ORIGIN OF TOl'OGEAPHIC FOEMS. 123 



Within the limits of this terminal moraine, the commonest character- 

 istic feature of glacial deposition is the drumlin, an oval mound of drift, of 

 height ranging from a few feet up to several hundred feet, and from one to 

 several square miles in area. They are extremely regular in shape and 

 their curves are round and smooth. In many localities they are so abun- 

 dant as practically to cover the surface, the intervals between them being 

 level and often marshy. The axes of these drumlins are commonly par- 

 allel, giving' a curiously artificial appearance to the map. In country other- 

 wise level, they determine the course of the streams, forcing them to wind 

 around their curves. PI. xvi shows a portion of the drumlin area of 

 southern Wisconsin, and PL xvn a part of the terminal moraine of the 

 same region. Pitted plains, which are level areas dotted with little pits, 

 are common features of glacial action. Osars, or long winding ridges, are 

 not uncommon, while numerous other forms, such as kettles, etc, are fre- 

 quently seen, but are of less importance as topographic features. 



Glaciers still exist in the Rocky mountains, the Sierra Nevada, and the 

 Cascade range, though they are by no means as extensive as in former 

 times. At the bases of many of the ranges of this region are found lateral 

 moraines reaching out from the mouths of mountain gorges and connected 

 at their ends by terminal moraines. 



The lateral moraines are of regular form, stretching in narrow ridges, 

 in some cases parallel, in others curving away from one another from the loot 

 of the canyon. The terminal moraines are like that of the continental glacier, 

 confused masses of material heaped up in disorder and consequently difficult 

 to sketch in the highest degree. 



GLACIAL EROSION. 



Glacial erosion is very similar in its laws and action to aqueous erosion, 

 or rather to that part of it which is called corrasion. The principal differ- 

 ence between them lies in the fact that ice is much less plastic and conse- 

 quently does not accommodate itself so readily to the form of its channel. 

 It moves, too, much more slowly and in far greater volume than water. 



The corrading effect of the continental glacier is shown in northern 

 New England, New York, Michigan, Wisconsin, and Minnesota very mark- 



