THE QUATERNARY PERIOD 



335 



rock fragments the power to erode is often pronounced, because 

 the work of abrasion is mostly accomplished by the rock fragments 

 embedded in the ice rather than by the soft ice itself. For instance, 

 when the great ice lobe moved up the St. Lawrence Valley it was 

 shod with many pieces of hard pre-Cambrian rocks, and the effects 

 of erosion are remarkably well shown in the Thousand Islands 

 region, where successions of great grooves cut in the solid rock 

 may often be seen. A little search will reveal polished and 

 scratched or grooved rock surfaces in almost any part of the 



' - : We:"': 



"■''*:. ; - : - 



iiBwygiBPjyk^^ * 



Fig. 211 



Smoothed and striated (glaciated) limestone. 

 (W. J. Miller, photo.) 



glaciated region of the continent (Fig. 211). Hard rock ledges 

 most frequently exhibit glacial marks, and the freshness and 

 hardness of such surface rock proves that the ice eroded all 

 of the deep residual soil as well as the zone of rotten rock, and an 

 unknown amount of live or fresh rock. 



In former years a very great erosive power was ascribed to 

 flowing ice, but today some glacialists consider ice erosion to be 

 almost negligible, while many others maintain that, under favor- 

 able conditions, flowing ice may produce very notable erosive 

 effects. During the long pre-Glacial time, rock decomposition 

 must have progressed so far that rotten rock, including soils, had 



