r893-l LfPHAM — ESKERS NEAR ROCHESTER, N. Y. 197 



inward from the retreating margin, its surface had. a considerable 

 slope, so that the upper currents of the ice, unsupported on the outer 

 side, would move much faster than its lower currents which were 

 impeded by friction on the land. There would be accordingly within 

 this belt a strong tendency of the ice to flow outward with somewhat 

 curved currents, tending first to carry the onwardly moving drift 

 gradually upward into the ice-sheet, and later to bear it downward 

 and deposit it partly beneath the edge of the ice and partly along the 

 ice boundary. The Niagara boulders, and others from the Clinton 

 and Medina formations farther north, having been borne upward as 

 englacial drift to a greater altitude than the Pinnacle hills, were 

 exposed on the surface of the ice-sheet by its ablation and were 

 swept by torrents bearing ice rafts, or probably sometimes by ava- 

 lanches, into the river channel. Their great profusion in certain 

 parts of this esker implies unusual abundance in and upon the contig- 

 uous portions of the ice-sheet, which may have resulted from conver- 

 gent glacial currents and perhaps from a temporary re-advance of the 

 thicker tract of the ice, massing its superglacial drift stratum in a 

 way analogous with the accumulation of terminal morainic hills, which 

 often are equally charged with boulders. 



The morainic ridge continuing westward from the Mt. Hope 

 cemetery seems probably to have been formed along the margin of 

 the ice, on the northern side of a re-entrant angle or embayment into 

 which the glacial river depositing the esker of the Pinnacle hills 

 debouched. Close south of this ridge, a brick yard beside the Buffalo, 

 Rochester & Pittsburgh railroad works the stratified clay which the 

 river discharged into the shallow glacial lake of the embayment. 



Finding so abrupt an end of this esker at Brighton, we are con- 

 strained to believe that the powerful river by which it was accumulated 

 suddenly ceased to flow here. The neighboring I'ittsford esker appar- 

 ently shows the site of the new glacial channel, previously the course 

 of some smaller stream, which then became the main avenue of drain- 

 age from the rapidly melting ice-fields of this region. l'.ut when the 

 Pittsford esker had gradually grown in its length from the west flank 

 of the Turk's hill range northward to the present site of Allen's creek, 

 the glacial river which formed it was again diverted ; or more prob- 

 ably thenceforward it emptied into a marginal lake so broad and deep 

 that no distinct esker was made, the gravel and sand being then laid 

 down in the valley which now holds Irondequoit bay. 



If the eroded drift from the area north of the Pinnacle hills was 

 carried upward by glacial currents having an average ascent of one 



