THE GLACIAL OR PLEISTOCENE PERIOD 221 



c. Age of the Drift Sheets 



The proofs of the great age of the earlier drift sheets are found in the great 

 depth of erosion and the amount of leaching or oxidizing (weathering) that 

 took place before the later sheets were laid down. The intercalated forest beds 

 and old soils also provide criteria of value and the fauna is now proving a 

 criterion of large importance. 



d. Centers of Ice Accumulation and Radiation 



In the central and eastern part of North America there were two centers of 

 ice accumulation from which the glaciers moved outward in all directions. 

 (1) the Keewatin, west of Hudson Bay, and (2) the plains of Labrador. From 

 the first came the ice sheets known as the Nebraskan, Kansan, and Iowan; 

 from the second came the ice sheets known as the Jerseyan, Illinoian, Earlier 

 and Later Wisconsin. The Jerseyan is correlated doubtfully with the Neb- 

 raskan. 



e. Effect of Glaciation on the Englaciated Territory 7 



"The great and unequal erosion of the ice-sheets, and especially the great 

 and unequal deposition of the drift, produced a profound effect upon the topo- 

 graphy of the planer parts of the area affected by glaciation. One of the conspi- 

 cuous results of this alteration of the topography was the derangement of the 

 drainage. One of the results is seen in the thousands of lakes which affect the 

 surface of the later drift, and to a less extent, the surface of the older. The ba- 

 sins of these lakes or ponds arose in various ways. There are (1) rock basim 

 produced by glacial erosion; (2) basins produced by the obstruction of rivei 

 valleys by means of the drift; (3) depressions in the surface of the drift itself; 

 and (4) basins produced by a combination of two or more of the foregoing. 

 The third class as above, may be subdivided into depressions in the surface of 

 (a) the terminal moraine, (b) the ground moraine, and (c) stratified drift. 

 Since the stratified drift in which the lakes of this last subclass lie is largely in 

 valleys, it would not be altogether inappropriate to class them with group 2. 



"In addition to the lakes and ponds now in existence, there have been others 

 of a more temporary character. Some of them have already become extinct 

 by reason of filling or by the lowering of their outlets since the ice melted; others 

 depended for their existence on the presence of the ice, which often obstructed 

 valleys, giving rise to basins. The ice also developed basins outside of valleys, 

 when the surface slope was favorable. 



"Another result is to be seen in the changes in the courses of the streams. 

 In many cases, pre-existing valleys were filled with drift, so that when the ice 

 melted the old channels were obstructed at many points, and surface drainage 



7 Chamberlin and Salisbury, Geology, III, pp. 379-381. 



