Page:The Journal of geology (IA journalofgeology11893univ).pdf/266
helps to enlarge the subglacial or englacial channels. But, in the case of ice sheets covering all the land, the only heat available for enlarging the tunnels (omitting the small amount of basal heat) is the heat absorbed by surface waters and carried by them beneath the ice. It is known that the waters of surface melting often collect in superficial brooks and torrents of considerable size. The absorption of radiant energy from the sunlight is instantaneous, or at least much more rapid than the conduction of this energy as molecular heat from the water to the ice with which it is in contact. Under sunlight all surface waters become warmed a little above 32°, and, as they plunge beneath the ice, they give up their surplus heat to help melt the walls of the subglacial channels. This, I infer, is the most efficient of all the agencies that help to enlarge the subglacial tunnels. Enlargement of the subglacial tunnels is not uniform. Thus, where a surface stream pours beneath the ice and brings a fresh supply of heat into the tunnel, there would be more rapid enlargement than elsewhere. For various reasons, not necessary to be discussed here, the enlargement of the tunnels proceeds unequally.
Given a tunnel gradually enlarging till sedimentation begins, this sedimentation will commence at the most favorable places, as at the local enlargements, or at an obstruction. If, now, the size of the tunnel, or rather the ratio between the tunnel capacity and amount of water increase, sedimentation will take place at more frequent intervals, and if the tunnel becomes large and rather uniform in size, the sediments will form a continuous ridge.
Various causes can be adduced why a glacial stream should deposit a diminishing quantity of sediment, but the controlling cause and almost the only one admissible under the peculiar local circumstances is the following: We grant that as we go southward toward the distal extremity of the glacier the supply of drainage water will increase, as in all drainage systems. But all these surface waters take with them heat as they pass beneath the ice to help enlarge the tunnels. Thus, as it were, each