DISCUSSION. 



T. C. Chamberlin : It is an undeniable fact that kames, eskers and 

 moraines contain a very considerable percentage of local material. 

 Sometimes by far the larger portion of it is derived from formations not 

 far distant. If these kames, eskers and moraines were formed from 

 englacial material which became, by ablation, superglacial material, it 

 must have been through some process of upward movement within the 

 ice, as urged by the author of the paper. If there were such upward 

 movement, modern glaciers should show upon their surfaces and ter- 

 minal sloi^es material of like nature. This they do not seem to present, 

 as a rule. There may, of course, be exceptions. I have examined very 

 carefully the material on the surface and slopes of a considerable num- 

 ber of glaciers and found it invariably of sharp, angular, unworn forms. 

 I would call attention particularly to the englacial material that comes 

 to the surface on the terminal slope of the Rhone glacier. This I found 

 to be altogether angular and entirely without any evidence that it had 

 been at the bottom of the glacier or had suffered the rounding which 

 the material of kames, eskers and moraines present. The basal material 

 of the same glacier was, however, well rounded, and the moraines just 

 below contained large quantities of this rounded material. 



Frank Leverett : It would seem as though a practically continuous 

 sheet of drift several feet in thickness should have been deposited during 

 the retreat of the ice-sheet if the amount of englacial drift were as great 

 as advocated by Mr. Upham, and I would call attention to the fact that 

 in districts where there has been but a single ice invasion, as in southern 

 Illinois and in portions of Illinois and Wisconsin bordering the driftless 

 area, there are large districts across which the ice-sheet retreated without 

 leaving a sheet of drift — the former existence of the ice-sheet there being 

 known only by the presence of scattering bowlders and occasional patches 

 of till of slight depth. 



H. F. Reid : Mr. Upham thinks that in a glacier occupying a canal- 

 shaped valley there is a component of the motion at right angles to the 

 shore in order to open the lateral crevasses. This is not necessary, for 

 if the motion is strictly parallel to the sides and is more rapid as the 

 center is approached, oblique marginal crevasses will be formed. In fact 

 the direction of motion cannot be inferred from the direction of a crevasse 

 without also considering the relation of the crevasse to the glacier itself — 

 that is, without knowing whether the crevasse is lateral, transverse or 

 longitudinal. 



(85) 



