374 



GEOLOGY. 



not confined to definite channels, their deposits probably took on the 

 form of irregular patches of silt, sand, or gravel; bul where definite 

 streams were confined to definite channels, their deposits were cor- 

 respondingly restricted. When the channels remained constant in 

 position for a long time, the aggradation may have been considerable. 

 In so far as the channel deposits wen; made near the edge of the ice 

 during the time of its maximum extension or retreat, they were likely 

 to remain undisturbed during its melting, after which they stood out 



Fig. 504. — An esker in Scandinavia, locality unknown. 



as ridges. These ridges of gravel and sand are known as .osars or eskers 

 (Figs. 504 and 505). It is not to be inferred that eskers never orig- 

 inated in other ways, but it seems clear that this is one method, and 

 perhaps the principal one, by which they came into existence. 



Eskers early attracted attention, partly because they are rela- 

 tively rare, and partly because they are often rather striking topo- 

 graphic features. They are often conspicuous, not so much because 

 of their height, as because of their abrupt slopes and their even and 

 narrow crests. They may be ten or several times ten feet high, but 

 their crests are generally no more than a few feet wide. They are, 

 for example, often so narrow, and their slopes so steep, that two wagons 

 could with difficulty pass each other on their tops. The angle of their 



