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THE OSAR GRAVELS OF THE COAST OF MAINE.
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region of the glacier furnishes the heat to enlarge the tunnels within its own limits. This is the natural career of ordinary ice sheets above the sea.

An important law of the enlargement of subglacial tunnels depends on the velocity of ice movement. Subterranean waters, as those of the limestone caves, go on enlarging their channels from age to age, because they act continuously on the same body of rock. But the subglacial tunnel cannot become thus enlarged, because of the constant renewal of the ice. Other things being equal, the enlargement of the subglacial tunnels is directly proportional to the time during which it is being enlarged, and inversely as the rate of ice flow. Obviously, when the flow is rapid the tunnel never becomes very much enlarged, for before this can happen the ice at any given part of the channel is pushed on to the distal extremity and disappears by melting or by berg discharge.

The details, here omitted, prove there was probably a small acceleration of the rate of ice flow as the coast of Maine was approached, hence the rate of enlargement of the ice channels would not increase so rapidly as the supply of water of local melting. But the surface of that region is much diversified with hills and valleys. The rate of ice flow would be most rapid in the deeper north and south valleys, and would be retarded in the lee of the higher transverse hills, of which there are several long systems. If differences in the rate of ice flow were the only cause of different rates of ice channel enlargement, then we ought, on such an uneven coast, to find evidence of the fact in the distribution of the gravels. Examination shows that this was a real cause of varying rates of enlargement, but it was a minor cause. This cause alone could not have enabled all the subglacial rivers to clear their tunnels of sediments at the same or nearly the same horizontal line. It would have acted at various levels, according to the conditions for rapid ice supply from the north.

We have seen that the ice in late glacial time flowed into the sea in the coastal region of Maine. It remains for us to inquire

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