High Terraces and Abandoned Valleys in Western Pennsylvania

Abstract

The terraces with which this paper has to do are the well-known gravel-covered rock shelves found along the Allegheny, Monongahela, and other large streams of western Pennsylvania, about 200 feet above present stream channels. The abandoned parts of valleys are closely associated with the terraces, being found at the same elevation, and in many places the two are connected. Fig. i shows the principal areas of high terrace. The region includes all the Ohio River basin above New Martinsville, where there was formerly a divide. There are, however, terraces and abandoned parts of valleys of the same age on the Kanawha, Guyandot, Big Sandy, Kentucky, and other streams. The impressiveness of these features is attested by the long list of names of eminent men who have studied and described parts of them. This list includes Stevenson, Leslie, Jilson, Chance, Wright, Chamberlin, Gilbert, I. C. White, Tight, Campbell, E. H. Williams, Leverett, and others. Some of the earliest workers believed that the terraces were due to a submergence and marine erosion. Stevenson in 1879 (Proc. Am. Phil. Soc., XVIII, 289-316) called attention to benches along the valley of the Monongahela and its tributaries. He divided them into a higher series of twenty benches, and a lower one of five. The higher series he attributed to marine action. They are probably entirely above those under discussion, and later work on them has shown that they are obscure and are probably due to hard layers of rock. The lower series of Stevenson seems to include those under discussion, and he refers them to stream action, without going into details of development.