PHYSICAL FEATURES. 1038 
flats and on the north side of the Whirlpool, are nearly or quite vertical 
in the upper part, which consists of the Niagara limestone, and are very 
steep below, where a talus is formed by the fallen debris of the limestone 
and of the underlying shales and the inclosed beds of limestone and 
sandstone. These strata are nearly horizontal, but have a slight dip 
southward, which, according to Spencer, amounts to about 75 feet in 
the whole extent of the gorge, being least (only 10 feet) in the distance 
of three miles between the Whirlpool and the falls. 
No appreciable difference in the amount of subaerial decay and change, 
or weathering, of the rock walls is found when we compare the older 
northern part of the gorge and the newer southern part. In avery long 
geologic period the precipitous walls would be reduced to gentle slopes 
and would become indented by wide tributary ravines ; but scarcely any 
perceptible progress toward this result has been made during the geolog- 
ically very short period since the gorge began to be eroded. 
FROM LEWISTON TO THE WHIRLPOOL 
In this older part of the gorge, measuring 32 miles, the river descends 
about 31 feet, from 280 to 249 feet above the sea; and in its further course 
of 7 miles, to lake Ontario, it descends, at the ordinary stage of water, 
about 2 feet. After its narrow egress from the Whirlpool the river flows 
some 50 rods with its surface ruffled by the force of its current ; next it 
is smooth for about half a mile; thence, adjoining the broken and irreg- 
ular northwestern side of the gorge (called Foster flats), the stream is 
narrowed (having a minimum width of about 300 feet) and runs a mile 
in foam-crested rapids; through the next mile it‘has a very strong cur- 
rent, but little or no foam and broken water; and along the remaining 
distance of. about a mile to the Niagara escarpment and Lewiston the 
current is less and can be stemmed by the river steamers. 
Owing to the formerly much lower level of the western part of lake 
Ontario and its gradual rise to its present height, to be explained in Jater 
pages in treating of the Laurentian glacial lakes and the Champlain and 
Recent epeirogenic uplift of the region, the river eroded a channel at 
Lewiston and northward far below its present surface, its depth of water 
at the mouth of the gorge being 96 feet. Its depth in the heaviest part 
of the rapids adjoining the Foster flats is computed by Gilbert, from the 
velocity and volume, to be about 35 feet, and he estimates the depth to 
be about 100 feet for half a mile next above these rapids to the other very 
short rapids at the shallower and narrowed egress from the Whirlpool. 
The selection of this route by the Niagara river when it was first brought 
into existence by the melting away of the ice-sheet here and the reduc- 
tion of the glacial lake Warren to its successors, lakes Algonquin and 
