520 PROCEEDINGS OF PHILADELPHIA MEETING. 



drainin.2: into the Ohio river from northern Alabama, Tennessee, southwest Vir- 

 ginia and western Nortli Carolina; these streams head upon the Blue Ridge, flow 

 northwest through the Unaka mountains, southwest along the Great valley of Ten- 

 nessee and Virginia and northwest through the Cuml)erland plateau. Tlie second 

 group form the drainage of the Ohio river in southwest Virginia, AVest Virginia and 

 Kentucky ; these flow northwest in converging lines through New river and various 

 arms of the Ohio. Third are the streams of middle and northern Virginia, Mary- 

 land and Pennsylvania, such as the James, Potomac and Susquehanna. The 

 fourth, or Atlantic group, comprises streams rising east of the mass of the Appa- 

 lachians and flowing directly into the Atlantic and the gulf of INIexico through 

 eastern Virginia, North Carolina, South Carolina, Georgia and Alabama. Their 

 waters run southeast into the Atlantic and south into the gulf of Mexico. 



SrnFACE FORMS 



The typical surfaces produced by the cutting of these streams are well known 

 through literature and need only a l)rief mention here. In the Great valley a series 

 of long, straight valleys alternate with straight, narrow ridges. As compared with 

 their length, the valleys are narrow.even when most conspicuous in size. In the 

 Unaka mountains and Cumberland plateau the divides are irregular, the valleys 

 show little systematic arrangement beyond a normal convergence into the great 

 rivers, and the basins are broad in comparison with their width. Divides of the 

 Atlantic drainage have small relief except near the stream heads, and the streams 

 drain comparatively narrow, parallel basins. 



Two general types of divides exist — those whose sunnnits rise to nearly equal 

 heights and those which show great diversity. The latter juwails in the Unaka 

 mountains and along the borders of the chief river systems. The characteristic, 

 even crests prevail in the vicinity of the larger streams in all regions and are most 

 pronounced in the Atlantic drainage, the (ireat valley and the Cumberland plateau. 

 Tims upon a broad view the Appalachians are most uniformly reduced near the 

 larger streams and are most irregular near the major divides. Such a result is 

 normal in ordinary erosion and would be expected in this case. 



As deduced in theory and as exhibited in the Ai)palachians in hundreds of cases, 

 erosion produces a regular sequence of forms. Beginning on an unreduced sur- 

 face, a stream cuts a narrow trench steadily downward until it reaches its baselevel 

 of erosion. Then the sides of the canyon are attacked, the downward cutting ex- 

 tending meanwhile up tiie larger and smaller tributaries until an approximate base- 

 level grade is estal)lished, increasing in slope as the streams diminish in i)Ower. 

 Continued wear opens out the canyons into valleys and pene])lains, which in time 

 occupy the entire area adjacent to the larger streams and extend uj) the tiibutaries. 

 Toward the heads of the streams these peneplains contract into valleys with floors 

 rounding upward at their borders, and these in turn give i)lace to series of terraces 

 and bottoms. AVith the division and weakening of the streams, debris becomes 

 coarser near its source, the little falls over individual pebbles accumulate into 

 steeper grades, bottoms are replaced by pla'nation slopes, and these by ravines and 

 gullies. Above all project the unreduced masses or residuals forming the main 

 divides. The details and successive steps of this process can be seen to perfection 

 in the streams flowing into the Atlantic, and are there rendered especially clear 

 because the streams flow over rocks of (juite uniform powers of resistance. There 

 the uniformity and omnipresence of the concave curve establish it beyond a doubt 



