372 



900 feet or more. The region of their occurrence is quite rugged. Blue 

 River from the vicinity of FredericlvSburg to tlie Ohio River is intrenched 

 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 jyater 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 sinlv near the 

 margin of the uncovered limestone. I^ater, when more of the limestone 

 became exposed by the removal of the clastic material, a new sink would 

 api)ear 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 level 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 Oliio 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 coiHlition in its lower reaches, nor 

 does it have a lower gradient in its lower icachcs than it has much farther 

 up stream. It is in a graded condition, liowever. in its middle portion, as 

 for instance, Muddy Fork in the vicinity of I'ekin. 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 lilting as an explanatitm.'' 



There are tliree 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. 



