SPECIFIC DISCUSSION 25 



glacier in the process of valley widening, or in the production of the 

 theoretical U-form of glaciated valleys. 



(12) " Hanging " valleys have been the occasion in later years for pos- 

 tulating deep valley-cutting by stream glaciers. The assumption is made 

 of valley deepening in hard, crystalline rocks, of hundreds and even 

 thousands of feet, simply to explain a discordance of valleys. The phy- 

 siographic argument is based, in turn, on another assumption, that moun- 

 tain valleys, as those of Norway and Alaska, the Alps and Sierras, should, 

 under atmospheric and aqueous agencies, be graded to an accordant sys- 

 tem. With the highest personal regard and admiration for the masters 

 in physiographic science who have advanced the above ideas, the writer 

 yet believes that their conclusions are wrong. The genesis of topographic 

 forms involves a number of agencies which in their relative effects are 

 indeterminate. The students of these very theoretical and complex 

 problems should not expect to find the full explanation at once for all 

 puzzling features ; neither should they resort to short cuts which are mere 

 assumptions. Discordant valleys will be found to have some rational 

 explanation in non-glacial processes, probably as a natural stage of drain- 

 age under exceptional conditions. The assumption of enormous ice 

 excavation does not seem warranted. The further discussion of this in 

 reference to certain mountain districts and to central New York topog- 

 raphy will be found later in this paper. 



General negative argument. — Several elements or factors involved in the 

 mechanics of glaciers are decidedly unfavorable to great erosion, and 

 none are positively favorable. From the study of glacial phenomena 

 for fifty years the following principles of glacier physics seem to be well 

 established : 



(a) In the ordinary normal flow, the upper part of the glacier moves 

 faster than the lower or bottom portions. * 



(b) The fluency of the ice diminishes in proportion to the amount of 

 commingled rock-debris, f 



(c) Horizontal shearing occurs, the upper, more rapidly moving, layers 

 sliding over the lower, basal, and laggard layers. J 



(d) The typical form of the ice-eroded channel is U shape, as com- 

 pared with the V form of youthful stream channels. Glaciers, therefore, 

 widen their valleys. § 



(e) The condition of load most favorable to abrasion is a light charge 

 of coarse and hard rock fragments. || 



*H. F. Ried: "The mechanics of glaciers." Jour. Geol., vol. iv, 1896, pp. 912-928. 

 tl. C Russell : "The influence of debris on the flow of glaciers." Jour. Geol., vol. iii, pp. 823-832. 

 JH. F. Reid: Jour. Geol., vol. iv, pp. 920,925; I. C. Russell: Jour. Geol., vol. iii, pp. 823-832; 

 G. H. Barton, Tech. Quar., vol. x, p. 220; Chamberlin and Salisbury, Geology, I, p. 302. 

 I \V J McGee: "Glacial canons." Jour. Geol., pp. 350-364. 

 || I. C. Russell : Jour. Geol., vol. iii, p. 823-832; Chamberlin and Salisbury, Geology, I, pp. 271, 273. 



