NATURE 



537 



THURSDAY, OCTOBER 8, 1891. 



THE ICE AGE IN NORTH AMERICA. 

 The Ice Age in North America, and its Bearings upon 

 the Antiquity of Man. By G. Frederick Wright, D.D., 

 &c. With an Appendix on " The Probable Cause of 

 Glaciation," by Warren Upham, F.G S.A. With many 

 new Maps and Illustrations. (London : Kegan Paul, 

 Trench, Triibner, and Co., Limited, 1890.) 



SWITZERLAND has been called the playground of 

 Europe. The glacial epoch occupies a similar 

 position in geology. Here the student, wearied with the 

 precision of palaeontology or of mineralogy, may revel in 

 dreams of omnipotent glaciers, wrap himself in ice sheets, 

 throw mental somersaults, swallow self-contradictory 

 arguments, and be as blind to unpleasant facts as was 

 Nelson at Copenhagen, when he put the telescope to his 

 useless eye, and "spoke disrespectfully" of the signal 

 of recall. To any sarcastic historian of the progress of 

 geology, the literature of ice and its effects will be a 

 boon, since it is so rich in unsound inductions and un- 

 stable hypotheses. 



Dr. Wright's book, however, is, on the whole, a favour- 

 able exception to this general rule. Passages, no doubt, 

 may be found here and there, to which exception might 

 be taken — notably to his remarks on the subject of 

 cirques, in which he regards with favour opinions which 

 are hard to reconcile with expressions in other parts of 

 the book, and rest largely on an erroneous statement — 

 namely, that cirques " are confined to glaciated regions," 

 and " as a rule . . . occupy positions where glaciers first 

 appear." Still, in general his conclusions are supported 

 by facts, very clearly and carefully described, so that we 

 feel, even if occasionally not quite convinced, that his 

 view is worthy of careful and respectful consideration. 



But in the matter of ice the subject is long, and our 

 space is brief. It will be better to abstain from criticism 

 of details and give a short outline of those parts of Dr. 

 Wright's book which will be of most interest to readers on 

 this side of the Atlantic. As he states in his preface, his 

 work deals not only with the Ice Age in North America, 

 but also with the whole subject of the Glacial Period. 

 So in its earlier part a considerable space is allotted to 

 glaciers in general and their characteristics ; in its later, 

 to the effects of the Glacial Period in other parts of the 

 world, its cause, its date, and its relation to the history 

 of man. These, however, we shall pass over, and confine 

 ourselves to the section dealing with glacial action on the 

 North American continent. 



After a sketch of the existing glaciers on the Pacific 

 coast. Dr. Wright gives the results of a study of the 

 Muir glacier in latitude 58° 50', by the side of which a 

 small party, of which he was a member, camped out for a 

 month. This glacier is about a mile wide where it comes 

 down to the sea, terminating in ice cliffs 300 feet, and 

 sometimes a little above 400 feet in height. The rise in- 

 land is gradual— perhaps about 100 feet per m.ile— and 

 the main body of the glacier occupies a vast amphi- 

 theatre, with diameters ranging from 30 to 40 miles. 

 From a number of observations it appeared that the 

 stream of ice entered the inlet, where the cross section 

 NO. TI45, VOL. 44] 



was about five million square feet (5000 feet wide by 

 1000 deep), at an average rate of 40 feet a day (70 feet 

 in the centre and 10 near the margin). It was, however, 

 evident that this glacier, for some time past, had been re- 

 treating ; indeed, fresh striations and debris could be 

 traced to more than 2500 feet above its present surface. 

 Dr. Wright also found below the end of the ice the dead 

 stumps of a forest of cedar trees, erect, and rooted in a 

 clayey soil, but buried beneath glacial gravel. Probably 

 this was deposited by streams, flowing from the advancing 

 ice, which afterwards overrode the mass. 



Dr. Wright estimates the amount of sediment which is 

 now being washed down from the basin of the Muir 

 Glacier as equal to nearly one-third of an inch per annum 

 over the total area (1200 square miles) which it occupies. 

 In regard to the vexed question of the excavatory powers 

 of glaciers, Dr. Wright expresses himself, as a rule, 

 cautiously, ascribing to them the formation of true rock- 

 basins under favourable circumstances, but laying stress 

 upon the fact that, in the lower part of their course, where 

 they are beginning to spread out over the lowlands, they 

 can pass, as in the case mentioned above, over quite 

 incoherent materials, without disturbing them. It also 

 seems to follow from his remarks that he regards glaciers 

 as agents of abrasion rather than of erosion, in which we 

 have no doubt he is correct. As another indication of 

 his general caution and candour, we may note that he is 

 careful to point out that striated stones and rock surfaces 

 do not always prove the former presence of a glacier, and 

 may not even have been produced by the action of ice. 



A large part of the book is devoted, as a matter of 

 course, to a description of the glaciated area in North 

 America. The boundary of this, as Dr. Wright explains, 

 is sometimes distinctly marked by a terminal moraine, at 

 others it is less definite, being only vaguely indicated by 

 scattered debris. But in his opinion — and here he ex- 

 presses the opinion of the majority of American geo- 

 logists — there was a time when a large part of Northern 

 America east of the Rocky Mountains was buried beneath 

 a mass of ice. There is, indeed, a driftless area in 

 Wisconsin, which may have formed a kind of jardin 

 on a gigantic scale, in this huge mer de glace, but, speak- 

 ing generally, the whole region of the great lakes was 

 covered by an ice-sheet which came down to the sea at 

 Long Island and traversed the northern part of Penn- 

 sylvania ; thence its irregular frontal margin can be 

 traced to the south-west, until, in the valley of the 

 Mississippi, it reaches almost as far south as the 37th 

 parallel of latitude. Of the various indications of this 

 vanished ice-sheet, the smoothed and striated surfaces of 

 rock, the moraines and boulder clays, the " kames " and 

 "drumlins," Dr. Wright gives careful descriptions and 

 illustrations, usually taken from photographs, so that the 

 evidence is presented as clearly as is possible to the 

 reader. To the last-named phenomena — the " kames " 

 and " drumlins " — and some curious hollows which he 

 calls " kettle-holes," Dr. Wright devotes much attention. 

 The first he regards as indicative of lines of drainage 

 in the closing stage of the Ice Age ; the second, as 

 early terminal moraines, modified in shape by the sub- 

 sequent passage of the ice over them, and so anterior in 

 date to the kames. The kettle holes occur among 

 morainic deposits, and are thus explained : — As the 



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