700 



DYNAMICAL GEOLOGY. 



a valley leading down from the " Mountain of the Holy Cross," 

 one of the prominent summits (12,485 feet high) in the mountains of 

 Colorado. 



The thickness of the ice in the Alps nowhere exceeds 500 feet. Let 

 it be 2,000 feet, as now in some Greenland glaciers, or twice this, as 

 in many regions during the Glacier period, and the work of erosion 

 accomplished would be vastly greater, since it is directly proportioned 

 to the thickness. 



(3.) The grinding of the stones against one another, and those of 

 the bottom against the underlying rocks, produces very fine powder, 



Fig. 1107. 



View on Roche-Moutoim<Se Creek, a tributary of Eagle River, Colorado. 



which makes the waters of the nnderflowing stream milky, and forms 

 clay-like deposits, of which the bowlder clay of Drift regions is an ex- 

 ample. 



(4.) The glacier finds the rocks beneath usually jointed, laminated, 

 or foliated, or in upturned beds, and it thus has an opportunity to tear 

 off masses. This is in fact its most efficient means of erosion; for 

 hard rocks which might require a century for a foot of direct wear, 

 may, through the natural divisional planes, yield by scores of yards in 

 the same period. 



(5.) The subglacial streams, one of the prominent features of a gla- 

 cial region, and their continuation down the mountain valleys, are also 



