National Geographic : 1892 Mar 21
The Work of the Ice-mill. The erosion, of course, cannot be uniform, but must vary much with the nature of the rock and the thickness and rate of motion of the ice. Near the mouth of the glacier all of these conditions cooperate to increase the action, for the rock is slate, the motion more rapid, and the thickness of the ice probably greater than elsewhere. It does not seem excessive to consider the erosion here five or ten times as great as the average. The sudden fall between G and H probably marks the line between the harder tity of ice carried out per day would be a triangle of 7 feet base (see page 44) and 920 feet altitude. Near the sides the triangle would have a smaller base and a less altitude. Let us suppose a wedge having 7 feet base and 560 feet altitude (half the greatest depth plus the height of the ice above water) and a breadth equal to the whole ice-front, 9,200 feet, to be break ing off daily ; let us suppose this daily loss constant throughout the year (our ignorance of the law of glacial motion below the surface will not per mit a closer approximation-double this quantity would certainly be too much, and would still only slightly affect our result). The Signal service sends me as the average of six years' observation of rainfall at Sitka 105.62 inches, and the average at Juneau 89.30 inches. Our own observations at camp Muir for two months gave about the same rainfall as at Juneau for the same period. I have therefore adopted 90 inches for the yearly pre cipitation (see appendix ii). The rest of the data I have taken from Pro fessor Wright's account (Ice Age in North America, p. 64), viz: 708.48 grains of sediment in each United States gallon of water of the subglacial streams; specific gravity of this material, 2.5; loss by evaporation, one eighth of precipitation. We thus find for the total precipitation over the area drained by the glacier, 146,300,000,000 cubit feet; annual loss in bergs reduced to water, 5,906,000,000 cubic feet; loss by evaporation, 18,300,000,000 cubic feet; leaving, say, 120,000,000,000 cubic feet of water per year carrying off sediment, which gives an average of about 4 inch eroded from the whole bed of the glacier. It is assumed that all the water coming from the glacier is charged with sediment. This is in ac cord with observations so far as they go. The clean surface streams near the end of the glacier empty into the subglacial streams, from one of which the determination of the amount of sediment was made. It may be ob jected that much of this sediment comes from the surface moraines, the rocks either there disintegrating into fine material or falling through cre vasses to the glacier bed and being there ground up. The clearness of the surface streams show that the former is not the case; and the fact that all the moraines on the surface of the glacier would hardly be enough to supply material equal to the sediment carried out in a single year is conclusive evidence against them both. We have not taken into consideration the material pushed out from under the glacier before it has been ground fine, and this is probably of large amount (although we have no means of meas uring it) and would increase the above estimate of the erosion. 8-NAT. GEOO. MAO., VOL,.IV, 1892.
1892 May 15
1892 Feb 19