As the water worked its way over the precipice gradually, so it would gradually excavate its channel to Lake Ontario, and it is not probable that any great inundation of the lower terrace could have occurred.
Composition of the terrace cut through--Why retrocession is possible--Three sections from Lewiston to the Falls--Devil's Hole--The Medina group--Recession long checked--The Whirlpool--The narrowest part of the river--The mirror--Depth of the water in the chasm--Former grand Fall.
The water having laid bare the face of the mountain barrier from top to bottom, we are enabled to examine the composition of the mass through which it slowly cut its way. After removing the thin plates of the upper stratum, as we descend, according to Professor Hall, we find:
1. Niagara limestone--compact and geodiferous.
5. Gray and mottled sandstone, constituting with those below the Medina group.
6. Red shale and marl, with thin courses of sandstone near the top.
Before reaching the Whirlpool the mass becomes, practically, resolved into numbers three, four, and five, the limestone, as a general rule, growing thicker and harder, and the shale also, as we follow up the stream.
The reason why retrocession of the Falls is possible is found in the occurrence of the shale noted above as underlying the rock. It is a species of indurated clay, harder or softer according to the pressure to which it may have been subjected. When protected from the action of the elements it retains its hardness, but when exposed to them it gradually softens and crumbles away. After a time the superstratum of rock, which is full of cracks and seams, is undermined and precipitated into the chasm below. If the stratum of shale lies at or near the bottom of the channel below the Falls, it will be measurably protected from the action of the elements. In this case retrocession will necessarily be very gradual. If above the Falls the shale projects upward from the channel below, then in proportion to the elevation and thickness of its stratum will be the ease and rapidity of disintegration and retrocession. The shale furnishes, therefore, a good standard by which to determine the comparative rapidity with which the retrocession has been accomplished at different points.