33 



weathering and erosion of the long pre-glacial time. But 

 there is another interpretation. In the first place, the 

 widening is not so great as is stated by Grabau (two miles), 

 but is in fact one mile; and in the second place, the modi- 

 fication which appears to have produced the widening is 

 confined to the west and south side of the mouth of the 

 gorge and appears not to have affected the north side. 

 Supposing the gorge to have been of uniform width when 

 first made, the remnant of the plain on the west side of its 

 mouth would be a salient of the escarpment so situated 

 as to be exposed to the full force of the oncoming ice. On 

 this account it would have been torn away by the last ice 

 sheet and the cliff line would have been driven back and 

 straightened so as to present a more resistant front. The 

 north side of the gorge mouth was not so exposed and 

 would not have been modified in this way. 



There are other weighty reasons why this ancient 

 buried gorge should be regarded as inter-glacial rather than 

 pre-glacial. Its top width between the cliffs on the north 

 side of the whirlpool, and also its depth, are so closely 

 identical with the average width and depth of the Upper 

 Great gorge that one cannot resist the impression that 

 it was made by a cataract substantially identical in volume 

 with the present Niagara falls — by an inter-glacial Niagara. 

 So far as the characteristics of the buried gorge are known, 

 it is quite clear that they do not bear out the idea of 

 Pohlman, Grabau and others that it was originally made 

 by a relatively small stream and was widened to its ob- 

 served width by a long process of subaerial erosion. On 

 the contrary, they seem to show quite clearly that it was 

 made by a vertical cataract of large volume, substantially 

 identical with the present falls. 



At the south side of the whirlpool there is a strongly 

 marked reef which produces a sharp separation between 

 the whirlpool and the Eddy basin. This is the upper 

 reef of the whirlpool. When the water is clear the sub- 

 merged rocks of this reef can be seen for about one quarter 

 of the width out from each shore, and it produces a short 

 but sharply defined rapids. Directly over this reef the 

 top width of the gorge is narrower than over the central 

 part of the Eddy basin. The Eddy basin has not yet 

 been sounded, but the majestic swing of the great return 

 current which sweeps back on its west side shows plainly 

 that the water is deep. The top width and apparent 

 35065—3 



