2r<in and the (ihu-hij Period. 199 
run consisted of Ijare rock and rocky dehrin wbicli had fallen 
down from the face, on the north side the face of the rocks was 
entirely obscured by a sedimentary deposit which had evidently 
been brought in l)y a stream tlowing over the rocky bench from 
the north which here met the gorge when it was full of still water. 
It is, in short, a regular delta deposit, where for a limited time 
the coarse material had settled in tlie deep water as it was brought 
over the precipice. (}ranitic material was frequent in this de- 
posit; the stratification sloped downwards towards the axis of the 
valley; the depth of it exposed was fully SO feet; how much more 
we could not tell. 
To the south of (Mark's run as far as Wallace run, a distance 
of little over a mile, the bench was bare of drift material. 
1 see no explanation of these phenomena, except that which 
assumes the preglacial erosion of the under part of the troughs 
of these streams, and that the streams consequent upon the melt- 
ing of the glacier a])Ove were moving down the valleys wdien the 
lower depths of water were practically stagnant. This stagnation 
ma}' have been produced by a greater northerly depression of the 
land than I have thought to be probable, or by the Cincinnati ice 
dam, which might well enough account for the facts here, or b}' 
a combination of the ice dam with a more moderate amount of 
change in the land level. It is true, however, that it w'ould be 
possilile to suppose that this deposit at Clark's run was made at 
the climax of the second glacial epoch; thus allowing the erosion 
to have been interglacial : but in that case the causes assumed 
would have to be supplied during this second glacial epoch. 
Accumulating facts of this sort strengthen me in the conviction 
that the /^/^/-glacial erosion of the lower :>()0 feet of the troughs 
of the Ohio and its tributaries is far from being proved, and that 
the theory that the erosion was y*/v'glacial ma}' still l)e entertained 
by intelligent geolooists. 
The Cincinnati Ivv. Dam. 
-loKPii F. .TAME^^, Washington. 
While the j)r('senci' of an ice dam at Cincinnati has been dis- 
credited of late, there are features in the physical geography of 
tile vicinity of that city which make its presence mon' than prob- 
alilc. A few of those with which the writer is familial' from 
