44 UNIVERSITY OF COLORADO STUDIES 



that it widens the valley rapidly at the bottom while doing but little 

 work at the top, thus changing the V-shape to U-shape. 



Roches Moutonnees. — These are knobs of rock rounded by the 

 glacier, so called from their resemblance to the backs of sheep. 



Polished and Scratched Surfaces. — The ice, dragging sand and 

 boulders along its bottom, moves over the rocks, often giving them a 

 beautiful polish, and fluting, grooving and scratching them. The 

 roches moutonnees still stand out prominently in the upper parts of 

 our Colorado glaciated valleys, vegetation not yet having gotten 

 enough soil for a foothold, and in a few places polished surfaces and 

 scratches are visible, though the character of the rock in some places 

 is such that the latter are not well preserved, the surface of the granites 

 and gneisses weathering rapidly. 



Moraines. — Moraines afford the best evidence of glaciation 

 usually, in the regions examined by me, as they are so unmistakable 

 and so universally present, although in some places probably the 

 rapid retreat of glaciers has prevented the deposition of such moraines.^ 



Lakes. — Glaciers in their retreat in mountain regions have usually 

 left behind them two kinds of lake basins — true rock basins, scoured 

 out by the ice in softer rock lying back of harder zones, and basins 

 dammed by moraines. Lakes of both types are abundant in most of 

 our glaciated valleys, but rare or wanting in some. 



Hanging Valleys. — Hanging valleys are lateral valleys whose beds 

 where they enter the main valley are above the bed of the main 

 valley. They have been usually attributed to glaciers,^ but may 

 occur from causes not associated with glaciers.^ Such valleys are 

 common in our glaciated regions. 



Cirques and Benches. — Our glaciated mountain valleys head in 

 amphitheaters called cirques, and are usually terraced. The cirque 

 structure has often been looked upon as a cause, rather than an 

 effect, of glaciation, forming, as it does, a natural reservoir for the 

 accumulation of snow. Only recently have the cirques and terraces, 



' Lee, Willis T., "Note on the Glacier of Mount Lyell, California," Journ. Geol., Vol. XIII, p. 358, igos. 

 ' Davis, W. M., "Hanging Valleys in General," Science, N.S., Vol. XXV, pp. 833-36, igo;. 

 J Upham, Warken, "Fjords and Hanging Valleys," Amer. Geol., Vol. XXXV, pp. 3x2-15, igos; Crosby, 

 W. O., "The Hanging Valleys of Georgetown, Colorado," Amer. Geol., Vol. XXXII, pp. 42-48, igo3. 



