214 PHYSICAL GEOLOGY 



The drift is usually disposed irregularly, so that mounds 

 and hills without systematic arrangement are associated 

 with depressions of varying form and size, many of which 



have no outlets. The 

 streams of recently (geo- 

 logically speaking) gla- 



FIG. 225. Section of a lake lying in a ciated regions COm- 

 hollow of drift. 



and roundabout courses, and in many cases are interrupted by 

 lakes and marshes (Plate XI, Fig. A). All this is in contrast 

 with topographies due to river erosion. Since such topog- 

 raphies have resulted from the cutting of valleys, the ele- 

 vations are distributed systematically with reference to the 

 depressions, all of which have outlets (Plate XI, Fig. B). 

 As we have already seen (p. 207), it is in contrast, too, with 

 topographies due to stream deposition. The lake basins 

 and other surface hollows of drift areas have been formed 

 in several ways. Some are sections of preglacial river 

 valleys in which drift was deposited unevenly. Where the 

 ice deposited more material around than on a given area, 

 the latter came to stand lower than its surroundings (Fig. 

 225). Still other basins were gouged out of the underlying 

 rocks by the ice. The thousands of lakes in the northern 

 part of the United States are practically all of glacial 

 origin. 



The features described above as distinctive of drift sur- 

 faces are most pronounced in terminal moraines, which are 

 often characterized by notably hummocky topography (Figs. 

 226 and 227, and Plate XII). Numerous mounds, hillocks, 

 and short ridges, ranging in diameter from a few feet to a 

 half mile and more, and reaching occasionally a height of 

 100 to 200 feet, are associated with depressions varying in 

 depth from inches to scores of feet, and in area ranging up 

 to many acres. Many of the depressions contain ponds 

 or lakes. Elevations and depressions are huddled together 

 in confusion. Ground moraine surfaces are usually less 



