796 WALLACE W. ATWOOD 
limits of glaciation during the two epochs is shown in the table 
already given. 
The recognition of distinct glacial epochs among the mountains 
has associated with it some peculiar difficulties. When ice descended 
Fic. 3.—One of the sharp divides in the basin region. 
a canyon it was quite apt to obliterate all traces of any earlier ice 
advances through the same course. This was particularly true if 
the later ice advanced as far as any earlier ice. The chances of 
finding buried drift in these restricted courses of the ice are therefore 
much poorer than in the open country invaded by the continental 
