=. z 
_ THE LAKES OF IOWA,—PAST AND PRESENT. 145 
existed before any definite streams were formed. Where 
the depressions were longitudinal, or connected in chains, 
they gave initial direction to the courses of the streams into 
which the surface-waters were gathered and carried away to 
the sea. These are the streams of to-day, and their cease- 
less flow, aided by the rains and frosts of the unnumbered 
years that have passed since then, have worn their own chan- 
nels down, not only through the incoherent drift, but often 
also through solid stratified rocks, the edges of which we see 
protruding from their valley slopes. Thus all the valleys of 
this region are valleys of erosion, and it is meteorological 
erosion alone that has given it its most prominent physical 
features. 
As one stands upon the broad level prairies of Southern 
Iowa, and sweeps the well-defined ocean-like horizon with 
his level, he finds the bubble everywhere resting upon the 
cross-wire except where the distant dark line of forest foli- 
age reveals the presence of a stream. Approaching this, the 
surface becomes undulating like the smooth rolling of a sea; 
but looking closely he will see that every depression leads 
into a still deeper one until the upper branches of the streams 
are reached, the surfaces of which are often more than one 
hundred and fifty feet below the prairie level from which he 
started ; and the surfaces of the larger streams are some- 
times a hundred feet deeper still. The higher prairie-surface 
of to-day is the same surface which was left by the retiring 
Waters at the close of the Glacial epoch, and the time which 
passed since then—that during which the valleys were 
formed—is called by geologists the Terrace epoch, because 
the oscillations of the streams from side to side of their val- 
leys in the process of their erosion have left frequent terraces 
of material which successively constituted “flats” or “bot- 
toms” bordering the streams, but which are now far above 
the reach of their highest floods. The Terrace epoch verges 
upon the present time, because the same streams still flow, 
and earthy matter is still carried by them to the sea, as rap- 
AMER. NATURALIST, VOL. I. 

