Geology . rdof preexisting topography. (7) Contact of drift and underlying rock.—The plane of contactbetween the drift and the underlying rock is generally, though notalways, sharply defined, and the surface of the rock is likely to be freshand firm (Fig. 482). When this relation is contrasted with that be-tween the mantle-rock and the underlying formations where there is nodrift (Fig. 489), the conclusion is forced that in the regions of driftthe surface was stripped of all loose debris, and ground down to thesolid rock below, before the drift was left upon it. This is exactlywhat glaciers ar
Geology . rdof preexisting topography. (7) Contact of drift and underlying rock.—The plane of contactbetween the drift and the underlying rock is generally, though notalways, sharply defined, and the surface of the rock is likely to be freshand firm (Fig. 482). When this relation is contrasted with that be-tween the mantle-rock and the underlying formations where there is nodrift (Fig. 489), the conclusion is forced that in the regions of driftthe surface was stripped of all loose debris, and ground down to thesolid rock below, before the drift was left upon it. This is exactlywhat glaciers are now doing. (8) Striation and —The rock surface beneath the drift,and especially beneath the unstratified drift, is frequently polished,planed, striated (Fig. 482), and grooved (Fig. 483). These featuresare widespread throughout the drift-covered area, and they occur 1 7th Ann. Rept. U. S. Geol. Surv., pp. 155-248. An elaborate discussion of thistopic. THE PLEISTOCENE OR GLACIAL PERIOD. 347. Fig. 482.—Strise on bed-rock, Kingston, Des Moines County, la. (la. Geol. Surv.
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