INTERPRETATION OF PHENOMENA AS TO VOLUME. 69 
gorge characters alone, the gorge of the Whirlpool rapids may well be an 
old preglacial creek ravine; but if the Eddy basin is of postglacial age, 
then the gorge of the Whirlpool rapids must also be of the same age, and 
must have been made by a cataract of much less volume than those which 
made the greater sections above and below it. 
We have seen that there is a slight contraction and a shoal ledge be- 
tween the Whirlpool and the Eddy basin. Their position and character 
are substantially what would be expected if this were the site of the new 
cataract which began work while the basin of the Whirlpool was having 
its loose drift washed out. As pointed out above, the new cataract began 
as a feeble fall of shght descent. Hence, at first it did not bore deeply 
at its base, but as it grew in height by lowering the water in the Whir]- 
pool, it bored deeper and deeper, while at the same time it was slowly 
receding. It seems probable that the upper shoal ledge and the contrac- 
tion stand for the time of the feeble falls, and that by the time the cata- 
ract had attained sufficient height and power to bore deeply at its base 
it had receded so far that it no longer fell on the ledge, but back of it to 
the south, where it began to bore the deep hole of the Eddy basin. 
Two other factors may have contributed to the making of the upper 
shoalledge. If the great preglacial or interglacial cataract that made the 
Saint Davids gorge stopped its work suddenly at the south side of the 
Whirlpool basin, as it seems to have done, and if the overhanging ledge 
thus abandoned stood for any considerable time exposed to the weather 
before the last ice-sheet came upon it, then cliff recession by weathering 
must have taken place just as it has in all other parts of the gorge. The 
vertical face of the shales below would crumble away and the overhang- 
ing ledges of limestone would fall off, covering the face of the shales with 
a talus of fragments. This cause alone would probably have left the 
shoal ledge where it is now found, for the new cataract would not begin 
where the old one left off, but at some point several rods farther back— 
as much farther back as the cliff had receded by weathering. Then, too, 
the process of glaciation which smoothed and scored the west wall of 
Saint Davids gorge below the Whirlpool may have worn or torn away 
some part of the old cliff where the former cataract had been, and this, 
so far as effective, would also set the beginning place of the new falls 
farther back. All three of these factors may have contributed to the 
making, or rather the leaving, of the shoal ledge between the Whirlpool 
and the Eddy basin. 
Middle Great gorge.—It was pointed out above that the Eddy basin 
is apparently deep, and that it has a top width of about 1,200 feet. This 
is a trifle less than the width of the middle part of the Cove section, but 
it is the same as that of the Upper Great gorge opposite the end of the 
