APPALACHIAN BASELEVELS 271 



gards this region a common hight of summits over large areas has been 

 explained by the supposition that at one or more stages in the histor}^ 

 of the area the mountain-built rocks have been worn down to or near 

 their baselevel of erosion, subsequently elevated, and tliereafter subjected 

 to the stream erosion which has carved out the existing reliefs.* Con- 

 sidering first the general character of this field as regards the hight of its 

 elevation, we note that, despite the variations in the resistance to erosion, 

 there are sundry parts of it in which the elevation of the highest ridges 

 and peaks are tolerably uniform, so much so indeed as to make the sup- 

 position of an ancient baselevel a good working h37pothesis. Thus in the 

 hills of Nova Scotia, in the Green mountains and their southern exten- 

 sions in the Berkshire hills, and in the Alleghanies as far as Alabama we 

 find accordant bights which at first sight appear to be readily explained 

 by the supposition that the erosive work began after the areas had been 

 brought to near the baselevel and afterward uplifted to a hight not far 

 above that which the existing crests attain. For the purpose of our in- 

 quir}^ it will be well to begin with the Alleghanian district, taking account 

 of the areas to the east, including the Blue Ridge mountains and the 

 Piedmont area. More than any other known to me, this field of the 

 southern Appalachians affords good data for discussing the problem in 

 hand. 



The features of the area above designated which first claim attention 

 in the inquiry are, first, the tolerably accordant levels of the Alleghany 

 section with hight, which in a general way decline from near Cumber- 

 land gap to the plain of southern Alabama ; next the presence of a great 

 valley or system of valleys on the western side of the Blue Ridge ; then 

 the rise to the southward of the Blue Ridge in the Smoky mountains, 

 and, lastly, the uniform eastward sloping plateau of the Piedmont area, 

 composed of highly metamorphic mountain-built rock, presumably of 

 Cambrian or of Archean, but containing many areas of Mesozoic folded 

 strata. These three mountainous elements of the Appalachians I have 

 for convenience termed the western, the central, and the eastern divisions 

 of that great system. The central is the oldest in the order of formation, 

 owing its existence in part to orogenic actions which probably occurred 

 before the beginning of the Cambrian age. The western or Alleghanian 

 was dislocated nearly at the close of Paleozoic time. The eastern or Pied- 

 montwas in large part mountain-built after the closeof the Jurassic period, 

 though it doubtless shared with the Blue Ridge mountains in the much 

 earlier disturbance to which it owes its structure. 



* For admirable presentations of tliis view see Arthur Keith: Geology of the Catoctin Belt, 

 14th Ann. Rep. U. S. Geological Survey, p. 2d'.i et seq. ; also, Hayes (Willard) and Campbell 

 (Marius R.) : Geomorphology of the Southern Appalachians, National Geographic Mag., vol. vi, 

 p. 63 et seq. 



