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THE OSAR GRAVELS OF THE COAST OF MAINE.
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far from thirty miles wide. Without assuming any definite rate or rates of ice movement we can at least all agree that it would take many years for the ice to advance such a distance. An obstruction to the natural transfer of heat beneath the ice, and consequent enlargement of the tunnels, though it might be slow in its action, would, after a term of years, have a cumulative effect on the development of the tunnels, at least in cases where the subglacial rivers flowed in channels parallel with the ice flow.

We have seen that the three features of the coastal gravels above stated are associated together over a wide area, and would appear to have a common origin. Glacial rivers of different lengths, from five up to more than one hundred miles, all show the same development. At almost the same elevation they all were able to sweep their tunnels clear of sediments. We must seek for some cause capable of acting along two hundred miles of coast in lines approximately parallel to the surface of the sea. What but the sea itself could do this under so many varying topographical and glacial conditions?

Rightly interpreted, it would appear that the termination of the gravel systems north of the shore line is itself a proof of the former elevation of the sea. We may leave it as an open question how far the sea acted in other ways—such as by diminishing the effective "head" of the subglacial streams, etc., but that the sea was chiefly responsible for the peculiar development of the coastal gravels, I am persuaded, is the best interpretation of the facts. And of all the ways in which the sea or other body of water that submerges the base of a glacier affects the subglacial streams and they tunnels, I have been able to discover none so potent as that which is above described, whereby the enlargement of the tunnels is obstructed.

Where subglacial rivers flowed up and over transverse hills, as they often did in Maine, there would be a body of slack water in the tunnels, like a sewer trap, on the north side of the hills. Some of these bodies of slack water or dams on the north sides of hills were from five to ten or fifteen miles long, and in one

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