Deposition of the Drift.
1. Beneath the body of the ice.—During the advance of a glacier, deposition may take place both beneath the body of the ice and beneath its end and edges. Deposition beneath the body of the ice is liable to take place wherever the topography favors lodgment, or wherever the ice is overloaded. The topography favoring deposition is much the same as that favoring erosion, but the two processes are not favored at the same point. Erosion is greatest on the “stoss” side of an obstruction (the side against which the ice advances), and deposition on the lee side. The ice is likely to be overloaded (1) just beyond a place where conditions have favored the gathering of a heavy load, and (2) where the ice is rapidly thinning. On the whole, however, the deposition of material beneath the main body of a glacier is much more than balanced by erosion in the same position.
Fig. 275.—Glacier building an embankment. Southeast side of McCormick Bay, North Greenland.
2. At ends and edges of glaciers.—At and near the end of a glacier the conditions of deposition are somewhat different. Here deposition beneath the ice goes on faster than elsewhere, chiefly because of the more rapid melting and the more rapid thinning and weakening of the ice. If the end of the glacier be stationary in position, drift is being continually brought to it and left there, for though the end is stationary, the ice continues to move. If the glacier moves forward 500 feet per year, and if its end is melted at the same rate, all the débris in the 500 feet of ice which has been melted has been deposited, and all except that which has been washed away has been deposited at and beneath the end of the glacier. If the end of the glacier is retreating, the retreat means that the waste at the end exceeds the forward movement. If the ice advances 300 feet per year, and is melted back 500 feet in the same time, all the débris carried by the 500 feet which has been melted has been deposited, and largely in the narrow zone (200 feet) from which the ice has receded. Even in this case, therefore, there is a notable tendency to marginal accumulation. If the end of the glacier is advancing 500 feet per year while it is being melted but 300 feet, all the drift in the 300 feet melted has been deposited, and chiefly at or beneath the immediate margin of the ice. To the marginal and sub-marginal accumulations made in this way, the material carried on the ice is added whenever the ice is melted from beneath it. This addition is sometimes considerable and sometimes meagre. If the edge of the ice is without much fluctuation in position, the material dumped over its end may take the form of a narrow ridge or bowlder-wall (Geschiebe-wall). If a glacier pushes material in front of it, this, too, becomes a part of the general terminal aggregation of drift.
Fig. 276.—Embankment completed. Near the last.
TYPES OF MORAINES.
The terminal moraine.—The thick accumulation of drift made at the end of a glacier or at the edge of an ice sheet, especially where its end or edge is stationary, or nearly stationary, for a considerable time, is the terminal moraine. That part of the aggregation deposited beneath the ice is sometimes called the lodge moraine (Figs. [275] and [276]; see also [Fig. 235]); that carried on the ice and dropped at its edge, the dump moraine; and that pushed before the ice, the push moraine. Many moraines marginal to the ice appear to be push moraines, when they are really lodge moraines from which the ice has withdrawn ([Fig. 277]). The push moraine can rarely be distinguished, and the dump moraine by no means always, after the disappearance of the ice.
