THE PLEISTOCENE OR GLACIAL PERIOD 855 



agent which could have spared it, while covering its surroundings, 

 (f) Stratified drift often extends beyond the unstratified in the direc- 

 tion in which the ice was moving, especially in valleys and on low 

 land. This is the work of running water. 



5. Topography. Among the characteristic features of the 

 topography of the drift are: (a) Depressions without outlets, and 

 (b) associated knobs, hills, and ridges, similar in size to the depres- 



Fig. 567. A section of stratified drift. 



sions (Figs. 227 and 570). Many of the depressions contain ponds 

 or lakes. The surface of some parts of the drift, on the other hand, 

 is nearly plane. Neither planeness nor unevenness can be ascribed 

 exclusively to the stratified nor to the unstratified drift. Either 

 may be rolling, or either may be plane, though the phases of topog- 

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



6. Thickness. The drift ranges from zero to more than 500 

 feet in thickness, and the variations are often great within short 



