SURFACE GEOLOGY. 203 
clays are often accurately stratified, were apparently depos- 
ited in deep and generally quiet water, and mark a period 
when the glacial ice-masses, melted by a change of climate, 
retreated northward, leaving large bodies of cold fresh- 
water* about their southern margins, in which the mud 
produeed by their grinding action on the paleozoie rocks of 
the Lake District was first suspended and then deposited. 
On the shores of Lake Erie these clays contain no boul- 
ders and very few pebbles, while farther North and West 
boulders are more abundant. This is precisely what might 
be expected from the known action of glacial masses on the 
surfaces over which they pass. Their legitimate work is to 
grind to powder the rock on which they rest; an effect 
largely due to the sand which gathers under them, acting as 
emery on a lead wheel. The water flowing from beneath 
glaciers is always milky and turbid from this cause. Rocks 
and boulders are sometimes frozen into glaciers, and thus 
transported by them, but nearly all the boulders carried along 
by a glacier'are such as have fallen from above ; and a mo- 
raine ean hardly be formed by a glacier except when there 
are cliffs and pinnacles along its course. 
In a nearly level country, composed of sedimentary rocks 
passed over by a glacier, we should have very little débris 
produced by it, except the mud flour which it grinds. 
The Erie clays would necessarily receive any gravel or 
stones which had been frozen into the ice, either as RERE 
pebbles or stones, distributed to some distance from the gla- 
cial mass by floating fragments of ice, or as masses of frozen 
gravel, or larger and more numerous boulders near the gla- 
cier. In some localities torrents would pour from the sides 
and from beneath the glacier, so that here coarse material 
would alone resist the rapid motion of the water, and the 
stratification of the sediments would be more or less confused. 
* Cold, b ing from the melting glacier, and depositing v with its sediments 
iq f life; fresh, 1 eode wood— 
ver 11. *h 2. T sh - Laat shells. 
q Champlain” y 
