900 feet or more. The region of their occurrence is quite rugged. Blue 
River from the vicinity of Fredericksburg to the Ohio River is intrencled 
deeply in the Mitchell limestone group, and the adjacent hills are developed 
in the clastic Chester Series. The tributary streams upon approaching Blue 
River become subterranean. The intrenched condition of the main stream 
is responsible for this condition of the tributaries. The streams are sur- 
face streams in the clastic rocks of the Chester Series. but on coming down 
to the limestone below, the water disappears in the enlarged joints. The 
development of these streamless tributary valleys has been progressive. 
As the tributaries have grown and cut downward, they have progressively 
reached the limestone. The water then has developed a sink near the 
margin of the uncovered limestone. Later, when more of the limestone 
became exposed by the removal of the clastic material, a new sink would 
appear farther upstream, and the old one would be abandoned. In this 
way, too, the flat valleys in the Mitchell limestone have been developed. 
Probably much of the Mitchell plain itself has been developed in this 
manner. 
THE PECULIAR GRADIENT OF BLUE RIVER. 
Blue River with its several head streams each beginning at the crest 
of the escarpment, offers an excellent example of the southwestwardly 
flowing streams. The several sources of Blue River are well above 900 
feet above sea leyel on the remnantal portion of the Tertiary peneplain 
at the crest of the escarpment. The three main branches are down to 715 
feet at Salem, 730 feet at Farabee, and 700 feet at Pekin, respectively. 
From these places to the Ohio River the fall is a little better than five feet 
to the mile. This gradient is continued practically to the very Ohio itself. 
This condition is rather unusual. Normally a stream is well graded in its 
lower course, and possesses a much lower gradient in this portion of its 
course. Blue River is not in a graded condition in its lower reaches, nor 
does it have a lower gradient in its lower reaches than it has much farther 
up stream. It is in a graded condition, however, in its middle portion, as 
for instance, Muddy Fork in the vicinity of Pekin. Here, in one of the three 
branches of Blue River, the gradient is as low as in the many times larger 
lower portion of the stream. Ashley noted this peculiarity of Blue River, 
and offered rejuvenation by uplift or tilting as an explanation.” 
There are three different explanations which may be offered in interpre- 
tation of this rather unusual gradient of Blue River and other similar 
streams of the region. First, the condition may be the result of the differ- 
ence in the hardness of the rock in different reaches of the stream. In the 
region of Pekin, Muddy Fork of Blue River is at grade in a wide valley 
which it has developed in Knobstone strata. The Salem and Farabee forks 
are in a similar condition where they are developed in the Knobstone. 
Farther down in the course the stream is intrenched in the mechanically 
°G. H. Ashley, Geology of the Lower Carboniferous Area of Southern Indiana, 
27th Annual Report, Indiana Department of Geology and Natural Resources, 1902, 
pp. 58-61. 
