250 



DYNAMICAL GEOLOGY. 



ongm. 



Subsequent abrasion by a sub-glacial or glacier-fed stream may, how- 

 ever, remove the scratches from the stones. The ledges underneath, or 

 especially their harder portions, are often made, by glacial abrasion, into 

 rounded, grooved knolls, called sheep-backs {rocJies moutonnees) in allusion 

 to their forms. They are a prominent feature of all glacial regions ; and 

 those of the Glacial period, when they were formed over a vast extent of 

 country, are sometimes preserved to the present time in great perfection. 

 The view (Fig. 216), from a photograph obtained by the Hay den expedition 

 in 1873, represents a portion of a great crouching flock of them, extending 



216. 



View on Roche-Moutonnee Creek, a tributary of Eagle River, Colorado. 



for 2000 feet along 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. 



A glacier, too, may have vjater-falls in crevasses (and sometimes in well- 

 like shafts, formed in crevasses), which not only carry down moraine material, 

 but excavate the rocks underneath. They may thus make broad basins or 

 channels in the rocks as the glacier moves on its way ; but without stopping 

 its march for a few centuries the fall cannot bore out a " pot-hole " like the 

 pot-holes of river origin ; for these require a stationary tool, they being ordi- 

 narily as well-centered as if bored by a revolving bit. 



Deposition from the glacier takes place through the melting of the ice, as 

 in the making of the deposit of terminal moraine, above explained. Deposits 

 are also made through crevasses, and the waters of any super-glacier rivers 

 or lakes may add to the contributions. The descending waters carry down 



