198 ROCHESTER ACADEMY OF SCIENCE. [J^"- 9. 



degree, it would rise within one mile 92 feet and within two or three 

 miles would be higher than the tops of these hills. Currents ascend- 

 ing at this rate, or even two or three degrees or more, may very prob- 

 ably have existed in the lowest part of the ice sheet, on account of the 

 acceleration of its upper currents, within distances from 20 to 50 

 miles or more back from its boundary. By these currents much drift 

 eroded from the land surface would be gradually incorporated in the 

 comparatively sluggish lower part of the ice, reaching altitudes 100 

 to 1,000 feet above the ground within a few miles from its sources. 



It is also to be remarked that the rounded or at least subangular 

 forms of the greater part of the pebbles and small rock fragments in 

 the esker gravel do not necessarily imply wearing by the stream dur- 

 ing a long transportation. Daubree placed angular fragments of 

 granite and quartz, ranging from the size of one's fist to that of a hazel 

 nut, with water in slowly revolving cylinders and found that they 

 became perfectly rounded when the revolutions amounted to 25 kilo- 

 meters or about 15 miles. (') Within a third of this distance probably 

 some of the fragments had been well rounded, and in a less distance 

 nearly all would be worn to subangular forms. 



Many features of the modified drift, comprising glacial flood 

 plains, eskers and kames, show that the melting of the ice-sheet at the 

 close of the (llacial period was mostly very rapid. In the vicinity of 

 Rochester it was hastened by the laving action of the glacial lakes on 

 its southern border. T.ake Warren had formed a beach which extends 

 to the south side of the east end of Lake Erie, where its altitude is 

 860 feet above the sea. (') .A^t the time of formation of the Pinnacle 

 hills and Pittsford esker series, the ice-border in New York appears to 

 have receded so far that the water of the upper Laurentian lakes was 

 no longer held up to the level of Lake Warren, which had outfiowed 

 at Chicago, and avenues of drainage seem already to have been opened 

 eastward along the ice-border past the northern ends of the Finger 

 lakes to the Mohawk valley. Undoubtedly the deposition of these 

 esker gravel and sand beds took place above the level of such fring- 

 ing lakes, which from the Genesee and Irondequoit basins could have 

 no place of outflow eastward lower than by the way of Victor and 

 Mud creek. 'J'he divide at Victor is somewhat higher than the gen- 

 eral surface on which these eskers lie ; hence it seems probable that 

 when the esker beds were laid down in their ice-walled channels a 

 depth of some 100 feet, more or less, of ice still remained unmelted 



(i.) Etudes Synlh<''li(|ucs dc Geologic Expt'Timt'nt.ilc, 1879, pp. 248-250. 

 (2.) Bulletin, G S. A., Vol. U, pp. 258-265 ; Vol. Ill, pp. 484-487. 



