FIG. 305.—_a_, Map showing pit excavated by the current below the opening in a dam. _b_, Nunataks and surface moraines on the Greenland ice. Dalager’s Nunataks (after Suess).]
Any obstruction upon the pavement of the glacier apparently exerts a larger or smaller tendency to elevate the bowlders and pebbles and incorporate them within the ice. Rock débris thus incorporated is described as _englacial_ drift. In the case of Greenland glaciers this material seems at the ice front to be largely restricted to the lower 100 feet (plate 13 A).
Near the front of the inland ice the increased slope of the upper surface greatly increases the flow of the upper ice layers in comparison with those nearer the bottom, so that the upper layers override the lower as they would an obstruction. The englacial drift is either for this reason or because of rock obstructions brought to the surface, where it yields parallel ridges corresponding in direction to the glacier margin. Such transverse surface moraines are thus in many respects analogous to those which appear about the lower margins of scape colks. In contrast to the longitudinal or medial surface moraines the materials of the transverse moraines are more faceted and rounded—they have been abraded upon the glacier pavement.
=Melting upon the glacier margins in Greenland.=—During the short but warm summer season, the margins of the Greenland ice are subject to considerable losses through surface melting. When the uppermost ice layer has attained a temperature of 32° Fahrenheit, melting begins and moves rapidly inward from the glacier margin. In late spring the surface of the outer marginal zone is saturated with water, and this zone of slush advances inward with the season, but apparently never transgresses the inner border of what we have generally referred to as the marginal zone of the ice characterized by relatively steep slopes, crevasses, and nunataks. Upon the ice within this zone are found streams large enough to be designated as rivers and these are connected with pools, lakes, and morasses. The dirt and rock fragments imbedded in the ice are melted out in the lowering of the surface, so that late in the season the ice presents a most dirty aspect. At the front of the great mountain glaciers of Alaska, a more vigorous operation of the same process has yielded a surface soil in which grow such rank forests as entirely to mask the presence of the ice beneath.