120 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



pebbles makes it certain that the j were deposited by water moving south- 

 ward. There is additional evidence that these deposits were derived from the 

 Illinoian drift in the fact that the glacial pebbles show more weathering 

 than is commonly displayed by similar rocks found in the Wisconsin gravel. 

 Although these Illinoian gravels have not been traced from the Scioto down 

 thp Ohio past the old divide, that course appears to have been the only 

 available one for the stream which transported the gravel southward through 

 the Scioto Valley. The gravels show a decided slope toward the Ohio from 

 the glacial boundary. From an altitude aboiit 700 feet above tide, at the 

 point where the Scioto crosses the glacial boundary, just south of Chilli- 

 cothe, their surface declines to about 650 feet near the mouth of the Scioto, 

 30 to 35 miles farther south. With the data now at command the writer is 

 unable to make a definite estimate of the amount of trenching that had been 

 accomplished in this part of the Ohio prior to the Illinoian stage of glacia- 

 tion, but there was probably enough to give a downward slope from the 

 mouth of the Scioto past the old divide near Manchester to the lower Ohio. 

 There are features in the region which indicate that the trenching of 

 the gradation plain had extended up the Scioto beyond the glacial boundar}- 

 before the close of the Glacial epoch, but it is not entirely certain that the 

 trenching had been accomplished prior to the Illinoian glaciation. The 

 features referred to are found in the valley of the South Fork of Salt 

 Creek. This valley, and also the abandoned portion of the old Kanawha, 

 in southern Ohio, was greatly tilled by silt deposits, which are apparently of 

 glacial derivation, for they are in a sandstone region and yet are quite cal- 

 careous. Borings in the vicinity of Jackson, 20 miles above the mouth of 

 the stream, show that the silt deposits on South Salt Creek extend below 

 the level of the postglacial valley excavation, and are 40 or 60 feet lower 

 than the rock floor of the abandoned part of the old Kanawha west of 

 Jackson, less than 10 miles distant. The correlative rock floor of this val- 

 ley should stand a few feet higher than the rock floor of the old Kanawha, 

 since it had its discharge into that valley. The fact that it stands so much 

 lower is a clear indication that the valley had been deepened fully 50 feet 

 between the reversal and partial abandonment of the old Kanawha and the 

 deposition of the silt. That this amount of trenching occurred here on a 

 small tributary of the Scioto is a matter of far more consequence than if 

 it had been found along the present Ohio, in the vicinity of the old divide. 



