Fig. 277.—End of a glacier a few miles west of Kaslo on Lake Kootenai, B. C. A lodge moraine from which the ice has withdrawn, giving it the appearance of a push moraine. It is possible that the lodge moraine material has been pushed up a little by re-advance of the ice. (Atwood.)
The ground moraine.—When a glacier disappears by melting, all its débris is deposited. All the drift deposited beneath the advancing ice and all deposited from the base of the ice during its dissolution constitutes the ground moraine. The thickness of the ground moraine is notably unequal. In general, it is thicker toward the terminus of the glacier and thinner toward its source, but considerable portions of a glacier’s bed are often left without débris when the ice melts. In general, the terminal moraine is not only thicker, but more irregularly disposed than the ground moraine.
The lateral moraines.—The surface lateral moraines of valley glaciers are let down on the surface beneath when the ice melts out from under them; but the lateral moraines in a valley from which the ice has melted are not merely the lateral moraines which were on the glacier at a given time. They are often far more massive than any which ever existed on the ice itself at any one time ([Fig. 278]). As a glacier retreats, its lateral moraine material is more or less bunched. Thus if the ice advances 200 feet while its end is being melted back 300 feet, the lateral moraines on the 300 feet melted are concentrated into 100 feet, as they are delivered on to the land by the melting of the ice from beneath. If the retreat of the end of a glacier be very slow, the bunching may be great. But even this cannot explain the massiveness of some lateral moraines. Furthermore, the materials of which many lateral moraines are composed are nearly as well worn as those of the ground moraine. The massive lateral moraines of which this is true are often made up chiefly of the drift accumulated beneath the lateral margins of the glaciers. This accumulation is the result of the lateral motion of the ice from center to side. Such sublateral accumulations are akin to terminal moraines. Some of the lateral moraines of ancient valley glaciers, such as those of the Uinta, Wasatch, and Bighorn mountains are several hundred feet high, and in one case about 1000 feet. In northern Italy lateral moraines are said to be 1500 to 2000 feet high.[132]
Fig. 278.—A lateral moraine from which the ice has retreated. Bighorn Mountains, Wyo. (Blackwelder.)
Fig. 279.—Glacial drift, coarse and fine together. (Geol. Surv. of N. J.)
Most of the material which was englacial during the transportation becomes either subglacial or superglacial before deposition, for it ordinarily reaches the bottom or the top of the ice before being deposited. Where the ends or edges of a glacier are vertical or nearly so, as in the high arctic regions, deposition may take place from the englacial position directly.
Distinctive nature of glacial deposits.—The deposits made by glaciers are distinctive. In the first place the ice does not assort its material, and bowlders, cobbles, pebbles, sand, and clay are confusedly commingled ([Fig. 279]). In this respect, the deposits of ice differ notably from those of water. Furthermore, many stones of the drift show the peculiar type of wear which glaciers inflict. They are not rounded as the stones carried by rivers, though they are notably worn. Many of them have subangular forms with planed and beveled faces, the planes being striated and bruised ([Fig. 254]). The absence of stratification, the physical heterogeneity, and the striation of at least a part of the stones are among the most distinctive characteristics of glacial drift. A not less real though less obvious characteristic is the constitution of the fine material, for it is in general not the product of rock decay, but of rock grinding. The fine material handled by streams (except glacial streams) on the other hand, is usually the product of rock decay.
