42 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xii. 



Appalacliians were first formed by a great uplift and foldings. The slopes of the surface thus formed 

 were in a condition of unstable equilibrium, for the rains which fell upon them and the resultant 

 streams developed gulleys, ravines, and valleys, which cut down the land and formed, by the 

 close of the Cretaceous, an extensive surface of low relief near sea level, a peneplain. Upon 

 this peneplain there remained certain unreduced mountains (monadnocks and unakas) which 

 perpetuated the slopes and continued the unstable equilibrimn as an inherited condition untU 

 the renewed uplift of the Tertiary and later periods. It is thus seen that uplift or slope has 

 been accumulating from the Cretaceous monadnocks, from the Tertiary uplift, the unreduced 

 tracts upon the Tertiary peneplain, and finally by the later uplifts. It is therefore evident that 

 there has been a ciimidative elevation and a lag of degradation, thus showing that upward 

 crustal movements have been a dominant environmental factor. 



This cumulative uplift has had a dominant influence upon the development of the drainage 

 lines. The residuals and axeis of uplifts being in a condition of unstable equilibrium, have thus 

 continuously changed, as the drainage lines have developed upon the surface, except where 

 (relatively) continuous uplift has maintained the old divides. The persistence of the uplifts 

 in certain areas, as the south slope of the New Kanawha divide and in the large residuals, as 

 the higher Southern Appalachians, has given certain divides and streams remarkable relative 

 stability. Campbell ('96, p. 580) has shown that divides tend to migrate up slopes toward 

 the axis of uplift. Such relations show the intimate dependence existing between the devel- 

 opment of drainage lines and the axes of uplifts. Depressions due to local crustal movements 

 are difficult to recognize in a region in which uplift is prominent, because of their liabUity to 

 obliteration by later uplifts. 



d. Baseleveling processes. — As previously mentioned, uplifts which raise a land surface 

 relative to the sea and bring it into contact with the atmosphere and running water, produce 

 upon it a condition of tension which wiU continue as long as it possesses a slope down which 

 the rainfall may run with the burden of surface waste produced by weathering and erosion. 

 The Southern Appalachian province is an area which, as is testified by the character of the 

 baseleveling which it has undergone in the past, and its proximity to the warm sea, has had 

 a relatively abundant rainfall. The height of the mountains has no doubt been an important 

 factor in influencing the amount of this supply, which continues in abvmdance to the present 

 day. The composition and structure of the rock is stiU another fundamental factor in the 

 history of these conditions. The present mountain area consists largely of metamorphic rocks, 

 the folded Appalachian Valley of limestone, shales, and sandstones, and the Cumberland Plateau 

 of sandstones. It is thus evident why the VaUey area is more responsive to erosion. 



e. Some principles of drainage development. — The drainage development resulting from the 

 unstable equilibrium initiated by the crustal uplifts of a peneplain of relatively homogeneous 

 surface rocks, and the erosion of the land sm"face, presents a succession of conditions to be 

 expected somewhat as follows: 



1. Upon the peneplain, which is relatively a stage of equilibrium, the drainage acquires 

 a corresponding condition of equilibrium, and the streams become relatively uniformly adjusted 

 and symmetrical on account of the nearly equal chances for each one afforded by relatively 

 uniform or homogeneous conditions. On such a surface of low relief the similar minor streams 

 flow down the slopes of the peneplain, from the low divides or the monadnocks, at right angles 

 to the axes of slope, and tend to have separate mouths, as on a coastal plain near the sea. But 

 any inequality that will give advantage to some stream will tend to favor its capturing others 

 and thus ultimately to concentrate the drainage into an axial stream in a depression, at right 

 angles to the minor streams (Campbell, '06, pp. 657, 665) and parallel with the divide. 



2. Uplift such a peneplain, the slope is increased, the velocity and erosive power of the 

 streams are increased, and they entrench themselves on the surface. The heads of streams will 

 become extended up the slope, and the lateral tributaries will encroach upon their minor divides, 

 erode and completely remove them, and thus capture adj acent drainage. This etching process 

 will continue until the level interstream areas are worn away and a period of maximum rough- 



