302 THE ANDES OF SOUTHERN PERU 



The first stages in the upward migration of the bergschrund 

 will not effect a marked change from the original profile, since 

 the converging slopes, the great thickness of neve and ice at this 

 point, and the steep gradient all favor powerful erosion. When, 

 however, stage C is reached, and the bergschrund has retreated 

 to c", a broader terrace results below the schrundline, the gradient 

 is decreased, the ice and neve (since they represent a constant dis- 

 charge) are spread over a greater area, hence are thinner, and we 

 have the cirque taking on a compound character with a lower, less 

 steep and an upper, precipitous section. 



It is clear that a closely jointed and fragile rock might be 

 quarried by moving ice at c'-c" and the cirque wall extended un- 

 broken to x; it is equally clear that a homogeneous, unjointed gran- 

 ite would offer no opportunities for glacial plucking and would 

 powerfully resist the much slower process of abrasion. Thus 

 Gilbert 10 observed the schrundline in the granites of the Sierra 

 Nevada, which are "in large part structureless" and my own ob- 

 servations show the schrundline well developed in the open- 

 jointed granites of the Cordillera Vilcapampa and wholly absent 

 in the volcanoes of the Maritime Cordillera, where ashes and cin- 

 ders, the late products of volcanic action, form the easily eroded 

 walls of the steep cones. Somewhere between these extremes — 

 lack of a variety of observations prevents our saying where — the 

 resistance and the internal structure of the rock will just permit 

 a cirque wall to extend from x to c " of Fig. 199. 



A common feature of cirques that finds an explanation in the 

 proposed hypothesis is the notch that commonly occurs at some 

 point where a convergence of slopes above the main cirque wall 

 concentrates snow discharge. It is proposed to call this type the 

 notched cirque. It is highly significant that these notches are 

 commonly marked by even steeper descents at the point of dis- 

 charge into the main cirque than the remaining portion of the 

 cirque wall, even when the discharge was from a very small 

 basin and in the form of snow or at the most neve. The excess of 

 discharge at a point on the basin rim ought to produce the form 



10 Op. cit., p. 300; reference on p. 582. 



