The Development of Mlssissipp) Valley . — Hersheij. 265 
or more away from the river. In the northern part of Du- 
buque, several ridges of nearl}^ even hight form a dissected 
plain or terrace at least 100 feet beneath the Tertiary pene- 
plain. The3^ in conjunction with the 160-foot bluff on the 
east side of the valley, represent the basin valleys of north- 
western Illinois. This basin valley, here not averaging over 
two miles in width, trends from northwest to southeast. It 
sends long narrow arms up the Little Maquoketa and its trib- 
utaries. The caiion valley of the Mississippi enters it on its 
northeastern side and passes obliquely across it at Dubuque 
touching its southwestern side, thus again producing a 1250- 
foot bluff. Southward from Dubque this basin valley is plain- 
ly discernible as far as the topographical features of the land 
surface can be distinguished. 
Now. the point to which I wish to chiefly attract attention 
is the comparative narrowness of this basin valley. It is carved 
entirely in the Galena limestone, a formation which here is 
substantially identical in every particular with its outcrop- 
ping portions throughout northwestern Illinois. Remembering 
that the basin valleys in the Galena limestone are always pro- 
portional in size to the streams, and making allowance for a 
greater depth and therefore naturally less width of the valley 
at Dubuque than others with which it may be compared, we 
will conclude that a stream no larger than the Pecatonica 
river at Freeport was engaged in excavating it. Furthermore 
the distance between the edges of the dissected Tertiary pene- 
plain on either side of the canon valley, a few miles above Du- 
buque, is scarcel}' sufficient for the basin valley of a stream as 
large as the Pecatonica river. If now we pass down the river 
we shall find this basin valley gradually increasing in width, 
especially where it crosses the Hudson River shales, but no- 
where as large as the basin valley of a stream thrice as large 
as the Pecatonica river. Adopting a sufficient factor, of safe- 
ty, to cover all the possible causes of error in making compar- 
isons, I feel confident in making the following assertion. The 
present course of the Mississippi river between Dubuque and 
Le Clare, Iowa, was occupied, up to the close of the epoch 
during which the basin valleys were being eroded (Lafayette?) 
by no stream larger than the present Rock river between 
Rock ford and its mouth. 
