344 Warren Upham. — Pleistocene Climatic Changes. 



was melting away from the basins of the Upper Mississippi, of the 

 northern Red River, and of the Laurentian lakes. 



The entire departure of the North American ice-sheet, therefore, 

 probably occupied only two or three thousand years ; and half of this 

 time may measure the duration of Lake Agassiz, with the accumulation 

 of the numerous adjacent moraines whose courses intersect the lake 

 area, and with the formation of its beaches marking about thirty 

 successive stages in the concurrent subsidence of the lake surface 

 and rise of the earth's crust, which amounted together to 700 feet 

 on the latitude of the north part of Duck Mountaia and the middle 

 of Lake Winnipeg. But even these short estimates may be too long. 

 The shores of Lake Michigan, similar with those of Lake Agassiz, 

 in the Drift of which they are formed, in their north and south 

 trends and in the adjoining depths of water, have suffered an 

 amount of erosion by the lake waves during post-Glacial time 

 which very far exceeds the total erosion that was effected upon 

 the shores of Lake Agassiz during all its stages, the proportion 

 between them being surely not less than ten to one ; and Lake 

 Michigan has a similarly greater amount of beach deposits, which 

 npon a large area about its south end are raised by the wind in 

 conspicuous dunes. This contrast, indeed, suggests that the duration 

 of Lake Agassiz, and the recession of the ice-sheet from its area 

 and from the greater part of the area of Hudson Bay, may have 

 been included within less than one thousand years.^ 



Having thus reviewed the Glacial period, we come to inquire — 

 What were the causes of its grand climatic changes, leading first to 

 the accumulation, and then to the rapid departure, of the ice-sheets ? 

 Upon their areas warm or at least temperate climates had prevailed 

 during long foregoing geologic ages, and again at the present time 

 they have mostly mild and temperate conditions. The Pleistocene 

 continental glaciers of North America, Europe, and Patagonia have 

 disappeared ; and the later and principal part of their melting was 

 very rapid, as is known by various features of the contemporaneous 

 glacial and modified drift deposits and by the beaches and deltas of 

 temporary lakes that were formed by the barrier of the receding 

 glaciers. Can the conditions and causes be found which amassed the 

 thick and vastly extended sheets of land-ice, and whose cessation 

 suddenly permitted the ice to be quickly melted away ? 



Two principal theories of the causes of the Ice-age seek to answer 

 these inquiries. The one which we will first consider is the astronomic 

 theory of Croll, Geikie, and Ball. In accordance with Dr. Croll's 

 theory. Glacial periods would be expected to recur with geologic 

 frequency, whenever the earth's orbit attained a stage of maximum 

 eccentricity, during the very long Tertiary and Mesozoic eras, which 

 together were probably a hundred times as long as the Quaternary 

 era in which the Ice-age occurred. But we have no evidence of any 

 Tertiary or Mesozoic period of general glaciation in circumpolar 

 and temperate regions, although high mountain groups or ranges are 



^ Geol. and Nat. Hist. Survey of Canada, Annual Report, new series, vol. iv. for 

 1888-89, pp. 50, 51 E. 



