572 



NA TURE 



TApril 14, 1898 



obstacles in its course rather than aids to its accumulation. It 

 is, indeed, probable that in Eastern Europe too much stress has 

 been laid on the importance of the hill-ranges as glacial centres, 

 since there is much evidence to indicate that, at any rate 

 during the maximum glaciation, the movement, and probably 

 therefore the growth, of the great sheets was more or less 

 independent of the orographic features. In this connection the 

 explorations of Mr. J. B. Tyrrell, of the Canadian Survey, in 

 that birthplace of ice-sheets the desolate region to the westward 

 of Hudson Bay, are of the highest importance. In his paper 

 on the glaciation of North-western Canada, Mr. Tyrrell stated 

 that no evidence was discovered of any great elevation of this 

 central area in Glacial, or immediately Pre-glacial times, and 

 it would seem not improbable that the land then stood at about 

 the same height above the sea as at present ; and that the 

 moisture giving rise to the immense precipitation of snow would 

 probably be derived from the adjacent waters of Hudson Bay 

 and the Arctic Ocean. 



In the region immediately west of Hudson Bay, the earliest 

 glaciation of which he could recognise any traces flowed outwards 

 from a gathering-ground which lay north or north-west of 

 Doobaunt Lake. Subsequently this gathering-ground moved 

 south-eastward, until it centred over the country between 

 Doobaunt and Yath-kyed Lakes. From one or other of these 

 centres the ice seems to him to have flowed westward and 

 south-westward to within a short distance of the base of the 

 Rocky Mountains ; southward, for more than 1600 miles to the 

 States of Iowa and Illinois : eastward, into the basin of Hudson 

 Bay ; and northward, into the Arctic Ocean. 



He applies the name Keewatin Glacier to this central con- 

 tinental ice-sheet, which in general character appears to have 

 been somewhat similar to the great glacier of north-western 

 Europe, with a centre lying near the sea-coast, a steep and 

 short slope seaward, and a very much longer and more gentle 

 slope towards the interior of the continent. But, remarked 

 Mr. Tyrrell, there was this difference between the two, that the 

 centre of the latter was over a high rocky country ; while the 

 centre of the former was over what is now, and was probably 

 also then, a low-lying plain, on which the snow accumulated to 

 such depths as to cause it to flow ovei' country very considerably 

 higher. 



This great glacier, in the different stages of its retirement 

 down gradually descending slopes, caused many temporary 

 extra-Glacial lakes to be formed, which were drained one after 

 another as it retired to still lower country. Before it had with- 

 drawn from the Winnipeg basin, Mr. Tyrrell thinks that it was 

 joined by an advancing glacier from the east, and in front of the 

 two, Lake Agassiz, one of the largest of the extra-Glacial lakes, 

 was formed. During the final stages, its general gathering- 

 ground is believed to have moved still nearer to the coast of 

 Hudson Bay, and to have broken into several separate centres ; 

 and Mr. Tyrrell notes that after its retirement the land in the 

 vicinity of Hudson Bay stood from 500 to 600 feet below its 

 present level, and gradually rose to its present height. 



The shifting of the centres of glaciation at different stages of 

 the Great Ice Age, to which Mr. Tyrrell referred, seems to be a 

 well-recognised characteristic in North America, though it is 

 diversely explained. Dr. G. M. Dawson, in the admirable 

 summary of Canadian Geology in the new Handbook for 

 Canada prepared for the Toronto meeting, notes that the 

 western part of the Great Plains was invaded at an early stage 

 by large glaciers issuing from the Cordilleran ice-sheet through 

 the main valleys of the Rocky Mountains, while at a later 

 period, when 'this ice had shrunk back, a newer series of glacial 

 deposits was .spread out in the same area, largely composed of 

 Laurentiari airid'Huronian debris transported from the north- 

 east. ' " ' ■ 



Dr. Dawson, while acknowledging that the evidence is not 

 satisfactory, is still inclined to think that these latter deposits may 

 be in part of marine origin, and that they indicate great relative 

 and absolj^te changes of level in this region in Glacial times. 



In eastern Canada also "it has been found by Mr. Chalmers 

 that when the Laurentide Glacier invaded the lowlands to the 

 west of Quebec, the Appalachian glacier had either greatly 

 decreased or had vanished " (Handbook, p. 30). 



In England the drift deposits of the eastern and midland 

 counties show many similar indications of successive glaciation 

 from diff"erent centres, and until recently the tendency has been, 

 as in America, to ascribe the facts to the intervention cf warm 

 inter-glacial periods. But since it is now generally acknow- 



ledged that, given a sufficiently low temperature, the prime 

 factor in the accumulation of the ice-sheets has been the exces- 

 sive snowfall rather than the extreme cold, may not the whole of 

 the phenomena have been due to the gradual shifting of the 

 areas of greatest precipitation, brought about, as a secondary 

 effect, by the growth of the ice-sheets themselves ? 



It will be observed that Mr. Tyrrell, in common with all 

 American glacialists, has recognised an up-hill movement of the 

 ice sheet. The possibility of such movement has frequently been 

 questioned on our side of the Atlantic, in spite of the occurrence 

 of transported boulders in various parts of the British Islands at 

 levels considerably higher than their source. But the extent of 

 the uplifts in such instances is slight as compared with that 

 described by Prof. C. H. Hitchcock in his paper on the Southern 

 Lobe of the Laurentian Ice-sheet. 



Prof. Plitchcock pointed out that one great lobe of the Lauren- 

 tide Glacier went southward through the Champlain and 

 Hudson valleys, moving from a plain near sea level, over the 

 highest mountains in New England and New York, 6000 and 

 4000 feet in altitude, as shown by the transport of the boulders 

 and by the direction of the glacial strise. As indicative of the 

 distinctiveness of this lobe he described how the strise diverge 

 from the central line " much like the barbs of a feather from the 

 central shaft," and how the terminal moraines are looped round 

 the area rudely at right angles to the direction of the ice move- 

 ment. The initial gathering ground for this portion of the ice- 

 sheet seems to have lain to the eastward of Hudson Bay ; 

 hence it is sometimes termed the Labradorian Glacier. 



The lobate character of the southern termination of the ice 

 and the tendency of these lobes to spread outwards from a centre 

 is strongly insisted upon by all the American glacialists who 

 have studied the peripheral portions of the glaciated area, and 

 the delimitation of these lobes and the discussion of their 

 chronological relations has been made the subject of much recent 

 research. The mattei* is worthy of more attention than it has 

 yet received with us, for as was shown by Mr. H. B. Woodward 

 in a paper, read at Toronto, on the Chalky Boulder-clay of our 

 West-midland counties, some at least of the characteristic pheno- 

 mena can be recognised in this country also. The fact that in 

 America these distinct lobes did not reach their maximum 

 development at the same time, and that the overlapping of one 

 great tongue upon the area previously occupied by another is 

 frequently indicated, hg^given rise to much difference of opinion 

 as to the length of the»time-interval separating these different 

 stages of growth and retreat. Prof. T. C. Chamberlin, who 

 gave a lucid demonstration of his views ^ at Toronto, is of 

 opinion that the glacial phenomena of the northern United 

 States indicate two or more successive and distinct periods of 

 glaciation separated by mild interglacial intervals, while other 

 observers are inclined to agree with Dr. G. F. Wright, who 

 though acknowledging wide oscillations of the ice-margin, 

 regards the growth and wane of a single sheet as sufficient to 

 account for all the facts. 



In America, therefore, as in Europe, in spite of the prolonged 

 discussion, not only is the number of the supposed warm inter- 

 glacial periods still unsettled, but the evidence for even one such 

 interval is challenged. The whole question is largely a legacy ' 

 from the brilliant theorising of the late Dr. J. Croll, and with 

 the breakdown of his captivating generalisations it has become 

 necessary to reconsider the whole evidence which has been 

 adduced, in support of them, on both sides of the Atlantic, before 

 a safe conclusion can be reached. It is suggestive that while the 

 explorers of the peripheral areas of the old ice-sheets are usually 

 steadfast in their belief in such periods, those whose investiga- 

 tions lie more centrally to the regions of accumulation, both in 

 Europe and in America, are more frequently in favour of the 

 unity of the great glaciation. Yet even this localisation of 

 opinion is capable of two opposite applications. 



One of the strongest threads in the evidence for an inter-glacial 

 period in North America is furnished by the sections in the 

 vicinity of Toronto. Since Dr. G. J. Hinde described these 

 deposits in 1877, fresh excavations in the Don Valley have 

 revealed new facts of importance. Prof. A. P. Coleman, in his 

 paper on the subject at the British Association meeting, stated 

 that in the Don Valley a lowest till is seen, upon which rest 18 

 feet of sand and clay containing many unios and other shells, 

 as well as leaves aud pieces of wood. Some of the unios do 



1 These are ably stated by Prof. Chamberlin in the chapter on the 

 Glacial phenomena of North America in the third edition of Prof. Jas.- 

 Geikie's " Great Ice Age." 



NO. 1485. VOL. 57] 



