Fig. 265.—A glaciated hill. Southeastern Carey Island. About 30 miles west-northwest of Dalrymple Island.
Most of the débris gathered by ice is acquired at its bottom. While such material is basal at the outset, some of it may find itself above the bottom a little later. Thus when ice passes over a hill ([Fig. 267]) the bottom of the ice rends débris from the top of the hill. When it descends from one level to another there is a similar result ([Fig. 268]). To the lee of the hill the ice from either side may close in under that which came over the top, in which case the débris derived from the top of the hill by the bottom of the overriding ice will be well up in the ice. It has passed from an initial basal to a subsequent englacial position. The change does not usually involve an actual rise of the material, but rather a decline. If carried upward at all, the upward movement is temporary only, and incident to the passage of the ice over the hill, or to other local causes. The englacial débris may be little or much above the basal zone according to the height of the elevation overridden.
Fig. 266.—Alaskan fiords. The shaded areas represent land. (From charts of the C. & G. Surv.)
Fig. 267.—Diagram to illustrate the taking of débris from a hill-top. It also illustrates how englacial débris may become superglacial as the result of surface ablation.
Fig. 268.—Taking débris from a protuberance of the bed.
Superglacial débris may obviously become englacial by falling into crevasses or by being carried down by descending waters. Either superglacial or englacial débris may become basal by the same means.