E&G – Quaternary Science Journal - Recent advances in North American Pleistocene stratigraphy

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Recent advances in North American Pleistocene stratigraphy Richard F o s t e r F l i n t Yale University, N e w H a v e n , Connecticut, U.S.A. Introduction T h e Glacial Map of N o r t h A m e r i c a (FLINT and others 1945) assembled the essentials of w h a t was k n o w n of N o r t h American Pleistocene s t r a t i g r a p h y as of 1 9 4 2 ; t h e m a p data w e r e amplified by FLINT (1947). Since the m a p appeared, a l a r g e v o l u m e of field w o r k by m a n y geologists and some other scientists has resulted in much n e w information. T h e discussion t h a t follows is a r e v i e w of some of t h e more i m p o r t a n t advances m a d e during t h e last decade, b u t chiefly d u r i n g t h e last five y e a r s .

Extent

of N e b r a s k a n

drift

sheet

T h e outermost drift in s o u t h e a s t e r n Nebraska, n o r t h e a s t e r n K a n s a s , and n o r t h e r n Missouri had b e e n t h o u g h t to be probably N e b r a s k a n , t h e oldest of the four A m e r i c a n drift sheets. This opinion was based p a r t l y on the inference t h a t because much of that drift consists only of scattered boulders, the drift must be the r e s i d u u m of a sheet of till from which the fines had been r e m o v e d b y erosion, and t h a t such removal m u s t h a v e r e q u i r e d a long time.

This a r g u m e n t loses force w h e n it meets the fact t h a t much of t h e drift in question cocupies a much-dissected belt of country w i t h steep slopes, fringing the Missouri River, a n d some geologists h a v e suspected t h a t the outermost drift is not N e b r a s k a n (First glacial) b u t K a n s a n (Second glacial). This v i e w has been s t r e n g t h e n e d by test borings m a d e in e a s t e r n N e b r a s k a (E. C. REED, unpublished) a n d n o r t h e a s t e r n K a n s a s (FRYE & WALTERS 1 9 5 0 ) w h e r e the o u t e r m o s t drift consists largely of continuous till. T h e borings show t h a t the outer limit of the N e b r a s k a n drift lies well inside t h e b o r d e r of the K a n s a n drift, which overlaps and b u r i e s the N e b r a s k a n drift border. By analogy it seems likely t h a t the outermost drift in n o r t h e r n Missouri is likewise not N e b r a s k a n but K a n s a n .

Borings through t h e Illinoian (Third glacial) drift in Illinois showed still earlier t h a t in that S t a t e the N e b r a s k a n drift sheet is less extensive t h a n the K a n s a n , although both a r e buried b e n e a t h the Illinoian drift. Hence in a wide sector of t h e glaciated region the N e b r a s k a n drift is less extensive t h a n one or m o r e y o u n g e r drifts, a n d in no case is it the most extensive. In e a s t e r n United States w h e r e the outermost drift is variously Wisconsin (Fourth glacial), Illi足 noian, a n d Kansan, the N e b r a s k a n drift has not been identified.

These data lead to t h e probability t h a t the N e b r a s k a n drift sheet is the least extensive of the drift sheets laid d o w n by the L a u r e n t i d e Ice Sheet, although in some districts, as in n o r t h e a s t e r n Iowa, it extends beyond the y o u n g e r drifts.

This probability takes on new i n t e r e s t in the light of the relations recently described (WIRTZ & ILLIES 1 9 5 1 ) from the island of Sylt, off the w e s t coast of D e n m a r k . T h e r e are exposed coarse deposits of S c a n d i n a v i a n origin, possibly rafted on icebergs, and therefore suggesting a glacier t h a t stopped s h o r t of Sylt.

But t h e deposits lie in continuous s t r a t i g r a p h i c sequence w i t h fossil-bearing s t r a t a t h a t are a p p a r e n t l y u p p e r m o s t Pliocene; hence, if glacial, they d a t e from

6

Richard Foster Flint

the First glaciation. A First glacial ice sheet t h a t fell short of t h e limits reached by its successors is w h a t seems to be recorded i n t h e United States, and if t h e reasoning leading to this view is accepted, then t h e related climatic fluctuation was common to b o t h America a n d Europe.

Nonglacial s e q u e n c e in Kansas, Nebraska, and South Dakota Recent extensive studies h a v e clarified t h e Pleistocene nonglacial sequence in t h e t e r r i t o r y stretching w e s t w a r d from t h e Missouri River. T h e r e t h e rivers flowing e a s t w a r d from t h e Rocky Mountains across t h e G r e a t Plains built a series of alluvial fills a l t e r n a t i n g w i t h erosional excavations. T h a n k s to t h e fossil-vertebrate content of t h e fills, t h e w i d e s p r e a d presence of a distinctive layer of volcanic ash, a n d t h e s t r a t i g r a p h i c relations of fills to till sheets, c o r r e ­ lation in considerable detail h a s been possible. T h e sequence recognized in Nebraska and South D a k o t a is s h o w n in Table I. Details for K a n s a s h a v e been published b y FRYE, SWINEFORD & LEONARD (1948), a n d for N e b r a s k a by CONDRA,

REED & GORDON (1950). T h e S o u t h Dakota column is detailed in a report by FLINT, n o w in course of publication b y t h e United States Geological Society.

T h a t t h e various s e d i m e n t a r y bodies differ conspicuously from each other as to grainsize is evident i n Table I. However, n o consistent correlation between these differences, a n d glacial- interglacial climatic fluctuations, exists. This suggests (although it does n o t prove) that grainsize changes a r e t h e result, a t least in part, of factors other t h a n climatic changes. T h e y m a y be in p a r t t h e result of broad c r u s t a l m o v e m e n t s .

T a b l e I — C o r r e l a t i o n of P l e i s t o c e n e a l l u v i a l d e p o s i t s in N e b r a s k a a n d S o u t h D a k o t a w i t h t h e glacial s e q u e n c e Glacial sequence

Nonglacial sediments in Nebraska and South Dakota

Wisconsin (Fourth glacial)

Various glacial, alluvial a n d loess d e p o s i t s

Sangamon (Third interglacial)

(Deep

weathering)

Illinoian (Third glacial)

Loveland formation (valley p h a s e ) (silt a n d clay) Crete sand a n d gravel

Yarmouth (Second interglacial)

Sappa silt a n d clay (including Pearlette volcanic ash horizon)

Kansan (Second glacial) Aftonian (First interglacial) Nebraskan (First glacial)

Grand Island sand and gravel Red Cloud sand a n d gravel Fullerton silt a n d c l a y Holdrege sand and gravel

A n interesting fact e m e r g i n g from study of t h e fossils in t h e alluvial fills in N e b r a s k a is t h a t t h e most conspicuous faunal change occurred at t h e time of m a x i m u m e x t e n t of t h e K a n s a n ice sheet. This is n o t p r i m a r i l y a change from glacial to nonglacial, or vice-versa, b u t consists simply of strikingly n e w ele-

Recent advances in North American Pleistocene stratigraphy

7

m e n t s in t h e regional m a m m a l - f a u n a . (See faunal chart in CONDRA, REED & GORDON 1 9 5 0 ) . T h e u n d e r l y i n g cause of t h e change is not a p p a r e n t .

E x t e n t a n d s t r a t i g r a p h y of I l l i n o i a n d e p o s i t s W h e n t h e Glacial Map of North A m e r i c a (FLINT and others 1945) w a s p u b ­ lished, Illinoian drift had n o t been recognized west of t h e Mississippi River except in e x t r e m e s o u t h e a s t e r n Iowa.

Because of this, some geologists believed t h a t in Illinoain time the ice sheet w a s confined to the n o r t h e a s t e r n part of N o r t h America, a n d deduced climatic a n d meteorologic consequences that seemed to be inconsistent with the p r o b a b i ­ lities. Recently, however, it has been s h o w n to be p r e s e n t in South D a k o t a (WARREN, 1 9 4 9 ) and Minnesota (R. V. RUHE, unpublished). I n South D a k o t a the Illinoian drift border is e v e r y w h e r e b u r i e d beneath t h e m o r e extensive Wis­ consin drift; this fact d e l a y e d recognition of the presence of the older drift sheet. It n o w seems likely, therefore, t h a t t h e Illinoian drift sheet continues n o r t h w e s t w a r d , with an a r e a l extent s i m i l a r to that of t h e Wisconsin drift. If so, the climatic anomalies implicit in e a r l i e r , erroneous concepts of its e x t e n t disappear ,and the distribution of the Illinoian in America becomes m o r e n e a r l y c o m p a r a b l e w i t h the d i s t r i b u t i o n of the S a a l e drift in E u r o p e .

In this connection it h a s come to be recognized t h a t t h e extensive L o v e l a n d loess sheet is not S a n g a m o n (Third interglacial) as was f o r m e r l y supposed, b u t Illinoian. T h e areal distribution of the L o v e l a n d loess is consistent w i t h the newly recognized, extended distribution of the Illinoian drift. T h e r e can be little doubt as to the genetic relationship of t h a t loess to t h a t drift.

T h e discovery of b u r i e d weathering profiles within t h e Loveland loess in Nebraska suggests that t h e r e w e r e two o r m o r e episodes of loess sedimentation, well s e p a r a t e d in time. A s bursts of loess deposition w i t h i n the Wisconsin (Fourth Glacial) sequence a r e correlated r a t h e r generally w i t h glacial m a x i m a , t h e implication of repeated expansions of t h e Illinoian ice sheet is plain. If in fact t h e r e w e r e two such expansions, the Illinoian s t r a t i g r a p h y would r e s e m b l e t h a t of G e r m a n y according to the present v i e w of WOLDSTEDT ( 1 9 5 0 , cf .p. 2 5 6 , 3 0 2 ) , w i t h the W a r t h e drift sheet interpreted a s t h e product of a second e x p a n s i o n of t h e ice sheet d u r i n g the S a a l e glacial age.

It is w o r t h noting also t h a t the Illinoian ice sheet, r a t h e r t h a n an e a r l i e r one, w a s responsible for the m a j o r drainage r e a r r a n g e m e n t t h a t created t h e p r e s e n t Missouri R i v e r in North D a k o t a and S o u t h Dakota. The u p p e r Missouri, in p r e Illinoian time, flowed east a n d north to t h e region of H u d s o n Bay; the Illinoian blockade a n d diversion, w h i c h has become p e r m a n e n t , h a s shifted the continen­ tal divide b e t w e e n the A r c t i c Sea w a t e r s h e d and the Gulf of Mexico w a t e r s h e d , far to the s o u t h of its f o r m e r position.

S t r a t i g r a p h y of t h e W i s c o n s i n s t a g e D u r i n g t h e past few y e a r s great a d v a n c e s have b e e n m a d e in our u n d e r ­ s t a n d i n g of t h e Wisconsin drift. When t h e Glacial Map of North A m e r i c a was published t h e Wisconsin drift between t h e Mississippi River and t h e Rocky Mountains w a s very imperfectly known. O n l y two Wisconsin substages h a d been identified, a n d these w e r e tentatively r e g a r d e d as I o w a n a n d Mankato, respec­ tively. A g a i n this led to deductions c o n c e r n i n g the e x t e n t of the Wisconsin ice sheet and t h e climatic conditions controlling it, which, it is n o w evident, w e r e erroneous. Coordinated m a p p i n g of the Wisconsin drift b y several geologists in South Dakota, western I o w a , s o u t h w e s t e r n Minnesota, a n d eastern N e b r a s k a

8

Richard Foster Flint

has established t h e presence, in t h a t combined region, of four Wisconsin s u b stages, t h e Iowan, Tazewell, Cary, a n d M a n k a t o substages identified e l s e w h e r e on t h e Glacial M a p of N o r t h America. T h e a r e a l relations of these four s u b stages a r e n e a r l y identical in t h e two g r e a t glacial lobes—the Des Moines l o b e in Iowa a n d Minnesota and t h e J a m e s lobe in S o u t h Dakota, indicating t h a t t h e ice sheet responded in t h e same w a y to climatic fluctuations t h a t affected b o t h lobes simultaneously.

These relationships a r e described for Iowa by R. V. RUHE and for S o u t h Dakota by R. F. FLINT, in discussions n o w in course of publication. Subdivision of t h e Wisconsin drift is also in progress in N o r t h Dakota a n d Montana, b u t owing to thinness of t h e drift sheets, erosional dissection, a n d distance from t h e region in which t h e s t r a t i g r a p h y is best k n o w n , correlation is still s o m e w h a t uncertain In S o u t h D a k o t a a n d Iowa t h e four Wisconsin substages a r e separated f r o m each other by sheets of loess. T h e loess b e t w e e n t h e Iowan a n d Tazewell drifts is, however, very t h i n and wholly unaltered; h e n c e it is inferred that the I o w a n Tazewell i n t e r v a l w a s probably very short a n d witnessed only moderate w i t h ­ d r a w a l of t h e ice sheet from t h e region. As b o t h t h e Tazewell drift and its o v e r ­ burden of loess a r e much thicker t h a n t h e I o w a n drift a n d loess, t h e I o w a n glacial m a x i m u m can be t h o u g h t of as a relatively short-lived advance pulse of a longer-lived t h o u g h less extensive m a x i m u m of Tazewell d a t e .

At m a n y places west of t h e Mississippi R i v e r t h e Tazewell-Cary i n t e r v a l is m a r k e d by a zone of m o d e r a t e w e a t h e r i n g a n d soil development (although w i t h a far less m a t u r e w e a t h e r i n g profile than t h a t produced d u r i n g t h e S a n g a m o n interglacial age). I n contrast, t h e C a r y - M a n k a t o interval lacks t h e w e a t h e r i n g zone, although in t h e region of subhumid climate it is characterized by a C h e r ­ nozem soil. F r o m these facts it seems likely t h a t there is only one s u b s t a n t i a l break w i t h i n t h e Wisconsin sequence, and t h a t it intervened b e t w e e n t h e T a z e ­ well and t h e C a r y glacial sub-ages. During this b r e a k t h e ice-sheet margin m a y have w i t h d r a w n altogether from n o r t h - c e n t r a l United States. As t h e I o w a n Tazewell i n t e r v a l a n d t h e C a r y - M a n k a t o i n t e r v a l indicate a less m a r k e d c h a n g e in conditions, t h e fourfold Wisconsin s t r a t i g r a p h i c sequence m a y be v i e w e d best, perhaps, as consisting of t w o pairs of t w o glacial m a x i m a .

This view is supported a n d amplified by t h e results of r e c e n t study of loess s t r a t i g r a p h y i n Illinois, Kansas, a n d Nebraska, outside t h e limits of t h e W i s ­ consin drift. I n Illinois (LEIGHTON & WILLMAN 1 9 5 0 ) the section is fivefold. T h e four u p p e r units a r e correlated w i t h Wisconsin glacial substages through t r a c i n g into t h e drift region. Respectively they u n d e r l i e t h e Tazewell till, overlie t h e Tazewell till, overlie Cary outwash, a n d (as s a n d y deposits) overlie M a n k a t o outwash, a n d hence a r e Iowan, Tazewell, C a r y , and M a n k a t o . The fifth a n d lowest u n i t overlies t h e S a n g a m o n (Third interglacial) zone of w e a t h e r i n g , underlies t h e I o w a n loess (from which it is s e p a r a t e d by a zone of slight leaching), and is areally r e l a t e d to o u t w a s h valley t r a i n s of major r i v e r valleys. T h e s e facts imply t h a t this lowest Wisconsin loess — n a m e d F a r m d a l e loess — r e p r e ­ sents a brief p r e - I o w a n glacial expansion t h a t failed to reach as far south, i n Illinois, as t h e Tazewell ice sheet. On this basis t h e first group of glacial m a x i m a should be t h o u g h t of not as a p a i r b u t a trio.

I n K a n s a s (LEONARD 1 9 5 1 ; FRYE & LEONARD 1 9 5 1 ) t h e section is fourfold, a n d

consists of t h r e e zones, distinguished principally on an i n v e r t e b r a t e - f a u n a l basis, s e p a r a t e d from a fourth, overlying zone by a p r o n o u n c e d horizon of

Wisconsin

Eolian sand & ? loess Outwash

Loess Drift

substage

Loess Drift

Loess Outwash

B r a d y Soil zone

Loess

Loess

Tazewell

Drift

Drift

Cary

Iowan

substage

substage'

Loess — leaching zone — F a r m d a l e loess

Loess Drift

(& ? p r e - I o w a n )

Nebraska

Kansas (FRYE; LEONARD)

(SCHULTZ, REED & o t h e r s )

Loess (undifferentiated)

Bignell loess

(LEIGHTON & WILLMAN)

loess

substage

Illinois

(FLINT; RUHE)

Bignell

stage

Mankato

South Dakota & Iowa

Loess (undifferentiated)

Loess

Peorian loess

Correlation

CA CA CD

Loess

2 § 'C 0 cu

Loess (undifferentiated)

Loess

S a n g a m o n soil z o n e

Illinoian

stage

Loveland Drift

loess

Loveland

loess

Loveland

L o v e l a n d loess

Drift

T a b l e 2. — A p p a r e n t c o r r e l a t i o n of l a t e r W i s c o n s i n d r i f t a n d l o e s s s h e e t s t o i n f o r m a t i o n a v a i l a b l e a t t h e e n d of 1951

according

loess

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Richard Foster Flint

w e a t h e r i n g a n d soil development. T h e r e is reason to believe t h a t t h e intervening horizon, t e r m e d B r a d y s o i l , represents t h e Tazewell-Cary interval. T h e lowest of t h e five zones in t h e loess is believed to correlate w i t h t h e F a r m d a l e loess in Illinois. However, u n l i k e t h a t loess it is n o t separated from t h e over­ lying zone by a leaching horizon, a n d this m a y reflect t h e fact t h a t t h e Kansas sections a r e m o r e distant from t h e border of t h e Wisconsin drift t h a n a r e t h e sections in Illinois. T h e four overlying zones a r e believed to be respectively, Iowan, Iowan-Tazewell transition, Tazewell, a n d C a r y - M a n k a t o . T h e fact t h a t in K a n s a s t h e I o w a n loess is transitional into t h e Tazewell loess supplements the evidence in Iowa a n d South D a k o t a t h a t t h e Iowan-Tazewell i n t e r v a l w a s brief and w a s not characterized b y great s h r i n k a g e of t h e ice sheet. T h e fact that t h e Cary a n d M a n k a t o loess sheets seem to form a single unit in Kansas probably reflects t h e considerable distance b e t w e e n K a n s a s a n d t h e outer limits of t h e Cary a n d M a n k a t o drift sheets, a m i n i m u m of 2 0 0 miles traced along outwash routes.

I n N e b r a s k a (cf. SCHULTZ, LUENINGHOENER & FRANKFORTER 1 9 5 1 ) t h e Wisconsin

loess is subdivided into a lower p a r t (Iowan a n d Tazewell) a n d a n upper p a r t (Cary and Mankato), t h e t w o p a r t s separated b y t h e B r a d y soil zone. Detailed e x a m i n a t i o n of t h e i n v e r t e b r a t e faunas very likely will lead to a m o r e complete subdivision of t h e section, as h a s been done in K a n s a s .

T h e correlations mentioned above a r e shown in Table 2 . According to them the Wisconsin stage is subdivided by t h e B r a d y soil into a n u p p e r p a i r and a l o w e r pair of substages; if t h e fluctuation implied b y t h e leaching of the F a r m dale loess proves to be i m p o r t a n t , t h e n t h e lower p a i r would become threefold, at least in Illinois.

A w e a t h e r i n g zone at t h e s t r a t i g r a p h i c position of t h e B r a d y soil -— t h a t is, b e t w e e n t h e Tazewell a n d Cary drift sheets — h a s been identified in Indiana (WAYNE & THORNBURY 1 9 5 1 , p. 1 6 ) .

T h a t t h e ice-sheet margin, d u r i n g t h e Tazewell-Cary interval, stood farther n o r t h t h a n Aylmer, Ontario, is suggested by w i d e s p r e a d lacustrine deposits, a p p a r e n t l y d a t i n g from t h a t i n t e r v a l , in t h e A y l m e r district (A. DREIMANIS, unpublished).

U n d o u b t e d l y t h e subdivisions a r e determined b y climatic fluctuation, and speculation as to correlation w i t h t h e F o u r t h Glacial sequence in Europe is unavoidable. T h e question arises w h e t h e r t h e Rosenthaler Randlage (WOLDSTEDT 1 9 5 0 , p. 2 1 3 ) a n d t h e p r e - B r a n d e n b u r g Weichsel drift in J u t l a n d (K. MILTHERS, unpublished), r e p r e s e n t i n g a p p a r e n t l y a n early F o u r t h Glacial expansion of t h e Scandinavian Ice Sheet, m a y be t h e correlative of t h e ice advance sug­ gested by t h e F a r m d a l e loess, or w h e t e r it m a y r e p r e s e n t t h e I o w a n advance — or perhaps both.

T h e question arises also w h e t h e r in central E u r o p e t h e w e a t h e r i n g horizon t h a t separates t h e Younger Loess I from t h e Younger Loess II —• t h e horizon r e p r e s e n t i n g t h e so-called „Aurignacian interval" (cf. WOLDSTEDT 1 9 5 0 , p. 3 6 5 ) — is t h e correlative of t h e B r a d y soil zone. No fact presently k n o w n seems to s t a n d in t h e w a y of such a correlation.

I n South Dakota a n d Iowa t h e I o w a n drift sheet is t h e most extensive of t h e Wisconsin (Fourth Glacial) substages. I n contrast, in Illinois, Indiana, a n d Ohio t h e most extensive Wisconsin s u b s t a g e is t h e Tazewell. T h a t t h e I o w a n is p r e ­ sent, at least in Illinois, is indicated b y t h e presence of t h e I o w a n loess; p r e ­ s u m a b l y t h e I o w a n drift b o r d e r passes beneath t h e Tazewell drift n e a r t h e Mississippi River a n d continues e a s t w a r d b e n e a t h t h e younger drifts. Regional

Recent advances in North A m e r i c a n Pleistocene stratigraphy

11

variations in the extents of successive ice sheets seem t o find logical explanation in t h e atmospheric circulation p a t t e r n deduced from analogy w i t h p r e s e n t - d a y short-period

fluctuations

(cf. FLINT & DORSEY 1 9 4 5 ) ; specifically

t h e deduced

shift from a low-index p a t t e r n in I o w a n t i m e to a h i g h - i n d e x in Tazewell time is consistent with the p r e s e n t l y known b o r d e r s of t h e t w o respective drift sheets.

In this connection i t is interesting t h a t t h e Wisconsin drift of t h e Green River lobe in n o r t h w e s t e r n Illinois, l o n g of debatable correlation, h a s been established as Tazewell. Also this drift is more extensive t h a n is s h o w n on t h e Glacial M a p of North A m e r i c a , as it e x t e n d s west to t h e bluffs of t h e Mississippi River t r e n c h in Iowa (LEIGHTON & SHAFFER 1949).

In I n d i a n a and Ohio t h e Tazewell a n d Cary drift sheets have been identified stratigraphically and t h e a p p r o x i m a t e positions of t h e i r borders d e t e r m i n e d . In Indiana, as already noted, t h e interval b e t w e e n t h e m is represented b y a zone of w e a t h e r i n g . In Ohio t h i s interval is r e p r e s e n t e d b y fossil logs of forest trees incorporated in Cary till (R. P . GOLDTHWAIT, unpublished; cf. FLINT & DEEVEY 1951,

p . 2 8 6 ) ; in w e s t e r n P e n n s y l v a n i a (SCHOPF & CROSS 1 9 4 7 ; FLINT & DEEVEY

1 9 5 1 , p . 2 8 6 ) i t is r e p r e s e n t e d probably b y peat.

It h a s been shown b y BRETZ (1951, p . 4 2 2 ) that t h e P o r t H u r o n e n d m o r a i n e in Michigan, formerly believed to r e p r e s e n t t h e outermost position of t h e icesheet m a r g i n during t h e M a n k a t o substage, actually w a s built d u r i n g t h e later part of C a r y time, a n d t h a t t h e M a n k a t o m a x i m u m is represented b y t h e M a ­ nistee endmoraine, which lies farther n o r t h . This discovery brings t h e s t r a t i ­ graphic sequences on t h e t w o sides of L a k e Michigan i n t o a g r e e m e n t T h e M a n k a t o m a x i m u m w a s s e p a r a t e d from t h e earlier, Cary m a x i m u m b y a pronounced shrinkage of t h e ice sheet. T h e ecology of peat a c c u m u l a t e d n e a r Two Creeks, Wisconsin (see references i n FLINT & DEEVEY 1 9 5 1 , p. 2 6 2 ) during the s h r i n k a g e , and evidence of a p r o n o u n c e d lowering of t h e w a t e r in t h e Lake Michigan basin as it found a n e w outlet t h r o u g h t h e S t r a i t of Mackinac (BRETZ 1 9 5 1 ) indicate that t h e c o n t e m p o r a n e o u s glacier m a r g i n stood n o r t h of t h e existing G r e a t Lakes. Radiocarbon d e t e r m i n a t i n o s on t h e Two Creeks p e a t date the p e a t a t 1 1 , 4 0 0 ± 3 5 0 years, and f r o m t h e radiocarbon dates cf o t h e r m a ­ terials it is inferred t h a t L a k e Algonquin I I I was in existence a r o u n d 5 , 0 0 0 years ago a n d t h a t t h e early Nipissing G r e a t L a k e s w e r e i n existence a r o u n d 3 , 5 0 0 years a g o (FLINT & DEEVEY 1 9 5 1 , p. 2 8 3 — 2 8 5 ) . All these dates a r e r a t h e r sur­ prisingly small; probably t h e y indicate t h a t t h e r a t e of melting of t h e L a u r e n tide Ice Sheet accelerated d u r i n g the l a t e r p a r t of t h e Wisconsin Glacial age.

Evidence has continued to accumulate t h a t two successive „postglacial" seas occupied t h e St. L a w r e n c e lowland. T h e first carried a dominantly cold fauna; the second seems to r e c o r d t e m p e r a t e conditions. T h e r e a r e indications t h a t a n episode of expanded glaciers followed t h e t w o m a r i n e incursions, a n d t h a t in t h e St. L a w r e n c e valley u p s t r e a m from Quebec City t h e glacier ice w a s not the L a u r e n t i d e Ice S h e e t b u t an ice c a p centered i n t h e Appalachian region south of t h e river (cf. OSBORNE 1951).

T h e l a t t e r fact is t y p i c a l of events i n t h e highland region of n o r t h e a s t e r n North America, in which m a n y local i c e caps and cirque glaciers s u r v i v e d t h e general ice-sheet deglaciation (FLINT 1 9 5 1 ) , thus greatly complicating t h e lateWisconsin sequence of e v e n t s .

A s t u d y b y DEEVEY ( 1 9 5 1 ) has s h o w n t h a t pollen sequences in Aroostook County, Maine, a r e r e m a r k a b l y similar t o those in n o r t h e r n E u r o p e , w i t h a n Alleröd-like zone succeeded b y evidence of colder climate. DEEVEY ( 1 9 5 1 , p . 1 9 7 )

12

Richard Foster Flint

suggested the „ r e m o t e possibility" t h a t the Alleröd-like zone represents t h e C a r y - M a n k a t o i n t e r v a l , and t h a t although t h e M a n k a t o glaciation influenced the vegetation in Maine, M a n k a t o ice did not reach t h a t vicinity. Should this possibility be s u b s t a n t i a t e d by f u r t h e r pollen studies, it would aid in fixing t h e still-unknown position of the M a n k a t o glacial limit in the New E n g l a n d region.

It has been k n o w n for several y e a r s that t w o drift sheets of Wisconsin age are present in s o u t h e a s t e r n N e w England. This knowledge has been advanced by a recent detailed study (JUDSON 1949) of t h e Boston area, in which a y o u n g till and outwash a r e found in s e v e r a l places to overlie an older sequence of till and m a r i n e sediments, the older sequence h a v i n g been oxidized and eroded before the a d v e n t of the younger.

Within the n o r t h e a s t e r n region mention should b e m a d e also of the valuable attempt by LEIGHLY (1949) to reconstruct the glacial-age climatic conditions along the 49th p a r a l l e l of l a t i t u d e from the Rocky Mountains to Newfoundland, and the application b y HARE (1951) of the study of p r e s e n t - d a y snowfall to t h e g r o w t h of the L a u r e n t i d e Ice Sheet. Both papers h a v e advanced our knowledge of the meteorologic circumstances of ice-sheet development.

Cordilleran

region

Pleistocene studies in the Cordilleran region h a v e been fewer t h a n in t h e region covered by t h e ice sheet, b u t progress has b e e n made. Detailed work in the Wind River M o u n t a i n s in W y o m i n g and the L a Sal Mountains in U t a h h a v e revealed a fourfold sequence of Wisconsin glacial deposits which, after f u r t h e r study, it should b e possible to correlate with the Wisconsin s t r a t i g r a p h y of t h e Mississippi Valley region (G. M. RICHMOND, unpublished).

F r ozen-ground

phenomena

The study of frozen-ground p h e n o m e n a associated with t h e glacial stages is less advanced in N o r t h A m e r i c a t h a n it is in Europe. Publications thus far consist mainly of local descriptions; little a t t e m p t at regional synthesis has been made. One of t h e m o r e extensive studies is t h a t of PELTIER (1949) which des­ cribes flow e a r t h in t h e valley of t h e S u s q u e h a n n a River and its tributaries in Pennsylvania, a n d a t t e m p t s to d a t e this m a t e r i a l with respect to bodies of glacial outwash w i t h which it is in contact.

Conclusion The foregoing account a t t e m p t s to s u m m a r i z e briefly the chief advances made d u r i n g t h e p a s t few y e a r s in the s t r a t i g r a p h y of the N o r t h A m e r i c a n glacial deposits. T h e coverage is v e r y incomplete, because a space limitation forbade mention of a n u m b e r of valuable papers in this general field. However, the r e a d e r interested in further s t u d y will find t h a t t h e bibliographies contained in t h e papers listed below provide an extensive a n d up-to-date reading list.

R e f e r e n c e s BRETZ, J . HAHLEN (1951) T h e s t a g e s of L a k e C h i c a g o : t h e i r c a u s e s a n d c o r r e l a t i o n s . Am. J o u r . Sei. 2 4 9 , p. 401-429.

CONDRA, G . E . , REED, E . C . & GORDON, E . D . (1950) C o r r e l a t i o n of t h e P l e i s t o c e n e d e p o ­ s i t s of N e b r a s k a [ O r i g i n a l l y p u b l i s h e d 1 9 4 7 ; r e v i s e d b y G. E . CONDRA & E . C .

REED 1950]. - N e b r a s k a G e o l . S u r v e y , B u l l . 1 5 A , 74 p a g e s .

DEEVEY, E , S . , J r . (1951) L a t e - g l a c i a l a n d p o s t g l a c i a l p o l l e n d i a g r a m s f r o m M a i n e . Am. J o u r . Sei. 2 4 9 , p . 177-207.

Recent a d v a n c e s in N o r t h A m e r i c a n Pleistocene

stratigraphy

13

FLINT, R . F . & DOHSEY, H . G., J r . (1945) I o w a n a n d T a z e w e l l d r i f t s a n d t h e N o r t h A m e r i ­ c a n Ice S h e e t . - A m . J o u r . S e i . 243, p . 627-636.

— a n d o t h e r s (1945) G l a c i a l M a p of N o r t h A m e r i c a . - G e o l . S o c . A m . , S p e c . P a ­ p e r 60, P a r t I - G l a c . m a p ( i n c o l o r ) ; P a r t I I - E x p l a n a t o r y n o t e s , 37 p a g e s .

FLINT, R . F . (1947) G l a c i a l g e o l o g y a n d t h e P l e i s t o c e n e e p o c h . N e w Y o r k : J o h n W i l e y & S o n s , 589 p a g e s . — (1951) H i g h l a n d c e n t e r s of f o r m e r g l a c i a l o u t f l o w i n n o r t h e a s t e r n N o r t h A m e r i c a . - B u l l . G e o l . S o c . A m . 62, p . 2 1 - 3 8 .

FLINT, R . F . & DEEVEY, E . S . , J r . , (1951) R a d i o c a r b o n d a t i n g of l a t e - P l e i s t o c e n e e v e n t s . A m . J o u r . S e i . 249, p . 2 5 7 - 3 0 0 .

FRYE, J . C , SWINEFORD, ADA & LEONARD, A . B . (1948) C o r r e l a t i o n of P l e i s t o c e n e d e p o s i t s o f t h e c e n t r a l G r e a t P l a i n s w i t h t h e g l a c i a l s e c t i o n . - J o u r . G e o l . 56, p . 5 0 1 - 5 2 5 .

— & WALTERS, K . L . (1950) S u b s u r f a c e r e c o n n a i s s a n c e of g l a c i a l d e p o s i t s i n n o r t h e a s t e r n K a n s a s . - K a n s a s G e o l . S u r v e y , B u l l . 86, p t . 6, p . 1 4 1 - 1 5 8 .

— & LEONARD, A . B . (1951) S t r a t i g r a p h y of t h e l a t e P l e i s t o c e n e l o e s s e s of K a n ­ s a s . - J o u r . G e o l . 59, p . 2 8 7 - 3 0 5 .

HARE, F . K . (1951) T h e p r e s e n t d a y s n o w f a l l of L a b r a d o r - U n g a v a . - A m . J o u r . S e i . 249, p . 654-670.

JUDSON, S h e l d o n (1949) T h e P l e i s t o c e n e s t r a t i g r a p h y of B o s t o n , M a s s a c h u s e t t s a n d i t s r e l a t i o n t o t h e B o y l s t o n F i s h w e i r . - p . 7-48 i n : T h e B o y l s t o n S t r e e t F i s h w e i r I I . A s t u d y of t h e g e o l o g y , p a l a e a b o t a n y , a n d b i o l o g y of a s i t e o n S t u a r t S t r e e t i n t h e B a c k B a y d i s t r i c t of B o s t o n , M a s s a c h u s e t t s , b y E . S . BARGHOORN, a n d o t h e r s . - P a p e r s of t h e R o b e r t S. P e a b o d y F o u n d a t i o n f o r A r c h a e o ­ l o g y 4, n o . 1.

LEIGHLY, J o h n (1949) O n c o n t i n e n t a l i t y a n d g l a c i a t i o n . - G e o g r . A n n a l e r , p . 1 3 3 - 1 4 5 .

LEIGHTON, M . M . & SHAFFER, P . R . (1949) N e w l y d i s c o v e r e d e x t e n s i o n of t h e L a b r a d o r e a n i c e s h e e t i n t o e a s t e r n I o w a d u r i n g t h e T a z e w e l l s u b s t a g e of t h e W i s ­ c o n s i n s t a g e [ A b s t r . ] . - B u l l . G e o l . S o c . A m . 6 0 , p . 1904.

— & WILLMAN, H . B . (1950) L o e s s f o r m a t i o n s of t h e M i s s i s s i p p i V a l l e y . - J o u t .

G e o l . 58, p . 5 9 9 - 6 2 3 .

LEONARD, A . B . (1951) S t r a t i g r a p h i c z o n a t i o n af t h e P e o r i a l o e s s i n K a n s a s . - J o u r .

G e o l . 59, p . 3 2 3 - 3 3 2 .

OSBORNE, F . F . (1951) P a r e d e s L a u r e n t i d e s i c e c a p a n d t h e Q u e b e c s e a . - N a t u r a l i s t e C a n a d i e n 78, p . 221-251.

PELTIER, L . C. (1949) P l e i s t o c e n e t e r r a c e s of t h e S u s q u e h a n n a R i v e r , P e n n s y l v a n i a . P e n n s y l v a n i a G e o l . S u r v e y , 4 t h s e r . , B u l l . G 2 3 , 158 p a g e s .

SCHOPF, J . M . & CROSS, A . T . (1947) A g l a c i a l a g e p e a t d e p o s i t n e a r P i t t s b u r g h . - A m .

J o u r . Sei. 245, p . 421-433.

SCHULTZ, C . B . , LUENINGHOENER, G . C . & FRANKFORTER, W . D , (1951) A g r a p h i c r e s u m e of t h e P l e i s t o c e n e of N e b r a s k a ( w i t h n o t e s o n t h e f o s s i l m a m m a l i a n r e m a i n s ) . B u l l . U n i v . N e b r a s k a S t a t e M u s e u m 3, n o , 6, 4 1 p a g e s .

WARREN, C . R. (1949) P r o b a b l e I l l i n o i a n a g e of p a r t of t h e M i s s o u r i R i v e r [ A b s t r . ] . B u l l . G e o l . S o c . A m . 60, p . 1 9 2 6 .

WAYNE, W . J . & THORNBURY, W . D . (1951) G l a c i a l g e o l o g y of W a b a s h C o u n t y , I n d i a n a . B u l l . I n d i a n a G e o l . S u r v e y 5, 39 p .

WIRTZ, D a n i e l & ILLIES, H e n n i n g (1951) L o w e r P l e i s t o c e n e s t r a t i g r a p h y a n d t h e P l i o P l e i s t o c e n e b o u n d a r y i n n o r t h w e s t e r n G e r m a n y . - J o u r . G e o l . 59, p . 463-471.

WOLDSTEDT, P a u l (1950) N o r d d e u t s c h l a n d u n d a n g r e n z e n d e G e b i e t e i m E i s z e i t a l t e r .

S t u t t g a r t , K . F . K o e h l e r , 464 p a g e s .

M s . e i n g e g . 1 3 . 5 . 52 A n s c h r . d . V e r f . : P r o f . R . F . F l i n t , Y a l e U n i v . , D e p . of G e o l o g y , N e w H a v e n , C o n n . , U S A .