322 Notices of Memoirs — W. 0. Crosby — 



fields and ice-caps into a climatic zone wliere permanent ice cannot 

 form ; and hence they are moving over unfrozen soil, and the ice is 

 wasting by melting on its under as vi^ell as its upper surfaces. The 

 great desideratum is a shaft or boring in the interior of Greenland 

 extending through the entire thickness of the ice-sheet and a hundred 

 feet into its rocky bed. The Greenland studies of Professor Cham- 

 berlin show the facility with which ice shears along innumerable 

 lines of debris ; it slides over the debris instead of dragging it along, 

 as it would if the debris were firmly frozen into the ice. Granting, 

 hovpever, that tbe frozen soil would be more rigid and indifferent to 

 gravitative stresses than the clear ice above it, the fact remains that 

 the unfrozen soil at the base is more yielding and plastic than 

 either ; and hence, although we may reasonably conceive definite 

 shearing-planes as distributed through a considerable thickness of 

 the ice-sheet, the lowest plane, and the true base of the ice-sheet, 

 will still be at the lower limit of frost. 



Eectilinear strias, often scores of feet in length, are an impossi- 

 bility unless we conceive the entire mass of the ground-moraine as 

 enclosed in the moving ice. These would be produced during the 

 period of active glaciation when the ground-moraine was very scanty 

 or wholly incorporated with the moving ice. A true ground-moraine 

 between bed-rock and ice, and distinct from both, belongs to the 

 waning stage of the ice sheet and to lobes of ice (glaciers) descending 

 from an ice-cap or neve field into a climatic zone where permanent 

 ice cannot form. 



All this appears to be quite consistent with the local origin of the 

 greater part of the till or ground-moraine of the Pleistocene ice- 

 sheet, and the well-established fact that the total movement of the 

 ice amounted to hundreds of miles, if we simply suppose that through 

 the granular plasticity of the ice, or a series of shear-planes, the 

 basal, drift-laden layer moves much more slowly than the overlying 

 clear ice. 



Absorption of Drift by the Pleistocene Ice- sheet. 



As the initial shearing-plane of the ice-sheet is normally at the 

 lower limit of frost, a considerable amount of detritus is englacial 

 from the beginning of the movement of the ice-sheet. When 

 a sedentary ice-sheet is overridden by a Piedmont glacier, or by 

 the readvance of an earlier ice-sheet, the conditions must be 

 favourable for the transfer of drift from the base of the earlier 

 sheet to a somewhat elevated position in the composite sheet which 

 results from the overriding. The overriding sheet will carry 

 with it not only its own englacial drift, but will drag along a part 

 of its ground-moraine or subglacial drift. 



It is altogether probable that each important recession of the 

 ice-sheet, and not only the final recession, was characterized by 

 numerous glacial rivers and lakes, and an extensive development 

 of modified drifts in the well-known forms of kames, eskers, deltas, 

 with abrupt northern margins, etc. Such loose deposits would be 

 taken up by the succeeding ice-sheet. 



