Other conditions which influence erosion by ice are (1) the amount of loose or slightly attached débris on the surface, (2) the resistance of the rock, (3) the slope of the surface, (4) the thickness of the ice, (5) the rate of movement, and (6) the abundance and character of the débris which the ice has to work with. The effect of the first five of these conditions is evident. The effect of the last is less simple. Clean ice passing over a smooth surface of solid rock has little effect upon it; but a rock-shod glacier will abrade the same surface notably. The effect of this abrasion is shown in the grooves and scratches (_striæ_) which the stones in the bottom of the ice inflict on the surface of the rock over which they pass (Figs. 253, 255, and 256). At the same time the stones in the ice are themselves worn both by abrasion with the bottom, and with one another (Fig. 254). It does not follow, however, that the more material in the bottom of the ice the greater the erosion it effects; for with increase of débris there may be decrease of motion[131] and, beyond a certain point, the decrease of motion seriously interferes with the efficiency of erosion. When any considerable thickness of ice at the bottom of the glacier is full of débris, this loaded basal portion may approach stagnancy, and the lower limit of considerable movement may lie between the loaded ice below and the relatively clean ice above. A moderate, but not an excessive load of débris is, therefore, favorable for great erosion. Something depends, too, on the character of the load. Coarse, hard, and angular débris is a more effective instrument of erosion than fine, soft, or rounded material. The adverse influence of the overloading of the ice on its motion has been likened to the stiffening of a viscous liquid by the addition of foreign matter, but it may better perhaps be referred to the destruction of the granular-crystalline continuity on which glacier motion probably depends. [Illustration: +Fig.+ 253.—Glacial striæ and bruises. The block to the right shows two sets of striæ: that to the left shows the peculiar curved fractures known as _Chatter Marks_.] [Illustration: +Fig.+ 254.—Bowlders showing glacial striation. (Drawn by Miss Matz.)] [Illustration: +Fig.+ 255.—Striæ on bed rock, Kingston, Des Moines Co., Ia. (Iowa Geol. Surv.)]