ESKERS IN THE VICINITY OF ROCHESTER, NEW YORK. 171 



are striae preserved upon their surfaces, if once present water 

 action has completely removed their traces. They are rounded not 

 like those of ordinary stream beds, but like those of pot holes or 

 beach shingle (99). The boulders are for the most part subangular, 

 and many preserve striae upon their surfaces. However at times 

 boulders 2 to 4 feet in diameter occur that are well rounded. 



The ridges are entirely unfossiliferous, the waters in which 

 they were formed being apparently destitute of life. However the 

 individual component rock fragments may carry fossils indicative of 

 the time which they were formed. 



The material may be compact, even firmly cemented into a true 

 conglomerate, again it may be so loose as to be readily dislodged by 

 a stroke of the hammer. Davis has described the esker material as 

 "open work gravels" (27). In certain places fine material has all 

 been carried out from between the gravels, however, adjoining 

 layers may contain plenty of fine material, such as sand. 



A few ridges have been described that are composed entirely of 

 till (29, 126, 62. 88, 63). The till has been more or less washed 

 apparently, for the fine material is largely removed, and the coarser 

 material locally shows some degree of water action. The famous 

 Bird's Hill esker contains till incorporated within its mass (112). 

 This will probably be found to be true of a large number of eskers 

 when sufficient exposures are available to examine their interiors 

 fully. Till interbedded with the sand and gravel may be considered 

 then a rather common feature in many eskers. 



Near the point of origin of eskers the materials are coarser, and 

 less rounded. Proceeding toward their termination the materials 

 became finer, and well rounded. In the longer ^vlaine eskers Stone 

 states that their north ends are composed of material barely water- 

 worn, the finer material having been entirely removed (99). Toward 

 the crest of the esker the materials are apt to be less rounded than 

 in the lower portion. 



One part of an esker may be composed of sand, another part of 

 the ridge may be gravel. Steeper sided ridges are composed of coarse 

 gravel, possibly blocks, angular and subanguler debris mixed with 

 sand and earthy grit (40). The size and distribution of material 

 are influenced by several factors, the natvire of the underlying rock. 



