346 GEOLOGY. 



drift. Either may be rolling, or either may be plane, though the phases 

 of topography assumed by the two sorts of drift are somewhat unlike. 

 The significance of the topography of the drift at this point lies 

 in the fact that no agent of deposition, except glacial ice, makes deposits 

 of such topography over great areas, in measurable disregard of the 

 topography of the underlying rock. That glaciers develop such topog- 

 raphy is shown by the fact that the drift deposited by glaciers in 

 recent times, has a topography similar to that possessed by the drift. 

 It is to be noted, however, that no very recent glacial deposits, com- 

 parable in. area to the drift, are now accessible. Negatively, it may 

 be added that no other -agent of deposition except land ice is believed 

 to be capable of developing such topography as that possessed by 

 much of the drift. 



(6) Thickness of the drift. — The thickness of the drift ranges from 

 zero to more than 500 feet, and the variations are often great within 

 short distances. One hill may be composed of drift, while the next 

 has no more than an interrupted mantle of drift (Figs. 478 and 479). 

 The drift may be thick on hills and thin in valleys, but more com- 

 monly the reverse is the case. These facts are of significance in this 

 connection in that the thickness is often independent of the topog- 

 raphy of the underlying surface. No agent besides glaciers so habitu- 

 ally leaves its deposits so unequally distributed, and in such disregard 

 of preexisting topography. 



(7) Contact of drift and underlying rock. — The plane of contact 

 between the drift and the underlying rock is generally, though not 

 always, sharply defined, and the surface of the rock is likely to be fresh 

 and firm (Fig. 482). When this relation is contrasted with that be- 

 tween the mantle-rock and the underlying formations where there is no 

 drift (Fig. 489), the conclusion is forced that in the regions of drift 

 the surface was stripped of all loose debris, and ground down to the 

 solid rock below, before the drift was left upon it. This is exactly 

 what glaciers are now doing. 



(8) Striation and planation. 1 — The rock surface beneath the drift, 

 and especially beneath the unstratified drift, is frequently polished, 

 planed, striated (Fig. 482), and grooved (Fig. 483). These features 

 are widespread throughout the drift-covered area, and they occur 



1 7th Ann. Rept. U. S. Oeol. Surv., pp. 155-248. An elaborate discussion of this 

 topic. 



