THE LAKES OF IOWA,— PAST AND PRESENT. 153 
river would now fill with the same kind of sediment any de- 
pression, however large, if such existed in its course. 
The great northern lakes are not thus tilled, because their 
tributary streams are pure; and their streams are pure be- 
cause they flow over geological formations that are not easily 
disintegrated ; while the main tributary of that ancient lake, 
the Missouri River, is even now one of the muddiest stream 
on the globe. In the earlier portion of the Terrace epoch it 
was, if possible, more so; for then as now, it gathered up its 
sediment from that broad region occupied by the friable 
rocks of the Tertiary and Mesozoic ages, stretching far away 
toward the Rocky Mountains, at that time strewn with the 
grindings fresh from those “mills of the gods”—the glaciers. 
he formation of the basin in which the lake rested is 
known to have taken place during the Glacial epoch, because 
the drift, with its striated bowlders, now covers its bottom 
beneath the lacustrine deposit, and because the cutting out 
of the river valley has exposed, in a number of places, the 
stratified rocks which the drift rests upon, whose surfaces 
were scored and striated by the moving glaciers of that 
epoch. It is known that the filling of the lake with sediment 
occurred in the early part of the Terrace epoch, because it 
was filled up even with the prairie surfaces, which would not 
have been done if the Missouri River had first eroded its 
valley to any considerable depth below the lake. We know 
that the lake was so far filled with sediment before it was 
drained, that it was little else than a marsh, because the top 
of that deposit of sediment is now nearly even with the 
higher prairie surfaces, and because the river bluffs which 
it forms are as high as those formed of the usual materials, 
—the drift and stratified rocks. 
The physical characters of this lacustrine deposit are so 
Peculiar, that they attract the attention of every person who 
becomes acquainted with it, although a stranger might pass 
Over the formation without observing more than its peculiar 
Cutline of bluffs. It is perfectly uniform in character and 
AMER, NATURALIST, VOL. IL. 20 

