REE VT GHOLOGICAL VEISTORY TOR INEW YORK SAGs; Tass 
shale, and Medina shale and sandstone. These formations show 
only a slight southward dip or tilt. 
The Genesee river from its source to Portageville, Wyoming 
county, appears to be in a mature preglacial valley. Near Portage- 
ville, however, the river plunges into a deep, narrow, rock gorge 
of postglacial origin, which continues for 25 miles to Mount Morris. 
This gorge has been cut through soft Devonic shales and shal 
sandstones, and its walls are mostly nearly vertical, often rising to 
heights of several hundred feet. The three noted Portage falls 
(see plate 51) are situated just below Portageville, the upper falls 
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Fic. 38 Section at Niagara Falls, showing 
the character and position of the rock forma- 
tions and the depth of water below the falls. 
After Gilbert 
plunging 66 feet, the middle falls 110 feet, and the lower falls 96 
feet. According to Grabau, the preglacial course between Portage- 
ville and Mount Morris was farther westward along the present 
Oatka creek. A second postglacial gorge is entered at Rochester, 
and this continues for 7 miles to the mouth of the river. Here, 
also, are three falls, the first over Niagara limestone being 98 feet, 
the second over Clinton shale and limestone being 20 feet, and the 
third over Medina sandstone being 105 feet. The preglacial chan- 
nel here was probably a little to the east and through Irondequoit 
bay. 
