1 82 PHYSICAL GEOLOGY 



the glacier. As the ice melted these would be inverted to form 

 mounds. Sand and gravel carried into moulins (p. 149) whose sub- 

 glacial passage had been closed would produce such hills. When 

 stagnant ice occupies deep valleys, drainage along the sides may give 

 rise to large deposits of sand and gravel, which may be left in some- 

 what the form of a terrace with a kame topography when the ice has 

 disappeared. Outwash plains and valley trains sometimes begin in 

 kame areas. 



Relation between Stratified and Unstratified Drift. — It should 

 not be understood that stratified and unstratified drift always have 

 topographic forms which distinguish them, or that they can always 

 be clearly separated. The mingling of the unstratified and stratified 



Fig. 167. — Diagram showing the relation between stratified and unstratified drift. 

 In this figure the rough moraine leads to a sandy outwash plain on the left. On the 

 right is stratified drift which was laid down in a temporary pond between the terminal 

 moraine and the retreating ice front. 



deposits (Fig. 167) is readily comprehended when it is remembered 

 that the edge of the ice sheet probably seldom remained in the same 

 position long, but oscillated back and forth during its advance and 

 retreat. In this way till has been covered with sand and gravel 

 which in turn has been overridden by the ice and covered with till. 

 Moreover, when temporary lakes existed between the ice front and 

 its moraine, stratified deposits were laid down in the midst of the 

 unstratified. 



Erosion by Continental Glaciers 



The amount of erosion formerly ascribed to continental glaciers 

 was probably excessive. There is no doubt that the ice sheets 

 modified the topography over which they passed, — in some cases 

 profoundly, — but in general the more pronounced features of the 

 landscape were little changed by erosion, although large areas were 

 altered to a greater or less degree by the irregular deposition of drift. 



Effect on the Underlying Rock. — Previous to the appearance of 

 the continental ire sheets the surface of the rock of North America 

 was deeply weathered (p. 651), much as it is now in the southern 

 states. Consequently, when the ice covered and moved over this 

 " rotten " rock (p. 27) and soil, it found an abundance of material 



