308 SURFACE GEOLOGY, 



very fine clayey detritus in which its glaciated pebbles and boulders are 

 embedded, indicate that this portion of the drift has been mostly derived 

 by erosion from the rocks, and pulverized under the grinding action of 

 the ice-current. 



The level layers of gravel, sand, or clay that are occasionally found in 

 this lower till appear to have been formed by streams, which in summer 

 found their way through crevasses to the bottom of the ice. These 

 seams and beds of modified drift in the till frequently occur upon len- 

 ticular hills and slopes, where they could not have been deposited by 

 ordinary streams, if the ice-sheet was withdrawn. The usually horizon- 

 tal position and considerable extent of these beds show that after their 

 formation they lay undisturbed, while the ground-moraine continued to 

 be deposited above them. Where similar seams or beds are nearly ver- 

 tical, inclined, or contorted, as they are more rarely observed, it shows 

 that a large mass of the lower till was lifted up before the ice-current, 

 and pushed forward with its included layers of modified drift. 



The accumulation of these hills and slopes seems to have been by slow 

 and long continued addition of material to their surface, the mass remain- 

 ing nearly stationary from the beginning of its deposition. Obviously 

 this was the case with the lenticular slopes gathered behind the shelter 

 of higher ledgy hills, or upon their opposite sides. Except in their loca- 

 tion, these slopes are like the lenticular hills, which seem to contain no 

 ledge, being simply heaps of the ground-moraine 50 to 200 feet in height. 

 This resemblance suggests that both hills and slopes alike increased 

 slowly in extent and depth without much change in place, new material 

 being lodged upon their surface from the ice-sheet which swept over 

 them. 



The obscure lamination or cleavage, which is one of the characteristic 

 features of the lower till, was probably produced by this mode of its ac- 

 cumulation. In this deposit from the ice-sheet, it corresponds to the 

 stratification of sediments from water, but it is less distinct ; and the fine 

 detritus in which it appears contains glaciated pebbles and boulders in- 

 discriminately mixed through its whole mass. This structure was at first 

 thought to be a true cleavage, caused by the pressure of the glacial 

 sheet. If we take this explanation, it still proves, like the hardness and 

 compactness which also mark the lower till, that this deposit was not 



