282 E. C. ANDREWS. 



Conversely if the quantitative value of heavy ice corras- 

 ion in Alpine regions be known, then corrasion of senile 

 topography by similar ice volumes will be found to be but 

 relatively slight. 



(c) "Steps" and " Interstep Treads." — It is a fact of 

 common observation that the main Alpine valleys possess 

 huge "steps" rising one above the other with heads more 

 or less cup-shaped. By some investigators 1 these are con- 

 sidered as Hanging Valleys in the main channel. Tributary 

 streams also exhibit this feature. They originate as basins 

 on declivities and by headward recession they form "steps" 

 with "interstep treads" along the channel (See Bigs. 8 and 

 9, Part I). Ordinary stream channels are full of such forms 

 on steep channel slopes. The "treads" appear to be 

 basined in proportion as the local valley cross-section has 

 favoured increase of stream velocity, or as heterogeneous 

 structures have favoured variable corrasion. The initial 

 nips, which by recession give rise to the "steps," were 

 determined by conditions favouring local increase of velocity 

 on a declivity or by the differential strength of the struc- 

 tures forming the slope. Headward recession of some 

 "steps" will progress more rapidly than that of others. 

 Thus some will appear to pursue, some meanwhile lagging 

 behind others. 



fd) Cirques. — In geographical discussions the term 

 "cirque" has been generally restricted to the amphi- 

 theatres situated along present or recent glacial divides. 

 As will be shown presently, the valley "step" is closely 

 related to the cirque, but the cirque proper is a more simple 

 form than the " step." A glance at a cirque and the valley 

 of which it is the head shows that the cirque proper is not 

 merely a phase of corrasion peculiar to the start of a 

 glacier, but rather that it marks the method of headward 



1 Garwood. 



