240 The American Geologist. October. i90i'. 
reduced in size, retains its ang-ular shape. Daubree has 
shown "^^ that the sand v/hich appears at the end of an 
Alpine glacier is in the form of sharp broken grains, rather 
than that of water worn ])articles. Frequently the erosive 
rock material at the bottom of a glacier reaches its position 
by falling from the surface into the crevasses, as is evidenced 
notably by the Muir glacier. In the central part of this glacier 
most of the rock material falls into the crevasses. After a 
crevasse is formed, the sun melts back the north wall rapidly 
converting a broad ridge of ice into a sharp one, and thus 
causes the moraine load to be dropped in. Again the erosive 
material may fall into the narrow space that sometimes exists 
between the glacier and the sides of the valley. Geikie gives the 
Mer De Glace, Chamounix, as an example, t where he say^, 
"Blocks of granite are jammed between the mural edge of 
the ice and the precipice of rock along wdiich it moves, and 
wdiich is scored and polished in the direction of motion of 
the blocks." Such material acting with the slow, continuous 
movement of the ice becomes an enormously powerful erosive 
agent, grinding down the hardest rocks until they are left in a 
polished, smooth or striated condition. Such striated rocks 
are often scored by deep grooves, as well as by fine lines 
made by quartz pebbles, and are frequently of an undulating 
or hummocky nature, in which case the well-known term 
roches moutonnees is given to them. 
Erosive action is often valuable in determining the hi'Story 
of a given locality ; e. g., the sides of the mountains bordering 
the Aluir inlet are polished and striated with \er\' fresh marks 
up to a height of 2000 feet, and above this altitude striae 
are occasionally seen up to 3500 feet. This indicates that the 
whole region must have been covered at an earlier date with 
a mass of ice wherein only the highest mountains projected 
through. Further, on the tops of all the low mountains 
bordering the ]\Iuir glacier over which ice has swept, di- 
minutive lakes occur. All of them are small only a few yards 
across and not deep. "The conclusion cannot be avoided," says 
Gushing, ''that these hollows are the work of ice." 
It is to be remembered, however, that all debris charged 
glaciers do not necessarily produce important erosive results. 
♦"Geologic PJxper." p. 254. 
tGeikie, Text-book 1893, p. 428. 
