THE LAKES OF IOWA,—PAST AND PRESENT. 151 
This is proven by the existence there of terraces composed 
of very fine sedimentary material such as could have been 
deposited only in comparatively still waters, and also by the 
existence in that sediment of shells which inhabit still waters 
only,—the same species which now inhabit fresh-water 
lakes. River shells, such as now exist in the river, are 
found on the sides of the bluffs near the rapids at a height 
of seventy feet above the present high-water mark; and 
since such beds of shells exist only at low-water mark when 
alive, upward of eighty feet must be estimated as the height 
of the river above its present level at the time they lived. 
It will be observed that river, and not lacustrine shells are 
found near the rapids. This is accounted for by the fact 
that the obstruction which caused them, being a flinty forma- 
tion, and not so easily disintegrated as the other rocks are 
over which the river runs, has existed as such from its ear- 
liest history. Consequently the water there always had a 
considerable current, while farther to the northward there 
was too little current to produce a congenial habitat for 
those shells. The estimated eighty feet is doubtless only 
a part of the actual height from which the erosion of 
the Mississippi Valley has reached, because it now aver- 
ages about two hundred feet deep from the general prairie 
Surface. Thus we see that when that lake-like expansion 
existed in the Mississippi River, its valley had already been 
eroded to a considerable depth, and the Terrace epoch was 
Well advanced. But on the other side of the State we have - 
Proof of the existence, in the early part of that epoch, of a 
lake which was larger and deeper than Lake Erie. This 
Proof consists principally in the presence there of a peculiar 
“Acustrine deposit extending at least from the Big Sioux to 
the mouth of the Kansas River, and from twenty to thirty 
Miles on each side of the Missouri River, through which the 
tter has cut its present valley, in some places to a depth of 
More than two hundred feet before it reached the drift which 
Was deposited there during the Glacial epoch. That mate- 

