1879.1 "05 [Stevenson. 



these terraces fall down stream, though not so rapidly as do the present 

 stream beds. 



But a second series of benches or terraces appears throughout this whole 

 region and seems to be characteristic of a very much wider area than that 

 in which observations were made. The members of this upper series differ 

 in many respects from those of the lower series ; their coating of debris 

 contains little clay and no polished fragments ; they are almost absolutely 

 horizontal and parallel ; they do not merge into the lower series, though 

 as the higher benches often form divides between the streams, the lower or 

 stream terraces always end up in one of them. These horizontal benches 

 begin within the area west from the Alleghanies at 1100 feet above tide ; 

 they line the faces of the mountains, they curve round the conical hills and 

 often they are indicated only bjr the leveled crowns of the higher knobs. 



Let us look at these series separately, beginning with higher one. 



The Horizontal Benches. 



Standing on the highest point crossed by the National road between 

 Chestnut Ridge and the Monongahela river, one finds himself on an island 

 of bench 14, or 1350 feet above mean tide.* Below him an almost continu- 

 ous plain of the fifteenth bench stretches for a long distance north and 

 south and is broken only by gaps through which the larger streams cross 

 Brush Ridge. He sees also that this plain is the divide between two val- 

 leys, one at the east between Brush and Chestnut Ridges, and the other at 

 the west, through which the Monongahela river flows. The latter is uninter- 

 rupted, but the former is crossed by strips of the fifteenth bench as well as 

 by lower benches of the series, which breaks its continuity and convert it 

 into a succession of basins. On each side the surface from the summit of 

 Brush Ridge falls off in regular steps. 



If now the observer turn his attention to the region lying west from the 

 Monongahela river he will see that the fifteenth bench is a broad continuous 

 plain beyond that river ; but that still further back toward the west, the 

 fourteenth, on an island of which he is standing, forms a similar plain, 

 while still further west, the thirteenth, with an altitude of 1380 feet, 

 stretches northward and southward, and is broken only by narrow valleys, 

 in which the larger streams flow. 



Should the observer's position be changed to Hillsborough, on the 

 National road nearly midway between the Monongahela river and the 

 borough of Washington, where the elevation is about 1500 feet above tide, 

 he will see that this thirteenth bench is of great extent north and south, 

 while back of it the country rises to a still higher level, again and again, until 

 it reaches to 1445 feet above tide. This last bench is the ridge, which practi- 

 cally separates the benches of the Monongahela from those of the Chartiers 

 Valley further west, though near the National road that ridge is broken by 

 Pike run, along which all the lower benches of the series are shown. From 

 the river westward to Hillsborough, or rather to the ridge passing nearly three 



* The numbers are those on the last tabic. 



