Page:The Journal of geology (IA journalofgeology11893univ).pdf/268
what is the effect of the flowing of a glacier down into a body of water upon the enlargement of the subglacial tunnels. In such a case the tunnels and all crevasses opening into them are permanently filled with water up to the level of the surface of this body of water. But it is by the crevasses that the waters of local melting get down into the subglacial tunnel. The permanent water in the crevasses is at the temperature of 32°. As the waters of surface melting in the region whose basal ice is submerged in the sea or other body of water pour into the crevasses they cannot at once fall to the ground and enter the tunnels, but they fall into the water in the crevasses that already fills them to the level of the permanent body of water. The large streams find their way pretty directly into the tunnels, but all the smaller streams and trickles become so mixed with the cold waters in the crevasses that their heat, instead of being consumed in enlarging the tunnels, is largely expended in melting the ice walls of the crevasses above the level of the tops of the roofs of the tunnels.
Thus the flowing of a glacier down into the sea interferes with the natural transfer of heat beneath the ice whereby the tunnels are enlarged in large part. But the supply of surface waters is the same over the area whose base is submerged as elsewhere. The conclusion follows, that as we go toward the distal extremity of a glacier that flows into a body of water, the supply of drainage waters would be increased more rapidly than the tunnel capacity. This would result in increased velocity of the rivers, with a corresponding increase in power of transportation. In other words, they would do just as the osar rivers of Maine did as they approached the coast—would deposit sediments at longer and longer intervals, and in smaller quantities, and finally would sweep their tunnels free from all sediments.
Now it is certain and inevitable that the submergence of the basal ice should restrict the enlargement of the subglacial tunnels, yet it is an open question whether this was sufficient to account for the peculiar development of the coastal gravels.
We have seen that these changes take place within a belt not