576 W. M. DAVIS — DATES OF TOPOGRAPHIC FORMS. 



wheu an uplift introduces a new cycle, so the resurrected pre-Triassic pene- 

 plain of New England may be taken as an example of a tilted structural 

 plain, the plain here marking an unconformity, and not simply a change in 

 composition. 



Tertiary Work in Virginia and Beyond. — It is not necessary to multiply 

 the examples of narrow valleys and wide lowlands, the Avork of the Tertiary 

 cycle, in Virginia and further southwest. They all accord closely with the 

 cases already given. The Potomac has its head-waters in portions of the 

 plateau, its middle course across the folded Appalachians, and it then enters 

 its deep gorge in the Blue Ridge at Harper's Ferry. Its chief tributary, 

 the Shenandoah, drains the great valley for many miles. In either the 

 Cretaceous or the Tertiary cycle the Shenandoah seems to have captured the 

 head-waters of smaller transverse streams that then headed in the limestone 

 valley belt and crossed the Blue ridge by independent gaps. The Staunton 

 is the last Atlantic river to cross the Blue ridge from the northwestward ; 

 farther southward the crystalline area is too wide and too high to have per- 

 mitted the development of an Atlantic drainage. 



In West Virginia the plateau is so perfectly dissected that it may be 

 taken as the very type of a maturely developed region. Its relief is strong, 

 for its hills are high and its valleys are deep; it has no uplands, for they are 

 all consumed ; it has no lowlands, for they are not yet opened ; it has so 

 great a number of streams that there is no room for more ; its slopes every- 

 where lead the rainfall by the most rapid routes to the water-courses, allow- 

 ing the least loss by evaporation, and the area of the slopes must exceed the 

 area of the original peneplain surface by twenty or thirty per cent., so great 

 is the variety of relief Consequently, with rapid discharge of rainfall and 

 with large area of slopes from which waste is furnished and delivered most 

 effectually, the further development of the mass must be progressing at the 

 highest possible rate. The sheets of the Geological Survey named Hinton, 

 Raleigh, and Tazewell, in Virginia and West Virginia, illustrate all this to 

 a nicety. 



The Cumberland plateau of Tennessee and Alabama is less completely 

 dissected; it still preserves considerable areas of uncut uplands; perhaps 

 owing to its rocks being harder. The Stevenson and Scotsboro, Alabama, 

 sheets give a striking representation of this region. Near Chattanooga, 

 Tennessee, the linear plateau of Waldens ridge may be taken as a typical 

 example of a hybrid between plateau and mountain forms; it is enclosed be- 

 tween two structural anticlines, but these are now followed by Tertiary val- 

 leys, the result of the disclosure of the weaker beds along their axes after 

 their crests had been baseleveled off in the Cretaceous cycle. The plateau 

 itself is but little consumed by its gnawing valleys. 



Occasional peculiar examples of rivers cutting across mountains and 



