800 



Imagine now an immense glacier to occupy the greater 

 portion of the preglacia I valley cross-section. If the valley 

 be constricted at B and U and along D E, then the glacial 

 velocity is increased at B and O, and along the whole 

 distance D B. 



There are now several points to be borne in mind in dis- 

 cussing the action of a large ice stream upon the channel 

 whose bed is indicated by A B O D E. 



(1) In descending the steep slope A K the glacier receives 

 great increase in velocity generally. 



(2) At the points B and C and along the whole length 



D E also, the glacial velocity is locally increased. 

 The kinetic energy of the stream therefore rises in a high 

 degree and strong corrasion of the general channel base 

 AE ensues, nevertheless, because the greatest resistance 

 to the stream motion is experienced at the points B', 0, 

 and E', the greatest percentage of stream energy which is 

 converted into corrasion occurs at these points. Therefore 

 the basins E , 0', and IS are deepened and broadened and 

 lengthened until the lateral, vertical and longitudinal 



are altogether expended. An important point may be 

 mentioned at this stage of the enquiry, (facial action 

 immediately down stream of the point E tends to compress 

 the structures, because they lie below the temporary base 

 level Fft. Corrasion will therefore take on the form of 

 abrasion mainly at this locality, quarrying action being 

 possible only in the cases of large ice streams. It is the 

 analogy with the geologist who hammers the centre of a 

 rock mass instead of striking at the unsupported edges. 



But in the cases of the basins and IV the action is dual. 

 At the bottom of the basin the stream action is compressive 

 and corrasion works mainly by abrasion to produce a rock 



