210 ELEMENTARY GEOLOGY 
valleys are deepened and basins of rock carved out. 
Since we cannot observe the work being done beneath 
the ice, we cannot be certain upon this point, though 
a mass of evidence, gathered in regions once occupied 
by glaciers, indicates that the latter view is the more 
nearly correct. 
In whatever way the materials are obtained and 
Fig. 114. 
Photograph of a bed of bowlder clay in Pennsylvania. 
transported, if the ice departs, the various moraines 
are left. behind as proofs of former ice presence. In 
the place once occupied by a valley glacier, there 
would therefore remain a general sheet of till or 
bowlder clay, representing the ground moraine; and 
thicker accumulations would be found where the lat- 
eral, medial, and terminal moraines existed. 
