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GLACIAL BOUNDARY IN OHIO. 153 
In various ways earthy material becomes incorporated into the ice 
of an advancing glacier. When the ice moves past an exposed pre- 
cipice, fragments of rock fall down upon the ice, and landslides bring 
down from time to time a promiscuous mass of material. It is not 
improbable that the moving ice also breaks off projecting fragments of 
rock, and encloses them to be carried on by it in its onward motion. 
It is certain, also, that stones are picked up by the moving ice from the 
floor of the glacier, and by some process raised to a higher level. The 
ice being more or less plastic, and the stones unyielding, pebbles seem 
to work up in the moving mass as the larger marbles in a basket rise to 
the surface when the whole is shaken; or, since the upper strata of 
glacial ice move faster than the lower (owing to the effect of friction in 
retarding the movement at the bottom), the result is that the upper side 
of the boulder, which is embedded in the ice, is constantly subjected to 
a greater degree of onward pressure than the lower side. The effect of 
this must be to give an upward as well as an onward motion to the 
boulder in the ice. The course of such a boulder would be up a very 
gently inclined plane, the slower moving strata of ice beneath it forming 
the incline, and the more rapidly moving upper strata being the force 
to push it along. Once upon the surface, if the motion were to con- 
tinue long enough, and the front were not too far away, the boulder 
might be transferred to the front, and deposited before the moving 
mass ; and if the glacier were still advancing, it would stand a chance 
to be covered again with ice, and to be incorporated into the moving 
mass to repeat another cycle. But, whatever be the explanation, a great 
deal of earthy material was in and upon the continental ice-sheet, and 
moved with it southward. ‘The effect would be to dump the material 
along the southern terminal line as the supporting ice was melted under 
it; and thus vast piles would accumulate. In many cases it can be 
demonstrated that boulders have been carried upon the back of the 
glacier hundreds of miles. There are hill tops in Western Pennsylvania. 
and Southern Ohio completely covered with large granite boulders 
whose native place is far beyond Lake Erie, in the northern part of 
Upper Canada. ; 
So far as I know, Pres. Edward Hitchcock was the first to intimate 
that if the glacial theory were true, the backbone of Cape Cod was a 
real terminal moraine (see Geol. Report Mass., Postscript); and I 
understand that Prof. Agassiz was accustomed in his lectures to speak 
AS G, 7" 
