i8o 



PHYSICAL GEOLOGY 



loaded glacial streams or by streams which were overloaded as a 

 result of the erosion of till soon after it was uncovered by the ice. 



Deltas. — When glacial streams entered bodies of water they 

 rapidly built out deltas (Fig. 122). Ancient deltas of this origin 

 often afford the best evidence of the former existence of a glacial 

 lake. In western Massachusetts, for example, a glacial lake of large 

 extent was formed by the damming of the Hoosic River by the ice 

 sheet. Although it did not exist a sufficient length of time to permit 

 the waves to cut back the shores to form cliffs, yet the heavily loaded 

 streams which flowed into it built conspicuous deltas. 



Eskers. — Eskers (Fig. 165) are narrow, usually winding ridges of 

 gravel and sand, ten or more feet wide at the summit and from a few 

 feet to fifty or more feet high, and resembling abandoned railroad 

 grades. They usually follow valleys but sometimes extend across 

 the country with little regard to the topography, even when the hills 

 stand 200 or more feet above the valleys. Single esker ridges in 

 Ireland, Scandinavia, Finland, Maine, and elsewhere have been 

 traced many miles. Usually, however, they are less than a mile in 

 length. 



Eskers are believed to have been formed beneath glaciers by sub- 

 glacial rivers which flowed in tunnels beneath the ice, and are most 



FlG. [65. I he long, narrow, winding ridge is an esker. It is composed of strati- 

 fied sand and gravel. (Photo. F. B. Taylor.) 



