The Rivers and Valleys of PennsyVoania. 189 



and Williamsport, the East Branch from Wilkes-Barre in the 

 Wyoming basin, and the Juniata from the Broad Top region, 

 south of Huntingdon. The Anthracite basins lie on the right, 

 enclosed by zigzag ridges of Pocono and Pottsville sandstone ■; 

 the Plateau, trenched by the West Branch of the Susquehanna is 

 in the northwest. Medina sandstone forms most of the central 

 ridges. 



3. The drainage of Pennsylvania. — The greater part of the 

 Alleghany plateau is drained westward into the Ohio, and with 

 this we shall have little to do. The remainder of the plateau 

 drainage reaches the Atlantic by two rivers, the Delaware and 

 the Susquehanna, of which the latter is the more special object 

 of our study. The North and West Branches of the Susque- 

 hanna rise in the plateau, which they traverse in deep valleys ; 

 thence they enter the district of the central ranges, where they 

 unite and flow in broad lowlands among the even-crested ridges. 

 The Juniata brings the drainage of the Broad Top region to the 

 main stream just before their confluent current cuts across the 

 marginal Blue Mountain. The rock-rimmed basins of the anthra- 

 cite region are drained by small branches of the Susquehanna 

 northward and westward, and by the Schuylkill and Lehigh to 

 the south and east. The Delaware, which traverses the plateau 

 between the Anthracite region and the Catskill Mountain front, 

 together with the Lehigh, the Schuylkill, the little Swatara and 

 the Susquehanna, cut the Blue Mountain by fine water-gaps, and 

 cross the great limestone valley. . The Lehigh then turns east- 

 ward and joins the Delaware, and the Swatara turns westward to 

 the Susquehanna ; but the Delaware, Schuylkill and Susquehanna 

 all continue across South Mountain and the Newark belt, and 

 into the low plateau of schists beyond. The Schuylkill unites 

 with the Delaware near Philadelphia, just below the inner margin 

 of the coastal plain ; the Delaware and the Susquehanna con- 

 tinue in their deflected estuaries to the sea. All of these rivers 

 and many of their side streams are at present sunk in small 

 valleys of moderate depth and width, below the general surface 

 of the lowlands, and are more or less complicated with terrace 

 gravels. 



4. Previous studies of Appalachian drainage. — There have 

 been no special studies of the history of the rivers of Pennsyl- 

 vania in the light of what is now known of river development. 

 A few recent essays of rather general character as far as our 

 rivers are concerned, may be mentioned. 



