296 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



not been known so long that a similar silt extends outside the limits of 

 the Illinoian drift. Wright has called attention to a silt on Beech Flats, in 

 Pike County, Ohio, which was cited as an extra glacial material, but it now 

 appears to be underlain throughout by glacial deposits.-^ In the writer's 

 examinations in southeastern Ohio and neighboring parts of West Virginia 

 and Kentucky, in 1896, it was foiuid that the silt occurs at least as far east 

 as Parkersburg, W. Va. Examinations in the Beaver Valley, in Pennsyl- 

 vania, in 1898, as indicated on page 262, have raised the suspicion that it 

 occurs there. It vasij also occur on the Monongahela and its tributaries, 

 for the terraces there are in many places capped by a compact silt several 

 feet in depth. This, however, is merely a conjecture. Its limits will not 

 be easy to determine, for it is so thin that it is likely to be preserved only 

 on comparatively flat areas where erosion has been very slight. Wherever 

 flat uplands appear in southeastern Ohio from the glacial boundary east- 

 ward to Parkersburg, W. Va., the silt capping is clearly recognized. It 

 is especially noticeable in parts of Morgan County, near McConnelsville, 

 which are underlain by limestone, for the silt contrasts more strikingly in 

 color with the residuary products of the limestone than with those of 

 sandstone. 



The extent of this silt northward beneath the AVisconsin drift is unde- 

 termined. It has been found at some distance back fi-om the border, both 

 in southeastern Indiana and in southwestern Ohio. 



THICKNESS OF THE SILT. 



The thickness of the silt on the uplands of southeastern Indiana, 

 northern Kentucky, and southern Ohio, where it overlies the Illinoian di-ift, 

 averages scarcely 5 feet, and seldom reaches 10 feet. As the exposures are 

 mainly on slopes where more or less removal has occurred, the thickness 

 seems to be only 2 or 3 feet, but the wells or excavations on flat uplands 

 correct this interpretation and show it to be about 5 feet. The thickness 

 varies but little from place to place, though it seems to be less on uplands 

 near the Ohio Valle}' than farther north near the limits of the Wisconsin 

 drift. This difference may be due to greater erosion on the borders of 

 the Ohio, where the sm-face is more completely dissected than at points 

 some distance back. But there is also a strong probability that some silt 

 has been deposited as an outwash along portions of the Wisconsin border. 



iBuU. U. S. Geol. Survey No. 58, 1890, pp. 92-96. 



