344 
GEOLOGY OF THE FOURTH DISTRICT. 
that this valley communicates with the valley of the Canisteo river, and thence with the 
Chemung and Susquehannah. The Genesee river, beyond Portage, flows in a valley more 
than five hundred feet above the same, after leaving the gorge at Mount-Morris. The 
northern part of this valley, from Rochester to Dansville, maintains nearly the same elevation 
throughout, or with a gradual descent to the north. It is one of the most ancient valleys of 
excavation ; and its sloping sides, covered with superficial accumulations to the height of 600 
or 800 feet above its base, show an immense period of time to have elapsed since its forma¬ 
tion. Long subsequent to its formation it has been partially filled with water, having a bar¬ 
rier on the north, and extending over the whole plain of the “ Genesee flats,''' and south as far 
as Dansville, in one great shallow lake. At the same time the valleys south of Dansville and 
south of Mount-Morris, by way of Cashaqua creek, were discharging their waters into this 
lake. With these streams was brought down a large quantity of coarse and fine materials, 
which we now find about Dansville, and below the junction of the Cashaqua creek, while the 
great extent of the valley is spread over with a fine sandy loam. The materials are precisely 
like those which are carried into modern lakes, by their tributaries ; and when they are filled 
nearly to the surface, a growth of vegetation ensues, changing as the whole becomes drained. 
An examination of this deep deposit, on the Genesee flats, shows conclusively that it has 
been made in a lake, such as described, with a current passing through it from south to north. 
The deposit was evidently carried forward in that direction, as indicated by the lines of lami¬ 
nation. The coarser materials, at the points mentioned near the embouchures of the streams 
into this lake, are, in considerable proportion, of southern origin.* 
The former outlet of this valley appears to have been by the Irondequoit; but this becoming 
obstructed with the vast accumulations of superficial detritus, it sought another course, and 
excavated a channel by way of Rochester, which, as it has been worn down, has gradually 
drained the lake, leaving the present flats as its ancient bed. 
The valley of the Genesee south of Portage seems to have been in a similar condition with 
that portion on the north, except that from the proximity of the hills on either side, a larger 
proportion of coarse materials has been distributed over the bottom. At the junction of the 
Angelica creek with this valley, there is the clearest evidence of its condition at that period. 
From the direction at which that valley joins the valley of the Genesee, the stream flowing in 
would extend across to the western side, depositing its coarser materials, while the finer 
sediment of clay and sand would be deposited in the eddy formed by the junction of the two 
streams. Accordingly we find below the junction of this stream an extensive deposit of sand, 
loam and clay, chiefly of the former; and its extent is marked in some degree by the growth 
of evergreen timber. 
* As an example of this kind, may be noticed the accumulation of gravel and sand, resting on regularly stratified clay, at 
Squakie hill, near Mount-Morris. The excavation of the Genesee Valley canal has exposed a deep section at this place, 
showing the lower deposit of fine clay horizontally stratified, and succeeded by a stratum of coarse pebbles and gravel, and 
above this loose sand and gravel the ruins of rocks on the south. This example shows an inundation of these materials after 
the deposition of clay and loam forming the Genesee flats. 
