THE PINNACLE HILLS ESKER. 75 



finer gravel and sand, brought by the same streams, slowly descends at 

 the rate often or twenty feet per mile southward to the shore, and con- 

 tinues onward beneath the sea level. Many broad stream courses, now 

 dry or occupied by insignificant brooks, extend across this plain from 

 the morainic hills to the sea ; and in some cases they are traceable below 

 the present sea level, passing across the bottoms of the enclosed bays to 

 the beach-ridge which divides them from the open ocean. It is thus 

 known that when the ice-front stood at its moraine and the streams of 

 its melting formed the kames and ran across this plain, the land here 

 stood somewhat higher above the sea than now. 



Which of these views is the true explanation of the origin of the kames, 

 whether they were formed by streams flowing down from the melting 

 surface of the ice-sheet, or flowing upward from its bottom, I am will- 

 ing to leave to the decision of physicists and of engineers who know the 

 great hydraulic pressure of the head of water under which the supposed 

 subglacial streams must have risen to the tops of these high morainic 

 kames, which often stand as isolated hills with heights of 100 to 250 feet 

 above the lower ground surrounding them on all sides. 



The Pinnacle Hills Esker, Rochester, New York. 



Through the southeast edge of the city of Rochester, New York, a very 

 interesting esker, called the Pinnacle hills, extends about four miles in a 

 nearly straight west-southwesterly course. This esker and the adjoining 

 country, with another esker series several miles distant to the southeast, 

 I examined carefully during a few days following the summer meeting 

 of this Society in Rochester last year, and have presented a description 

 of these ridges of drift gravel, with discussion of their origin, in the cur- 

 rent volume of the Proceedings of the Rochester Academy of Science.* 

 The Pinnacle hills rise from a nearly flat region to a height which varies 

 from 75 to 200 feet. Their highest point is 229 feet above the University 

 of Rochester, 502 feet above lake Ontario, and 749 jfeet above the sea. 

 Gravel and sand, irregularly bedded and containing no bowlders, con- 

 stitute their principal mass, as is usual for the entire accumulations of 

 our eskers and kames ; but in some very limited portions the stratified 

 beds of the Pinnacle hills enclose exceedingly abundant boAvlders of all 

 sizes up to 8 or 10 feet in diameter, and they are less plentifully but yet 

 frequently found in and upon other considerable parts of the range, in- 

 cluding its highest summit. The difficulty of accounting for the trans- 

 portation of the bowlders to their present places in the stratified, river- 

 deposited esker is increased by the proximity of the rock formation 



* Vol. ii, Jan. 9, 1S93, pp. 181-200. 



