1893.] UPHAM — ESKERS NEAR ROCHESTER, N. Y. I95 



this direction, from the heaping of the coarse gravel upon the fine 

 sand." 



Mr. Charles R. Dryer, in the article before cited, calls both the 

 Pinnacle hills and the Pittsford series kames, implying their deposi- 

 tion in the channels of glacial rivers. He especially notices that on 

 the area where, if prolonged to the northeast and north, they would 

 intersect, the valley of Irondequoit bay has been apparently filled 

 with stratified sand and gravel to a height of 150 feet or more above 

 the lake, as indicated by narrow terraces at such height left on each 

 side of the bay. The level of the glacial Lake Iroquois during a late 

 stage of its history, according to Mr. Ciilbert, sank here considerably 

 below the shore of Lake Ontario, and the depth of the Irondequoit 

 bay suggests that the depression of this southern part of the glacial 

 lake, permitting erosion of the former plain of modified drift in the 

 Irondequoit valley, reached at least 80 feet beneath the present water 

 level. 



The discussion concerning the origin of the Pinnacle hills after 

 the excursion to them last summer by members of Section E of the 

 American Association was opened by Mr. Gilbert, who drew on the 

 blackboard a sketch map of the esker series and the region about it 

 and called attention to the narrowness of the east and west belts of 

 outcrop of the several geologic formations. The Niagara limestone 

 occupying a belt that ranges from 2 to 7 miles in width through this 

 part of New York, underlies the Pinnacle hills and much of the city 

 of Rochester. Ne.xt northward the Clinton formation has a similar 

 width, and beyond this the Medina sandstone outcrops on a somewhat 

 wider belt which adjoins Lake Ontario. Each of these formations 

 and the Archaean rocks of Canada are represented in the gravel and 

 boulders of this hill range, and it is especially notable that usually 

 the Niagara limestone is very plentiful, both as gravel and as 

 boulders, which vary in size up to ten feet in diameter. Evidently 

 this limestone drift can have been transported only a few miles, and 

 its occurrence in the highest portions of the Pinnacle hills must be 

 taken into account in inquiring how they were accumulated, for which, 

 however, Mr. Gilbert had not framed any complete and detailed 

 explanation. 



Professor G. F. Wright and Mr. C. W. Hayes spoke of their own 

 observations and those of Prof. I. C. Russell on glaciers in Alaska, 

 where much superglacial drift is exposed on the wasting borders of 

 the ice-fields and portions of it are washed away by rains and streams, 

 which in most cases carry it finally into crevasses and subglacial 



