





















150 THE LAKES OF IOWA,—PAST AND PRESENT. 
now very insignificant compared with the vastness of its 
bulk ; but, in such a case, there would be no mountains, 10 
islands, no continents. All would be an endless and shore- 
less sea, The erosion. of the river valleys, and the conse | 
quent drainage of a majority of the primitive lakelets, my | 
be regarded as the first steps in this levelling process, after 
the glaciers had ceased from the Great Valley ; for its post- 
glacial geology seems to warrant no subdivision into epochs | 
such as are made for other regions. Therefore the wholes 
here referred to the Terrace epoch. Long before this levee 
ling process can approach completion, beg elevations aul 
ao hidio will be formed upon the changing surface. . Set, 
then, how small a part of such a ailé hag been accom: 
plished eyen by the erosion of the valleys of the great Mis 
sissippi and its branches. A part of the primitive lakeles $ 
and a part of the original surface of the drift still remain | 
almost unchanged since their formation. The prairies hae 
still their ocean-like surfaces, and the greatest change th 
lakelets have undergone in that immense Japee of time ste 
formation of their insignificant embankments, if aught 
nature may be called insignificant. Let us look a little 
what has been accomplished by erosion in the Great Me 
during the Terrace epoch as before defined. 
‘Along the courses. of what are now the Mississippi 
Missouri Rivers, large depressions formerly existed 
formed lake-like expansions of those rivers. Thus after 
Mississippi had made for itself a definite valley, hut a 
it had cut its channel down to its present level t tie f 
rocky obstruction at the Keokuk rapids, that portion of 
which borders a large part of the eastern side of Towa” 
little else than a lake : which averaged about five miles 
and filled the space between what are now the bl 
horder each side of its broad flat valley. 

R PRR O a ; 
“It will be oe that,the word valtey is used with two separate sig 
one applied to the hydrographic basin drained by a certain prin nip stream streath, 
tributari mits an the other to the depression occupied by any particular 
Which its waters have cut out of the general surface. 
