Grabau.l 860 I May .6. 
from six liundred to about eiglit liinidred feet, and its sides are 
mostly perpendicular, rising in ])laces to a height of four hundred 
feet or more. In this gorge are situated the three celebrated 
Portage Falls, at a distance from the head of the gorge of one 
fourth, three fourths, and two and one fourth miles respectively, 
the first having a height of sixty-six feet, the second of one 
hundred and ten feet, and the third of ninety-six feet. The 
greater portion of the gorge is cut into the Portage sandstone 
and shales, only the lower portion, near Mt. Morris, being cut 
into the Genesee slate. 
The third portion of the channel extends from Mt. Morris to 
Rochester, and is another preglacial valley in all respects similar 
to the one south of Portageville, except that it is wider and has 
its sides less distinctly defined. Its width at Moscow is just two 
miles, while at Avon it is even more. Rock was found near 
Moscow at a depth of one hundred and eighty feet below the 
valley bottom, while four miles south, near Mt. Morris, it was 
found at a depth of one hundred feet.^ The bottom of the valley 
is an almost uniformly level plain composed of fine material, with 
only here and there a hill of coarser sand and gravel, constituting 
what is commonly knov/n as the "Genesee flats," and including 
some of the richest agricultural soil in the state. The supposition 
that this was an ancient lake bottom scarcely admits of any 
doubt. Professor Hall, arguing on this point, says-: — 
"An examinatiou of this deep deposit, on the Genesee flats, shows con- 
clusively 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 lamination. 
The coarser materials, at the points mentioned near the embouchures of 
the streams into this lake, are, in considerable proportion, of southern 
origin." 
Professor Davis thinks that the cutting of this goi-ge from 
Portage to Mt. Morris and the filling of the lake from Mt. Morris 
to Rochester occurred at the same time, the one furnishing mate- 
rial for the other.^ The sides of this valley have a gentle slope 
and are deeply drift covered, the rock being exposed oidy in the 
' Kindly coiiiinuiiicated by Mr. Clark Nichols of Moscow, N. Y. 
2 Report 4thgeol. dist. N. Y., 184-3, p. 344. See also 4th aim. report. 1840, p. 4;!8. 
3 W. M. Davis, I'roc. Bost. soc. nat. hist., v. 21, p. .359. 
