340 Warren Upham — Pleistocene Climatic Changes. 



II. — Pleistocene Climatic Changes.^ 

 By Warren Upham, F.G.S.A. 



THE most interesting and difficult climatic problem presented in 

 all the geologic record is that of its latest period, immediately 

 preceding the present, to discover the causes, first, of the accumu- 

 lation, and later, of the rapid final melting of its vast sheets of 

 land-ice. The fossil floras of Greenland and Spitzbergen indicate 

 that those far northern latitudes enjoyed a temperate climate in the 

 Miocene period ; and, from the absence of glacial drift through the 

 great series of Tertiary and Mesozoic formations, we infer that 

 climates as mild as those of the present day had prevailed during long 

 eras before the Ice-age. With suddenness, geologically speaking, 

 there came during the Quaternary era a very exceptional and unique 

 period of great refrigeration of the climate of northern regions, 

 overwhelming the Siberian herds of Mammoths, and covering the 

 surface of the northern half of North America and of North- Western 

 Europe with snow and ice which increased to thousands of feet in 

 depth. The conditions that seem requisite for the formation of 

 these ice-sheets are long-continued rather than excessive cold and an 

 abundant supply of moisture by storms, giving plentiful precipitation 

 of snow during more of the year than now, so as to include in the 

 time of snow accumulation not only the present winter but also 

 the autumn and spring months. The summers, too, were probably 

 cooler than now, for their heat was not sufficient to melt away the 

 accumulated snow, which gradually increased in thickness from year 

 to year, its lower part being changed to ice. When large portions 

 of continents became thus ice-covered, the storms sweeping over 

 them would be so rapidly cooled that the greater part of their snow- 

 fall would take place upon the borders of the ice-sheet, within 

 probably fifty to two hundred miles from its margin ; but the 

 snowfall during the advance of the ice was probably in excess of 

 the amount of evaporation and melting over the whole of the ice- 

 covered areas. 



Upon British America and in the northern part of the United 

 States the directions of the glacial strias and transportation of the 

 drift show that there were two general ice-sheets — oae reaching from 

 New England, Nova Scotia, Newfoundland, and Labrador west to 

 the Rocky Mountains, and north to the Arctic Ocean, having its 

 greatest thickness over the Laurentide Highlands and James Bay, 

 with outflow thence to the east, south, west, and north ; and the 

 other west of the Eocky Mountains, covering British Columbia, 

 attaining a maximum thickness of about one mile, and outflowing 

 south into the United States, west into the Pacific Ocean, and north- 

 ward into the upper part of the Yukon Basin. These ice-sheets, 

 named by Dr. Greorge M. Dawson the Laurentide and Cordilleran, 

 are shown, by the characters of the drift deposits along the eastern 



1 Presented at the World's Congress on Geology, auxiliary -ndth the Columbian 

 Exposition in Chicago, August 26th, 1893. 



