STUDIES FOR STUDENTS. 839 



the result of any post-drift alterations of level. The topographic 

 irregularity of the drift border was original. It follows that the 

 activities of the drift-producing agencies were not confined to a 

 horizontal line along the outermost limit of their reach. 



Topography of the Drift. 



The topographic expression of a drift-covered country can 

 hardly be said to be identical with the topographic expression of 

 the drift, since the former is largely dependent on the topog- 

 raphy of the subjacent rock. Strictly speaking, the topography 

 of the drift is the topography which it possesses independently of 

 its bed. It is the topography which it might assume if deposited 

 on a plane surface. It should not be understood, however, that 

 the topography of the underlying rock exerts no influence on 

 the topography of the drift. Suffice it here to say that the 

 topography of the drift varies within wide limits. Both stratified 

 and unstratified drift may have surfaces which are nearly plane, 

 though such surfaces are rather more characteristic of the former 

 than of the latter. A plane surface of stratified drift is almost 

 sure to have a slight inclination, tolerably constant both in 

 degree and direction for any limited area. This cannot be said 

 of the plane surfaces of unstratified drift, where such exist. By 

 all degrees of gradation plane surfaces of either phase of drift 

 may depart from planeness by taking on shallow depressions ; 

 but a plane topography interrupted only by depressions is much 

 more characteristic of stratified than of unstratified drift. Again, 

 the depressions in the surface of the stratified drift are more likely 

 to be circular in form and more sharply defined than those in the 

 surface of the unstratified, where such exist. In the latter case 

 depressions are almost always accompanied by slight elevations, 

 the counterparts of the depressions, the profiles of which are very 

 gentle. If, along with the depressions, swells affect the surface 

 of the stratified drift, as they sometimes do, they are likely to be 

 more abrupt than the corresponding features of the unstratified 

 drift. They generally have smaller bases for equal heights. 



Either the stratified or the unstratified phase of drift may 



