STUDIES FOR STUDENTS. 84 1 



thousands of these depressions are marked by the swamps, 

 ponds, and lakes of the northern part of the United States. 

 Indeed, these features are so nearly co-extensive with the drifts 

 that the line marking the limit of the latter may almost be said! 

 to be the line marking the limit of the former. The principal 

 exceptions to this statement are the marshes and shallow ponds 

 along coasts and sluggish streams outside the drift. 



If it be remembered that the depressions in the surface of the 

 drift have a wide range in the matter of area, shape, depth, and 



Fig. 2. One type of drift topography. Drawn from photograph of drift sur- 

 face, near Hackettstown, N. J. 



abruptness of slope, a rough sort of idea of drift topography may 

 be acquired by imagining at least an equal number of hills, of 

 equally diverse sizes, shapes, and slopes, interspersed between the 

 depressions. It is not to be understood that the depressions and 

 hills are everywhere existent in the drift-covered area, or that they 

 are everywhere striking, where they exist. It is to be remem- 

 bered that there are areas of drift of great extent, the topography 

 of which is essentially plane, and that there are other areas of equal 

 extent where the topography is but gently rolling, and where the 

 elevations and depressions are therefore not obtrusive. It is to 

 be remembered further that in many parts of the drift- covered 

 territory, the controlling element in the surface topography is. 



