Stevenson.] OUU [Aug. 15, 



iniles from thai place, the surface rises in a succession of steps which are 

 beautifully marked. From the hill-top at BealJsville, throe miles east from 

 Hillsborough, the descent of these to the river is well seen. There is here, 

 then, a repetition of the conditions shown east from the .Monongahela. The 

 diagram rudely represents the succession. 



Profile of benches from Chestnut Ridge to Hillsborough. 



In the valley between Brush Ridge and Chestnut Ridge, benches lower 

 than the fifteenth fringe the several drainage areas, while the sixteenth and 

 eighteenth form subordinate divides. 



Two general questions present themselves here, the one relating to the 

 time when the valleys were scooped out, the other to the age and origin of 

 these benches by which the sides of the valleys are marked. 



Erosion of the Valleys. The present drainage system was outlined at a 

 very early date, in part no doubt before the elevation of any of the great axes ; 

 the Conemaugh river taking its rise on the western slope of the Alleghany 

 Mountains of Pennsylvania, breaks through both Laurel and Chestnut 

 Ridges, as well as the Blairsville, Saltsburg and Waynesburg axes, which 

 are west from Chestnut Ridge ; Loyalhanna, Jacob's and Dunbar creeks cut 

 through Chestnut Ridge ; the Youghiogheny has worn its way across all 

 the main axes west from the Alleghanies of Pennsylvania ; Cheat river 

 heads against the Alleghanies of Virginia, and its deep channel-way passes 

 by deep gaps through the Alleghanies of Pennsylvania and all other axes 

 at the west, until it enters that of the Monongahela midway in the Salts- 

 burg arch ; while the main fork of the Monongahela river splits the 

 Tygart's Valley axis in West Virginia, flowing for miles in a broad anticli- 

 nal valley, and after breaking through the west slope of that axis, runs 

 rudely northwest and north, until, having cut through every axis to the 

 Brady's Bend, it joins the Alleghany river at Pittsburgh. 



It seems altogether probable that this system developed itself gradually as 

 the land increased toward the central part of the basin, and that the great 

 streams flowed very near their present lines before any marked elevation of 

 the axes had taken place. 



