286 



GEOLOGY, 



most effective at the Bergschrund because the slope here favors " pluck- 

 ing/' Here, notable amphitheatres (cirques) are sometimes excavated. 

 After the glacier disappears, the bottom of the cirque is often seen to 

 contain rock basins (Fig. 257). Glacial cirques abound in mountains 

 where glaciers once existed, but from which they have now disappeared. 



Fig. 256. — Striae, grooves, etc., in a canyon tributar}^ to Big Cottonwood Canyon, 

 Wasatch Mountains. (Church.) 



The cirques of the Bighorn mountains of Wyoming (PI. XIX) are 

 examples. 



Summary. — In summary it may be said that rapidly moving ice 

 of sufficient thickness to be working under goodly pressure, shod with a 

 sufficient but not excessive quantity of hard-rock material, passing 

 over incoherent or soft formations possessing a topography of sufficient 

 relief to offer some resistance, and yet too little to retard seriously 

 the progress of the ice, will erode most effectively. 



Varied nature of glacial debris. — From its mode of erosion it will 

 readily be seen that the bottom of a glacier may be charged with various 

 sorts of material. There may be (1) bowlders which the ice has picked 

 up from the surface, or which it has broken off from projecting points 

 of rock over which it has passed; (2) smaller pieces of rock of the size of 



