390 



NA TURE 



[August i6, 1906 



glariation. And iillhough it is certain that the great 

 extension of the Alpine glaciers was due to the same 

 glacial conditions that gave rise to the lowland ice-sheets 

 of Northern Europe, 1 do not regard it as a necessary 

 consequence that advances and retreats of the ice should 

 occur simultaneously in both regions. \'ariation in the 

 relative amount of snowfall over the glaciated areas during 

 the course of the Glacial Period, for which there is much 

 evidence, would be likely to produce great effects in the 

 high-lying reservoirs of the Alps ; and at the latitude of 

 this region we should expect rapid recession of the low- 

 level glaciers in response to diminished supply. To dis- 

 tinguish between the effects of oscillations in precipitation 

 and of oscillations in temperature under such conditions 

 iiuist be peculiarly difficult. 



'North America. — In North .America, where both the 

 drifts and their literature attain gigantic proportions, the 

 state of opinion is closely analogous to that among our- 

 selves. It is agreed by all that during the Glacial Period 

 there were very extensive oscillations in the borders of the 

 ice-sheets ; and by some geologists some of the stages of 

 recession are supposed to represent mild epochs of actual 

 " deglaciation " ; while others, fewer in number, among 

 whom Mr. Warren Upham and Dr. G. F. Wright have 

 been the most active, regard these stages as of minor 

 consequence, and advocate the essential unity of the 

 glaciation. ;\nd between the two extremes stand the great 

 majority of the workers in American glacial geology, who 

 refrain from expressing positive opinions, but mostiv lean 

 toward the idea of at least one great interruption in the 

 glaciation. Some of the suggested schemes of classifi- 

 cation ' are fully as elaborate and complex as that pro- 

 posed for Europe, but it seems to be recognised that these 

 are only of local value. Prof. T. C. Chamberlin and his 

 fellow-workers in the North-Central States have, however, 

 adopted a sequence based on the successive advance of 

 ilifferent ice-lobes, which is believed to be of wider appli- 

 cation ; and Prof. Chamberlin has tentatively suggested 

 that some of these divisions may have their counterpart in 

 the European scheme, but is careful to show that the 

 correlation must at present remain entirely hypothetical, " 

 especially as the proposed American grouping may itself 

 require modification. 



It is well established that the American ice-sheets, like 

 their European equivalents, radiated from several distinct 

 centres that attained their maximum influence consecu- 

 tively, and not simultaneously. Of these the " Lauren- 

 tide " and the " Keewatin " sheets had their radiants 

 over comparatively low ground east and west of Hudson 

 Bay, while the " Cordilleran " sheet spread outward from 

 ihe Western Mountains. In his general discussion of the 

 glacial phenomena of North-Western Canada, Mr. J. B. 

 Tyrrell ' concludes that the Cordilleran sheet had reached 

 its greatest extent and had retired before the boulder-clay 

 of the Keewatin sheet was laid down ; and that the Kee- 

 watin sheet, in turn, had gone south to its farthest limit, 

 and had retired for many hundreds of miles — more than 

 half-way to its gathering ground — before the Laurentide 

 sheet had reached its greatest extension. 



If these conclusions be accepted, they must imply that 

 at least in some cases the recession of the ice-lobes was 

 due to causes acting locally, and not to mild interglacial 

 periods affecting the whole hemisphere. The phenomena 

 of invasion by successive ice-lobes in the peripheral regions 

 might thus be readily explained without recourse to the 

 Interglacial hypothesis. 



Most of Ihe detailed evidence brought forward in 

 America to support the Interglacial idea is as fragmentary 

 .-md unconvincing as that of our own countrv. But there 

 is one notable exception, to which I must particularly 

 refer, as it has been investigated by a Research Committee 

 of the .Association, and has, moreover, come under mv 



> eg , "The Diversity of the Glacial Period in Long Island," by A. C. 

 Veatch. Jflurn. GeoL, vol. ,\i. (igo;^!, pp. 762-776. 



- "Classification of American Glacial Deposits." lourn. Geol..\o\.\\\. 

 (189=). pp. 270-277. and in T. Geikie's " Great Ice Age," ^ird. ed., chap. xli. 

 See also Chamberlin and Salisbury's recent text-book, "Geology: Earth 

 History," vol. iii. chap, xix- (London, igofi). 



3 "The Glaciation of North-Central Canada." Jou>-n. Ccol, vol. vi. 

 (1898), pp. i^7-i6i; and "The Genesis of Lake Agassiz," ibid., vol. iv. 

 (1896), pp. S, - 



personal observation. In this case the interglacial deposits, 

 first described by Dr. G. J. Hinde, are magnificently ex- 

 posed in cliff sections at .Scarboro' Heights, on the shores 

 of Lake Ontario, near Toronto. When I visited these 

 sections under the guidance of my friend Prof. A. P. 

 Coleman, in 1897, they impressed me strongly, inasmuch 

 as they afforded the kind of evidence for which one had 

 sought in vain in Britain. The section around Scarboro' 

 Heights reveals a great mass of fossiliferous stratified 

 deposits, more than iSo feet thick, consisting in the lower 

 part of slightly peaty clays, and in the upper part of 

 sands ; and these deposits are overlain by a complex series 

 of boulder-clays, with intercalated beds of sand and gravel, 

 attaining a thickness of at least 200 feet. The fossiliferous 

 clays are the lowest beds seen in the cliff section, but beds 

 belonging to the same series, that are exposed in the Don 

 Valley, on the outskirts of Toronto, are underlain by a 

 few feet of boulder-clay, so that it seems to be beyond 

 question that the Scarboro' beds were deposited in an 

 interval between two epochs of glaciation.' In their upper 

 part these beds contain a flora and fauna indicating a cool 

 climate, but in their lower portion some of the plants and 

 freshwater shells no longer exist so far north as Canada, 

 and are therefore considered to denote a climate warmer 

 than that of the present day. On this and other evidence 

 it is clear that during the course of the Glacial Period 

 the whole of the district was for a considerable time re- 

 leased from the ice-sheets which previously and after- 

 wards covered it. Moreover, in the opinion of Prof. Cole- 

 man, some of the plants and shells of the warm-climate 

 beds denote conditions that would be incompatible with 

 the persistence of ice-sheets anywhere in Canada " : and if 

 this be so, then we here have proof for at least one inter- 

 glacial epoch. But I still permit myself to feel doubt re- 

 garding this last-mentioned deduction, as the shells and 

 plants in question, which have their present habitat in the 

 Middle L'nited States, even yet endure winters of consider- 

 able severity ; and there are certain factors in the com- 

 position of the beds and their altitude above Lake Ontario 

 that justify caution. It is, however, mainly from my 

 knowledge of this "Toronto formation," and of the 

 Kirmington section in England, presently to be discussed, 

 that I still maintain an undecided attitude in respect to 

 the Interglacial hypothesis in its simpler form. 



Further support to the probability of an interglacial 

 epoch has been adduced from the history of the great lakes 

 which formerly existed in the Interior Basin of the Western 

 States. It has been shown by the researches of G. K. 

 Gilbert in the "Lake Bonneville" basin' and of I. C. 

 Russell in that of "Lake Lahontan,"* that there were 

 two separate epochs, during which these enormous basins 

 were filled with water, and an intervening arid epoch, 

 during which they were dried up. The region is one in 

 which the actual glacial phenomena are restricted to the 

 mountain valleys ; but as it seems evident that the lakes 

 were associated in some way with the Glacial Period, the 

 two stages of extension arc supposed to represent two 

 distinct epochs of glaciation, separated by a long inter- 

 glacial drought. The correlation, however, has difficulties, 

 which are very impartially discussed by Gilbert and 

 Russell ; and it will not aclniit of more than one inter- 

 .glacial episode. 



T/ic Interglacial Problem in the British Islands. 



Let us now consider the application of the Interglacial 

 hypothesis to our own land. 



The task of following up the evolution of Prof. Geikie's 

 scheme through its various phases, though instructive, is 

 very confusing — one might even say irritating — bv reason 

 of the continual changes of correlation which its author 

 has suggested in sorting out the British drift deposits into 

 this orderly sequence. Our East Coast bouider-clays, for 

 example, were at one time held to cover four glacial epochs, 



1 Prof. A. P. Coleman, Reps. British Assoc, for 1898, pp. 522-29; for 

 pp. 411-414; for iqoo, pp. 328-40; also (s\immary and discussion) 



'Glacial 



/"■ 



NO. 1920, VOL. 74] 



d Interglacial Beds near Toroni 

 (1901), pp. 285-310. 



- "The Duration of the Toronto Interglacial Period. 

 Geologist, vol. xxix. (1902). p. 79. 



3 "Lake Bonneville." Monogr. U.S. Geo!. Survey, vol. i. (1890). 



■> " Lake Lahontan." .Moiiogr. U.S. Geo!. Stir-.'ey, vol. xi. (1S85), 



