THE QUATERNARY PERIOD 337 



is a structure section across it showing the rock terraces and the 

 relations of the various rock formations. The conditions for ice 

 erosion there were unusually favorable, because the ice, in its great 

 sweep around the Adirondacks, was heavily shod with hard rock 

 fragments and entered the deep valley by striking with greatest 

 force against the soft rocks on the west side. The soft shales were 

 worn back more than the harder limestones, while the very hard 

 pre-Cambrian rocks were but little affected. If soft shales had 

 made up the valley bottom, ice erosion would have caused consid- 

 erable deepening, as was, no doubt, the case in the valleys of the 

 Finger Lakes region of western New York. Even in places so 

 favorably situated as those just mentioned there is no reason to 

 believe that ice erosion did any more than to modify the profiles 

 of the pre-Glacial channels. 



It is also a singular fact that glacial deposits left by one ice 

 sheet may actually have been overridden by a later advance of 

 ice with little erosion of even such soft material. This probably 

 happens only near the margin, where the ice is rather thin and 

 hence would not be expected to have much erosive power. 



In conclusion we may say that while many comparatively 

 small, local features were produced by ice erosion, the major 

 topographic features of the great glaciated area were practically 

 unaffected by the abrasive effects of the passing ice sheets. 



Ice Deposits 



The vast amount of debris transported by the great ice sheet 

 was carried either on its surface, frozen within it, or pushed along 

 beneath it. It was heterogeneous material ranging from the finest 

 clay, through sand and gravel, to boulders of many tons' weight. 

 The deposition of these materials took place during both the 

 advance and retreat of the ice, but chiefly during its retreat. Most 

 of the deposits made during the ice advance were obliterated by 

 ice erosion, while those formed at the time of the retreat have been 

 left intact except for the small amount of post-Glacial erosion 

 and weathering. The term " drift," applied to all deposits of glacial 

 origin, was given at a time when they were regarded as flood or 

 iceberg deposits. Drift covers practically all of the glaciated re- 

 gion except where bare rock is actually exposed, and its thickness 

 is very variable, ranging from nothing to some hundreds of feet. 



