Giabau-l '-^68 [May lO, 
tlius made tributary to it, having probably been reduced in size 
by the diversion of the drainage south of Dansville into tlie 
Chemung. 
Outlets of the ancient rivers. — The former outlet of the Caneseraga 
seems to have been by way of the Irondequoit channel. ^ This 
channel, w^hich is cut into Upper Silurian strata, extends ten 
miles backward from Lake Ontario and is in all respects similar 
to the other preglacial valleys, appearing to be of the same age ; 
but, unlike them, it has probably been deepened by ice action. - 
The ancient Genesee, likewise, may have emptied into the 
Ontario Valley by means of this channel, joining the Caneseraga 
somewhere north of Avon, as is the case now with tlie Oatka or 
Allen's Creek. 
Development of the loiver gorge. — When the Genesee began to 
occupy the northern half of the valley of the Caneseraga, it found 
the outlet of this valley barred by drift ; and, again overflowing 
the sides of the valley, which here were scarcely defined, it reached 
Lake Ontario by way of Rochester. As Lnke Ontario subsided, 
a fall came into existence over the Niagara escarpment, which, 
travelling southward, was soon followed by a second and a third, 
as Lake Ontario continued to subside. The recession of these 
falls has left the goi-ge which noAV extends from Rochester to 
Lake Ontario. 
A possible inter-glacial channel. — At the head of the groat gorge 
opposite Portngeville, we find another ancient drift-filled channel 
less than seven hundred feet in width and of unknown de])th. 
This channel enters the Genesee Valley at an angle of 110°, its 
mouth being shown in the drift hill on the right bank of the river 
opposite Portageville. Almost at its beginning the present gorge 
cuts across this channel, exposing a complete section. The 
channel has the character of a drift-filled gorge, with nearly per- 
pendicialar sides ; and this, together with the fact that it enters 
the Genesee Valley against the current, would seem to indicate 
that, at some time during a temjjorary recession of the ice sheet 
beyond Lake Ontario, it formed an outlet for the waters of 
Genesee Lake, probably carrying them into the Wyoming Lake 
and thus re-establishing the old drainage. Tlie re-advance of the 
ice sheet would then block this gorge, and the outlet by way of 
1 A suggestion first made by Professor Hall. 
2 Dryer, Amer. geol., v. 5, p. 202. 
