National Geographic : 1892 Mar 21
46 H. F. Reid-Studies of Muir Glacier. As one approaches the end of the glacier the surface of the ice becomes steeper and steeper, and frequently becomes too pre cipitous to allow one to stand on it. The diagram (figure 1) shows the form of the surface cut by a longitudinal section. Now, why does the glacier assume this shape ? We know that the surface of a drop of water or of a small quantity of honey on a plate will assume some such shape; but this is the result of molecular forces which can not have any appreciable effect on large bodies like a glacier. The end of a flowing lava stream will have a somewhat similar form, but this is a case of contin ued flow and not one of equilibrium. These analogies throw no light on the question. If we divide the glacier into layers by a series of surfaces parallel to the direction of flow, the con dition that the end shall be stationary requires that the ice sup plied by each layer shall be melted at its end. Now, the upper layers move more rapidly than the lower ones; therefore their ends must melt more rapidly. A glance at the diagram will show that, on account of the form of the end of the glacier, the ends of the upper layers expose a larger surface than the lower to the air and sun, resulting in their more rapid melting. This, although undoubtedly a part of the explanation, is not the whole of it, for the form of the glacier's end would be one of unstable equilibrium. If anything should cause the surface to become somewhat steeper, the exposed ends of the upper layers would become smaller, and these layers would no longer melt away rapidly as they advance; the surface would continue to grow steeper until the upper part would break off and thus restore the slope. Although glaciers have been observed to advance I have never heard of it occurring in this manner. A series of meas urements of the rate and direction of motion, and the rate of melting at the end of some glacier, such as the Gorner or Morte ratsch, in Switzerland, would undoubtedly throw light on this problem. At the end of the valley of Norris glacier, Taku inlet, there is a broad expanse of gravel, etc, on which the glacier, after issuing from its gorge, spreads itself like a great fan, thus presenting a large surface to the air and sun; so that the melting of the ice is as rapid as the supply.* If it were prevented from spreading it would extend much further than it does, and would undoubt * This level expanse must be either the accumulation of glacial debris or a delta formed when the glacier was less extensive than now.
1892 May 15
1892 Feb 19