372 
GEOLOGY OF THE FOURTH DISTRICT. 
left, and pursuing, for a short distance, an almost opposite direction, again gradually assumes 
a northerly course; but here, instead of the alluvial bottom of a broad valley, with sloping 
sides, it flows through a deep narrow gorge, and in the space of less than two miles pre¬ 
cipitates itself over three falls, descending about four hundred feet. 
It is to be recollected that in all the narrow portions of this gorge the cliffs are nearly or 
quite perpendicular, the chasm is narrow, and there is no deposition of drift or alluvium in 
its bottom. The bed of the stream presents a vast accumulation of fragments, fallen from 
the sides of the chasm, and there are few or no rocks of foreign origin. From the very 
aspect of this kind of channel, when compared with the broad deep valleys, we would at 
once decide, that it is of more recent origin. 
In examining the counter north of Portage, in the direction of the river, on the south, we 
find a valley filled with an accumulation of sand and gravel. This has been sounded, in many 
places, to such a depth, and over an extent so great as to authorize the conclusion that it was 
an ancient valley which received these deposits. The uppermost of these is very modern, as 
was before shown (page 345), and doubtless made after the river was formed; indeed, proba¬ 
bly after it had followed this channel for some length of time. The inundation of gravel and 
sand closed this former valley, and the Genesee on the south became a lake. It then took a 
course westward, over the lowest barrier, which appears to have been in the direction of its 
present course Here it has formed for itself a channel in the rocky strata, worn in some 
places to the depth of three hundred and fifty feet. 
The following diagram shows the respective positions of the ancient valley, now filled, and 
the present river channel: 
177. 
a. Present channel of the Genesee. 
c. Ancient channel, filled with modern detritus. 
b. Strata of shale and sandstone of Portage group. 
