POSTGLACIAL CHANNELS OF THE GENESEE. 427 



The origin of the drift material has not been carefully studied. The 

 great bulk in any section seems to have been derived from the terranes 

 contiguous on the north, but some percentage is far- traveled material, as 

 Medina, or even hypogene waste from the crystalline terranes, north and 

 northeast. 



The ancient Genesee Valley. 



postglacial channels of the genesee. 



Throughout the greater part of its course the river flows in its old 

 valley. It may be said in general that the ancient valley is the present 

 valley from the source to Portageville. Some unimportant divergences 

 of the river from its preglacial bed probably occur above Portageville. 

 The few, but very important, diversions below Portageville are found 

 where the river has been compelled by drift dams to abandon its former 

 valley and to cut new rock channels. To trace the old buried valleys 

 will require a brief description of the new rock channels. 



The postglacial channels of the river are really only two — one from 

 Portageville to Mount Morris, and the other from Rochester to lake 

 Ontario. In both cases these have produced fine canyons and noted 

 cataracts. 



At Portageville an impregnable barrier of drift was left by the glacier, 

 blocking the whole valley, which here was probably two miles wide. 

 The local morainal lake thus formed by the drift dam found its outlet 

 over the western rock-wall of the ancient valley, and the downcutting 

 through the Portage shales has resulted in the famous Portage ravine and 

 falls. 



Between the Portage ravine and the rock-cutting at Mount Morris the 

 river, while in a channel new to itself, does not occupy a postglacial exca- 

 vation, but the narrow preglacial valley of some tributary of the ancient 

 river. In curving from one side to the other of this narrow valley the 

 river has undercut the rock-wall in several places, producing vertical ex- 

 posures of the strata. Near Mount Morris this valle}^ also was closed by 

 morainic drift and another local morainal lake was formed in the river, 

 which we will call Saint Helena lake, lower and smaller than the Portage- 

 ville morainal lake. The outlet was cut down through the Hamilton 

 shales, producing the ravine known as the " high banks." By erecting 

 an artificial dam in the narrow channel above Mount Morris it is pro- 

 posed to impound the Genesee water in this valley, thus partially restor- 

 ing the Saint Helena lake, and so create a storage reservoir for equilizing 

 and controlling the flow of the lower river. 



At Rochester the river has again departed from its old channel and 



