852 
Culver—The Erosive Action of Ice. 
which destroy it, and where by common belief it is least ef¬ 
fective in erosion. 
Prof. W. H. Niles 26 at the close of his interesting paper 
on the relative agency of glaciers and subglacial streams in the 
erosion of valleys says: 
. . “the observations of three summers among the glaciers 
of the Alps have led me to estimate the relative agency of 
glaciers and of subglacial streams in the erosion of valleys as 
follows, viz. : the subglacial streams are of primary importance 
in working in advance of the ice, in deepening and enlarging 
these valleys. The glaciers abrade, modify, and in a measure 
reduce the prominent portions left by the streams, and give 
them the well known glaciated surfaces. ” 
Long continued observation in the Alps has given Edward 
Whymper 27 a very similar view as to the erosion accomplished 
by glaciers. He says: 
“Moraines are largely composed of matter which falls upon 
the glacier, or is washed down the sides of the mountain, aDd 
only to a very limited extent of matter that is ground, rasped, 
or filed off by the friction of the ice. Moraines illustrate the 
transporting power rather than the erosive power of ice. ” 
It is evident that by moraine he means terminal or end mor¬ 
aine. Mr. Douglas Freshfield said: 28 
“For the last twenty five years I have had constant oppor¬ 
tunities for observing glaciers at work in the Alps and else¬ 
where. The period has been one of retreat, following one of 
advance. What have some of the thickest — the Brenna, Lower 
Grindelwald, left behind? Not hollows, but hummocks. 
“Burnishing I have seen glaciers often; scraping or pushing 
back soft protuberances in their path sometimes; but scooping 
or excavating, never. ” 
He doubts that the thicker the ice the swifter its base will 
travel, and is of opinion that shearing will occur as a result of 
the increased friction on the bed, so that while as a result of 
the increased thickness the surface may move with accelerated 
velocity, the basal layers may not be affected in the same way 
or to the same degree, yet he agrees that the erosive power is 
increased by added thickness. 
The studies of Dr. Albrecht Heim of Zurich, on the glaciers of 
26 Proc. Bost. Soc. Nat. Hist., vol. 19, pp. 330-336, Mar. 1878. 
27 Scramble Among the Alps, pp. 101-102. 
28 Proc. Royal Geog. Soc., Dec. 1888, pp. 779 et seq. 
