main valley heads mid cirques over ledges or ''steps'' each 

 representing a recession along a tiny thalweg at the glacial 

 gathering ground. It now remains to be seen by what 

 activities the typical cirque obtains its peculiar proiiles. 

 In the first place it will be evident to students of dynamics 

 that the "tread" of the valley which forms the lower por- 

 tion of the cirque will be fiat in proportion to the volume 

 of the glacier which has caused the "step" recession, and 

 also that the "step" which is formed by the development 

 of the "tread" will be of great height in proportion to the 

 steepness of the thalweg along which the recession has 

 taken place. That is, all other thing- being equal, a cirque 

 slmuld he deep in proportion both to the volume of the 

 glacier and to the steepness of the declivity (thalweg) 

 along which it has been developed by recession. In regions 

 of heavy snowfall therefore and in steep Alpine countries 

 one must expect the grandest development of the cirque 

 form. And this is evident from what has been adduced in 

 the present discussion on stream mechanics, no matter how 

 insignilicant the initial "step" and "tread" may have been. 

 Another point into which we cannot enter fully here, 

 because of lack of space, is that every stream tends to form 

 a channel of such shape and size as to satisfy the demands 

 of the lateral and vertical measures of its strength. That 

 is to say, under similar condition-, streams even of different 

 material will form similar profiles, nevertheless the more 

 voluminous stream will form profiles altogether larger than 

 those of the stream of smaller volume. So long as the 

 thalweg is steep ami the channel deep and narrow, the 

 vertical factor of corrasion is dominant; so long as the 

 channel walls are deep and strong so long will the lateral 

 factor of corrasion be cramped in its movements because 

 of the compression of the structures (by the overlying rock 

 weight) which opposes the lateral cutting action; so long 

 as a stream is of great relative depth, so long will the 



