


















152 THE LAKES OF IOWA,— PAST AND PRESENT. 
rial is known to have been deposited in fresh water, because | 
only fresh-water shells are found in it, and they are found in | 
it from top to bottom. It is known to have been deposited | 
in still water, because the same kinds of shells are now living 
in still water only, and because the whole deposit is a fine | 
homogeneous material without sand, gravel, bowlders, or any 
thing else, except what would have been deposited in a lake | 
of muddy water. é | 
It has been claimed by a few geologists that at the clot | 
of the Glacial epoch a shallow fresh-water lake occupie : 
the whole hydrographic basin of the Mississippi, and that the 
fine soil and subsoil of the prairies and other lands of the f 
whole region, as well as the peculiar deposit just referred to, {f 
are identical in their formation, and had their origin in one | . 
and the same broad lake. Upon this hypothesis some have l 
accounted for the origin of the prairies and for the absen gi 
of trees upon them; but the fact is, prairies exist upon e l 
these deposits, and it would require direct effort to kep | 
all kinds of indigenous trees from encroaching upon i 
prairies if there were no annual fires. . 
It is not improbable that such a wide-spread ee 
physical characters from the deposit under discussion, © 
evidently had a different, as well as a subsequent Or 
These circumstances seem to leave no room to doubt t 
well-defined lake existed there after the continent 
great part become dry land, but before the great rivers, 
cut their valleys down to any considerable depth. 
lake, although so large and deep, was doubtless filled ' 
sediment to the general prairie level within a comp 
short time after the glaciers ceased, just as the sedi r 
the same river which then flowed into and from iti 
speedily fills the reservoirs of the St. Louis Water 
so that they must often be reéxcavated. Just as ther 
