proceedings: geological society 255 



« 



That the ice masses accumulating in a cirque are competent to gen- 

 erate through corrasion alone a bowl characterized by smooth, concave 

 slopes, and hollowed out so as to hold a lake basin, can be demonstrated 

 by analysis of the mechanics involved. Similarly it can be shown that 

 the ice masses flowing out of the cirque are competent by their corrasive 

 action alone to produce a U-shaped trough. Frost sapping, it is true, 

 does take place along the margins of the outflowing glacier, but it does 

 not contribute to the elaboration of the U-trough any more than it 

 contributes to the elaboration of the cirque bowl. It is effective only 

 to the shallow depth to which the marginal crevasses open, and gives 

 rise to special marginal features above the edges of the U-trough; 

 namely, to a shelf or shoulder, and a cliff or scarp. In fact, the mar- 

 ginal shoulder and scarp of the outflow canyon are but the continuations 

 of the shelf and cliff of the cirque, while the U-trough, properly speaking, 

 is the continuation of the cirque bowl. It is not contended that these 

 sculptural features are characteristic of cirques and outflow canyons in 

 all parts of the world. They can acquire prominence only in cirques 

 and canyons carved from prevailing massive rocks, such as those of the 

 Sierra Nevada, for such rocks do not lend themselves well to plucking, 

 and in them mass corrasion must perforce consist mainly of abrasion, 

 which is productive of smoothly curving surfaces, contrasting strongly 

 with the hackly faces of frost-riven cliffs. In densely jointed rocks, 

 on the other hand, mass corrasion may consist of both plucking and 

 abrasion, and the surfaces produced by it are likely to be more or less 

 hackly and therefore not greatly different in appearance from those 

 resulting from frost sapping. In cirques laid in thin-bedded, finel}'' 

 jointed sedimentary rocks, accordingly, one should not expect to find 

 an}^ pronounced contrast in appearance between the bowl slopes and 

 the cliff. 



Again, the features of some Sierra cirques are particularly^ clean-cut 

 because the uniformity of the rock structure makes for uniformity and 

 continuity of the individual sculptural units, and as a consequence the 

 boundary between those units — between bowl and cliff — is sharply 

 defined and regular. Local variations in structure and in resisting 

 qualities, on the other hand, commonly occur in the rocks of most moun- 

 tain regions. As a consequence, irregularities in cirque sculpture are 

 the rule. Rocks of locally varying structure and resistance, further," 

 are likely to develop ragged surfaces, giving rise to several successive, 

 and more or less discontinuous bergschrunds, and in them clean-cut, 

 simple cirque forms can not be realized. No doubt it is the prevailing 

 irregularity of the cirque forms, due to interfering structures, that has 

 prevented observers in most mountain regions from clearly apprehend- 

 ing the relative importance of the two processes involved in cirque 

 erosion — mass corrasion, and bergschrund sapping. Another circum- 

 stance, finally, that appears' to have favored the production of a pro- 

 nounced sap line in the cirques of the High Sierra is indicated by a 

 study of the level reached in the cirques and canyon by the ice of each 

 of the two pleistocene glacial epochs that have occurred in the Sierra 



