Il6 PHYSICAL GEOLOGY 



Monadnock, Mt. Greylock, Mt. Wachusett, and some others, as has 

 been stated, rise as monadnocks (Fig. 103) several hundred feet 

 above the ancient plain. 



(2) The Appalachian Peneplain. — The Appalachian Mountain 

 region, from the Hudson River to Alabama, is underlain by rocks 

 differing in their resistance to erosion, which have been bent into 

 broad folds and, in places, broken by faults (p. 25) (Fig. 104). In 

 ancient times (Cretaceous, p. 516) the folds were planed off by ero- 

 sion, leaving the outcropping strata in long, more or less parallel 

 lines, resistant beds alternating with weaker ones. The surface of 

 this (Cretaceous) peneplain is now seen in the approximately level 

 crests of the ridges, showing that the base-leveling of the region had 

 been almost completed. Upon this plain the rivers took their 

 courses to the sea : the Delaware, Susquehanna, and Potomac flowing 

 to the Atlantic across the strata without regard to their structure; 

 the New River of Virginia and the French Broad of North Carolina 



Fig. 104. — A generalized section across the southern Appalachian Mountains. 

 Peneplains are shown by the dotted lines. 



flowing to the west; while the southern part of the region was 

 drained to the south by the large Appalachian River. An up- 

 warping along a north-south axis occurred which diminished the ve- 

 locity of some streams and increased that of others, thus favoring 

 stream capture (p. 108). The Potomac, Susquehanna, and Dela- 

 ware rivers, continuing to flow in approximately their old channels, 

 cut the deep gorges or water gaps at Harpers Ferry, the Delaware 

 Water Gap, and near Harrisburg. The tributaries of these rivers, 

 such as the Shenandoah and Lehigh, cutting more rapidly in the 

 weaker limestone beds, have excavated broad, subsequent valleys, 

 more or less at right angles to their mains, leaving the resistant strata 

 standing up as mountains (Fig. 105). The gradient of the south- 

 west wind, as well as that of the eastward-flowing streams was in- 

 creased and resulted in the headward cutting of one of these until it 

 captured the headwaters of the southward-flowing Appalachian 

 River. A later warping in northern Alabama and Mississippi along 

 an east-west line caused a tributary of the Ohio to cut headward 

 and capture the stream which had formerly robbed the Appalachian 



