62O 



THE PLEISTOCENE PERIOD 



roundings. (f) Stratified drift extends beyond the unstratified in 

 the direction 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- 

 sions (Figs. 168 and 514). Many of the depressions contain ponds 



xT^ ' ^ -"' 



&f& : .5-^ 



i^^"- '-^-'-*'' * ' '. ip- ' 



W^x^ ^.--s^H',..^^*^-,/- * '. , .-J^- 



^^t^^^*^ : '^^^ 



^&^^ J 4St&rJ^^J?&!^%a 



?ij*?, 

 iik^^^vVii -V' 



J:^ 

 ^ 

 Wfl 



Fig. 510. Foliated till. (Photo, by Jefferson.) . 



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

 is nearly plane. 



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

 feet in thickness, and the variations may be great within short 

 distances. The drift may be thick on hills and thin in valleys, or, 

 more commonly, the reverse. No agent besides glaciers habitually 

 leaves its deposits so unequally distributed, and in such disregard of 

 pre-existing topography. 



7. Contact with underlying rock. The plane of contact be- 

 tween the drift and the rock beneath is generally, though not always, 



