348 
Culver—The Erosive Action of Ice. 
surface as though it were a rock mass ^instead of a bed of till. 
On this glaciated surface rested ten or twelve feet of newer, 
more bouldery till, which was in turn covered by six or eight 
feet of gravel containing rounded cobbles and boulderets up to 
eight or ten inches in diameter. 
The three strata of drift were perfectly distinct and the sec¬ 
tions were at that time (1888) perfectly fresh. The section 
showing the plane of separation between the old and the new 
till was an eighth of a mile in length and the plane itself was as 
marked as that between a bed of granite and a bed of schist. 
During the summer of 1893 two similar cases were observed 
on the Big Fork river in northern Minnesota. The best case is 
about seven miles above Big Falls on the left bank of the river. 
In an air line the distance is about forty miles from Rainy Lake 
river. The till was compact blue clay resting on gravel. It 
was not very bouldery. Its upper surface has been planed off 
smooth by ice. It is covered by ten feet of sand. 
Fifty miles farther south on the same stream a bed of boul¬ 
ders was seen over which the ice had moved striating the upper 
surfaces in a common direction. 
These three cases are mentioned because they were observed 
so far to the north. It would seem that the ice must have 
moved over the region for many centuries at least. 
J. Ruskin said: 
“Try to saw a piece of marble through (with edge of iron for 
saw, and sharp flint sand for feldspar slime) and move your saw 
at the rate of an inch in three quarters of an hour, and see what 
lively and progressive work you will make of it. 
“The Glacier Du Bois has not done more these 4,000 years 
against some of the granite surfaces beneath it, than the drift of 
desert sands have done on Mt. Sinai. ” 
Mr. Wurtemberger 21 observes that the falls of the Rhine at 
Schaffhausen did not exist in the glacial era. The river (bed) at 
the falls is cut out of the upper Jurassic limestone, and its right 
bank is covered by well characterized drift. 
Along a course cutting off the present bend in the stream, 
there is no trace of the limestone and only deposits of pebbles 
20 Am. Jour., vol. 40, pp. 98-99. 1865. 
21 Jahrb. Min.,1871, p. 582. 
