GENESEE DRAINAGE BASIN. 203 



It lias been pointed out by Fairchild that the valley followed by the 

 Grenesee between Portage Falls and the deflection at Mount Morris is the 

 prelacial valley of some tributary of the old river.' The preglacial 

 course from Portage was apparently eastward, past Nunda, into the valley 

 now occupied by Kishawa Creek, and thence north to the present Grenesee 

 just below Mount Morris. Grabau has suggested a westward course, past 

 Castile, to the valley of Oatka Creek near Warsaw, and thence northward 

 down that valley to the lowlands near Leroy, beyond which its course is not 

 given.^ In suggesting this course he apparently overlooked the broader 

 and more direct line past Nunda. It is also a mere conjecture that a buried 

 valley connects the Grenesee past Castile with Oatka Valley. The present 

 divide, it is true, carries a larger deposit of drift there than at points farther 

 north and south, but it seems likely to be the site of one of the low passes 

 or cols that characterize this region. 



At Rochester the Genesee enters another gorge, which extends to Lake 

 Ontario, a distance of 7 miles. In this gorge there are three falls made in 

 passing over the Niagara, the Clinton, and the Medina formations, with 

 heights of 90, 20, and 94 feet, respectively. Concerning these falls, Hall 

 has given the following interpretation:^ 



The different rates of recession in waterfalls is shown when the successive rocks 

 are of different degrees of hardness, producing a series of falls. This happens when 

 the highest are more destructible than the lower, and by this means the upper fall 

 outruns the others. The Genesee River at Rochester presents an example of this 

 kind, where the Medina sandstone, the rocks of the Clinton group, and the Niagara 

 group have each produced a distinct fall. This, at one period, was doubtless a single 

 cascade; but the upper shale wearing away faster than the rocks below, allowed the 

 fall to travel rapidly southward till it came to the limestone surmounting the shale, 

 where its progress was somewhat arrested. At the present time it seems probable 

 that the lower fall is receding faster than the upper, which is thus protected. 



The upper fall is now upon the northern edge of the limestone, which increases 

 in thickness for 2 miles south, being a medium of constantly augmenting resistance, 

 while the Medina sandstone and the limestone of the Clinton group are no thicker 

 and no more difficult to wear away than they have been for centuries past. Thus it 

 is plain that, under otherwise equal circumstances, the lower falls will advance upon 

 the upper until the whole will become one. It will not then, however, be of the 



1 Bull. Geol. Soc. America, Vol. VII, pp. 427-429. 



^The preglacial channel of Geneseft River, by A. W. Grabau: Proc. Boston Soc. Nat. Hist., 

 Vol. XXVI, 1894, pp. 359-369. 



' New York Geol. Survey, Fourth District, 1843, pp. 381-382, fig. 184. 



