Stevenson.] X4D [May 20, 



In some portions of South- west Virginia, notably in the region embracing 

 much of North-west Tazewell county, Virginia, and of Mercer and Sum- 

 mers counties of West Virginia, which is known as " Flat Top," there are 

 high benches or fragmentary plains which are of considerable extent, but 

 do not appear to carry many water-worn stones in their thin cover. They 

 are very like the higher benches observed by the writer* in South-west 

 Pennsylvania, and very possibly they are due to the same causes. 



But what these causes were is still an open question. The most natural 

 explanation is that which regards them as erosion planes, such as were 

 termed "base levels of erosion " by Major Powell. But in discussing 

 these benches as they occur in Pennsylvania and adjacent States, the 

 writer showed that as they line valleys and form irregular rings about iso- 

 lated hills, they are merely incidental modifications of a topography due 

 to prior and long continued erosion ; and that they are too well preserved 

 to be regarded as fragments of great erosion planes. More than this. The 

 deposit on these benches is marked by the absence of water-worn 

 and rounded fragments ; the absence of such fragments cannot be 

 accounted for by the supposition that they have been broken up by 

 exposure, for the greater part of the deposit has been protected from 

 atmospheric agencies ttntil exposed by the plough or in excavations for 

 roads. The Coal Measures of Western Pennsylvania contain sandstones, 

 whose fragments should resist disintegration eqitally with the Medina and 

 Potsdam pebbles of South-west Virginia ; while in Bedford county of the 

 former State, where high benches are as conspicuous as in the Coal Meas- 

 ures counties further west, the Chemung conglomerates and the Medina 

 sandstones are present and the stream beds are loaded with their frag- 

 ments ; yet no rounded pebbles were seen on the benches. 



The absence of these boulders militates against any application of the 

 base level process, as generally understood, and equally against the sup- 

 position suggested by the writer, that the benches were produced by shore 

 erosion between tides. The problem of their origin is not simplified by 

 denying their existence, for, unfortunately, the benches are "here to 

 stay." A ride along the National road in Pennsylvania, from Uniontown 

 in Fayette county to Washington in Washington county, enables one to 

 secure a key to the whole succession. 



Returning to Virginia. That the great erosion has occurred since the 

 faulting took place is sufficiently shown by the contrast between the up- 

 throw and downthrow sides of the faults ; for on one side is seen the 

 highest portion of the Lower Carboniferous, while on the other are the 

 lowest beds of the Knox limestones. It may be that the main streams 

 antedated the faults and possibly the folds. The course of New river, 

 which rises far beyond the axis of the Blue Ridge crosses at least six great 

 faults as well as all the folds, great and small, from the Blue Ridge to the 

 Ohio river, suggests that it may be following in a general way the original 

 direction, 



*Proc. Amer. Phil. Soc, Vol. XVIII, Aug. 15th, 1879. 



