simplified representation and the correlation of the main pollen diagrams of the sections Ciega I and Ciega III. After this description of the" Ciega diagrams, all other available information, published or unpublished, relating to the vegetational history of the last 12,500 years in the area, will be briefly discussed. Most of the relevant information m a y be f o u n d in Gonzalez et al. (1965), already cited. The most important diagrams are VaUe de Lagunillas III and V, at 4000 m and 3925 m altitude, respectively. In VL-V (a sequence of sediments in the Drift 4 area, just outside the large Drift 5 end moraine; see below), there is a basal layer of varved clay and sand, deposited shortly before 12,320 + 100 B.P. (GrN 3247) and with the same pollen composition (viz., low density, arboreal pollen dominating, high Dodonaea percentage) as the short section VL-X, in the very y o u n g Drift 6 area (see below), in the superp~ramo at 4220 m

277 and practically devoid of vegetation. Accordingly, at ca. 3900 m there was at that time practically no vegetation in the part of Valle de Lagunillas recently vacated by the glaciers. At 12,320 + 100 B.P., however, there was clearly an amelioration of the environmental conditions so that grasses became dominating elements. As Cruciferae (cf. Draba) are also well represented, we believe that the area lay within the lower part of the superp~ramo belt. Soon afterwards (at ca. 12,300 B.P.) a rise of PolyIepis, of other arboreal pollen (Alnus, Quercus, Podocarpus and even Weinmannia) and of Dodonaea t o o k place. The site probably lay within the uppermost part of the grass p~ramo belt. At approximately 12,000 B.P. (between the 14C dates 12,140 + 120, GrN 4002, and 11,900 -+ 120, GrN 4036) the pollen of Gramineae increased again at the expense of that of arborescent species, Cruciferae also being represented again. Shortly thereafter, around 11,900 B.P., an increase of arboreal pollen began again, but the lake definitely turned into a peat bog, and the pollen content is from then on mainly determined by the local bog vegetation (of grasses and Compositae), which obscures the regional picture. What is reflected between ca. 11,900 and ca. 12,350 B.P. in the pollen diagram, is apparently the early (and middle) part of the Guantiva interstadial (Van der Hammen and Gonzalez, 1965): in the type area (and in Laguna Ciega) this interstadial shows a cooler climatic interruption in the middle.

Some most informative local phenomena (in section VL-V) are worthy of being mentioned. In the lower part of the section, as soon as the contact with the glacier became lost (and the varve deposition ceased), Distichia muscoides, a cushion bog plant, became very abundant, judging by its pollen representation. There are also some thin peaty layers intercalated in the grey lake clay apparently to some extent related to the maxima of Distichia and to some very sudden peaks of the Lysipomia curve (formerly taken for "Valeriana stenophylla"; see Van der Hammen and Cleef, 1978). This vegetation type disappears around 11,000 B.P., when (between ca. 11,000 and 10,000 B.P.) the glacier was probably standing nearby, and sand and gravel became deposited in the bog at irregular intervals. Shortly after 10,000 B.P. (by the beginning of the Holocene), however, apparently another cushion bog plant, Plantago rigida, suddenly extended in the bog (see also above). Plantago rigida cushion bogs abound in the upper grass pfiramo, whereas Distichia cushion bogs abound in the lower superpfiramo, although there is of course some transgression of their limits (Cleef, 1978). This provides a nice confirmation of the conclusion that the Valle de Lagunillas site lay within the lower superp~ramo (or uppermost grass pfiramo) belt from ca. 12,320 B.P. to at least 11,000 B.P., and in the upper grass pfiramo from ca. 10,000 B.P. onward.

In Valle de Lagunillas III (alt. 4000 m) the Holocene is developed in a lacustrine facies. It is possible to discern some of the more important zone boundaries of the Andean pollen zones. Zone I and II (the Guantiva interstadial) show higher percentages of forest elements (Alnus, Podocarpus,

278

Quercus, Polylepis), and there is an increase of grass pollen in the middle.

Zone III (the E1 Abra stadial) contains a higher percentage of grass pollen; the fall of Podocarpus at the II/III boundary is characteristic. Hedyosmum pollen is relatively abundant in I I and III, Urticaceae become abundant during II, and the forest elements increase again in the Holocene. Urticaceae decrease and Hedyosmum increases at the b o u n d a r y VI/VII. Plantago shows higher percentages from zone IV onwards to zone VII inclusive. Zone VII shows the highest percentages of tree pollen of the Holocene, and zone VIII a considerable increase of grass pollen and a decrease of arboreal pollen.

Plantago and Caryophyllaceae are poorly represented in zone VIII. These Andean pollen zones were dated elsewhere in Colombia; approximate dates are: 10,000 B.P. for the" boundary III/IV, 5000 B.P. for the boundary VI/ VII, and 3000 B.P. for the boundary VII/VIII. In general terms we may say t h a t a higher forest limit during the Guantiva interstadial was followed by a lowering of the forest limit during the cooler E1 Abra stadial; afterwards there is a gradual upward shift of the forest limit, until some 3000 years ago when it shifted downward and the climate apparently became cooler.

As to the local development of vegetation in the lake, Isoetes abounded till within zone III, and did n o t return before zone VIII. In the meantime (ca. 10,000--3000 B.P.), Cyperaceae, Lycopodium (foveolate spores) and in parts Compositae were well represented, and some Plantago occurred. Probably the lake level was lower during that period, so that a broad marshy zone could develop around the lake. This may have been caused mainly by the higher temperatures and c o n c o m i t a n t higher evaporation.

A new diagram (Fig.4) from Rio Corralitos (Corralitos I, in the SW part of block A3 on the map Fig.18) from a section taken at an elevation of ca.

3860 m, in the area of Drift 5 came to hand. It corresponds with 250 cm of peat on sandy clay. There is at first a slight but significant rise of Weinmannia pollen, then of Alnus pollen in the middle, but otherwise it is dominated by grass and composite pollen all the way. Plantago pollen is present from the b o t t o m upwards, but shows a steep fall in the upper part. The appearance of Dodonaea in the uppermost samples doubtlessly indicates the influence of man on the stands of vegetation farther down the slopes. The diagram represents a major part of the Holocene, and apparently shows the influence of the well-known cooling after ca.3000 B.P. Up to now, we have only discussed pollen diagrams from the actual p~ramo region in the Sierra Nevada del Cocuy. A diagram from well below the forest line on the wet eastern slopes south of the Sierra is now available.

It is from the area of the Lagunas de Ocubi, at ca. 2800 m elevation (in the extreme SW of block B7 on Fig.18). The lakes are situated in a broad, open marshy plain surrounded by forest of the "Weinmannietum rollottii" community. The diagram is from the marshy plain and is of considerable importance for the understanding of the influx of tree pollen in the p~ramo zones.

The diagram (Fig.5) is based on 525 cm of sediment: below ca. 330 cm are

279 lake sediments, above t h a t depth peat. The local influence of Gramineae and Compositae in the peat is so dominant, that a diagram based on the sum of arboreal pollen only had also to be made (Fig.6).

The lower part of the deposit of lake sediments shows a higher pollen percentage of grasses and Compositae, up to some 80% and up to 6% Polylepis pollen and Isoetes spores. All this indicates that at that time the area of Ocubi was in the p~ramo belt (zone Oc.1). After a short temporary increase of the pollen of Weinmannia, Alchornea, Cecropia, Acalypha, Miconia and Urticaceae, and a sharp increase of Botryococcus and Isoetes, apparently representing an amelioration of the climate and a rise of lake level, respectively (zone Oc.2), the above-mentioned arborescent species all decrease strongly or disappear altogether and there is a very marked rise of Alnus, Quercus, Podocarpus, Hedyosmum and Myrica (zone Oc.3). Somewhat later, Quercus and Podocarpus decreased, and Myrica, Hedyosmum, Ericaceae and the fern Jamesonia increased (zone Oc.4). There is a strong increase of the pollen of Alnus and a decrease of all other arboreal elements, followed by a sharp fall of Alnus and increase of Compositae, Gentiana and Lycopodium (fov.) (Oc.5). In this zone the open water changed into marsh.

There is a rise of the pollen of Miconia, Urticaceae, Cecropia, Alchornea and Weinmannia at the beginning of the next zone (Oc.6). At the beginning of zone Oc.7, Cecropia disappears again and the percentage of Urticaceae pollen is low in this zone, Weinmannia slightly lower, and Ilex pollen relatively abundant. The sediment in this zone contains layers of sand and even pebbles. All this seems to indicate somewhat cooler conditions. Zone Oc.8 shows a considerable rise of the pollen of Weinmannia, Cecropia, Miconia and Urticaceae, and Alchornea and Clusia reach rather high values in the upper part. It seems t h a t the highest thermal conditions prevailed at that time interval. Zone Oc.9 shows a slight rise of Myrica pollen, whilst Cecropia pollen is disappearing and Alchornea becomes lower again; the Urticaceae curve falls steeply and Hypericum increases. This seems to indicate a slight cooling of the climate.

Although there are no 14C dates of section Ocubi, there can be little d o u b t that the boundary between Oc.5 and Oc.6 represents the beginning of the Holocene, and that Oc.2-~Oc.5 represent the Late Glacial (sensu lato; Van der Hammen and Vogel, 1966) and probably include the Guantiva interstadial and E1 Abra stadial. The arboreal pollen content of this interval very much resembles that of the Guantiva interstadial in the pfiramo, especially that of the zones Oc.3 and Oc.4 (Alnus, Quercus, Podocarpus, Hedyosmum, Myrica, etc.). It seems as if these anemophilous species were the widespread pioneers of the re-afforestation of the pfiramos of glacial times, the pollen of which could easily have been blown upwards by ascending air-currents, and give a clear reflection of this ascending pioneer forest in the pollen diagrams of the pfiramo lakes.

In Fig.20 the interpretation of the diagrams of the Cocuy area is summarized and compared with the sequence in other parts of the Cordillera Oriental, and with the sequence of glacial events.

280 THE GLACIAL SEQUENCE An aerial photographic map of the glacial morphology of a major part of the Sierra Nevada del Cocuy (Fig.18) was prepared by the last two authors, making use of the earlier work of Gonzalez et al. (1965). The aerial photographs used for mapping were obtained from the Instituto Geogr~fico "Agustin Codazzi" in Bogota. The map corresponds to three runs of aerial photographs, taken from a flying height of 30,000 ft. Run 80 (numbers 8057--8072) was taken on January 6, 1959, run 41 (numbers 4100--4119) on January 26, 1955 and run 62/63 (numbers 6294--6309) on February 8, 1959. The principal point of each photograph is indicated on the map (+), with the corresponding number. At the time of preparation of the map, the only existing topographical maps were from the southern part of the Sierra (1:25,000, Inst. Geogr. "Agustin Codazzi", 1963). The Intemational Institute for Aerial Survey and Earth Sciences (I.T.C.) in Ensched~ (The Netherlands) made up at our request a slotted templet of the area, which was used for drawing the drainage system. This system forms the basis in respect of which the p h o t o interpretation was adjusted. The entire area is now covered by the topographic maps 1:100,000 numbers 137 and 153 (Inst. Geogr. "Agustin Codazzi", 1973).

The original map was made on the scale of the photographs (i.e., approximately 1:40,000); later it was photographically reduced to the approximate scale of 1:80,000. It covers an area of 70 km long (north--south) and 18 km broad, with a surface of about 1250 k m : . The highest point in the area studied is the Ritacuba, 5490 m altitude (map square A3); the lowest point is in the valley of the Rio Casanare, ca. 1300 m altitude (map square A7).

The central chain is covered with a nearly continuous chain of ice caps, and forms the divide between the Magdalena and Orinoco drainage systems. This central chain is formed by one of several sandstone layers, with intercalations of shales, of Cretaceous age, striking north--south and dipping westward.

At least during the Last Glacial period, the glaciers in the area had a much greater extension. The origin of the ice cap was the highest part of the mountain ridge, as it is today, but the source area was greatly extended during m a x i m u m glaciation. The landscape was transformed both by glacial erosion and by accumulation. The most obvious glacial features are visible around the area of perennial snow. Erosional forms are cirques, transfluence passes, glacial valleys, homs, ar~tes, valley steps, roches moutonndes, truncated spurs and plucked rock faces. To avoid a for our purpose unduly complicated map, only the first three types have been drawn in. Glacial deposits that occur are regression moraines, till, and fluvioglacial deposits.

Farther down, older glacial features become less pronounced, effected as they are by later denudation and erosion. The glacial valley walls have been covered by more recent talus slopes, and in the regions covered by till, landslides and other mass movements frequently occur. The moraine ramparts have been dissected by gullies. The watersheds farther away from

C 0 R RALITO

S

E

E O N

>-

o

~

:]C

O_

pp. 281--286.

0CUBI

2800 rn a.b.s.I.

o,

~

% 10 20 30 4D 50 60 70 80 90 100

o (])

A.P.

ALT.

ca.

3860

m

ABOVE

SEALEVEL

~

o_

:z a0

E

._a O

A.R D I A G R A M a

~:

__

L) ~

_~

a) __

® 0a

O

E

F

o o

NA£

._o o_

2 o

o_ >"

-~

o "-

~

o

×

o

cr

~ o

t o°

to

D t' - n ¢'~

~r

< w

ANAL.

<

d. Barelds n w,

W Z 0

~w ~<

~[ld w

,,,,,,

u~,,,

(j~

W

--

~

W ~ _z

t-m O

cm O-

...Jw

STRATI- u) GRAPHY 0

zd~EE<u z .J <

10

20

30 40

50

60

70

5 °

:E

u o o ~ o - m o ~ < o o

w_

z

~

w

~

~

O~

~ < m <Jx

~ -I W o

'

:E

wO

~.,

a.-I"-:3--I

F-

z o ~

0 Z

-- >- U:bJ

<=z~o~ou~ >w<<~om~_

80 90100%

I~

o

:E

0 Z

o

:E

~w ~. m

o,,,~-~-oo -r W w o-°~ I- -.IJ O O

~_~.~uu

~/_/./i....- ~

]

7

]

@ Z

,.~u~-~

•

~

J

4

50

'ii ~8 ,-

100.

150.

200

' [

250

30C %

(_ i...-.-.:1

r

TREES C O M P O GRAMINEAE A C A E N A / AND SITAE POLYLEPIS SHRUBS

•]]]]

PEAT

10

]

I

20 30 40 I

[

I

50 60 I

I

70

80

I

I

go 1OO% I

il

[]

ALNUS

0

I

SCALE FOR ALL CURVES UNLESS OTHERWISE INDICATED

•

ONE POLLENGRAIN FOUND

Fig.4. Pollen diagram of section Corralitos I (Sierra Nevada del Cocuy). Alt. ca. 3 8 6 0 m.

CLAY

SAND

O

100 90 80 70 60 50 40 30 20 10 0 i i i i i i i i i J J % % peat ~

~

sand wit]: p,antrema~ns

i

Polylepis

clay ~

~

clay with plantreraams

~

Gramineae

peat with clay ~

~

peat with sandand clay

L ~ r ~ Compositce

sandy peat ®

~

gravel

F-R

sand

. . . . . . 0 hrub,

Alnus o A chorneo Acalypha, Cecropio x su['a of other forest elements

I

I

%

10 20 30 40 50 60 70 80 90 100 S C C I e for a l l c u r v e s

e

Fig. 6. Pollen diagram o f section Ocubi. The general cumulative diagram (left) is the same as that of Fig. 7, but the separate curves are calculated on the base of the arboreal pollen sum only. Depth b e l o w water le~-el.

pp. 2 8 7 - - 2 9 2 . ~n (b E

OCUB ALl'. ca.

ANAL:

m+

2 8 0 0 m A B O V E SEALEVEL

=F

3~ <Ld O -

y

3

STRAT(-

¢~

GRAPHY

~

,9

J. B u r e l d s

~0

10

20 30

40

50

60

~J

o

o

.j~ U'~

x . j < a_O W ~a'EU

•

U o I--

_

Z z,zI u

<

DO :i:< O w ~:a.

~z ~ ..Jm uu ~

5o

J J

0 ~z

~b

70 8 0 9 0 1 0 0 %

ARAGOA

05

CROTON

05

25

CORIARIA 05

50.

RUMEX O . 5 , V A L E R I A N A O . 5 , ARCYTOPHYLLUM 0 5 , CRUC. 0.5

0C

9

R E L B U N I U M - T 0.5, GAIADENDRON 0.5, A R T E M I S f A T. 0,5

Itllllll

~

75, AMARANTHACEAE

;HiliH I~llllil

HYSTRJCHOSPHAERIDAE 9.8, A R A L I A C E A E 0.5, BILLIA 0 5 , L E G U M INOSAE 0.5 CRUC. OA, L O R A N T H A C E A E OA

FIIIlll 0c 8

125.

A N A C A R D I A C E A E 0.4, P O L Y G O N A C E A E 0.4

|lllllll'

LEGUMINOSAE

150.

.

:f~:t~

05,

CHENOPOOIACEAE

t75"

:rag=It.-+.

0.5

MARATTIA 0.8, SAXIFRAGACEAE 0.8, A N A C A R D I A C E A E 0.8

EOO.

RUMEX

05

0.5

200 225.

0C

7 ~50

DODONAEA

275"

SAPIUM

Oc 6

D O D O N A E A 0.6

300

AMARANTHACEAE

LABIATAE t

PANOPSIS

375"

0.5

0.5

MALVACEAE 0.5,

t50'

0c 4

10

C H E N O P O D i A C E A E 0.5, PALMAE 0 5 , A R A U A C E A E

~25"

0c 5

0.5

0.8

P A N O P S [ S 0.5

0.5

CRUC. 0 . 5 CRUC. 0 5 , S T E P H A N O P O R A T E - T Y P E D 0.5

~00'

CRUC. 0 5

0c 3

CRUC

425"

1.0,

MYRIOPHYLLUM

0.5

T I L I A C E A E 0,5 ESCALONIA 0 5 , G A I A O E N D R O N O 5 , A N A C A R D I A C E A E 0 5

~ I

OC

2

Oc 1

450

475'

5O0" SAPIUM 525-

INCLUDED IN THE SUM

r'

r'

TREESiCOMPOSITAE GRAMINEAE A C A E N A AND I , ,

i 0

SHRUBS

10

20

30

4050__

60

70 80 90 100 %

SCALE FOR ALL CURVES, UNLESS OTHERWISE INDICATED PEAT E====~

CLAY

SAND WITH P L A N T R E M A I N S

PEAT WITH CLAY

CLAY WITH P L A N T R E M A I N S

=

ALNUS

SANOY PEAT

PEAT WITH S A N D AND CLAY

o

ALCHORNEA+ACALYPHA*CECROPIA

SANO

GRAVEL

X SUM OFOTHER FOREST ELEMENTS

Fig. 5. Pollen diagram o f section O c u b i (Sierra N e v a d a del C o c u y ) . Alt. ca. 2 8 0 0 m. D e p t h s b e l o w surface of solid segments.

0.5

pp. 293--296.

Fig.7. Panorama of part of the central range o f the Sierra Nevada del C o c u y , f r o m slightly S o u t h of E1 Concavo (left) to Pan de Azucar and Pulpito del Diablo (right). Laguna de la Sierra (or Laguna Grande) in foreground (alt. ca.4500 m). A l m o s t the entire area on t h e p h o t o g r a p h (outside the glaciers ) is covered with Drift 6. The huge y o u n g moraine f r o m which the p h o t o g r a p h was taken (seen best at the right of the p h o t o ) is a very r e c e n t " N e o g l a c i a l " recession moraine. P h o t o T. van der H a m m e n , March 1977.

pp. 297--300.

Fig.8. Corralitos moraine in the upper part of the Bocatoma Valley (or Chorro Aguabendita; Sierra Nevada del Cocuy). This is the outermost bordering moraine of the "Neoglacial" Drift 6. At the upper left the present-day glacial lobe, at the lower right a glacial lake associated with the endmoraine. View from ca.4450 m altitude towards the south. Foreground: upper Calamagrostis bunch grass p~ramo. Photo T. van der Hammen, March 1977.

Fig.9. Laguna de la Sierra (Laguna Grande) and glacier in contact with lake, forming high ice-wall (Sierra Nevada del Cocuy). Photo Erwin Kraus, March 1938.

Fig.10. Laguna de la Sierra (Laguna Grande) and E1 Concavo. The ice has disappeared and the glacier has retreated considerably. Photo Erwin Kraus, March 1948.

301 the present ice cap have also been lowered by denudational processes, and glacial features can only be observed in some places.

Tarns and o t h e r lakes behind the moraine ramparts and in overdeepened valleys are numerous. Some of t he m are partly or com pl et el y filled with deposits, the mor e so farther down. T h e y are present at all levels of glaciation.

Some o f these deposits were used for pollen analysis and 14C dating (see above), and gave besides data on the vegetational and environmental history, i m p o r t a n t evidence for the dating of the glacial phases. Gonzalez et al. (1965) distinguished f o ur drifts of the ice cap in the valley of the Rio Lagunillas (map squares A 4 - - B 4 on Fig.18), mainly on the basis of differences in geographical distribution and o f characteristics of the moraine ramparts deduced from aerial p h o t o g r a p h y and f r om field studies. These drifts could then be dated by palynological studies and radiocarbon analysis.

When the aerial photographic study of the area was e x t e n d e d across a major part o f the Sierra, and to m o r e low-lying areas, the same criteria were used. Practical criteria used in the photographic interpretation are: (1) the closer a moraine r am pa r t in a valley is t o the present ice cap, the y o u n g e r it is; (2) a light-toned drift is less overgrown by lichens or herbs and must be y o u n g er than a darker-toned one; (3) clearly curved moraine ramparts in a river valley, in some places becoming broad and high, point to a stagnation of the icefront; and (4) a lake behind a moraine ram part may point to the same; (5) a lower position with respect to sea level m ay be an indication o f an older age; (6) moraine ramparts m ore disintegrated by erosionaldenudational processes, m a y be older than less disintegrated ones. Although no one o f these criteria m a y be conclusive by itself, their com bi ned indications in relation to spatial distribution and overall characteristics of the moraine ramparts m a y lead to a correct interpretation.

Starting from the four glacial phases distinguished by Gonzalez et al.

(1965) and using the above-mentioned criteria, the limits of the drifts were traced on the glacio-morphological map. One, or possibly two, older drift(s) could be recognised. The drifts were n u m b e r e d 1 to 6 in such a way, t hat Drift 3 corresponds with Drift I of Gonzalez et al. (1965) (hence, Drift 4 = Drift II; Drift 5 = Drift III and Drift 6 -- Drift IV). The older the drifts are and the farther down, the vaguer and less manifest their limits become. On the map each drift has its own colour shade. This does n o t necessarily mean th at a shaded area is entirely covered by t hat drift. The older the drift, the m o r e i nt e r r upt e d the cover is supposed to be.

The different drifts in their sequence f rom young to old, their principal moraine ramparts, and the related "glacial stades" (and their age) will be treated in this order. It is i m p o r t a n t to n o t e in this c o n n e c t i o n t hat b o t h the terms " d r i f t " and " s t a d e " are used here in respect of the m o r p h o glacial sequence. "Moraine r a m p a r t " , end moraine, etc., are morpho-glacial units, " d r i f t " is a m or e lithostratigraphical unit in relation to the morphoglacial sequence and includes all genetically related moraine deposits from one glacial phase. "Sta'de" is a chronological unit in relation to the

302 morpho-glacial sequence corresponding with a time period of stagnation of the ice front (generally resulting in well-developed moraine ramparts). The terms "stadial" and "interstadial" are used here in a chrono- or climatostratigraphical sense, and are n o t related to the morpho-glacial sequence.

In the following descriptions both data from a study of aerial photographs and from field observations will be given. The dating and correlation of the drifts and stades is summarized in Fig.20.

Drift 6 and the Corralitos stade Drift 6 is the youngest drift. It is quite easily recognisable on aerial photographs and in the field. The moraine ramparts are very conspicuous and complete. There is very little vegetation on them or none at all, and it has a very clear, white tone; for that reason it is sometimes more difficult to distinguish it on aerial photographs from the present-day ice cap than from Drift 5. Drift 6 was already mapped in an area covering a considerable part of the Sierra and published in Gonzalez et al. (1965) (Plate 1, map of Anita van der Hammen-Malo). There are often several groups of end moraines including very y o u n g ones.

The Corralitos stade is defined here as the time interval during which the ice stood at, and formed the outer group of, end moraines, the Corralitos moraines (the designation Upper Corralitos moraines and Late Corralitos stade might be used for the inner group of end moraines, if they are welldeveloped). Type area for Drift 6: between the ice border and the Drift 5 deposits, between the Bocatoma Valley and Rio Corralitos. Type moraines and site of the Corralitos stade: the outer moraines of the same area, especially those of the higher part of the Bocatoma Valley, the valley of Quebrada Concavito, and Rio Corralitos.

Drift 6 deposits and the Corralitos moraines are illustrated in Figs.7 and 8. The lower limit of Drift 6 is found in the southern half of the map area between ca. 4500 m and 4300 m altitude. There is no direct date for this drift, nor for the stade. On the basis of a comparison of the thickness of the sediment in a little glacial lake behind the Corralitos end moraines in the Bocatoma Valley with t h a t of other dated lake sediments, Gonzalez et al.

(1965) calculated a possible age of 2000 years B.P. This should be considered as a m a x i m u m date, as the sedimentation rate may have been higher in the prevailing superp~ramo conditions. Radiocarbon analysis from sediments behind different end moraines of the very similar Bolivar stade (named

by Raasveldt, 1957) in the Sierra Nevada de Santa Marta yielded dates within the last three hundred years (Van der Hammen, 1979 and unpublished). Herd (1977) found for the similar Neoglacial moraines in the Colombian Central Cordillera ages within the last 500 years; about 1845 A.D. the glaciers may still have been near their Neoglacial m a x i m u m . It seems, therefore, t h a t we may rather safely date the Drift 6 deposits as Neoglacial ("Little ice-age"), formed within the last 500 years or so, the ice retreating

303

from the outer moraines possibly after ca. 1850 A.D. The Corralitos stade proper may have lasted from ca. 1500 to 1850 A.D. Fresh Drift 6 sediments are still appearing as the ice is still retreating, a p h e n o m e n o n observed in m a n y places on earth. This recent retreat is dramatically illustrated by the photographs of Figs.9, 10 and 11.

Drift 5 and the Bocatoma stade

As stated above, the boundary between Drifts 5 and 6 is very clear (tone, vegetation cover, etc.). Drift 5 has often fine, high, broad and curved bordering moraines, usually rendering the delimitation quite easy. Only around the isolated ice cap in the northern part of the map area was it more cumbersome to indicate a limit between Drifts 4 and 5, as the moraine ramparts are poorly developed because of the small size of these ice caps and the often isolated position of the moraine ramparts in this region. Very conspicuous bordering moraines are shown in map square A 4 (Fig.18).

The Bocatoma stade is defined here as the time interval when the ice stood at and formed the large bordering moraines, the Bocatoma moraines.

The type area for Drift 5 is the area from Quebrada Concavo to the Valle de la Bocatoma. The type moraine and area for the Bocatoma stade: the outer moraines of Drift 5 in this area, especially the one at the junction of the Bocatoma Valley and Valle de Lagunillas. The Bocatoma moraine is illustrated in Fig.12. The lower limit of Drift 5 deposits is, in the southern half of the map, found between ca. 4200 m and 3900 m altitude.

The Bocatoma moraine was formed after the glacier retreated from the nearby Valle de Lagunillas (shortly before 12,320±100 B.P., GrN 3247, when sedimentation of varves started there: Gonzalez et al., 1965), while the varved sediments in the small filled-up lake behind this moraine most probably were deposited near the transition from the Late Glacial to the Holocene (Andean pollen zones III to IV; ca. 10,000 B.P.: Gonzalez et al., 1965). As we know that the climate was warmer from ca. 12,400 B.P. till about 11,000 B.P. (Guantiva interstadial; with a cooler interruption around 12,000 B.P.), the Bocatoma moraine was probably built up mainly during the following cooler phase between ca. 11,000 B.P. and 10,000 B.P. (the E1 Abra stadial). Sand and gravel were deposited during t h a t period as intercalations in the peat formed in the Lagunillas valley. The possibility that a glacier end was in the same position already for some time during the cool phase of the Guantiva interstadial (probably ca. 12,100--11,900 B.P.) cannot be excluded, however.

As to the later border moraine, above the Bocatoma moraine, in a lake behind the rather conspicuous moraine 2 (see fig.7 in Gonzalez e t al., 1965), we f o u n d sediments whose deposition may have started by the beginning of the Andean pollen zone VI (ca. 7500 B.P.). On the other hand, peat formation was interrupted in the Valle de Lagunillas to be replaced by the depcsit±on of gravel and sand, shortly after 8190 i 100 B.P. (GrN 3598). Conceiv-

304 ably this moraine was formed by an ice advance some 8000 years ago (Gonzalez et al., 1965).

Hence, we believe that the Bocatoma moraine was formed mainly between ca. 11,000 B.P. and 10,000 B.P. (but possibly a moraine had accumulated at approximately the same spot by ca. 12,000 B.P.). Accordingly the Bocatoma stade would correspond in time approximately with the E1 Abra stadial.

Drift 5 might be dated as from between 12,000 B.P. or 11,000 B.P. to at least 7500 B.P. A part of it in the higher levels may, however, be of younger Holocene age.

Drift 4 and the Lagunillas stade(s) This drift has an especially conspicuous boundary of moraine ramparts along the Rio San Pablin (B3 on map Fig.18), but clear bordering moraines are also present in the valley of the Rio Lagunillas (B4). In the same Lagunillas valley, but farther upstream, there is another group of well-developed moraines (A/B4).

An isolated area covered with Drift 4 has developed as a result of the presence of valley glaciers on three sides of this area, west of Laguna de la Sierra, in A4. Another example of the isolation of Drift 4 is present in square B5/6 (Fig.18), near the Rio Negro. A local ice cap seems to have developed here and the moraine ramparts are better preserved than those f o u n d in the surrounding area. During this stade an ice-free valley appears to have existed in square A 1/2. Local glaciations may at that time have cut off the drainage through this valley. Possibly a lake existed in the valley behind this dam of glacier ice. Lake deposits are probably present in most parts of the valley, incised by the river farther downstream. A side-valley glacier from the south left behind moraine deposits that obstructed the course of the main river.

In the Lagunillas Valley, below the Bocatoma moraine, there are two principal groups of moraine ramparts (see above). We call the upper group the Upper Lagunillas moraines (near principal photo point 4109 on the map) and the lower ones (bordering Drift 4) the Lower Lagunillas moraines; the corresponding glacial stades are called the Late Lagunillas stade and the Early Lagunillas stade, respectively.

We chose the Valle de Lagunillas as type area for Drift 4 moraines and stades because the two groups of moraines are best differentiated here. A photograph of Lagunillas moraines is shown in Fig.13. In the southern half of the map the lower limit of Drift 4 is f o u n d between 4000 and 3300 m altitude.

The earliest sediments deposited in the lake formed behind the Upper Lagunillas moraines most probably are very slightly older than 12,320 + 100 B.P. (GrN 3247; section Valle de Lagunillas V: Gonzalez et al., 1965). Ice must, therefore, have been standing at these moraines shortly before that date, during the time interval immediately prior to 12,400 B.P. This renders

pp. 305--306.

Fig.11. Laguna de la Sierra (Laguna Grande). The glacier has retreated completely from the lake and is considerably reduced. Photo T. van der Hammen, March 1977. Comparison of Figs.9, 10 and 11 (all taken from the same spot) dramatically illustrates the retirement of the glaciers during the past forty years.

pp. 307--308.

Fig.12. Bocatoma moraine, bordering Drift 5. Junction of Bocatoma Valley (Chorro Aguabendita) and Lagunillas Valley (Sierra Nevada del Cocuy). Grass p~ramo with Espeletia lopezii in foreground. Altitude ca.3950 m. The Bocatoma stade is Late Glacial, and ice may have stood at this place between c a . l l , 0 0 0 and 10,000 B.P. (the E1 Abra stadial). Photo A. M. Cleef.

pp. 309--310.

Fig.13. U p p e r Lagunillas moraine (part o f Drift 4). Valle de Lagunillas, altitude ca.3900 m. The Late Lagunillas stade seems to fall within the Ciega stadial, and m a y have lasted from ca.13,000 to 12,400 B.P. P h o t o T. van der H a m m e n .

pp. 3 1 1 - - 3 1 2 .

Fig.14. L a n d s c a p e in Drift 3 area. Moraines parallel t o t h e Rio Concavo, looking d o w n s t r e a m f r o m an elevation o f ca.3450 m.

P h o t o T. van der H a m m e n .

Fig.15. R e c e s s i o n m o r a i n e s o f Drift 3 in the Rio C o n c a v o Valley, near its j u n c t i o n w i t h Valle de Lagunil]as. View f r o m H a c i e n d a La Pajita (alt. ca.3500 m ) t o w a r d s the ENE. The a l t i t u d e o f t h e m o r a i n e s is b e t w e e n 3200 and 3400 m. P h o t o T. van der H a m m e n .

pp. 3 1 3 - - 3 1 4 .

Fig.16. P o z o Negro, a glacial lake at a p p r o x i m a t e l y 3000 m altitude in t h e u p p e r m o s t part o f t h e Drift 2 area. The lake is c o m p l e t e l y s u r r o u n d e d b y m o n t a n e A n d e a n forest. The p h o t o was t a k e n f r o m t h e b o r d e r i n g m o r a i n e , at ca. 3050 m. P h o t o T. van der H a m m e n .

pp. 3 1 5 - - 3 1 6 .

Fig.17. M o r a i n e b o r d e r i n g P o z o V e r d e , a glacial lake at a p p r o x i m a t e l y 2925 m in t h e u p p e r part o f t h e Drift 2 area. P h o t o T. van der H a m m e n .

317 it very probable that the Upper Lagunillas moraines were formed by the stagnating ice during the cold phase between the Susac~ and Guantive interstadials (ca. 13,000--12,400 B.P.; see above). The Late Lagunillas stade is, therefore, probably of t h a t same age. We have no direct dating of the Lower Lagunillas moraines. They should at any rate be younger than Drift 3 (dated as older than ca. 25,000 B.P., see below). As they should be older than the Upper Lagunillas moraines, they are probably also older than the Susac~ interstadial. Their age would accordingly be between ca. 25,000 and 14,000 years, and it seems logical to suppose that the Early Lagunillas stade corresponds with the Fflquene stadial (or possibly with only a part of it). The best estimate is, therefore, 21,000--14,000 B.P. For a further discussion, see below. Drift 4 as a whole could then be dated as between ca. 21,000 B.P. and 12,400 B.P. Drift 3 and the Rio Nevado stade This drift has a great extension, but sometimes it is not easily recognizable owing to the fact that some moraine ramparts act as watersheds. The latter were probably more extensive when t h e y were formed, and have been worn down since. The boundary between Drift 3 and Drift 2 is vague and cannot be readily located, so t h a t it is indicated on the map by smooth curves. The timber-line tends to run parallel to it. Clear bordering moraines near this limit are scarce; we call those situated near that border SE of Guican, close to the Rio Concavo, the (Rio) Concavo moraine, and the corresponding glacial stade the Rio Concavo stade. The type area of Drift 3 is the area east of Guican unto the boundary with Drift 4; it includes the moraines along the Rio Concavo and the area of Laguna Ciega. In Figs.14 and 15 some moraines of Drift 3 are shown. The lower limit of Drift 3 is found in the southern half of the map at elevations between 3400 and 3000 m.

The sediments from Laguna Ciega are most important for the dating (see above). As several drillings were made in this filled-up lake at ca. 3510 m (NE of Guican; see B3 on map Fig.18), we feel pretty sure that sediments from the deepest part are represented in section Ciega I. The oldest ~4C date we have from Laguna Ciega is 20,840 + 140 B.P. (GrN 6907), corresponding with the limit of local pollen zones C2 and C3. Zone C3 corresponds with the Fflquene stadial and zone C2 with the Saravita interval (Van Geel and Van der Hammen, 1973). Zone C1 probably corresponds with the local pollen zone W-III of Fflquene, and possibly with W-II, the Suta interval (ibid). The base of W-II was calculated by extrapolation of ~4C dates as having a possible age of 24,500 B.P. (base W-III: 23,500 B.P.); extrapolation of the combined ~4C dates in section Ciega I and III yields a possible age of ca. 27,000 B.P. for the base of the section Ciega I; however, in the lower part of the section (which contains some sand) the rate of sedimentation was probably higher. For the time being we may conclude t h a t the age of the beginning of lake sedimentation is somewhere between ca. 24,500 B.P.

318 and 27,000 B.P.; this would correspond with the time when glacier ice disappeared from the lake area. This must also be the age of the end of the Rio Concavo stade. This surprisingly great age of a Last Glacial extension, much greater than t h a t of the Fflquene stadial, will be discussed in the final section of the present paper.

Drift 2 and the Rio Negro stade On the aerial photographs, Drift 2 could only be recognized with some difficulty. Isolated moraine ramparts seem to be present at various heights.

Between ca. 2600 and 2800 m altitude in the neighbourhood of the Rio Negro (square A B 6 of Fig.18) the presence of moraine ramparts is relatively clear; these moraines are covered with m o n t a n e forest. Lakes, part of them filled up, are also relatively frequent in this area. One might wonder whether the difference between Drift 2 and Drift 3 is accentuated by the presence of forest on most of Drift 2 and by the fact t h a t glaciers extended farther down into the valleys in this wetter and east-exposed area than t h e y did on the west side. However, the glacial features are relatively c o m m o n in a large area there and at least some glacial features were also recognized at similar altitudes in the west.

Field observation proved t h a t there is no d o u b t about the glacial origin of the Pozo Negro and Pozo Verde lakes (in the extreme SE corner of B5 and the extreme NE corner of B6 on the map) and the associated morainic deposits (Figs.16 and 17), and a clear end moraine was observed at ca. 2840 m altitude in the Quebrada Los Osos. Further down, field observation becomes very difficult because of the dense forest cover.

The Las Risaguidas lakes (immediately east of point 4104 in B6 on Fig.18), associated with moraines, have elevations of ca. 2600 and 2400 m, and some remnants of moraine ramparts may be present as far down as ca. 2200 m altitude.

The age of Drift 2 and the Rio Negro moraines must be greater than that of Drift 1, and probably exceeds 30,000 B.P. However, the fact that several lakes are still open renders it probable that t h e y date back from after the last major interglacial.

Possible Drift 1 or unglaciated In the area indicated on the map as such, some remnants of possibly glacial deposits or signs of glacial morphology seem to occur, such as near the cirque-like formation in the SW corner of A 6 (Fig.18; ca. 2200 m) and near the Ocubi lakes (ca. 2700 m). Their doubtful glacial origin is indicated by question marks. They might belong to Drift 2, or be of non-glacial origin. The pollen section from Ocubi (see above) is from the surrounding bog, and c a n n o t be expected to date the lakes.

In Fig.20 the sequence of drift bodies and stades and their (partly tenta-

319 tive) dating and correlation with other events is given. This table will be further discussed in the next section.

DISCUSSION AND CONCLUSIONS One of the most interesting results of the present study is that in the Cocuy area the m a x i m u m glaciation during the Last Glacial period antedates in any case ca. 25,000 B.P.; which is earlier than the time of m a x i m u m glaciation in most places of the present northern temperature zone of the earth, and also earlier than the age of the minimum temperatures and the maximum of 180 (= maximum ice volume on the continents) in the deep-sea curves.

The annual rainfall in the Cordillera having dropped drastically in the period between ca. 21,000 and ca. 13,000 B.P. (Van Geel and Van der Hammen, 1973), this might constitute the main cause of this phenomenon: the annual precipitation must have been considerably higher in the periods between ca.

30,000 B.P. and 21,000 B.P. and between 36,000 B.P. and ca. 45,000 B.P. (Van Geel and Van der Hammen, 1973; Schreve-Brinkman, 1978). As we have only a t e r m i n u s a n t e q u e m for Drift 3 and Drift 2, based on the disappearance of the ice from Laguna Ciega and on the '4C and pollen-analytical dating of the sediments of this lake, one might ask if there are any additional data relevant to the dating of glaciation in the Cordillera Oriental outside the Cocuy area. Stratigraphical data and associated '4C dates from the area of the high plain of Bogotfi ("Sabana de Bogota") and surroundings, from between ca. 2500 and 3000 m altitude, appear to be of importance for this problem of dating (Van der Hammen et al., 1980).

In the valley of the Embalse del Neusa (60 km NNE of Bogotfi, at ca.

3000 m) we found (above the level of the present-day artificial lake), moraine deposits overlain by some 2 m of lake clay. Both deposits were eroded and the erosion surface is overlain with an irregular, up to 1 m thick layer of coarse gravel, in its turn overlain by a sequence of (paleo) soils. There was a thin peat layer in the middle of the lake clay, dated as 33,810 +_ 770 B.P. (GrN 5843). Remnants of an end moraine are present at the end of the valley (near the present dam site). These data show that a glacier was lying in the Neusa Valley and a lake was formed which persisted for some time after this glacier had disappeared. A renewed advance, possibly not extending to the lower end of the valley, must have been responsible for the irregular gravel layer. The earlier and farther extending advance took place before, and the later one after, the above-mentioned date.

North of Subachoque (a site ca. 40 km NNW of Bogotfi at an elevation of 2840 m), moraine deposits overlain by a paleosol are present. The latter was dated 35,800 _+ ,100900B.P. (GrN 5681). The soil was overlain by loam and by gravel, respectively, and this sequence is covered by the recent soil.

Near La Calera (a site some 10 km NNE of Bogot~i at 2835 m alt.), we f o u n d a black paleosol overlain by sandy deposits, in its turn overlain by recent soil. The top of the paleosol was dated 43,100 _+ 12001,00B.P. (GrN 4680).

320 Possibly morainic material (or solifluction?) with large blocks is present below the soil.

In the Sabana de Bogot~ proper (ca. 2580 m alt.) we found extensive (probably fluvio-glacial) gravel deposits, near the former glacial nucleus east of Zipaquir~, overlying a peaty clay layer dated 27,905 + 410 (GrN 5836) and 32,890 -+ 660 B.P. (GrN 5838). There are also gravel deposits below the layer, separated by another peaty layer. Near Tabio a similar situation was found: gravel overlying a peaty layer dated 31,500 + 610 B.P. (GrN 5837), while a clay with charcoal overlying (probably fluvio-glacial) gravels was dated 39,650 -+ l°ss° B.P. (GrN 6002). Near Mosquera a fine-grained silty deposit and soil, dated 30,245 -+ 520 (GrN 5922) and 29,760 + 610 (GrN 5835) was directly overlain by a'coarser colluvial deposit.

In the 14C dated pollen diagram of a new and more than 40 m thick section of lake deposits from Ffiquene (III), very cold conditions began at ca. 45,000 B.P., and there was a clear interstadial interruption at ca. 38,000 B.P. These are only a few examples of available data, confirmed by several more to be published together in the near future and apparently all confirming the interstadial character of a relatively long period before ca. 45,000 B.P., the presence of a shorter period around 38,000 B.P. and of a distinct period between ca. 28,000 and 36,000 B.P. (with a possible interruption, the period around 30,000 B.P. probably being the warmest). They also seem to indicate a glacial advance below 3000 m, downwards to possibly 2800 m, before 36,000 B.P., b u t probably after ca. 45,000 B.P. This period contains an interstadial, and t w o cold periods between ca. 44,000 and ca. 39,000 B.P. and between ca. 38,000 and ca. 36,000 B.P. seem to have existed. During another cold period (ca. 28,000--25,000 B.P.) there was probably another ice advance, extending downwards to 3000 m, and leaving behind coarse fluvio-glacial deposits, etc., in many places in the high plain of Bogot~ and the surrounding areas.

Taking all these data into account, one m a y accept that Drift 3 and the Rio Concavo glacial stade may be dated as lying between 25,000 B.P. (or a somewhat earlier time) and 28,000 B.P., and it is not at all unlikely that Drift 2 and the Rio Negro glacial stade may be dated somewhere between 36,000 B.P. and 45,000 B.P. (between 36,000 and 38,000 and/or between 39,000 B.P. and 45,000 B.P:.; of course the possibility cannot be excluded that Drift 2 is still older; however, because of the presence of many still open lakes, it does n o t seem to be probable that it dates back from before the last major Interglacial). The reason for this early maximum extension of the ice is probably the circumstance that the very cold period between 20,000 B.P. and 14,000 B.P. was very dry in the Cordillera, and that during these earlier periods the effective rainfall was considerably higher, while the climate was reasonably cold. If the maximum extension of the Cocuy ice during the Rio Negro glacial stade t o o k place in the period between 36,000 B.P. and 45,000 B.P., it corresponds most probably with the time of the highest level and maximum extension during the Last Glacial period

pp. 3 2 1 - 7 3 2 6 .

GLACIO MORPHOLOGICAL MAP OF MAPA G L A C I O M O R F O L O G I C O DE LA

LEGENDA L E YENDA

\

lee

%

.)

+ 8064

!

I

\\

limit of former icesap, phase 6

+ 8063~

~

l/mite del casquete glacial, lase 5

limit of former icecap, phase 4

~

limite del casquete glacial, fase 4

R. Blanca

approx limit of former iceeap, phase 3 limite ap~ox, del eaequete glacial, lose 3

estimated limit of former icecap, phase 2

\ \ \

possible drift I or unglaciat ed

Iimite eetimado de/ casquete glacial, lose 2 possible "drift " 1, o nunca cub/otto per glaciares

.

T

.

borde(of present-day glacier tongue

~

~

moraineramparts (? uncertain)

L O C A T I O N IViAP -MAPA I N D I C E DE LOCAL.ISACION

SEA

I

VENEZUELA =======================::::::+~ ======== . . .~.~ . .i_, . ~/ i-

S

.,.-, ~:i:,ii:]ii::iii::~i/~!!FDel Coeuy::?::!::i::i:::L, ~ ~~ .:.::::: :.::-:-~.i' '.','~ •:.:+:.-. ::::::::::::::::::::::::::::.

OCEAN

. -. - ~. - . . .

~ii::iiii::!:~ S og ota :::::::::::::::::::::::::::::::::::::::::::: ::iiiii::ii~"~ ~i~!~i~i::~i!i!i~:i!~:~!:~:i~i~{:~:i~::::l~i~::!:~:[i~!!i)!~!:~i :!:ii i~i !i!i!!i:~i~i!~i~~:i~l)

'- "<~'

., 0~r ':~!~)ii)))))i!))i!i!ii!il)ii!))):i!))?)i)i))iii:i',

/'~

~ ',

",

" /

BRAZIL

ATLANTIC

,' ,--.L

,,'"

OCEAN

." /"

~#~'..~

.5 TOPOGRAPHY/

TOPOGRAF[A

NORTHERN PART 1 P A R T E NORTE--' ITC, ENSCHEDE, HOLLANO SUR: iNST. GEOGR. AGUSTIN CODAZZI SOUTHERN P A R T / P A R T E

Fig.18. Glacio-morphological map o f the Sierra Nevada del C o c u y , based o n aerial p h o t o graphs and field observations. T h e general d e l i m i t a t i o n o f "drifts" indicate the areas where each glacial drift m a y be f o u n d (the corresponding drift m a y be absent in smaller or larger parts o f the area as indicated o n t h e map).

COLOMBIA

upperedge borde superior /

~5490

lower edge

/ de valle glacial "

.~ ~ ~'~ ~

lake deposit sedimento de laguna

watershed, first order, sharp

,.

divorcio de aguas, primer crden, agudo

watershed, first order, rounded or vague

divorcio de aguas, primer orden, suave o rage

~

circe ~

laguna

picas

~

cirque

corriente de agua con ealto

lake

summit

of glacial valley

borde interior

stream with fall

~

pass paso

)(

morrenas terminales y taterales (? dudosa)

-

paso ,de deebordamiento del hielo

~

~

path (acc. to interpretation~ with village "-~. . . . . - -

X._.-~-

watershed, second order

-(:~. +4100

divorcio de aguas, segunde orden

i

CARIBBEAN

DELCOCUY

transflaenee pass

~

I/mite de lengua glacial actual

"~

limit of former icecap, phase 5

la Sierra

.//

'-.~., ~ - J/

l/nitre del casquete glacial, lose 6

\

NEVADA

based on interpretation of aerial photographs by H. DE JONG and A.A. DE VEER basado en interpretaci6n de fotograffas a@reas per y

limit of present-day icecap t/mite de/ casquete glacial actual

k

SIERRA

cam/no (segun interpretacioa) con pueblo

path (acc. to topogr, map) camino (segun mapa topogr.)

principal point of photograRh

punto principal de lotograffa a~rea

scale ca. I"S0000 Escala

// :

,. "~"

J

.... i~dicaiion di mapsquare

pp. 327--332.

F

m 0CHRONO STRATIGRAPHICAL UNITS

1-

Tentative curve of r e l a t i v e f l u c t u a t i o n s of l a k e l e v e l s on the h i g h p l a i n s ( S a b a n a de Bogot~ and L a g u n a de Fuquene )

Tentative

a[titud

e

curve

of r e l a t i v e GLACIAL fluctuations

"

O

SEQUENCE

E

(Drift b o d i e s

and stades)

of the level of L a g u n a C i e g a • >. ~.*

m

o

Rio C o r r a l i t o s

- - 0 ~ f ~

2-

h I

I I

Drift

I

3

10

I-EL A B R A S T A D I A L

5 1 Bocatoma

~i~-L _/a- 7---

GUANTIVA INTERSTADIAL CIEGA STADIAL SUSAC~ INTERSTADIAL

4-

FUQUENE

5"

-

r-

-

-

J~

~ , L a t e

~

a

g

stade -

u

-

-

n

~

Early Drift

STADIAL

4a

Lagunillas stade

20

m

SARAVITA INTERSTADIAL -- -- -----%~INTERVAL)

6

GUICAN STADIAL

7'

S

Drift -

3 L !.

Rfo C 6 n c a v o

--I

stade

M o v e m e n t s Of SABTUARIO

(ROCAS V)

'3(

vegetation INTERSTADIAL \ (INTERVAL) \ ?

8"

belts in the area

% -L- . . . . .

of L a g u n a C i e g a Drift (right)

STADIAL 9"

4(

(left)

\

\ k

INT.' -- ----

INTERSTADIAL

Fig.19. Simplified representation and correlation of the main pollen diagrams o f sections Laguna Ciega I and III.

50

I

ow high [low high

t

Rio N e g r o

Lagunillas

STADIAL

10-

2 1 i

V a l l e de

J

I N T E R S T A D I A L ( R O C A S IVb

and

)

)

• ¢i Flg.~,0.

Table showing the chronostratigraphical and glacial sequence in The Cocuy area, curves for displacement of vegetational zones and estimated relative fluctuations of lake level;s.The absolute dating in the lower half of the table is tentative.

i

stade

333 of the large lake of the Sabana de Bogota. The decrease of volume of the Cocuy ice mass from the Rio Negro moraines to the Early Lagunillas moraines, is then explicable by the general decrease of the a m o u n t of effective precipitation.

The pollen diagrams from Laguna Ciega show very clear changes in the composition of the pollen rain and may be easily correlated and subdivided into zones. The 14C dates enable us to correlate the zones with those from other pollen diagrams from the Cordillera. However, as we have seen above, it is not always easy to translate the pollen data more or less precisely in terms of a place in the sequence of vegetation belts. On the one hand one would expect that the site lay in the Andean forest belt during the Holocene, but on the other that the site lay in the upper superp~ramo during the coldest phase of the last 25,000 years. At first sight there is no evidence substantiating the anticipated results, however.

In the first place, influences of very local stands of vegetation seem to play a role, as more than the upper half of the section consists of peat and clayey peat deposits, and lake sediments only occur in the lower part (but even there sometimes with intercalated peaty layers). Cyperaceae (sedges), Gramineae (grasses), Espeletia and shrubby Compositae may have grown on the spot in these bogs or marshes, and especially the last three groups may have determined the aspect in the general diagram. A tree pollen diagram seems to be rather meaningless in this case, so that it has been tried to interpret the diagrams by taking into account the possible contribution of both regionally and locally produced pollen. In the case of the Holocene deposits, the pollen produced in situ in the bog seems to be dominating; but remnants of forest still persist in the same area, up to 3600 m and locally more than 3800 m. However, conceivably the forest that surrounded the bog principally consisted of Hesperomeles, an extremely poor pollen producer. It is, for all these reasons, very difficult to determine the position and movements of the tree line during the Holocene. It is very striking t h a t a very similar Late Glacial and Holocene pollen sequence was found in the P~ramo de Guantiva (Van der Hammen and Gonzalez, 1965). In the corresponding diagram, there is a very marked Guantiva interstadial with the highest tree-pollen percentages of the entire diagram followed by a sequence from the E1 Abra stadial to the present showing almost no differentiation in the diagram, but with continuously very high percentages of herbaceous pollen types. This curious sequence (at ca. 3300 m a l t . ) was explained as probably being caused by a relatively high forest limit during the wet Guantiva interstadial, which became lower afterwards because the climate became much drier (the effective precipitation became much lower).

This is probably an important factor, but the influence of the development of an extensive bog and the presence of a forest consisting of poor pollen producers seem to have accentuated this effect at least in the case of the Laguna Ciega area.

During the coldest phase the typical situation of the higher superp~iramo,

334 characterized by a very low pollen density combined with a relatively high percentage of pollen of anemophilous trees, is never found in the Laguna Ciega diagrams, as was the case in, e.g., the lowermost part of the pollen diagram of Valle de Lagunillas V. Pollen is abundant and grass pollen is mostly dominating. It seems to represent a lower superp~ramo or higher grass p~ramo vegetation (see the table in Fig.20). The consequence would be that there was a m a x i m u m altitudinal displacement of vegetation belts of no more than ca. 900 m. However, the forest limit in the Cordillera Oriental was found to be at the time of the Fflquene stadial 1200--1500 m lower than t o d a y (Van der Hammen, 1974). However, most probably the present position of the upper s u p e r p ~ a m o is influenced by soil factors: this area corresponds principally with the very young morainic area, which became ice-free mostly n o t more than 150 years ago. We consider it more than likely that, if soil conditions were at an optimum, the uppermost grass p~iramo vegetation could thrive at altitudes several hundreds of metres higher than elsewhere. This is confirmed by the local presence of grass p~ramo vegetation up to ca. 4500 m on the nunatak-like sites (which were probably ice-free for thousands of years) on the Lagunillas--Concavo divide (Cleef, in press), while there are some indications from aerial photographs that grass p ~ a m o m a y extend to even greater altitudes. As the area around Laguna Ciega was (since the Rio Concavo stade) ice-free for many thousands of years, the above-mentioned factor might very well provide the correct explanation. It follows that in order to deduce the lowering of vegetation zones and the corresponding decrease of temperature from the pollen spectra, an altitudinal value of presumably at least 4500 m (and possibly more) has to be accepted for the grass p~ramo/superp~ramo boundary. More field studies in the highest areas are evidently required before the altitude (and the corresponding average annual temperature, respectively) of the proper climatic (zonal) boundary between grass p~ramo and superp£ramo, and between the lower and the upper superp~ramo can be more accurately determined.

The Late Glacial (and more generally the interstadial) pioneer vegetation is of special interest. The most conspicuous element is Dodonaea viscosa; an abundance of Dodonaea pollen in the Guantiva interstadial was found t h r o u g h o u t the Cordillera Oriental, in sites as far apart as Laguna de Fflquene (Van Geel and Van der Hammen, 1973), P~ramo de Guantiva (Van der H a m m e n and Gonzalez, 1965) and the Sierra Nevada del Cocuy, and at altitudes between 2500 m and 3800 m. Although Dodonaea pollen seems to become air-borne easily, the feature is so manifest and so general that it can only be attributed to a considerable extension of this shrub in Late Glacial time. Today Dodonaea is abundant in xerophytic vegetation and in degraded vegetation types on eroded soils between 2000 m and 3000 m, and in the Cocuy mountains even up to 3400 m. When the climate became w a r m e r at the end of the Last Glacial, Dodonaea seems to have been one of t h e shrubby pioneers that invaded the former p~ramos of lower elevations (above 2000 m), favoured by light and soil conditions. Arborescent taxa that

335

concomitantly started to invade the lower part of this glacial p~ramo area, apparently included Alnus, Quercus, Podocarpus and Hedyosmum, and locally Myrica and Rapanea, but Weinmannia, nowadays one of the dominant trees of the Andean forest, extended only little by little during the Holocene.

This p h e n o m e n o n can very clearly be observed in the pollen diagram from Fflquene (Van Geel and Van der Hammen, 1973), where even during the Early Holocene, when Dodonaea had already disappeared, Quercus seems to have been the d o m i n a n t tree, only to become gradually replaced by Weinmannia. We think, therefore, that most Weinmannia species are typical of climax vegetation, and require shade and a mature, humic soil to develop and extend 1. Of both Alnus and Hedyosmum we know that they are relatively heliophilous; some Hedyosmum species may extend considerably in disturbed forests when light increases. We found Quercus forests often on relatively bare soils poor in humus, whereas it seems to become richer in other arboreal species when a thicker humus layer is present. The approximate order from a pioneer species to a climax tree of the species of the above-mentioned genera is presumably: Dodonaea--Alnus--Myrica--Hedyos-

mum--Rapanea--Quercus--Weinmannia.

During some earlier interstadials, about the same soil and light conditions apparently existed as during the Guantiva interstadial, and the pollen percentages of the same genera increase during the Susac~ interstadial (Alnus, Quercus, Dodonaea) and the Saravita interval (Alnus, Quercus, Myrica, Dodonaea). Pollen of Acalypha, an anemophilous tree of the Subandean forest, may also show a noticeable increase. This rise may be related to the fact that Acalypha was possibly one of the pioneers of the Subandean forest zone and invaded first the lower part of the glacial pfiramo zone, between ca. 2000 and 2500 m altitude. A temporary increase of Acalypha in the vegetation cover, in conjunction with its anemophily, may, then, explain the temporary increase of its pollen percentages in, e.g., the Guantiva interstadial.

In Fig.19 the main pollen diagrams of the sections Laguna Ciega I and III have been drawn in a somewhat simplified form, and the correlation of the two diagrams is indicated. In Fig.20 some of the results of the present study are combined with some earlier data in one table. At the left the absolute time scale in thousands of years before present is indicated. From ca. 21,000 B.P. to 50,000 B.P. these dates become increasingly more unreliable, and have to be considered as only tentative, especially as far as the dating of the glacial drift bodies and stades is concerned. Limits are indicated by interrupted lines if their correspondence with the indicated date is uncertain; sometimes two possible positions of a limit have been indicated on the absolute time scale. The chronostratigraphical sequence shows the Holocene and the upper part of what we call here the Fuquenian. This unit is based on the pre-Holocene sequence in the basin of Fflquene, and will be more amply defined in a forthcoming publication. It presumably corres1 T h i s d o e s n o t h o l d f o r all W e i n m a n n i a s p e c i e s ; r e c e n t l y w e f o u n d in t h e C e n t r a l C o r d i l lera a s p e c i e s t h a t b e h a v e d as a p i o n e e r .

336 ponds in time approximately with the Last Glacial period in other parts of the world (the upper part, representing the last 21,000 years, certainly does).

The Fuquenian is subdivided into a number of stadials and interstadials.

Most of them have already been published and defined. Two new units are proposed here. The first is the Ciega stadial, corresponding with the cooler interval between the Susac~ and Guantiva interstadials (type section and pollen diagram Laguna Ciega III, zone C5). The second is the Guican stadial, corresponding with the cold interval between the Santuario and Saravita interstadials (type section and pollen diagram of the upper part: the Laguna Ciega I section and pollen diagram of zone C1; lower limit: the top of the Santuario interval in section Ffiquene II). The Guican stadial apparently includes the " S u t a interval". For the Saravita and Santuario interstadials we originally used the more neutral word "interval", but their nature seems to be sufficiently substantiated now. So far no formal names have been given to the stadials and interstadials older than the Santuario interstadial. Some o f these interstadials were provisionally named Rocas IV and Rocas V (Schreve-Brinkman, 1978); t h e y will be formally named later, when the extensive pollen diagram Ffiquene III will be published (Van der Hammen and Noldus, in prep.).

The tentative curve of fluctuations of lake levels on the high plains is based on the curve of F6quene (Van Geel and Van der Hammen, 1973) and the curve of E1 Abra (Schreve-Brinkman, 1978). The tentative curve of relative fluctuations of the level of Laguna Ciega is based on an evaluation of the curves of elements from the hydro- and hygroseries in both diagrams I and III. The curves of the algae Botryococcus and Mougeotia, and the pollen curves of Isoetes and of Cyperaceae contributed important evidence in this respect. The lake level curve at first runs more or less parallel with that of the high plains, but in the Susacfi interstadial it suddenly drops to zero; this is apparently attributable to the fact that the lake had by then become filled-in with sediments and turned into a marsh, and n o t to climatological influences. The vertical shifts of vegetation belts have been indicated in Fig.20 by two curves. The first shows the situation in Valle de Lagunillas (ca. 3900 m a l t . ) and is based on Gonzalez et al. (1965). The second curve is based on the interpretation of the pollen diagrams from Laguna Ciega (Figs.2, 3 and 19). If we accept that the climatological boundary between grass p£ramo and superp~ramo lies at 4500--4600 m, we find a m a x i m u m displacement to over 1000--1100 m below the present lower limits. Of course no estimates could be made of the time when the Laguna Ciega area was covered by glacier ice.

The glacial sequence is based on glacio-morphological features and their dating. The term " s t a d e " is used to indicate a time interval during which a glacier was stationary at a particular moraine or a group of moraines.

The exact dating of the Rio Negro stade is most probably older than ca.

35,000 B.P. Why this is so and why its suggested position is estimated at somewhere between 35,000 B.P. and 44,000 B.P. h~is been explained in the discussion at the beginning of this section. It is n o t e w o r t h y that this is a

337 relatively wet period of high lake levels on the high plains. The considerable extension of the glaciers during this period, locally reaching downwards to altitudes of 2500--2200 m, t o o k place very early as compared to the time of m a x i m u m glaciation in the Northern Hemisphere. This might be attributable to the apparently much higher a m o u n t of precipitation during that time than during the colder but much drier Ffiquene stadial. Even the next Rio Concavo stade, when the ice came down to elevations of 3400--3000 m, is older than the period of m a x i m u m extension of land-ice in the Northern Hemisphere, since it is in any event older than ca. 25,000 B.P. This is a most interesting phenomenon, also recorded in Mexico by Heine (1973). This author f o u n d that the m a x i m u m glaciation on the Mexican volcanoes occurred between 26,000 B.P. and 39,000 B.P. (moraine MI). The age of the next moraine there (MII) was dated as 12,100 B.P., however, and nothing comparable to the Early and Late Lagunillas stades was found.

The Early Lagunillas stade most probably corresponds with at least a part of the F~quene stadial (which was apparently the coldest, but very dry, part of the Last Glacial). In m a n y places on the continents and in the oceans the coldest part of the Last Glacial and the greatest extension of land-ice was found to be in the time interval represented by the Ffiquene stadial, i.e., at approximately 18,000 B.P. By that time the glaciers on the Cocuy mountains covered a much smaller area than they did during the earlier stades, reaching down to elevations not lower than 4000 m to 3300 m. The Late Lagunillas stade corresponds rather precisely with the Ciega stadial (ca.

13,000--12,400 B.P.). The Bocatoma stade should correspond approximately with the E1 Abra stadial (ca. 11,000 to ca. 10,000 B.P.), but the ice may conceivably have stood at the same place during the short, cold phase in the middle of the Guantiva interstadial (approximately between 12,100 B.P. and 11,900 B.P.).

The Rio Corralitos stade is very young (Neoglacial), and most probably the ice started to retire from the moraines after ca. 1850 A.D. This recent retreat of the ice still continues, which indicates that there is as yet probably no equilibrium between ice extension and climate.

If we now overlook all we know about the Last Glacial environmental and glacial history in the tropical northern Andes, we may conclude that the most striking features seem to be: (1) A relatively cold and very wet period before ca. 25,000 B.P. (between ca. 25,000 B.P. and ca. 45,000 B.P.; with climatic fluctuations), with relatively high positions of the altitudinal forest limit (which lay possibly ca.

800--1000 m lower than it lies today) and very low positions of the lowermost limit of permanent ice and snow (glaciers reaching down to elevations between 2700 m and 2200 m).

(2) A very cold and dry period between ca. 21,000 B.P. and ca. 14,000 B.P., with relatively low positions of the altitudinal forest limit (possibly ca. 1 2 0 0 - 1 5 0 0 m lower than today), and with relatively high positions of the lowermost limit of permanent ice and snow ( 4 0 0 0 - 3 3 0 0 m).

338

(3) As a consequence of (1) and (2): a narrow zone of relatively wet p~ramo (with abundant Polylepis) between ca. 45,000 B.P. and 25,000 B.P. and a broad zone of relatively dry p~ramo between 21,000 B.P. and 14,000 B.P. In the period before 25,000 B.P., during times of maximum glaciation, glaciers may have been locally in contact with forest.

(4) A relatively wet Late Glacial (ca. 14,000/12,400 B.P. to ca. 10,000 B.P.), with several fluctuations, leading via interesting pioneer vegetations with Dodonaea (also known from earlier interstadials) to a re-afforestation of the lower part of the glacial p~ramos, completed in the beginning of the Holocene and leading gradually to the establishment of the Holocene montane Andean climax for'ests.

APPENDIX: LIST OF GENERA AND FULL NAMES OF SPECIES OF PLANTS MENTIONED IN THE TEXT, AND THE CORRESPONDING FAMILY NAMES Acaena cylindristachya R. & P. (Rosaceae) Acalypha (Euphorbiaceae) Acaulimalva (Malvaceae) Ageratina tinifolium

(H.B.K.) K. & R. (Compositae) Alchornea (Euphorbiaceae) Alnusjorullensis H. B. K. (Betulaceae) Aragoa lycopodioides

Benth. (Scrophulariaceae) Arcytophyllum nitidum

(H. B. K.) Schlt. (Rubiaceae) Arenaria (Caryophyllaceae) Alsophila (Cyatheaceae) Baccharis (Compositae) Bartsia (Scrophulariaceae) Billia (Hippocastanaceae) Blechnum (Lomaria) (Blechnaceae) Botryococcus

(Tetrasporaceae, Chlorophyceae) Brunellia (Brunelliaceae) Buddleja lindenii Benth (Buddlejaceae) Bucq uetia (Melastomataceae) Calamagrostis effusa

(H. B. K.) Steud. (Gramineae) Carex acutata Boott. (Cyperaceae) Castilleja (Scrophulariaceae) Cecropia (Cecropiaceae) Cerastium (Caryophyllaceae) Cestrum (Solanaceae) Chusq uea (Gramineae) Clethra (Clethraceae) Clusia (Guttiferae) Cyathea (Cyatheaceae) Diplostephium (Cornpositae) Distich& muscoides

Nees & Meyen (Juncaceae)

Dodonaea viscosa

(L.) Jacq. (Sapindaceae) Draba (Cruciferae) Drimys (Winteraceae) Eleocharis macrostachya

Britton (Cyperaceae) Espeletia curialensis

Cuatr. (Compositae) Espeletia lopezii Cuatr. (Compositae) Espeletiopsis colombiana

(Cuatr.) Cuatr. (Compositae) Espele tiopsis jimenez-q uesadae

(Cuatr.) Cuatr. (Compositae) Eugenia (Myrtaceae) Fresiera (Theaceae) Geissanthus (Myrsinaceae) Gentiana (Gentianaceae) Geonoma weberbaueri

Dammer ex Burret. (Palmae) Guarea (Meliaceae) Gy noxys (Compositae) Hedyosmum (Chloranthaceae) Heliocarpus (Tiliaceae) Hesperomeles lanuginosa

R. & P. (Rosaceae) Hieronima (Euphorbiaceae) Hydrocotyle (Umbelliferae) Hymenophyllum trichophyllum

H. B. K. (Hyrnenophyllaceae) Hy pericum (Guttiferae) Ilex kundtiana Tr. & P1. (Aquifoliaceae) Isoetes (Isoetaceae) Jamesonia bogotensis

Karst. (Polypodiaceae) Loricaria complanata

(Sch. Bip.) Wedd. (Compositae)

339 L ycopodium (Lycopodiaceae) L ysipomia (Campanulaceae) Miconia (Melastomataceae) Monnina (Polygalaceae) Montia (Portulacaceae) Mougeo tia (Zygnemataceae) Myrica parvifolia Benth. (Myricaceae) Oco tea (Lauraceae) Oreopanax (Araliaceae) Parietaria de bilis Forst. (Urticaceae) Piper (Piperaceae) Plantago rigida H. B. K. (Plantaginaceae) Podocarpus (Podocarpaceae) Poly lepis q uadrijuga Bitter (Rosaceae) Prunus (Rosaceae) Psychotria (Rubiaceae) Quercus (Fagaceae) Rapanea (Myrsinaceae) Rapanea dependens

Sapium (Euphorbiaceae) Saurauia (Saurauiaceae) Senecio niveo-aureus Cuatr. (Compositae) Senocio vaccinoiides

(R. & P.) Mez (Myrsinaceae) Rhamnus (Rhamnaceae) Ribes (Saxifragaceae)

Weinmannia pinnata L. (Cunoniaceae) Weinmannia rollottii Killip (Cunoniaceae) X y losma (Flacourtiaceae)

(Sch. Bip.) H. & B. (Compositae) Stylogyne (Myrsinaceae) Styrax (Styracaceae) Sphagnum (Musci, Sphagnaceae) Swallenochloa tesselata

(Munro) McClure (Gramineae) Ternstroemia (Theaceae) Tibouchina (Melastomataceae) Urtica ballotaefolia Wedd. (Urticaceae) Valeriana plantaginea

H. B. K. (Valerianaceae) Vallea (Elaeocarpaceae) Viburnum (Caprifoliaceae) Weinmannia fagaroides

H. B. K. (Cunoniaceae)

ACKNOWLEDGEMENTS T h e p r e s e n t s t u d y was m a d e possible b y grants f r o m the N e t h e r l a n d s foundation for Tropical Research, WOTRO. We are v e r y g r a t e f u l to Drs. A n t o i n e M. Cleef a n d Dr. R o b e r t o J a r a m i l l o M. f o r i n f o r m a t i o n on ( t h e t a x o n o m y a n d e c o l o g y of) r e c e n t p ~ r a m o p l a n t s a n d s t i m u l a t i n g discussions, t o t h e I n t e r n a t i o n a l T r a i n i n g C e n t r e for Aerial S u r v e y I T C in E n s c h e d e for p r e p a r i n g t h e s l o t t e d t e m p l e t , t o Prof. dr.

A. D. J. Meeuse f o r t h e c o r r e c t i o n o f t h e English t e x t , to Mr. Gijs O o m e n f o r t h e p r e p a r a t i o n o f a n u m b e r o f figures, t o t h e N e t h e r l a n d s Soil S u r v e y I n s t i t u t e f o r t h e c a r t o g r a p h i c w o r k in r e l a t i o n to Fig.18, and t o Ms J o d y d o s S a n t o s f o r her c o n t i n u o u s a n d s t i m u l a t i n g assistance w i t h t h e p r e p a r a t i o n of the manuscript.

REFERENCES Cleef, A. M., 1978. Characteristics of Neotropical pfiramo vegetation and its subantarctic relations. In: C. Troll and W. Lauer (Editors), Geoecological Relations Between the Southern Temperate Zone and the Tropical Mountains. Erdwiss. Forsch., 11: 365-396.

Cleef, A.M., 1979. Altitudinal sequence of the vegetation of Pfiramos of the Cordillera Oriental, Colombia. Actas IV Symposium Internacional de Ecolog[a Tropical, Panama, 1977, I: 281--297.

Cleef, A.M., in press. The Vegetation of the Pfiramos of the Colombian Cordillera Oriental.

Dissertationes Botinacae. J. Kramer, Vaduz.

Gonzalez, E., Van der Hammen, T. and Flint, R. F., 1965. Late Quaternary glacial and vegetational sequence in Valle de Lagunillas, Sierra Nevada del Cocuy, Colombia.

Leidse Geol. Meded., 32 : 1.57--182.

340 Grabandt, R. A. J., 1980. Pollen rain in relation to arboreal vegetation in the Cordillera Oriental. Rev. Palaeobot. Palynol., 29: 65--147.

Heine, K., 1973. Zur Glazialmorphologie und pr~ikeramischer Arch~ologie des mexikanischen Hochlandes w~ihrend des Sp~itglazials (Wisconsin) und Holoz~ns. Erdkunde, 27(3): 161--180.

Herd, D. G., 1977. Neoglaciation in the tropical Andes. Abstracts X Inqua Congress, Birmingham, p.202.

Kraus, E. and Van der Hammen, T., 1960. Las expediciones de glaciologfa del A.G.I. a las Sierras Nevadas de Santa Marta y del Cocuy. Comit~ Nac. del Afio Geoffsico, Instituto Geogr~fico "Agustin Codazzi", Bogota, 9 pp.

Lauer, W., 1979. Die hypsometrische Asymmetrie der P~ramo-HShenstufe in den nSrdlichen Andes. Innsbr. Geogr. Stud., 5: 115--130.

Raasveldt, H. C., 1957. Las glaciaciones de la Sierra Nevada de Santa Marta. Rev. Acad.

Columb. Cienc. Exact. Fis. Nat., 9: 469--482.

Schreve-Brinkman, E. J., 1978. A palynological study of the Upper Quaternary sequence in the E1 Abra corridor and rock shelters (Colombia). Palaeogeogr., Palaeoclimatol., Palaeoecol., 25: 1--109. (Also in: The Quaternary of Colombia, Vol. 6.)

Smit, A., 1978. Potlenmorphology of Polylepis boyacensis Cuatrecasas, Acaena cylindristachya Ruiz et Pavon and Acaena elongata L. (Rosaceae) and its application to fossil material. Rev. Palaeobot. Palynol., 25: 393--398.

Van Geel, B. and Van der Hammen, T., 1973. Upper Quaternary vegetational and climatic sequence of the Ffiquene area (Eastern Cordillera, Colombia). Palaeogeogr., Palaeoclimatol., Palaeoecol., 14: 9--92. (Also in: The Quaternary of Colombia, Vol. 1.)

Van Geel, B. and Van der Hammen, T., 1978. Zygnemataceae in Quaternary Colombian sediments. Rev. Palaeobot. Palynol., 25: 377--391. (Also in: The Quaternary of Colombia, Vol. 5.)

Van der Hammen, T., 1962. Palinologfa de la regiSn de Laguna de los Bobos. Historia de su clima, vegetaciSn y agricultura durante los filtimos 5000 afios. Rev. Acad. Colomb.

Cienc. Exact. Fis. Nat., 11: 359--361.

Van der Hammen, T., 1968. Climatic and vegetational succession in the Equatorial Andes of Colombia. In: Geo-ecology of the Mountainous Regions of the Tropical Americas.

Colloq. Geograph., 9: 187--194.

Van der Hammen, T., 1974. The Pleistocene changes of vegetation and climate in tropical South America. J. Biogeogr., 1: 3--26.

Van der Hammen, T., 1978. Stratigraphy and environments of the Upper Quaternary of the E1 Abra corridor and rock shelters (Colombia). Palaeogeogr., Palaeoclimatol., Palaeoecol., 25: 111--162. (Also in: The Quaternary of Colombia, Vol. 6.)

Van der Hammen, T., 1979. Historia y tolerancia de ecosistemas parameros. In: M. L. Salgado-Labouriau (Editor), E1 medio ambiente p~ramo. Actas del seminario de M~rida, Venezuela, 5 a 12 de Noviembre de 1978. Caracas, Ediciones Centro de Estudios Avanzados, pp.55--66.

Van der Hammen, T. and Cleef, A. M., 1978. Pollenmorphology of Lysipomia H. B. K. and Rhizocephalum Wedd. (Campanulaceae) and the revision of the pollen determination "Valeriana stenophylla" Killip. Rev. Palaeobot. Palynol., 25: 367--376.

(Also in: The Quaternary of Colombia, Vol. 5.)

Van der Hammen, T. and Gonzalez, E., 1960. Upper Pleistocene and Holocene climate and vegetation of the Sabana de Bogota. Leidse Geol. Meded., 25: 261--315.

Van der Hammen, T. and Gonzalez, E., 1965. A Late-glacial and Holocene pollen diagram from Cienaga de! Visitador (Dept. Boyac~, Colombia). Leidse Geol. Meded., 32: 193-201.

Van der Hammen, T. and Vogel, J. C., 1966. The Susac~ interstadial and the subdivision of the Late-Glacial. Geol. Mijnb., 45(2): 33--35.

Van der Hammen, Duefias, H. and Thouret, J.C., 1980. Guia de excursion Sabana de Bogota. Primer Seminario sobre el Cuaternario de Colombia. Bogota, 49 pp.