In arctic glaciers, and probably in others, some material which has been basal becomes englacial by being sheared forward over ice in front of it. So far as observed this takes place chiefly where the ice in front of the plane of shearing lies at a lower level than that behind, as where the surface of an upland falls off into a valley, or where a boss of rock shelters the ice in its lee from the thrust of the overriding ice ([Fig. 268]).
Fig. 272.—Diagram illustrating the upturning of the layers of ice at the end of an arctic glacier as seen in end-section. The bottom line represents sea level.
At the borders of arctic glaciers the lower layers are not infrequently upturned, as shown in Figs. [269] to [272]. Where the layers turn up at the end of a glacier (Figs. [269] and [270]), basal and englacial débris is carried to the surface by actual upward movement, and a terminal moraine or a series of terminal moraines sometimes aggregated where the upturned layers of ice outcrop at the surface ([Fig. 271]). That the material of these moraines was originally basal is abundantly demonstrated by the bruised and scratched condition of the bowlders and pebbles, and sometimes by the nature of the material itself. For example, in two cases in North Greenland where glaciers descend into the heads of shallow bays and move forward on their bottoms, moraines formed by the upturning of the layers were seen to contain abundant molluscan shells derived from the bottom of the bay. The upturning sometimes affects the side-edges of ice-tongues ([Fig. 272]) as well as their ends, and the material thus brought to the surface gives origin to lateral moraines altogether different in origin from the lateral moraines formed by the falling of débris upon the glaciers. Sometimes also there is an upturning of the ice along a longitudinal zone well back from the lateral margins ([Fig. 273]), and the material so borne to the surface in such a zone gives rise to a moraine resembling the medial moraine formed by the union of lateral moraines, but of wholly different origin.
Fig. 273.—Diagram illustrating the same point as 272, where the structure is more complex. The bottom line of the figure represents sea level.
The phenomenon of upturning here referred to has been observed only at or near the terminus of the ice, and is perhaps due in most part to the resistance of frozen morainic or other material beneath and in front of the edge. To this should probably be added the effect of the increased rigidity of the ice at its borders, due to the low external temperature during the larger part of the year, while the interior, with its higher temperature, remains more fluent. But even this probably leaves the explanation inadequate. In not a few instances the upturning is associated with a notable thickening of the layers toward their edges ([Fig. 274]). This suggests that perhaps there is an exceptional growth of the granular crystals of the ice near the edge of the layers, owing to the penetration of the surface-waters which are much more abundant at the borders than elsewhere, and which in the arctic glaciers probably do not penetrate deeply before they reach a freezing temperature.
Wear of drift in transit.—Drift carried at the bottom of the ice is subject to notable wear. The materials in transportation abrade one another and are abraded by the bed over which they pass. Englacial drift is subject to less wear because it is commonly more scattered. Superglacial drift is worn little or none while it lies on the surface of the ice; but in so far as superglacial or englacial drift is derived from the basal load, it may show the same evidences of wear as the basal drift itself. Superglacial drift often reveals its history in this way.
Fig. 274.—Thickening of the upturned layers of ice.