EVOLUTION OF IRONDEOUOIT VALLEY. 147 



coastal plain flowed upon a now-vanished land surface hundreds of 

 feet above the present one and upon rock strata long since removed 

 by erosion, — they themselves being active agents in that erosional 

 process. Nevertheless in the long course of this denudation few if 

 any of these valleys could ever have entirely disappeared, since even 

 when stream-capture shifted their main flow elsewhere they would 

 still serve to collect and carry some run-off and thus continue to be 

 deepened at least as rapidly as the surrounding slopes until the}' 

 reached base-level. But there is scant evidence that our region was 

 ever base-levelled. The rock valley of the Irondequoit must there- 

 fore be regarded as such an inheritance from the remote past, for it 

 is not explicable as a subsequent adjustment to the rock structures 

 which it crosses nor as the escapement for glacially coerced waters. 

 Its location was determined by one of those throvigh-flowing rivers 

 of the coastal plain running southward to the Mississippian Sea. In 

 course of time this river was decapitated somewhere to the 

 north of Rochester by the headwaters of another stream working 

 in from the west along the belt of weak Ordovician shales ; and as 

 this invading stream grew by successive conquests and opened out 

 the great valley of present Lake Ontario it sent a small tributary 

 headward into the amputated end of the older valley to eat off the 

 head of the latter's river little by little. Thus by a slow but cease- 

 less shifting of the divide southward down the old consequent 

 valley the original flow was progressively reversed. Successive 

 uplifts of the land, acting from the north, merely gave to the On- 

 tario river greater capacity to excavate its basin, thereby imparting 

 greater vigor to this its Irondequoit tributary. Apparently there 

 was a revival of this invigoration not long before the end came, a 

 rather immature inner canyon through the Niagara escarpment east 

 of Rochester being the resultant as will be described in a subsequent 

 paper. Before it could be widened to full maturity the advance of 

 the ice-sheet put an end to further river work and inaugurated a 

 stage of glacial modification and erosion, of which the particulars 

 are narrated in another paper. 



Glacial occupation. So Irondequoit died, by a frigid fate, and 

 the glaciers came and went over its grave. Inasmuch as the main 

 ice movement was in a southwesterly direction diagonally across 



