TOPOGRAPHY OF THE REGION STUDIED. 283 



reth westward along the slate belt to the Lehigh, with divergence north 

 to Mauch Chunk and return, and an extension to a point north of Topton, 

 and a return to Easton along the southern border of the limestone and 

 around the Saucon valle}^, marks the oOO-foot contour, and includes the 

 lake formed by the damming of the -waters of the Lehigh by the glacier. 

 The legend on the map describes the various forms of cross-section of 

 the deposits, but it must be noted that the broad slate belt is left clear to 

 avoid confusion. 



The height of the crest of the Blue ridge will average 1,500 feet, and 

 the next ridge to the north reaches 800 feet, and is formed by the vertical 

 outcrop of the Oriskany. Between the two the Aquanchicola flows west- 

 w^ard to meet the Lehigh at the gap. The Ridge falls abruptly over the 

 basset edges of the Oneida and Medina sandstones and conglomerates 

 to the Hudson river slates at an average elevation of 750 feet. This 

 slate belt is from seven to ten miles wide, and consists of a broken and 

 hilly region, whose southern edge, where it meets the limestone plain, is 

 at 500 feet. This plain is eight miles wide east of Trexlertown, and 

 nearly four west of that phice. East of Topton the northern edge of the 

 limestone is 20 feet higher than the southern side, wdiile west of that place 

 the northern edge will average 80 feet lower than the opposite side. The 

 Archean highlands are variously called the South mountains and the 

 Durham and Reading hills. They are divided into two parts by the 

 Saucon valley ; that to the east is twelve miles long, and has but one eleva- 

 tion (the Hexenkopf ) above 1,000 feet, while that to the west stretches 

 to the Schuylkill at an average elevation of 800 feet, with forty peaks 

 rising above 1,000 feet, and eleven above 1,100 feet. It has an average 

 width of six miles, and is bordered, except on the extreme eastern end, 

 by Cambrian quartzite, and generally capped by it in patches to the west. 



RIVER SYSTEMS OF THE REGION. 



Between the lUue ridge and Riegelsville the Delaware falls from 298 

 feet to 124, wdiile between the Ridge and Reading the Schuylkill descends 

 from 402 feet to 195. The Leliigh enters the region shown by the map 

 at Mauch Chunk, with an elevation of 504 feet, and flows thence to Allen- 

 town slightly north of a line bearing South 40° P]ast between those places, 

 and, in 'IS) miles, falls. to 235 feet, or 9 feet per mile. At Allentown 

 South mountain turns it sharply to north 70° east, and in that direction 

 it flows to the Delaware at Easton at 160 feet, by a fall of 4.2 feet per mile, 

 though the greater part of the distance has a fall of but 3 feet per mile, 

 and it is only as it nears the Delaware tliat the slope of the bed becomes 

 great. This must l)e rememliered in connection with tlie discussion of 

 postglacial erosion presented later. 



