Quaternary Science Journal - Glaciation and Periglacial Morphology in the Northwestern Venezuel...

Eiszeitalter

u.

Gegenwart

Band

26

Seite

196-211

Öbringen/Württ.

1975

Glaciation and Periglacial Morphology in the Northwestern Venezuelan Andes Dedicated to the memory of Carl Troll by

C A R L O S SCHUBERT,

Caracas

With 5 figures and 2 plates

A b s t r a c t . Evidences of previous glaciation in the northwestern Venezuelan Andes, mainly in Paramo de Piedras Biancas, include a main morainic level at 3400 to 3700 m, and minor moraines at higher elevations up to about 4300. Glaciated valleys, hanging valleys, rock, steps, glacial channels, cirques, aretes, horns, striae and grooves on polished rock, roche mountonnes, and whaleback forms, are the main evidence of glacial sculpture and erosion. The main morainic level probably represents a Late Wisconsin glacial advance, and the higher moraines are either neoglacial or due to minor readvances during the main postglacial retreat. The main morainic level, 400 m higher than that in the southeastern Venezuelan Andes, indicates that the glacial climate was drier in the northwestern p a r t than in the southeastern part of this mountain range.

Present-day periglacial features include micro-terrassettes, turf exfoliation, talus, and striated ground, above 3500 m; and nonsorted stripes, sorted polygons or sorted circles, and sorted nets, above 4000 m. The present-day periglacial zone is located between approximately 3600 and 4700 m.

During glacial times it probably reached down to 2400 m in the southeastern, and 2900 m in the northwestern parts of the Venezuelan Andes.

Zusammenfassung.

Zu den wichtigsten Zeugen einer früheren Vergletscherung in den nordwestlichen venezolanischen Anden, hauptsächlich im Paramo de Piedras Biancas, zählen ein Hauptmoränenniveau (3400 bis 3700 m) und höhere Moränen bis zu 4300 m. Trog- und Hänge­ täler, Felsstufen, glaziale Schmelzwasserrinnen, Kare, schroffe Grate, Hörne, Gletscherschrammen und -furchen auf poliertem Gestein sowie Rundhöcker und Walrücken sind die wichtigsten Zeugen der Glazialskulptur und -erosion. Das Hauptmoränenniveau wurde wahrscheinlich durch einen Jungwisconsin-Gletschervorstoß gebildet, und die höheren Moränen sind entweder neoglazial oder sie wurden durch kleine Vorstöße während des postglazialen Hauptgletscherrückgangs gebildet. Das Hauptmoränenniveau, das 400 m höher liegt als in den südöstlichen venezolanischen Anden, deutet darauf hin, daß das Glazialklima im nordwestlichen Teil dieser Kordillere trockener w a r als im südöstlichen Teil.

Die heutigen Periglazialformationen enthalten: Hänge mit Bodenfließmarken, Rasenstufen, Schutthalden und Felsenmeere sowie Streifenböden über 3500 m; nicht-sortierte Streifen, sortierte Polygone oder sortierte Zirkel; über 4000 m sortierte Netze. Die heutige Periglazialzone liegt un­ gefähr zwischen 3600 und 4700 m Höhe. In der Eiszeit hat ihre untere Grenze wahrscheinlich bis 2400 m im südöstlichen Teil, und bis 2900 m im nordwestlichen Veil der venezolanischen Anden gereicht.

1.

Introduction

The P ä r a m o de Piedras Biancas, the most extensive high altitude zone in the n o r t h ­ western Venezuelan Andes (Sierra d e L a C u l a t a ) , is located (Fig. 1) between approximately 8° 50' and 8° 5 4 ' N. latitude, 7 0 ° 5 1 ' and 70° 5 5 ' W . longitude. It lies between the upper reaches of R i o Chama, which is Quebrada ( = creek) Mifaff, and Quebrada Piedras Biancas (Fig. 2). The R i o Chama flows longitudinally along the central, fault-controlled, v a l l e y of the Venezuelan Andes, in a southwest direction, and the latter flows directly n o r t h ­ west, both ending in the Lake Maracaibo Basin. The area of study lies between a p p r o x i -

Glaciation and Periglacial Morphology

197

m a t e l y 3 7 0 0 a n d 4 6 0 0 m e l e v a t i o n a b o v e sea l e v e l , i n the p ä r a m o z o n e , the w e t , t r e e ­ less, h i g h a l t i t u d e , a l p i n e zone of t h e n o r t h e r n A n d e s ( E c u a d o r , C o l o m b i a , a n d V e n e z u e l a ) .

T h e p r e - Q u a t e r n a r y g e o l o g y of t h e a r e a h a s b e e n s t u d i e d b y K O V I S A R S ( 1 9 6 9 , 1 9 7 1 ) .

It is u n d e r l a i n b y a P r e c a m b r i a n ( ? ) m e t a s e d i m e n t a r y t e r r a i n ( S i e r r a N e v a d a F o r m a t i o n ) , consisting of schist, gneiss, a n d a m p h i b o l i t e ( m e t a m o r p h o s e d to t h e a m p h i b o l i t e f a c i e s of r e g i o n a l m e t a m o r p h i s m ) . These r o c k s a r e o v e r l a i n b y l o w e r g r a d e (greenschist f a c i e s ) U p p e r P a l e o z o i c ( ? ) m e t a s e d i m e n t s (EI A g u i l a F o r m a t i o n ) , c o n s i s t i n g of a s e q u e n c e ( b o t t o m to t o p ) of q u a r z i t e , p h y l l i t e a n d s i l t s t o n e , a n d l i m e s t o n e . These rocks w e r e i n t r u d e d d u r i n g t h e T r i a s s i c b y t h e El C a r m e n G r a n o d i o r i t e a n d l a t e r s m a l l e r b o d i e s of p e g m a t i t e . S t r u c t u r a l l y , the m e t a s e d i m e n t a r y u n i t s c r o p out in b r o a d , n o r t h e a s t t r e n d i n g , a n t i f o r m s a n d s y n f o r m s . S e v e r a l n o r t h e a s t t r e n d i n g f a u l t s o c c u r i n the region, b u t t h e m a i n s t r u c t u r a l f e a t u r e is the B o c o n o F a u l t , a m a j o r geotectonic b r e a k w h i c h f o l l o w s the v a l l e y s of the R i o C h a m a a n d R i o S a n t o D o m i n g o , just southeast of the a r e a ( K O V I S A R S 1 9 7 1 , Fig. 5 ) . P o s t g l a c i a l r i g h t - l a t e r a l s t r i k e - s l i p m o v e m e n t h a s b e e n i n f e r r e d a l o n g this f a u l t from d i s p l a c e d g l a c i a l a n d f l u v i o - g l a c i a l f e a t u r e s (SCHUBERT & SIFONTES, 1 9 7 0 , 1 9 7 2 ; GIEGENGACK & G R A U C H , 1 9 7 2 a , 1 9 7 2 b ) . A n o r t h w e s t w a r d t h r u s t ( G a v i l ä n F a u l t ) h a s been m a p p e d b y K O V I S A R S ( 1 9 7 1 ) a l o n g the u p p e r R i o C h a m a v a l l e y , w e s t of the B o c o n o Fault trace.

W i l h e l m SIEVERS ( 1 8 8 5 — 1 8 8 6 , p . 1 0 1 — 1 0 3 ) , one of the first t o describe g l a c i a l f e a t u r e s in the V e n e z u e l a n A n d e s , v i s i t e d the s o u t h e r n b o u n d a r y of t h e P ä r a m o de P i e d r a s B i a n c a s a r e a , w h e n h e climbed t h e P i c o P a n de A z i i c a r ( 4 6 2 0 m ) , t h e highest p e a k of the

Fig. 1. Index map.

P a r a m o de L a C u l a t a region, n o r t h of the c i t y of M e r i d a a n d just s o u t h of the s t u d y a r e a (Fig. 1 ) . H o w e v e r , h e did not t r a v e l o v e r the M e r i d a - M u c u c h i e s - T o r o n d o y t r a i l , w h i c h t r a v e r s e s P a r a m o de P i e d r a s B i a n c a s , a l o n g Q u e b r a d a P i e d r a s B i a n c a s . M o r e r e c e n t l y , R O Y O Y GOMEZ ( 1 9 5 9 ) d e s c r i b e d b r i e f l y the m o r a i n e s a t the b o t t o m of the Q u e b r a d a M i f a f i v a l l e y ( F i g . 2 ) a n d g l a c i a l f e a t u r e s d o w n v a l l e y , a l o n g t h e R i o C h a m a , to M u c u chies. The g l a c i a l m o r p h o l o g y of t h e Q u e b r a d a M i f a f i v a l l e y w a s d e s c r i b e d b y CARDENAS ( 1 9 6 2 ) , w h o m e n t i o n e d the e x i s t e n c e of U - s h a p e d g l a c i a l v a l l e y s , a r e t e s , c i r q u e s , h o r n s ,

Glaciation and Periglacial Morphology

199

Xs'X* ARETE

MIFAFI MORAINES

I ^y>|ciRQÜE

TALUS

I Cu l

J CIRQUE UKE

'

BLOCK TALUS

FEATURE

PEAT BOG

L^y|GLACIAL CHANNEL

|"U"U|SOLIFLUCTIQN FEATURES

R

I t»?| OCK STEP ROCHE MOUTONNEE \ ^

[STRIAE, GROOVES

ROAD

PIEDRAS BLANCAS MORAINES

,«*JDIRT ROAD

m :

GLACIAL LAKE

LA PAILA MORAINES

JTRAIL

:

|V. -.v. |LOS NEVADOS F:-':'.VIMOR AINES

1CREEK

MINOR MORAINES

Explanations on figure 2.

rock, steps, striated and polished roches moutonnees, talus, flat v a l l e y bottoms probably representing ancient glacial lakes closed b y morainic deposits, and remnants of lateral moraines along the right side of the v a l l e y . K O V I S A R S ( 1 9 6 9 , p. 1 0 0 — 1 0 3 ) briefly des­ cribed the glacial features of the P a r a m o de Piedras Biancas area. This author mentioned the existence of terminal moraines in the Quebrada Piedras Biancas v a l l e y (Fig. 2 and Plate I a ) , described in more detail in the present report.

The area of study is w h o l l y within the paramo zone of the Andes, whose

physical

and botanical characteristics w e r e treated in detail by A Z O C A R ( 1 9 7 4 ) , is characterized in general b y high humidity and a great d a i l y range in temperatures (up t o a d a i l y freezet h a w cycle). Table 1 lists typical r a i n f a l l data of stations close to the area. From these data, and the distribution of the stations (Fig. 1 ) , it is clear that the southeastern part of P ä r a m o de Piedras Biancas is in a " d r y " belt (yearly r a i n f a l l between 6 0 0 and 8 0 0 m) lying along the central axis of the Venezuelan Andes (this w o u l d correspond t o the Subalpine P a r a m o , a climatic definition b y E W E L & M A D R I Z 1 9 6 8 , p. 2 2 3 ) . Both t o the north­ west and the southeast, y e a r l y r a i n f a l l increases considerably in a short distance, to over 1 2 0 0 m m in Santo Domingo (the climatic Pluvial P ä r a m o of EWEL & M A D R I Z 1 9 6 8 , p.

Table 1 R a i n f a l l data (monthly average in mm) for selected stations near the Päramo de Piedras Biancas area (After MINISTERIO DE OBRAS PUBLICAS, Direccion de Obras Hidraulicas, Caracas).

J 35.7 Päramo de la Culata (3000 m; 1961—1970) 29.6 Mucubaji (3550 m; 1969—1973) 8.6 Pico El Aguila (4090 m; 1953—1970) 8.5 Päramo de Mucuchies (3750 m; 1942—1970) 6.5 San Rafael de Mucuchies (3200 m; 1949—1970)

F

M

A

M

J

J

A

S

O

N

D

Year

22.5

44.5

180.3

162.2

101.5

75.3

109.3

118.0

173.4

129.0

34.7

1186.8

17.9

25.1

118.4

95.1

130.3

128.9

121.0

133.8

103.2

66.0

21.8

991.1

7 8

17.4

68.1

103.4

124.4

127.7

97.9

83.4

86.5

54.6

16.3

796.1

8.6

17.9

69.9

107.2

130.6

143.4

109.7

80.5

80.3

38.9

18.2

813.7

4.3

13.4

66.7

92.0

80.2

91.1

77.5

77.7

71.9

31.6

8.5

630.4

200

Carlos Schubert

2 2 3 — 2 2 9 ) . These climatic conditions a r e the result of the geographical location and orien­ tation of the Venezuelan Andes. W A L T E R ( 1 9 7 3 , p. 1 2 3 — 1 3 0 ) summarized the climatic conditions of Venezuela. During the w i n t e r months (northern hemisphere: November to A p r i l ) the main p a r t of Venezuela is unter the influence of the northeast Trade Winds which, over land, change to a more easterly direction o v e r the Llanos (east of the Andes), and a more n o r t h e r l y direction in the Maracaibo Basin (northwest of the Andes). As these winds reach the A n d e s and ascend, the scant moisture left in them after discharge when reaching the Caribbean Sea, is dropped in the foothills and flanks of the mountain range.

Little moisture reaches the internal v a l l e y s of the Andes (such as the R i o C h a m a v a l l e y ) .

During the summer months ( M a y to October), the main p a r t of Venezuela is under the influence of the Inter-Tropical Convergence Zone, and therefore receives abundant rain.

A climatic profile across the Venezuelan Andes, illustrating these climatic conditions, was published by FLOHN ( 1 9 6 8 ) . The vegetation of the p ä r a m o zone has been recently des­ cribed in detail b y V A R E S C H I ( 1 9 7 0 ) ; outstanding are several species of Espeletia.

This study of glacial evidence in P ä r a m o de Piedras Biancas is the first in the north­ western Venezuelan A n d e s (Maracaibo Basin side). U p t o n o w , earlier publications on glaciation ( T R I C A R T 1 9 6 6 ; SCHUBERT 1 9 7 0 , 1 9 7 2 a , 1 9 7 2 b ; SCHUBERT & V A L A S T R O

1974)

dealt with the central and southeastern parts (Sierra de S a n t o Domingo, Mucuchies area, Sierra N e v a d a de M e r i d a , and P ä r a m o de La Culata). A comparison between these areas and Päramo de Piedras Biancas is of interest, because there are indications of climatic differences during glacial times between the southeastern, central, and northwestern Vene­ zuelan Andes, just as there are differences at present (humid in the southeastern and central, and r e l a t i v e l y d r y in the northwestern Venezuelan Andes). Finally, periglacial features are described in a effort to define more precisely the contemporary periglacial zone in the Venezuelan Andes.

A e r i a l photographs w e r e essential in mapping the distribution of the glacial and peri­ glacial features, especially Photographs N o . 1 4 0 to 1 4 9 , 2 1 4 to 2 2 6 , and 5 3 3 to 5 3 6 of mission A - 3 4 ( C A R T O G R A F I A N A C I O N A L , Caracas). Elevations were determined with a Lambrecht altimeter, whose accuracy is at best within 2 0 m. The altimeter w a s frequently checked during the d a y at points of k n o w n elevation.

2. Glacial a n d fluvio-glacial

deposits

In the Quebrada Piedras Biancas v a l l e y (Fig. 2 ) , between a p p r o x i m a t e l y 3 4 0 0 and 3 7 0 0 m, there is an extensive complex of moraines forming a prominent level within the v a l l e y (Plate I a). W e l l - f o r m e d lateral moraines in the v a l l e y s of Quebradas Piedras Biancas and La Paila, a r e called in this paper the Piedras Biancas and La Paila Moraines.

The latter is preserved best, including an end moraine which has been dissected by the stream. In the l o w e r p a r t of the Quebrada Los Nevados v a l l e y (below Pico Los Nevados), another morainic complex (Los N e v a d o s Moraines) crops o u t : this one p a r t l y overrode the Piedras Biancas right lateral moraine (Fig. 2).

The lateral moraines, forming typical arcuate ridges, begin at the rocky outlets of the glaciated valleys (above 3 6 0 0 to 3 7 0 0 m) and descend to 3 4 0 0 m, where they end either as an end moraine o r as a gap cut b y the stream flowing w i t h i n the morainic valleys. The elevation of these morainic ridges ranges from 50 to more than 1 0 0 m above their central v a l l e y . Within the morainic v a l l e y s , the streams either h a v e eroded a w a y all the fluvioglacial deposits (Quebrada Piedras Biancas), or they are in process of eroding them (Quebrada La Paila), forming extensive, l o w terraces. Peat bog deposits, interlayered with fluvio-glacial sediments, are common in the valleys.

Plate

I

a. Panoramic view of the main morainic level. P채ramo de Piedras Biancas; P B ~ Piedras Biancas Moraines; LP = La Paila Moraines; LN = Los Nevados Moraines. Note the fluvioglacial terraces within Quebrada La Paila velley. b. Typical till outcrop, Mifafi Moraines, c. Roche moutonnee, upper Quebrada Mifafi valley. Note the grooves on its side. d. Striated and grooved whaleback form in banded gneiss, upper Quebrada Piedras Biancas valley.

202

Carlos Schubert

In the area between the Piedras Biancas and La Paila Moraines (Fig. 2 and Plate I a), at least t w o remnants of older lateral moraines occur. These form low, straight ridges, not more than 2 0 m high, extensively covered by vegetation and talus deposits. They are parallel or subparallel to the Piedras Biancas lateral moraine, and could represent older glacial advances in that valley. This is supported by the fact that they were completely overridden by the Los Nevados Moraines.

A b o v e the main morainic level, l o w ( 1 0 to 3 0 m) minor moraines f o r m arcuate ridges enclosing cirque outlets or rock steps. Several good examples occur in the Quebrada Mifafi valley (Fig. 2 ) , where t w o small end moraines (mentioned by CARDENAS 1 9 6 2 ) enclosed the lower t w o , elongated, flat bog areas, a b o v e the rock steps. These moraines (Mifafi Moraines) are p a r t l y covered b y talus derived from the steep v a l l e y walls. A t present, the stream has cut through these moraines and is actively cutting the rock steps.

In the upper rock step, it has cut a small gorge with numerous examples of large potholes in the basement rock. In the upper bog area (called L l a n o Redondo), just above the place where Quebrada M i f a f i turns upstream to the southwest, are three minor moraines (Fig. 2 ) , which closed the outlet of the glacial channel just above them. They form l o w ridges which barely overlook the flat bog deposits. A l l of these minor moraines in the higher parts of P a r a m o de Piedris Biancas may represent either small glacial readvances during the main postglacial retreat of the glaciers, or they represent neoglacial moraines. The latter is p r o b a b l y the case f o r the moraines above 4 0 0 0 m, because of their r e l a t i v e l y fresher appearance.

The moraines consist of till characterized by an unsorted and unbedded mixture of boulder- to clay-sized grains. Plate I a shows an outcrop of till from the Mifafi Moraine just west of Loma Cebolleta (Fig. 2 ) . The fresh, d r y till has a light gray (N 7 to 5 B 7 / 1 in the Munsell C o l o r System) color, and the main lithologic types among the pebbles and boulders are: granitic rocks, gneiss, mica schist, and minor amounts of amphibolite, peg­ matite, and quartz. These rocks are typical of the underlying basement and also make up the high aretes and peaks of the region, and they belong to the Sierra N e v a d a Formation ( K O V I S A R S 1 9 7 1 ) . The shape of the pebbles within the till is mostly prismoidal to ellypsoidal, usually f a i r l y angular. Facets are well developed, and some striated pebbles were also found. These characteristics are similar to those described for tills and diamictons in other regions of the Venezuelan Andes (SCHUBERT 1 9 7 0 ; SCHUBERT & V A L A S T R O 1 9 7 4 ) , such as the northern flank of the Sierra de Santo D o m i n g o and P ä r a m o de La Culata (Fig. 1 ) .

Fluvio-glacial deposits crop out mainly in the Q u e b r a d a La Paila v a l l e y (Fig. 2 and Plate I a), in the f o r m of t w o main terraces, approximately at 3 and 1 0 m elevation above the creek level. The highest terrace (whose elevation ranges between 5 and 1 0 m) ist best exposed in the l o w e r p a r t of the morainic valley, near the outlet through the end moraine, on the right bank of Quebrada La Paila. The sediment within this terrace consists of a sandy conglomerate w i t h little apparent bedding. For this reason, it m a y also be a morainic deposit, possibly from a younger glacial advance, later modified b y erosion. The sediment of the l o w e r terrace ( 1 to 3 m elevation) consists of beds of v a r y i n g thickness (a few centimeters to one meter or more) of conglomerates, conglomeratic sand, and sandy silt. In a few places, thin peat and carbonaceous layers occur within this terrace. Conglo­ meratic lenses, indicating former stream beds, are common within this terrace. The con­ glomerates contain pebbles of gneiss, mica schist, amphibolite, and granitic rocks (Sierra N e v a d a Formation and El Carmen Granodiorite; K O V I S A R S 1 9 7 1 ) . Imbricate structures, together with the coarse-grained nature of these deposits, indicates torrential deposition.

These sediments were deposited during and after deglaciation. Stratigraphically, the sedi­ ments of the upper terrace are older than those of the l o w e r terrace. The main evidence for this is the fact that they o n l y crop out along the l o w e r inside edge of the La Paila

Glaciation and Periglacial Morphology

203

M o r a i n e a n d d i s a p p e a r u p s t r e a m (either e n t i r e l y or u n d e r n e a t h the y o u n g e r t e r r a c e d e p o s i t s ) , a n d a l s o because of its r e l a t i v e l y g r e a t e r w e a t h e r i n g c o m p a r e d w i t h the y o u n g e r t e r r a c e s e d i m e n t s . T h e l a t t e r r e p r e s e n t recent v a l l e y fill which is a c t i v e l y b e i n g c u t a t present b y the s t r e a m .

A s m a l l o u t c r o p of f l u v i o - g l a c i a l sediments o c c u r s in the Q u e b r a d a M i f a f i v a l l e y , just b e l o w the t h i r d rock step u p s t r e a m . O n the left b a n k , this deposit is a b o u t 6 m t h i c k , a n d consists of t h i n (5 to 10 c m ) beds of c o n g l o m e r a t i c s a n d a n d silt. T h e b e d d i n g is h o r i z o n ­ t a l . This o u t c r o p is c o v e r e d b y s a n d y c o n g l o m e r a t e s t r o n g l y influenced b y s o l i f l u c t i o n , o r i g i n a t e d b y t h e influence of the steep v a l l e y w a l l on which it lies a n d b y the h i g h w a t e r c o n t e n t w h i c h freezes a n d t h a w s f r e q u e n t l y ( p r o b a b l y d a i l y ) . S o l i f l u c t i o n has p r o d u c e d l a y e r s p a r a l l e l to the slope b u t r e s t i n g u n c o n f o r m a b l y on the h o r i z o n t a l f l u v i a l s e d i m e n t s .

T h e l a t t e r c o n t a i n f r a g m e n t s of g r a n i t i c rocks, p e g m a t i t e , g r a y p h y l l i t e , a m p h i b o l i t e , gneiss, a n d q u a r t z - m i c a schist. T h e s e rocks a r e t y p i c a l of the S i e r r a N e v a d a a n d El A g u i l a F o r m a t i o n s , w h i c h c r o p out in t h e eastern p a r t of the P ä r a m o d e P i e d r a s B i a n c a s a r e a (KOVISARS 1971).

3 . Glacial sculpture and erosion A b o v e t h e m a i n m o r a i n i c l e v e l ( 3 4 0 0 to 3 7 0 0 m ) , the P ä r a m o de P i e d r a s B i a n c a s r e g i o n is c h a r a c t e r i z e d b y the t y p i c a l features p r o d u c e d b y g l a c i a l s c u l p t u r e . A m o n g these, the most i m p o r t a n t a r e the g l a c i a t e d v a l l e y s , h a n g i n g v a l l e y s , rock steps, g l a c i a l c h a n n e l s , c i r q u e s , a r e t e s , a n d horns. F i g . 3, a v i e w of the u p p e r Q u e b r a d a M i f a f i v a l l e y , s h o w s some of these f e a t u r e s .

T h e m a i n g l a c i a t e d v a l l e y s a r e those of Q u e b r a d a M i f a f i a n d Q u e b r a d a P i e d r a s B i a n c a s . T h e s e a r e t y p i c a l U - s h a p e d v a l l e y s , w i t h v e r y steep to v e r t i c a l p o l i s h e d rock w a l l s . In n u m e r o u s p l a c e s t a l u s deposits ( F i g . 2 ) p r o d u c e a m o r e g e n t l e slope. In o t h e r

Fig. 3. Panoramic view of upper Quebrada Mifafi v a l l e y , to the southwest from road to Pinango.

This view shows the glaciated valley, glacial channels, roche moutonnees, and the Llano Redondo bog area.

204

Carlos Schubert

p l a c e s , bogs c o v e r t h e b o t t o m s of t h e v a l l e y s , a n d t h e s t r e a m s a r e a c t i v e l y e r o d i n g . H a n g ­ ing t r i b u t a r y v a l l e y s a r e c o m m o n a l o n g the m a i n g l a c i a t e d v a l l e y s , e n d i n g u s u a l l y i n a v e r t i c a l d r o p of 5 0 to 100 m.

Rock steps a r e c o m m o n f e a t u r e s in the Q u e b r a d a M i f a f i v a l l e y . A sequence of t h r e e w e l l formed s t e p s in the s t r a i g h t (southeast o r i e n t e d ) l o w e r p a r t of t h e v a l l e y , p e r m i t a flat bog a r e a on t h e u p s t r e a m s i d e of each. T h e s e steps h a v e a h e i g h t of 20 m o r m o r e ; h i g h e r steps i n t h e u p p e r p a r t of Q u e b r a d a L a P a i l a a n d Los N e v a d o s m a y be m o r e t h a n 100 m h i g h . In t h e l a t t e r case, t h e steps r e p r e s e n t c i r q u e outles. G l a c i a l channels, l o n g a n d u s u a l l y s t r a i g h t t r o u g h s , a r e common in t h e Q u e b r a d a M i f a f i v a l l e y , w h e r e t h e y m a r k the u p p e r e r o s i o n a l l i m i t of the g l a c i e r s t h a t filled the v a l l e y . F r o m their h e i g h t , it is inferred t h a t t h e g l a c i e r s m a y h a v e been 100 m o r m o r e thick.

Cirques a r e ubiquitous above 4000 m elevation. T h e y are circular, bowl-shaped struc­ tures, w i t h steep to v e r t i c a l w a l l s s u r r o u n d i n g t h e m l a t e r a l l y a n d u p s t r e a m , a n d w i t h a n a b r u p t rock s t e p d o w n s t r e a m . T h e c i r q u e b o t t o m s often c o n t a i n g l a c i a l l a k e s , a n d s o m e cirques a r e c l o s e d b y s m a l l m o r a i n e s which w e r e f o r m e d a t the t i m e w h e n o n l y c i r q u e g l a c i e r s r e m a i n e d in the r e g i o n . In other a r e a s , such as the S i e r r a d e S a n t o D o m i n g o , S i e r r a N e v a d a d e M e r i d a , a n d P ä r a m o d e L a C u l a t a (SCHUBERT 1 9 7 0 , 1 9 7 2 a ; SCHUBERT & V A L A S T R O 1 9 7 4 ) , the l o w e s t c i r q u e s h a v e b o t t o m s a t a p p r o x i m a t e l y 3 5 0 0 m e l e v a t i o n .

In P ä r a m o d e P i e d r a s B i a n c a s , h o w e v e r , the l o w e s t c i r q u e b o t t o m is just b e l o w 4 0 0 0 m .

T h e possible s i g n i f i c a n c e of this difference in t e r m s of L a t e P l e i s t o c e n e snow l i n e s w i l l be d e a l t w i t h in t h e f o l l o w i n g section.

A b o v e the c i r q u e s , a t e l e v a t i o n s over 4 4 0 0 m a n d u p to the h i g h e s t p e a k s , t h e m a i n g e o m o r p h o l o g i c a l f e a t u r e s a r e t h e aretes a n d h o r n s . T h e aretes a r e n a r r o w , r o u g h , a n d v e r y p r e c i p i t o u s r i d g e s w h i c h j o i n behind the c i r q u e s , f o r m i n g t h r e e - t o f o u r - c o r n e r e d horns (GERBER 1 9 6 9 ) . T h e h i g h e r p a r t s of these f e a t u r e s w e r e a b o v e the g l a c i e r s s i n c e no e v i d e n c e for g l a c i a l a c t i o n is f o u n d on t h e m .

G l a c i a l e r o s i o n a l features a r e w i d e s p r e a d in t h e P ä r a m o de P i e d r a s B i a n c a s r e g i o n .

T h e m a i n ones a r e : s t r i a e , g r o o v e s , roches m o u t o n n e e s , a n d w h a l e b a c k forms. A l l of t h e s e a r e found in t h e u p p e r p a r t s of t h e g l a c i a t e d v a l l e y s , a b o v e the l i m i t of g l a c i a l a n d fluviog l a c i a l s e d i m e n t a t i o n . P l a t e I d s h o w s a w h a l e b a c k f o r m d e v e l o p e d in b a n d e d g n e i s s (upper Q u e b r a d a Piedras B i a n c a s ) , with prominent grooves and striae pointing in a d o w n v a l l e y d i r e c t i o n . P l a t e I c s h o w s an e x a m p l e of a s t r i a t e d a n d g r o o v e d roche m o u tonnee in the Q u e b r a d a M i f a f i v a l l e y . The d i s t r i b u t i o n of these f e a t u r e s is s h o w n in F i g . 2 .

T h e i r o r i e n t a t i o n is c l e a r l y consistent w i t h a d o w n v a l l e y g l a c i e r m o v e m e n t .

4. Extent and age of glaciation From the g l a c i a l features d e s c r i b e d a b o v e a n d t h e i r d i s t r i b u t i o n ( F i g . 2 ) , it is c l e a r t h a t the P ä r a m o d e P i e d r a s B i a n c a s region w a s f o r m e r l y g l a c i a t e d d o w n to a t l e a s t 3 4 0 0 m e l e v a t i o n . T h i s is the m i n i m u m e l e v a t i o n of e n d m o r a i n e s in the region, a n d r e ­ presents the m i n i m u m r e c o g n i z a b l e e x t e n t of the g l a c i e r s . In the S i e r r a d e S a n t o D o m i n g o (SCHUBERT 1 9 7 2 b , 1 9 7 3 a ) a n d P ä r a m o de L a C u l a t a (SCHUBERT & V A L A S T R O 1 9 7 4 ) , a l o w e r l i m i t of p o s s i b l e m o r a i n e s w a s described a t 2 6 0 0 to 2 7 0 0 m e l e v a t i o n . M o r a i n e s w e r e not f o u n d a t t h i s l e v e l in t h e P ä r a m o d e P i e d r a s B i a n c a s r e g i o n , in the o n l y a r e a t h a t reaches this l o w e l e v a t i o n , j u s t off the m a p ( F i g . 2 ) to the n o r t w e s t , n e a r M u c u m p a t e .

T h e absence of t h i s m o r a i n i c l e v e l d u e to p a l e o c l i m a t i c d r i e r c o n d i t i o n s should not be d i s ­ missed w i t h o u t a d d i t i o n a l e v i d e n c e . T h e h i g h e r e l e v a t i o n of the l o w e s t c i r q u e l e v e l in P ä r a m o de P i e d r a s B i a n c a s , as c o m p a r e d w i t h o t h e r r e g i o n s to t h e southeast, m e n t i o n e d in the p r e v i o u s section, m a y be e x p l a i n e d b y t h e e x i s t e n c e of d r i e r c l i m a t i c c o n d i t i o n s during g l a c i a l t i m e s in the c e n t r a l - n o r t h w e s t e r n p a r t of the V e n e z u e l a n A n d e s . T h i s

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w o u l d m e a n a L a t e P l e i s t o c e n e s n o w l i n e of j u s t u n d e r 4 0 0 0 m , c o m p a r e d to a p p r o x i m ­ a t e l y 3 5 0 0 m in the S i e r r a d e S a n t o D o m i n g o a n d P ä r a m o d e L a C u l a t a . M o r e c e r t a i n p a l e o c l i m a t i c d a t a r e q u i r e t h e results of r e c e n t l y b e g u n p a l y n o l o g i c a l i n v e s t i g a t i o n s , i n ­ c l u d i n g the d r i l l i n g of s e v e r a l p e a t bogs.

T h e a g e of the g l a c i a l f e a t u r e s in the V e n e z u e l a n A n d e s w a s p r e v i o u s l y r e v i e w e d a n d discussed (SCHUBERT 1972b a n d in p r e s s ; SCHUBERT & V A L A S T R O 1 9 7 4 ) . On t h e b a s i s of r a d i o c a r b o n d a t e s a n d c o m p a r i s o n s w i t h s i m i l a r f e a t u r e s in the S i e r r a N e v a d a d e l C o c u y ( C o r d i l l e r a O r i e n t a l of C o l o m b i a , 5 0 0 k m to t h e southwest a n d 4 ° of l a t i t u d e south of the c e n t r a l p a r t of the V e n e z u e l a n A n d e s ) (GONZALEZ et a l . 1 9 6 5 ) , it w a s c o n c l u d e d t h a t the m a i n m o r a i n i c level in t h e c e n t r a l a n d s o u t h e a s t e r n V e n e z u e l a n A n d e s ( S i e r r a d e S a n t o D o m i n g o a n d P ä r a m o de L a C u l a t a ) , a t e l e v a t i o n s b e t w e e n 3 0 0 0 a n d 3 5 0 0 m, is p r o b a b l y of L a t e W i s c o n s i n a g e ( L a t e G l a c i a l of GONZALEZ et a l . 1 9 6 5 ) . In t h e C o l o m b i a n C o r d i l ­ l e r a O r i e n t a l this w o u l d c o r r e s p o n d to t h e El A b r a S t a d i a l ( V A N DER H A M M E N 1 9 7 4 ) w h i c h w a s c o r r e l a t e d w i t h t h e Y o u n g e r D r y a s t i m e of E u r o p e . R a d i o c a r b o n d a t i n g of p e a t l a y e r s w i t h i n p o s t g l a c i a l f l u v i a l d e p o s i t s in m o r a i n i c v a l l e y s (SCHUBERT 1 9 7 0 , T a b l e 3 ; SCHUBERT & V A L A S T R O 1 9 7 4 , T a b l e 3 ) i n d i c a t e a m i n i m u m a g e of a p p r o x i m a t e l y 1 0 , 0 0 0 y e a r s B . P. for these m o r a i n e s . T h e s e r a d i o c a r b o n a n a l y s e s ( I V I C - 7 6 2 to - 7 6 7 ; T x - 1 4 3 6 to - 1 4 4 9 ) w e r e d o n e on s e d i m e n t s w h i c h w e r e deposited after the g l a c i e r s h a d r e t r e a t e d a t l e a s t from the l o w e r p a r t of the m o r a i n i c v a l l e y s . T h e r e f o r e , the a g e of the m o r a i n e s t h e m s e l v e s is s i g n i f i c a n t l y o l d e r t h a n t h e r a d i o c a r b o n d a t e s r e p o r t e d so far.

T h e l o w e r m o r a i n i c level ( 2 6 0 0 to 2 7 0 0 m ) w a s a s s i g n e d to t h e m a i n W i s c o n s i n g l a c i a l a d v a n c e ( P l e n i g l a c i a l of GONZALEZ et a l . 1 9 6 5 ) . T h e term M e r i d a G l a c i a t i o n h a s been p r o p o s e d for the g l a c i a l e v e n t w h i c h p r o d u c e d the p r o m i n e n t t i l l a c c u m u l a t i o n in the V e n e z u e l a n A n d e s (SCHUBERT 1 9 7 4 ) . A n i n v e s t i g a t i o n on the r a d i o c a r b o n c h r o n o l o g y of the M e r i d a G l a c i a t i o n w a s b e g u n r e c e n t l y ( i n c o l l a b o r a t i o n w i t h the U n i v e r s i t y of T e x a s R a d i o c a r b o n L a b o r a t o r y ) , t o g e t h e r w i t h p a l y n o l o g i c a l a n a l y s e s of p e a t a n d g l a c i a l l a k e s e d i m e n t s , in order to d e t e r m i n e the L a t e P l e i s t o c e n e a n d H o l o c e n e g l a c i a l a n d c l i ­ m a t i c e v o l u t i o n of the V e n e z u e l a n A n d e s .

T h e g l a c i a l features of t h e P ä r a m o d e P i e d r a s B i a n c a s r e g i o n c a n e a s i l y be i n c l u d e d w i t h i n the M e r i d a G l a c i a t i o n , because t h e m o r a i n i c levels a n d s e d i m e n t s , d e s c r i b e d in p r e v i o u s sections, a r e i d e n t i c a l to those d e s c r i b e d in other r e g i o n s of the V e n e z u e l a n A n d e s . T h e m a i n m o r a n i c l e v e l a t 3 4 0 0 to 3 7 0 0 m is the e q u i v a l e n t of the m a i n m o r a i n i c l e v e l in t h e S i e r r a de S a n t o D o m i n g o , S i e r r a N e v a d a de M e r i d a , a n d P ä r a m o d e L a C u l a t a . In these regions, t h i s m o r a i n i c l e v e l o c c u r s at e l e v a t i o n s b e t w e e n 3 0 0 0 a n d 3 5 0 0 m. T h e difference in t h e l o w e r l i m i t b e t w e e n these r e g i o n s a n d P ä r a m o d e P i e d r a s B i a n c a s ( a b o u t 4 0 0 m ) , s u p p o r t s the s t a t e m e n t m a d e above, t h a t the s n o w s l i n e in the l a t t e r r e g i o n d u r i n g the l a s t g l a c i a l a d v a n c e w a s h i g h e r , p o s s i b l y d u e to a d r i e r c l i m a t e ( s o m e w h a t s i m i l a r to the p r e s e n t - d a y c l i m a t i c difference b e t w e e n P ä r a m o d e P i e d r a s B i a n c a s a n d s u r r o u n d i n g r e g i o n s , as s h o w n in T a b l e 1 ) .

T h e m i n o r m o r a i n e s in t h e h i g h e r p a r t s of t h e P ä r a m o d e P i e d r a s B i a n c a s r e g i o n ( a b o v e 4 0 0 0 m ) a r e s i m i l a r to those in t h e S i e r r a N e v a d a d e M e r i d a (SCHUBERT 1 9 7 2 a ) a n d P ä r a m o d e L a C u l a t a (SCHUBERT & V A L A S T R O 1 9 7 4 ) . T h e y p r o b a b l y r e p r e s e n t p e r i o d s of m i n o r r e a d v a n c e s d u r i n g the m a i n p o s t g l a c i a l r e t r e a t of t h e g l a c i e r s , or t h e y m a y be n e o g l a c i a l m o r a i n e s . T h e last p o s s i b i l i t y h a s to be i n v e s t i g a t e d in m o r e d e t a i l , e s p e c i a l l y in t e r m s of finding m a t e r i a l s u i t a b l e for r a d i o m e t r i c d a t i n g .

5. Periglacial

morphology

T h e t e r m p e r i g l a c i a l is u s e d in the p r e s e n t r e p o r t f o l l o w i n g W A S H B U R N ( 1 9 7 3 , p . 2 ) , in t h a t " i t d e s i g n a t e s c o l d c l i m a t e , p r i m a r i l y t e r r e s t r i a l , n o n g l a c i a l processes a n d f e a t u r e s r e g a r d l e s s of d a t e a n d p r o x i m i t y of g l a c i e r s " . T h e V e n e z u e l a n A n d e s , b e t w e e n 7 ° a n d

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1 0 ° N . l a t i t u d e , c l e a r l y f a l l w i t h i n TRICART'S ( 1 9 7 0 , p . 2 6 — 2 7 ) p e r i g l a c i a l c l i m a t i c classification, as a n a r e a of m o u n t a i n c l i m a t e s of l o w l a t i t u d e s . These a r e c h a r a c t e r i z e d b y e x t e n s i v e frost a c t i o n d u e to f r e q u e n t d a y s of p a r t i a l frost; no p e r m a f r o s t a n d o n l y s l i g h t p e n e t r a t i o n of g r o u n d frost; a n d h i g h h u m i d i t y ( e x c e p t in a r i d r e g i o n s ) which a i d s soil movement.

In the V e n e z u e l a n A n d e s , c l i m a t i c d a t a of h i g h a l t i t u d e s a r e s c a n t , consisting m a i n l y of r a i n f a l l d a t a ( T a b l e 1 ) . T e m p e r a t u r e records of sufficient d u r a t i o n h a v e o n l y been r e c o r d e d at M u c u b a j i , El A g u i l a , a n d the stations a l o n g the M e r i d a - P i c o Espejo f u n i c u l a r (Fig. 1 ) . Fig. 4 s h o w s the a v e r a g e d i u r n a l v a r i a t i o n of t h e a i r t e m p e r a t u r e at these l o c a l i ­

ty

1

I

I

I

I

1 I

I

I

1

I

I

1

1

1

1 1

1

1

1

1

1 L

SI-

HOURS OF THE

Fig. 4 . Air temperatures at high altitude stations in the Venezuelan Andes (for location see Fig. 1 ) .

At El Aguila, the data was recorded between J u l y and December, 1 9 6 8 (INSTITUTO DE GEOGRAFIA 1 9 7 0 , p. 3 7 ) . At Mucubaji, the data w a s recorded between April and September, 1 9 6 8 (NIETO & ARROYO 1 9 6 8 , Fig. 2 7 ) . At the rest of the stations, along the Merida-Pico Espejo funicular, the data was recorded during 1 9 7 0 — 1 9 7 1 (GONZALEZ 1 9 7 1 , Figure 2 0 ) .

Table 2 Temperature of soil at 3 0 cm depth, Venezuelan Andes (after WALTER & MEDINA 1 9 6 9 ) Locality W . of Santo Domingo Mucubaji Paramo de Mucuchies El Aguila Pico Espejo Elevation (m)

Temperature ( ° C )

2915

3600

3940

9.6

5.0

3.9

4280

2.0

4765

—1.5

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ties. A t M u c u b a j i , in a d d i t i o n , the d i u r n a l t e m p e r a t u r e v a r i a t i o n a t g r o u n d l e v e l is i n c l u d e d . A t M u c u b a j i a n d El A g u i l a , t h e m e a n m o n t h l y m i n i m u m t e m p e r a t u r e reaches b e l o w f r e e z i n g o n l y d u r i n g the m o n t h s of D e c e m b e r to M a r c h (NIETO & A R R O Y O 1 9 6 8 , Fig. 3 0 ; INSTITUTO DE G E O G R A F I A 1 9 7 0 , p . 3 7 ) . This m e a n s t h a t M u c u b a j i ( 3 5 5 0 m ) is b e l o w t h e l i m i t of d a i l y freezing a n d t h a w i n g . Fig. 4 s h o w s t h a t this l i m i t is b e t w e e n 4 0 6 5 a n d 4 7 6 5 m. T a b l e 2 lists the s c a n t soil t e m p e r a t u r e d a t a ( a t 30 cm d e p t h ) for the V e n e z u e l a n A n d e s . F r o m t h i s it can be i n f e r r e d t h a t the p e r m a f r o s t region b e g i n s a b o v e a p p r o x i m a t e l y 4 6 0 0 to 4 7 0 0 m, a s s u m i n g t h a t the soil t e m p e r a t u r e r e m a i n s e s s e n t i a l l y c o n s t a n t b e l o w a few c e n t i m e t e r s d e p t h ( W A L T E R & MEDINA 1 9 6 9 ) . F u r t h e r m o r e , FAHEY ( 1 9 7 3 ) h a s r e c e n t l y s h o w n t h a t in the C o l o r a d o F r o n t R a n g e , the g e o m o r p h i c effectiveness of f r e e z e - t h a w c y c l e s is l i m i t e d to the u p p e r 10 c m . F r o m these c l i m a t i c d a t a , i t c a n be t e n t a t i v e l y c o n c l u d e d t h a t the p e r i g l a c i a l z o n e in the V e n e z u e l a n A n d e s is b e t w e e n a p p r o x i m a t e l y 3 6 0 0 m a n d the l o w e r l i m i t of the g l a c i e r s ( a b o u t 4 7 0 0 m ) , w h i c h a r e , at present, r e s t r i c t e d to the h i g h e r p a r t s of t h e S i e r r a N e v a d a d e M e r i d a (SCHUBERT 1 9 7 2 a ) .

In P 채 r a m o de P i e d r a s B i a n c a s , the l o w e s t o c c u r r e n c e of n e e d l e - i c e w a s o b s e r v e d at a p p r o x i m a t e l y 3 6 0 0 m. L o w e r o c c u r r e n c e s a r e p r o b a b l e , b e c a u s e n e e d l e - i c e is o b s e r v e d at Mucubaji (3550 m ) , especially between December a n d March. H o w e v e r , the lower l i m i t is n o t much l o w e r t h a n this, as f r e e z i n g t e m p e r a t u r e s a r e u s u a l l y not r e a c h e d b e l o w 3500 m at a n y time during the year.

O n e of t h e most c o m m o n p e r i g l a c i a l f e a t u r e s in the l o w e r p a r t s of P 채 r a m o d e P i e d r a s B i a n c a s a r e m i c r o - t e r r a s s e t t e s (Fig. 5 ) . T h e s e a r e formed b y n e e d l e - i c e solifluction on the steep v a l l e y sides, e s p e c i a l l y in the Q u e b r a d a M i f a f i v a l l e y , w h e r e m a n y slopes a r e c o v e r e d b y f i n e - g r a i n e d t a l u s a n d turf. I n e x t r e m e cases, turf e x f o l i a t i o n is c o m p l e t e ( P l a t e H a ) , f o r m e d b y t h e process d e s c r i b e d b y TROLL ( 1 9 7 3 ) : n e e d l e - i c e solifluction a i d e d b y d e f l a t i o n . M i c r o - t e r r a s s e t t e s b e c o m e less c o m m o n a b o v e a p p r o x i m a t e l y 4 2 0 0 m, because of the r a p i d d e c l i n e of the grass c o v e r .

Block t a l u s slopes a r e v e r y c o m m o n p e r i g l a c i a l f e a t u r e s a l s o , a n d f o r m e x t e n s i v e covers in b o t h Q u e b r a d a M i f a f i a n d Q u e b r a d a P i e d r a s B i a n c a s v a l l e y s (Fig. 2 ) . I n d i v i d u a l

Fig. 5. Turf micro-terrassettes on solifluction lobe, upper Quebrada Mifafi v a l l e y .

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blocks r a n g e from c o b b l e - s i z e t o s e v e r a l meters i n d i a m e t e r . T h e y a r e d e r i v e d from t h e h i g h v a l l e y w a l l s ( a r e t e s a n d h o r n s ) b y n i g h t l y f r e e z i n g a n d i c e - w e d g i n g of t h e rock. T h e p r o c e s s is v e r y a c t i v e a t present, a n d b r e a k i n g a n d f a l l i n g blocks c a n b e h e a r d f r e q u e n t l y during the night.

B y f a r t h e m o s t c o m m o n p e r i g l a c i a l p h e n o m e n o n i s t h e f o r m a t i o n of s t r i a t e d g r o u n d ( s o r t e d stripes, a c c o r d i n g t o WASHburn 1 9 5 6 ) . T h i s f e a t u r e is p r a c t i c a l l y u b i q u i t o u s a b o v e 4 0 0 0 m , a l t h o u g h i t i s a l s o c o m m o n l y f o u n d d o w n t o a p p r o x i m a t e l y 3 5 0 0 m. T h e best e x a m p l e s a r e i n t h e h i g h e r p a r t s , b e c a u s e of t h e m o r e frequent f r e e z i n g a n d t h a w i n g c y c l e s . P l a t e II b s h o w s a n e x a m p l e of s t r i a t e d g r o u n d . T h e s t r i a e a r e c o n t i n u o u s for 1 0 to 2 0 c m , a n d a r e s e p a r a t e d b y c h a n n e l s of 1 t o 3 c m w i d t h a n d 1 c m d e p t h . T h i s f e a t u r e w a s d e s c r i b e d p r e v i o u s l y i n t h e V e n e z u e l a n A n d e s (SCHUBERT 1 9 7 3 ) , a n d i t w a s c o n c l u d e d t h a t t h e o r i e n t a t i o n of t h e s t r i a e w a s e s s e n t i a l l y p a r a l l e l t o t h e p r e v a i l i n g w i n d d i r e c t i o n , s u p p o r t i n g TROLL'S ( 1 9 4 4 ) o b s e r v a t i o n s . I n t h e h i g h e r p a r t s of t h e Q u e b r a d a M i f a f i v a l l e y , n e e d l e - i c e w a s o b s e r v e d e v e r y m o r n i n g u n t i l a b o u t 1 0 or 1 1 h o u r s ( F e b r u a r y a n d M a r c h ) , b y w h i c h t i m e i t h a d m e l t e d c o m p l e t e l y . B y t h e e a r l y afternoon, t h e s t r i a t e d ground w a s d r y . There is c l e a r l y a g r a d a t i o n between the p o o r l y - d e v e l o p e d striated g r o u n d i n t h e l o w e r p a r t s of t h e v a l l e y , a n d t h e w e l l - f o r m e d s t r i a t e d g r o u n d in t h e h i g h e r p a r t s . T h e l a t t e r c a n o n l y f o r m after s e v e r a l f r e e z e - t h a w c y c l e s , b e c a u s e s o r t i n g p r o c e e d s s l o w l y , b e c o m i n g b e t t e r w i t h each p r o g r e s s i v e c y c l e ( C O R T E 1 9 6 3 ) .

L a r g e n o n s o r t e d s t r i p e s a r e a less c o m m o n f e a t u r e i n P a r a m o d e P i e d r a s B i a n c a s ( P l a t e II c ) . T h i s f e a t u r e is r e s t r i c t e d t o t h e highest p a r t s , a b o v e 4 2 0 0 m , w h e r e t h e g r o u n d is b a r r e n a n d subject t o d a i l y f r e e z e - t h a w c y c l e s . T h e g r o u n d m a t e r i a l i s s o r t e d i n t o s a n d a n d silt stripes, a n d p e b b l e s t r i p e s . P l a t e II d s h o w s a crossection of t h e stripes, w h i c h demonstrates that they a r e o n l y a f e w centimeters deep. T h e s a n d y stripes a r e approx­ i m a t e l y 4 t o 5 c m w i d e , w h i l e t h e p e b b l e stripes a r 3 t o 4 c m w i d e . T h i s f e a t u r e is v e r y s i m i l a r to t h a t d e s c r i b e d b y H A S T E N R A T H ( 1 9 7 1 ) i n t h e C o r d i l l e r a R e a l of B o l i v i a , a n d HASTENRATH

( 1 9 7 3 ) , a n d FURRER & FREUND ( 1 9 7 3 ) o n M o u n t s K e n y a a n d

Kilimanjaro

( E a s t A f r i c a ) . A s i n those r e g i o n s , t h e p e b b l e stripes a r e s e v e r a l c e n t i m e t e r s h i g h e r t h a n t h e s a n d y stripes. T h e g r a i n size, h o w e v e r , c a n v a r y w i d e l y , a s i n d i c a t e d b y t h e o c c u r r e n c e of l a r g e n o n s o r t e d s t r i p e s in P ä r a m o d e M u c u m p o s i t o (just n o r t h e a s t of t h e m a p a r e a of F i g . 2 ) , consisting of fine p e b b l e s t r i p e s a n d s t r i p e s of coarse p e b b l e s t o cobble-size.

N o n s o r t e d stripes o c c u r o n m o d e r a t e slopes, a n d t h e y w i n d a r o u n d b o u l d e r s o r rock outcrops, giving a r a k e d appearance.

I n c i p i e n t s o r t e d p o l y g o n s or s o r t e d c i r c l e s also o c c u r i n some of t h e flat a r e a s , in t h e u p p e r Q u e b r a d a M i f a f i v a l l e y ( a b o v e 4 0 0 0 m ) . T h e s e consist of r o u g h l y c i r c u l a r lines of pebbles a n d c o b b l e s , s u r r o u n d i n g a r e a s of fine e a r t h , u s u a l l y c o v e r e d b y g r a s s or m a d e i n t o l i n e a r f e a t u r e s b y n e e d l e - i c e a c t i o n ( P l a t e H e ) . T h e s e a r e s i m i l a r t o those d e s c r i b e d b y W A S H B U R N ( 1 9 5 6 ) . A s s o c i a t e d w i t h t h e m a l s o a r e i n c i p i e n t s o r t e d nets of pebbles ( P l a t e I I f ) . These a r e r e s t r i c t e d t o s h a l l o w d e p r e s s i o n s i n t h e h i g h p a r t s of t h e v a l l e y , w h e r e pebbles a n d cobbles a p p a r e n t l y h a v e c o l l e c t e d t h r o u g h m i g r a t i o n b y f r e e z e - t h a w a c t i o n from t h e s u r r o u n d i n g slopes. I n p a r t , t h e y a r e a l s o s i m i l a r t o t h e stone p a c k i n g s described b y HASTENRATH ( 1 9 7 1 ) i n B o l i v i a .

T h u s , t h e p r e s e n t - d a y p e r i g l a c i a l z o n e of t h e V e n e z u e l a n A n d e s , c h a r a c t e r i z e d b y m o s t of t h e c l a s s i c a l f e a t u r e s of a l p i n e p e r i g l a c i a l z o n e s , e x t e n d s t h r o u g h a p p r o x i m a t e l y 1 1 0 0 m v e r t i c a l d i s t a n c e (from a p p r o x i m a t e l y 3 6 0 0 t o 4 7 0 0 m ) . T h e s n o w line d u r i n g t h e L a t e P l e i s t o c e n e M e r i d a G l a c i a t i o n w a s e s t i m a t e d a t 3 5 0 0 m for t h e c e n t r a l a n d s o u t h ­ e a s t e r n V e n e z u e l a n A n d e s (SCHUBERT, i n p r e s s ) , a n d a t a b o u t 4 0 0 0 m i n t h e n o r t h w e s t e r n p a r t . I n t h e first a r e a s , this w o u l d c o r r e s p o n d t o a 1 2 0 0 m d r o p w i t h respect t o t h e p r e s e n t - d a y s n o w l i n e . I n t h e n o r t h w e s t e r n V e n e z u e l a n A n d e s this w o u l d c o r r e s p o n d t o a p r e s e n t d a y s n o w l i n e a t a p p r o x i m a t e l y 5 1 0 0 m . S i n c e n o p e a k in this r e g i o n reaches

Eiszeitalter und Gegenwart, Band 26

Plate II (C. Schubert)

Plate

II

a. Periglacial features, upper Quebrada Mifafi valley, a. Turf exfoliation, b. Striated ground, c. Nonsorted stripes, d. Close-up of section across nonsorted stripes of c. e. Incipient sorted polygons or sorted circles, f. Incipient sorted nets.

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such h e i g h t , i t i s n o t possible to check this inference. T h e r e f o r e , t h e p e r i g l a c i a l zone d u r i n g t h e m a x i m u m L a t e Pleistocene g l a c i a l a d v a n c e ( a s s u m i n g a s i m i l a r r e l a t i o n s h i p b e t w e e n t h e g l a c i a l a n d p e r i g l a c i a l z o n e s a s t o d a y ) p r o b a b l y reached d o w n t o a p p r o x i m a t e l y 2 4 0 0 m in t h e s o u t h e a s t e r n , a n d 2 9 0 0 m i n t h e n o r t h w e s t e r n V e n e z u e l a n A n d e s .

6. Conclusions 1. In t h e P ä r a m o d e P i e d r a s B i a n c a s region, n o r t h w e s t e r n V e n e z u e l a n A n d e s , e v i d e n c e of p r e v i o u s g l a c i a t i o n i n c l u d e a m a i n m o r a i n i c l e v e l a t 3 4 0 0 t o 3 7 0 0 m , w i t h m i n o r m o r a i n e s a t d i f f e r e n t l e v e l s a b o v e t h a t ; a n d e v i d e n c e of g l a c i a l s c u l p t u r e a n d e r o s i o n , such a s g l a c i a t e d v a l l e y s , h a n g i n g v a l l e y s , rock steps, g l a c i a l c h a n n e l s , c i r q u e s , a r e t e s , h o r n s , s t r i a e , g r o o v e s , roche m o u t o n n e e s , a n d w h a l e b a c k forms. T h e s e a r e s i m i l a r to those in o t h e r p a r t s of t h e V e n e z u e l a n A n d e s , a n d b e l o n g to t h e M e r i d a G l a c i a t i o n ( L a t e Pleistocene), w i t h higher moraines representing either minor r e a d v a n c e s during the m a i n p o s t g l a c i a l g l a c i e r r e t r e a t , or t h e y a r e n e o g l a c i a l m o r a i n e s . T h e m a i n m o r a i n i c a n d c i r q u e b o t t o m l e v e l s of P ä r a m o d e P i e d r a s B i a n c a s , a r e a p p r o x i m a t e l y 4 0 0 m h i g h e r t h a n those of t h e S i e r r a d e S a n t o D o m i n g o , S i e r r a N e v a d a d e M e r i d a , a n d P ä r a m o d e L a C u l a t a .

This indicates a drier glacial c l i m a t e in the northwestern part t h a n in the southeastern p a r t of t h e V e n e z u e l a n A n d e s .

2. P e r i g l a c i a l features i n P ä r a m o d e P i e d r a s B i a n c a s i n c l u d e m i c r o - t e r r a s s e t t e s , t u r f e x f o l i a t i o n , t a l u s , s t r i a t e d g r o u n d , n o n s o r t e d s t r i p e s , sorted p o l y g o n s o r sorted c i r c l e s , a n d sorted n e t s . A l l of these a r e r e s t r i c t e d to e l e v a t i o n s a b o v e 4 0 0 0 m , except t h e first f o u r , w h i c h a l s o o c c u r less p e r f e c t l y d e v e l o p e d d o w n t o 3 5 0 0 m , w h e r e freezing t e m p e r a ­ t u r e s a r e reached o n l y o c c a s i o n a l l y . D a i l y freezing a n d t h a w i n g i s e s s e n t i a l for the f o r m a ­ t i o n of t h e o t h e r features. T h e p r e s e n t - d a y p e r i g l a c i a l z o n e i n t h e V e n e z u e l a n A n d e s is b e t w e e n a p p r o x i m a t e l y 3 6 0 0 a n d 4 7 0 0 m e l e v a t i o n . D u r i n g t h e m a x i m u m of t h e l a t e s t g l a c i a l a d v a n c e ( M e r i d a G l a c i a t i o n ) , t h e l o w e r l i m i t s of t h e p e r i g l a c i a l zone w e r e p r o b a b l y a p p r o x i m a t e l y 2400 m in the southeastern, a n d 2900 m in the northwestern p a r t s of t h e V e n e z u e l a n A n d e s .

7.

Acknowledgements

R o b e r t GIEGENGACK r e v i e w e d a n d s a g n i f i c a n t l y i m p r o v e d t h e m a n u s c r i p t ; a n e a r l i e r v e r s o n w a s r e v i e w e d b y E . M E D I N A , Z . B A R U C H , a n d E . W A G N E R . I t h a n k M . MONASTERIO,

G . SARMIENTO, a n d A . A Z O C A R f o r i n t r o d u c i n g m e to t h e b o t a n i c a l a n d c l i m a t i c c o n ­ d i t i o n s of P ä r a m o d e P i e d r a s B i a n c a s , a n d for c o n t r i b u t i n g m a n y useful suggestion.

8.

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Manuscript received on 3. 12. 1974.

Address of author: Dr. C. Schubert, Centro de Ecologi'a, Instituto Venezolano de Investigaciones Cientificas, Apartado 1827, Caracas 101, Venezuela.

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