172 ROCHESTER ACADEMY OF SCIENCE. 



the supply of water in which the esker material was deposited, and 

 chiefly the slope on which the esker lies. Where the slope is steep 

 all of the fine material seems to have been washed out, where gentle 

 only the finer material was transported (102). 



In the broad places, the "plains" of the esker courses, the cen- 

 tral portion is of coarser material and more water worn. The lateral 

 portions are chiefly sand, in fact most of the sand and finest materials 

 are located in the wide flat-topped portions of the esker. Clay in a 

 few instances makes up the lateral portions, but in most cases it is 

 apparent that the water possessed sufficient current to carry out the 

 very fine materials. Coarser materials are also apt to be near the 

 top and the finer near the bottom in these broad areas. 



Reticulated ridges show little change in composition throughout 

 their length, on a whole their materials are coarser and not so well 

 rounded as in the single ridges. The material becomes finer where 

 the ridges grow broader and where they finally become coalescent 

 in a rolling plain (99). 



The gravel and sand, the materials of the esker, do not spread 

 out laterally over the adjacent lowland, but are strictly confined to 

 the ridge itself. 



Source of materials. The materials composing eskers are 

 largely local in origin, that is, they have been derived for the most 

 part from formations immediately beyond the point of origin of the 

 esker in the direction from which the ice came. The material has 

 been transported somewhat farther than the till adjacent to the esker, 

 but still it is largely local in origin. Hershey states that in the case 

 of the eskers he studied 90 per cent, of the materials have come from 

 less than 60 miles from the eskers themselves (48). The larger 

 material, such as boulders, is more apt to be far-traveled than the 

 smaller, yet a large proportion of these has been transported only a 

 short distance from the place of origin. It is safe to say that in 

 many eskers 90 per cent, of the materials have come from within a 

 few miles to the north of the esker in cjuestion, in a majority of 

 eskers 75 per cent, or more of the material is of similar local 

 derivation. 



Stratification. The materials of eskers,, being water-lain, are 

 stratified almost without exception. This stratification is rude. 



