STUDIES FOR STUDENTS. 843 



It is made obvious in another way by the definite and systematic 

 relations which exist between the elevations and depressions. In 

 non-mountainous driftless regions, the depressions are usually 

 river valleys, and the elevations inter-stream ridges. The com- 

 mon relationship of these features is such as to show that the 

 inter-stream ridges are but remnants of a surface which has been 

 roughened by the excavation of the valleys. In the drift-covered 

 areas, on the other hand, this relationship does not always exist, 

 and even where it can be made out, it is often obscure. 



It is also of consequence to distinguish between the topog- 

 raphy of drift-covered areas and the topography of the drift. The 

 topography of the drift, it will be remembered, is the topography 

 which it possesses independently of its bed. It is the topography 

 which it might have if deposited on a plane surface. The topog- 

 raphy of drift-covered areas is due partly to the topography of the 

 drift per se, and partly to the topography of the rock below the 

 drift. Either one of these elements may be the controlling one. 

 Where the drift is thin and uniformly distributed, the topography 

 of the drift-covered territory corresponds closely with that of the 

 rock beneath. Where the drift is thick and irregularly disposed, the 

 topography of the area it covers may be very unlike that of the sub- 

 jacent rock. In extreme cases, and for limited areas, the drift may 

 be so thick and so irregularly disposed that the surface affected by 

 it preserves none of the topographic features of the underlying 

 rock. In any case the existing topography is the resultant of the 

 two elements. If the topography of the underlying rock has a 

 relief greater than the average thickness of the drift, it will still 

 determine the larger features of the resultant topography, though 

 perhaps not its details. If the topography of the underlying 

 rock possesses a relief which is slight in comparison with the 

 average thickness of the drift, the latter may determine both the 

 major and minor features of the resultant topography. 



The drift and drift-free surfaces present many differences. 

 Where there is no drift the valleys are likely to be more nearly 

 straight. Here, too, the tributary valleys join their mains in a 

 more regular way, and at a more nearly uniform angle, forming 



