106. WwW. UPHAM—NIAGARA GORGE AND SAINT DAVIDS CHANNEL. 
ravine, though of smaller size, along its higher course for a considerable 
distance before reaching the site of the Whirlpool. Dr Pohlman sup- 
poses, and I think with sufficient reason, that the Saint Davids ravine 
reached along the part of the Niagara gorge occupied by the Whirlpool 
rapids, having a middle vertical fall over the Clinton limestone and 
terminating at an upper vertical fall over the Niagara limestone, beyond 
which, in its approach from the south, the stream was only a little lower 
than the adjoining country. 
EFFECT ON THE RECESSION OF THE FALLS 
Immediately after the departure of the ice-sheet and the withdrawal 
of the ice-dammed lake Warren, the Niagara river began to erode its 
gorge, and it has continued in this work, under varying conditions, to 
the present time. It found a lower passage along the course of the gorge 
to Lewiston than in the course of the preglacial channel, deeply drift- 
covered, between the Whirlpool and Saint Davids. Perhaps the erosion 
of the gorge below the Whirlpool had been in some part accomplished 
by small preglacial streams, one cutting into the escarpment at the north 
and another tributary to the Saint Davids channel at the Whirlpool ; 
but these streams, if any such existed, were much smaller and of less 
geologic age than that flowing past Saint Davids. There appears to have 
been no massive cataract like the present Horseshoe falls, but rather a 
series of rapids and low cataracts or cascades, along the greater part of 
the distance from Lewiston to the Whirlpool during the erosion of that 
part of the gorge, as is indicated by the shallowness and rapids of the 
present river. 
The action of a high waterfall, with great volume of water, precipitated 
over a hard rock stratum of which large blocks give way and fall because 
they are gradually undermined, as in the Horseshoe falls, is well com- 
pared by McGee to the deep wearing of potholes. The fallen blocks are 
moved under the powerful impact of the high cataract and wear a deep 
channel, attaining near the foot of the present falls the depth of almost 
200 feet under the river level. Such cataract action of deep channel 
wearing may be supposed to have produced the great depth of the 
Niagara river at the mouth of the gorge; but I think that this is better 
attributed to the usual process of stream cutting at the time of de- 
pressed level of this part of lake Ontario, which is otherwise known by 
its lower inclined beaches extending here under thelake. Deep cataract 
channellng is more surely indicated between the Foster flats and the 
Whirlpool for a distance of about a half mile, implying that any tribu- 
tary of the Saint Davids channel which may have aided toward the 
erosion of the gorge could not have cut down to the present river surface. 
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