_Deposition by the ice. Effect on topography._--On melting, glacier ice leaves its bed covered with the debris which it gathered during its movement. Had this debris been equally distributed on and in and beneath the ice during its movement, and had the conditions of deposition been everywhere the same, the drift would constitute a mantle of uniform thickness over the underlying rock. Such a mantle of drift would not greatly alter the topography; it would simply raise the surface by an amount equal to the thickness of the drift, leaving elevations and depressions of the same magnitude as before, and sustaining the same relations to one another. But the drift carried by the ice, in whatever position, was not equally distributed during transportation, and the conditions under which it was deposited were not uniform, so that it produced more or less notable changes in the topography of the surface on which it was deposited.
The unequal distribution of the drift is readily understood. The larger part of the drift transported by the ice was carried in its basal portion; but since the surface over which the ice passed was variable, it yielded a variable amount of debris to the ice. Where it was hilly, the friction between it and the ice was greater than where it was plain, and the ice carried away more load. From areas where the surface was overspread by a great depth of loose material favorably disposed for removal, more debris was taken than from areas where material in a condition to be readily transported was meager. Because of the topographic diversity and lithological heterogeneity of the surface of the country over which it passed, some portions of the ice carried much more drift than others, and when the ice finally melted, greater depths of drift were left in some places than in others. Not all of the material transported by the ice was carried forward until the ice melted. Some of it was probably carried but a short distance from its original position before it lodged. Drift was thus accumulating at some points beneath the ice during its onward motion. At such points the surface was being built up; at other points, abrasion was taking place, and the surface was being cut down. The drift mantle of any region does not, therefore, represent simply the material which was on and in and beneath the ice of that place at the time of its melting, but it represents, in addition, all that lodged beneath the ice during its movement.