Minor obstructions coming in contact with one side of the fragment only, may turn it in its own plane without overturning. Evidence of such interruptions can be later read in the different directions of striæ upon the same facet (plate 17 A). [Illustration: FIG. 304.—A glacier pavement of Permo-Carboniferous age in South Africa. The striæ running in the direction of the observer are prominent and a noteworthy gouging of the surface is to be noted to the right in the middle distance (after Davis).] The floor beneath the glacier is reduced by the abrading process to a more or less smooth and generally flattened or rounded surface—the so-called _glacier pavement_ (Fig. 304). To accomplish this all former mantle rock due to weathering processes must first be cleared away, and the firm unaltered rock beneath is wherever susceptible of it given a smooth polish although locally scored and scratched by the grinding bowlders. The earlier projections of the surface of the floor, if not entirely planed away, are at least transformed into rounded shoulders of rock, which from their resemblance to closely crowded backs in a flock of sheep have been called “sheep backs” or “_roches moutonnées_.” Thus the effect of the combined action of the processes of plucking and abrasion is to reduce the accent of the relief and to mold the contours of the rock in smoothly flowing curves, generally of large radius. =The lifting of the grinding tools and their incorporation within the ice.=—Wherever the ice is locally held in check by the projecting nunataks, relief is found between such obstructions, and there the flow of the ice has a correspondingly increased velocity (Fig. 305 _b_). If the obstructions are not of large dimensions, the ice which flows around the outer edges is soon joined to that which passes between the obstructions and so normal conditions of flow are restored below the nunataks. The locally rapid flow of the ice is, therefore, restricted to a relatively short distance, the passageway between the nunataks, and the conditions are thus to be likened to the fall of water at a raceway due to the sudden descent of its surface from the level of the reservoir to the level of the stream in the outlet. As is well known, there is under these conditions a prodigious scour upon the bottom which tends to dig a pit just above and below the dam—a _scape colk_—and carry the materials up to the surface below the pit. Such a tendency was well illustrated by the behavior of the water at the opening of the Neu Haufen dam below the city of Vienna (Fig. 305 _a_). In the case of ice, material from the bottom may by the upward current be brought up to the surface of the glacier at the lower edge of the colk and thus produce a type of local surface moraine of horseshoe form with its direction generally transverse to the direction of ice movement (Fig. 305 _b_). [Illustration: