GLACIERS =e > 909 
action, thus showing the great work of scouring that 
is being done. As one rock is thus ground against 
another, a fine clay or rock flowr 1s worn off, and to 
this mixture of clay and pebbles, is given the name 
of bowlder clay or till* (Fig. 114). 
* 
Fie. 113. 
A pebble that has been carried by ice and smoothed and scratched. 
There is a difference of opinion among geologists as 
to how much a glacier wears against its bed, some 
holding that little is accomplished, others believing that 
1 Many of the names and facts set forth in this section apply to other 
glaciers besides the valley. Hence some of the illustrations are from de- 
posits left by the extinct American continental glacier (p. 475). 
Fe 
