1870.] 199 [Shaler. 



and was dropped in the place where it lies by the melting of the iee 

 which held it. The only difficulty in this view is to conceive that such 

 a mass of detritus as that in question could have ever been contained 

 in a glacial stream. There can be little doubt that this drift must 

 have been, when originally deposited, at least one hundred and fifty 

 feet thick. It is very doubtful whether the thickest of the Swiss 

 glaciers, the Aletsch or the Mer de Glace would, if melted down, de- 

 posit a coating of more than twenty feet in average thickness upon 

 their floors, and yet these have immense feeding grounds, enormous 

 tracts of mountain side which are constantly throwing masses of de- 

 tritus into the glacial streams. Although the evidence entitles us to 

 suppose that the continental glacier, to whose action we would attrib- 

 ute these detrital hills, w r as immensely thicker than the valley glaciers 

 of Switzerland, yet the region projecting above the level of the ice 

 must have been small, — too insignificant indeed to have furnished 

 any considerable part of the drift materials. 



We are thus driven to suppose that the mass of this sheet of drift, 

 the relics of which alone we see in the hills we are studying, must 

 have been rent from the floor of the glacier as it moved along. The 

 riving power of the movement which scored our hills with the deep 

 grooves must have been sufficient to have torn up large amounts of 

 fragments from its bed. As we have evidence that the glacial sheet 

 was at many points over half a mile in depth, we may without diffi- 

 culty allow it the power of riving this supply of detritus from the 

 rock floors over which it moved; it is difficult, however, to perceive 

 how the supply of fragments could have been lifted into the body of 

 the ice sheet in order to have been carried along with it. We are 

 not prepared to see how it would be possible for a glacier to push 

 along with its advance a stratum of one hundred feet or more of 

 pebbles, mud and sand. We must believe that there was an admix- 

 ture of ice with the drift, so that it could move as ice. It is not easy 

 to see how a pebble could be lifted to a position in the glacier above 

 the point where it was torn from the bed rock. Yet that there was 

 some such lifting action, there is abundant evidence. Nothing is 

 more common than to find fragments of a peculiar bed rock many 

 feet above the base of the drift section. The admirable exposure in 

 the cutting made for a sewer in the College yard in Cambridge, shows 

 this feature very clearly, large masses of the clay slate grooved and 

 scratched by long working on the solid rock, were found at a height 

 of several feet above the bed from which they had been torn. 



