Fig. 255.—Striæ on bed rock, Kingston, Des Moines Co., Ia. (Iowa Geol. Surv.)
From the preceding statement, it is evident that erosion is not equally effective at all points beneath a glacier. So far as concerns the ice itself, erosion is not most effective at the end of a valley glacier, or at the edge of an ice sheet, for here the strength of movement is too slight and the load too great; nor is the most effective erosion at the source or near it, for though the ice may here be thick, the movement is slow and the load likely to be slight. Ice conditions only being considered, erosion is most effective somewhere between the source and the terminus, and probably much nearer the latter than the former. The conditions of the surface over which the ice passes may be such as to vary the place of greatest erosion widely. Thus in an Alpine glacier, erosion may be most effective at the Bergschrund because the slope here favors “plucking.” Here, notable amphitheatres (cirques) are sometimes excavated. After the glacier disappears, the bottom of the cirque is often seen to contain rock basins ([Fig. 257]). Glacial cirques abound in mountains where glaciers once existed, but from which they have now disappeared. The cirques of the Bighorn mountains of Wyoming ([Pl. XIX]) are examples.
Fig. 256.—Striæ, grooves, etc., in a canyon tributary to Big Cottonwood Canyon, Wasatch Mountains. (Church.)
Summary.—In summary it may be said that rapidly moving ice of sufficient thickness to be working under goodly pressure, shod with a sufficient but not excessive quantity of hard-rock material, passing over incoherent or soft formations possessing a topography of sufficient relief to offer some resistance, and yet too little to retard seriously the progress of the ice, will erode most effectively.
Varied nature of glacial débris.—From its mode of erosion it will readily be seen that the bottom of a glacier may be charged with various sorts of material. There may be (1) bowlders which the ice has picked up from the surface, or which it has broken off from projecting points of rock over which it has passed; (2) smaller pieces of rock of the size of cobbles, pebbles, etc., either picked up by the ice from its bed or broken off from larger masses; (3) the fine products (rock-flour) produced by the grinding of the débris in the ice on the rock-bed over which it passes, and similar products resulting from the rubbing of stones in the ice against one another; and (4) sand, clay, soil, vegetation, etc., derived from the surface overridden. Thus the materials which the ice carries (drift) are of all grades of coarseness and fineness, from large bowlders to fine clay. The coarser material may be angular or round at the outset, and its form may be changed and its surface striated as it is moved forward. Whether one sort of material or another predominates, depends primarily on the nature of the surface overridden.
PLATE XIX.
U. S. Geol. Surv.