SURFACE GEOLOGY. 51 
rocky bed. As the current of this river was checked by the still water 
which then set far up from the Gulf, the coarser materials transported 
by it were deposited, forming a flood-plain which once stretched across 
the whole breadth of the valley. A subsequent elevation of the conti- 
nent made the drainage freer, and caused the river to cut away all the 
vacancy between the terraces in which it now flows. It will be under- 
stood, from the allusions before made to them,'that the gravel terraces 
are the last formed of all the series of Quaternary deposits, and they 
were produced during the retirement or reflux of the water which filled 
the whole valley and caused the deposition of the sheets of Drift which 
overlie the Forest Bed. 
The probable equivalents of these are found beneath the gravel ter- 
races in the Ohio valley, and have been described by Prof. Orton in his 
report on the geology of Hamilton county. They have also been alluded 
to on a preceding page in this chapter, and it will be remembered that 
they include an old soil, with rooted stumps, prostrate trunks, leaves, 
and fruits of trees and plants now growing in the valley of the Ohio. 
But this old soil lies nearly down at low-water mark, the flood-plain of 
the present river being forty feet above it. We have evidence, therefore, 
that at a time previous to the last submergence of the Ohio valley, and 
previous to its being filled with the gravel beds which now form its ter- 
races, the Ohio river flowed full forty feet lower than now, and its bottom 
lands were overgrown with a dense forest similar to that which now 
grows ata higher level. The record of this sequence of events, surprising 
as it may be to many, seems to be clearly legible. 
The valley of the Ohio was, for the most part, cleared of water while 
yet the lake basin was filled, and formed a great fresh-water sea. This 
was gradually drained, first by the waste-weirs, already described, and 
subsequently by the opening of other outlets, until finally the water 
level has been reduced several hundred feet, and the old inland sea is 
represented by our chain of great lakes. This depression of the water 
level either took place in a number of paroxysms, or slowly, with long 
intervals of arrest. These paroxysms, or stops, are distinctly recorded 
in the remarkable series of ancient beaches (lake ridges), terraces, and 
cliffs which mark the old shore lines on all the declivities that surround 
the lakes. Just how this vast body of water was drained away we shall 
probably never fully ascertain, but we know that it flowed off by several 
outlets, and in several directions. As remarked elsewhere, it is probable 
that the glaciers which filled the lake basins for ages, constituted ice- 
dams, that obstructed the natural lines of drainage, and maintained a 
