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THE OSAR GRAVELS OF THE COAST OF MAINE.
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In the coastal region we find numerous marine glacial deltas deposited in front of the ice by glacial rivers that flowed into the sea, but we do not find such frontal or overwash sediments as naturally form in front of glaciers terminating above sea level. These and other facts prove that the ice had not all melted over the coastal region before the advance of the sea. The subsidence of the land (apparent advance of the sea) either preceded the retreat of the ice over the coastal region or accompanied it in such a manner that all the land free from ice was covered by the sea as fast as the ice melted, up to the time when the sea had advanced northward to the highest beach. That is, up to this time, all the subglacial streams poured into the sea at the ice front and not on land above the sea. It follows that the causes of the ending of the osar systems north of the shore not only acted parallel to the present and former surface of the sea, but also in a region where the basal ice was bathed in sea water.

The presence of deep glacial pot-holes in considerable numbers near the coast proves the existence of subglacial streams in that region. Since there are no glacial gravels near these pot-holes, we have proof that there were rapid subglacial streams that left no gravels. Evidently their velocities were such that they transported all their sediments beyond our field (out into the region now under the sea). For years my conclusion has been that the osar rivers of the coastal region of Maine were all subglacial. Assuming the subglacial streams, the problem now resolves itself into this: How happened it that as the subglacial rivers approached the coast, they all found themselves able to sweep their channels free from sediments at nearly the same elevation?

Without here pausing to consider the genesis of the subglacial tunnels, we confine ourselves to the question, how are the tunnels enlarged? Two physical agencies do most of the work. First, mechanical erosion; second, melting of the ice walls by surface waters. In the case of ordinary mountain glaciers there is usually considerable land on the mountains that is bare of ice, and thus water warmed on the land passes beneath the ice and

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