CORRASION BY GRAVITY STREAMS. 287 



series of " steps" and "treads," as lias been already dis- 

 cussed for the general stream in Part I of this Series. 



This introduces the subject of the similarities and dis- 

 similarities of form which should be expected to exist 

 between cirques proper and " steps." The "step" is really 

 a cirque form, but it involves a more complex corrasive 

 action than does the cirque proper. The typical cirque 

 approximates more to the ideal gravitative form — that form 

 produced by earth material falling freely towards a vertical 

 gash made in the earth's crust — because the form is pro- 

 duced typically at or near glacial divides. The "step" is 

 a result of all the processes at work in the formation of 

 the cirque, but involves also the passage of the stream 

 (glacier) over the lip of the cirque. 



The enormous quarrying action thus exercised completely 

 modifies the typical cirque form, namely, by squaring off 

 the profiles of the upper portions of the "gravitative " form. 

 Magnificent examples of such quarrying action and conse- 

 quent destruction of the typical cirque form occur in the 

 Yosemite, as at Nevada and Vernal Falls. These will 

 receive due attention later. 



The cirque under a col is an example of a " step " or 

 modification of the typical cirque. 



(e) Action in "Broads." — Where the flood glaciers, for 

 any reason, left a canon defile or entered a part of the 

 valley possessing a much greater cross-section, the velocity 

 of the ice suffered a check. This involved the dropping 

 out of a part of the ground moraine some distance in 

 advance of the canon mouth. 



If now the ice should suddenly vanish and water streams 

 should occupy the old glacial channels, they would here 

 form huge lakes dammed by moraines, which in many cases 

 would pass into rock basins higher upstream. The partial 



