168 ROCHESTER ACADEMY OF SCIENCE. 



in broad valleys (99). They are most liable to development near 

 the terminus of the individual eskers. 



Eskers find their fullest development on a long gentle slope. In 

 crossing divides they are apt to be low and their materials coarse 

 (92). They may be represented in such situations merely by scat- 

 tered pebbles or they may be absent with ridges on both the up and 

 down slopes on either side of the divide. On long gentle up slopes 

 there is a slight tendency toward increase in size, while on short 

 slopes there is no material change in size. On long steep down slopes 

 there may be no deposit, or only a string of large boulders, Avhile at 

 the base of the slope large ridges or, in some places, "plains" are 

 formed. 



They may be conspicuously developed in valleys, and then pass- 

 ing onto plains become so faint as to be difficultly traced. The 

 ridges are apt to broaden in the direction of their termination. In 

 Maine they may broaden southward to plains one-half a mile in 

 width. These very broad eskers behave as do the narrow ridges 

 and may change back again to the narrow ridge type (99). Their 

 sides tend to become pitted with small hollows, branches may diverge 

 from them, and the adjacent lowlands become covered with small 

 kames (99). These broad esker plains may constitute valley filling 

 for a distance, or even a narrow marine delta (99). 



"Buttress-like deposits" may lie against the base of an esker 

 ridge; sometimes a fan-like spreading of debris from a similar 

 position has been observed (88). 



Relation to surroundings. Eskers show no particular regard 

 for topography, they tend however to follow valleys, especilaly if 

 such valleys parallel the direction of ice movement. Rarely they 

 may follow the axis of a valley transverse to the direction of ice 

 movement, and in such cases are likely to lie along the side of the 

 valley toward which the ice moved. They may cross valleys and 

 pursue their direction across neighboring divides. Rarely do the\' 

 cross ridges more than 200 feet high that lie athwart their courses ; 

 in the case of higher ridges the eskers pass through gaps which are 

 not always the lowest or the most direct. One may turn aside ro 

 avoid a hill 100 feet high and in another part of its course cross a hill 

 of greater height. 



