THE PLEISTOCENE OR GLACIAL PERIOD 



375 



slopes is about the angle at which the drift will lie. Where they cross 

 marshes and swamps, as is sometimes the case, they are most con- 

 spicuous, sometimes resembling railway grades. Eskers no more 

 than a fraction of a mile in length are more common than longer ones, 

 but eskers scores of miles long are known. Long eskers sometimes 

 wind up and clown over low elevations and valleys, showing that the 

 water which made them must have been under great head, if they 



Fig. 505. — An esker 10 miles west of Aurora, 111. (Bastin.) 



are of strictly subglacial origin. They often lie along the lower slope 

 of a valley, though distinctly above its bottom. Eskers are likely 

 to be interrupted at intervals, probably at points where the deposit- 

 ing waters failed of confinement to definite channels, or their channels 

 were too constricted, or had too high gradient to permit of deposition. 

 The best-developed eskers in the United States are in Maine. 1 



Eskers are made up primarily of stratified gravel and sand. As 

 in kames, the stratification is often much distorted, probably as the 

 result of ice pressure. Bowlders are often present in them and on 



1 Stone, Mono. XXXIV, U. S. Geol. Surv. 



