Warren TJpham — Pleistocene Climatic Changes. 341 



side of the Eocky Mountains, to have become confluent at their 

 time of maximum thickness and extent. The Cordilleran outflow, 

 pouring eastward through passes of these mountains, and in the 

 Peace River region probably overtopping the highest summits, which 

 there are only about 6000 feet above the sea, pushed across a narrow 

 belt adjoining the mountains, to a maximum distance of nearly 

 100 miles, and there (on land about 2500 feet above the sea) was 

 merged with the Laurentide ice, the two united currents thence 

 passing in part to the south and in part to the north from the interior 

 tract, where the confluent ice was thickest. When the Glacial 

 period reached its culmination, nearly all the northern half of this 

 continent, excepting Alaska, was thus enveloped by a continuous 

 sheet of land-ice which stretched from the Atlantic to the Pacific, 

 and from the Ohio and Missouri Rivers to the Arctic Archipelago, 

 having an approximate area of 4,000,000 square miles. 



Almost half as large an area was ice-covered in Europe, with 

 the basins of the Irish, North, Baltic, and White Seas, the principal 

 centre of outflow being the plateau and mountains of Scandinavia, 

 whence the ice moved west and north into the Atlantic, southward 

 over Northern Germany, and eastward over a large part of Russia. 

 Smaller ice-sheets were formed upon Scotland and Ireland, and 

 these became confluent with each other and with the Scandinavian 

 ice which outflowed to the borders of Great Britain. Glaciers also 

 were far more extensive than now in the Alps, Pyrenees, Caucasus, 

 and Himalayas ; but Northern Asia seems to have had no general 

 ice-sheet. The most anomalous feature in this strange accumulation 

 of ice was its absence from the greater portions of Siberia and 

 Alaska, while so heavily massed in the same and more southern 

 latitudes of British America, the Northern United States, the British 

 Isles, and North-Western Europe. 



Both the growth and the decline of the ice-sheets were attended 

 by considerable fluctuations of the ice-border, which appear to have 

 been due to secular changes of moderate amount in the average 

 climatic conditions of temperature and precipitation of snow and 

 rain. Comparatively few records, however, of the glacial fluctuations 

 during the time of general advance and culmination are preserved, 

 since the drift deposits of those stages are mostly covered, unless 

 they suffered glacial erosion and were thus wholly lost, beneath 

 the deposits of later drift which tell the vicissitudes of the general 

 retreat and departure of the ice. 



Evidence of important glacial recession and re-advance in North 

 America, separated by centuries or probably thousands of years, has 

 been obtained only in the great interior expanse of the Mississippi 

 basin. Mr. Frank Leverett ^ has. reported there, in Southern 

 Illinois, Indiana, and Ohio, sections showing an old surface of Till, 

 deeply buried by the Till of another ice advance, which extended 

 far south of the retreatal moraines and on some areas formed the 

 extreme glacial boundary. The soil and the oxidized and leached 



* Proceedings of the Boston Society of Natural History, vol. xxiv. pp. 455-459, 

 January 1st, 1890. 



