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



The following outline indicates the general conditions and the processes which have 

 brought about base-leveling in the southern Appalachians : Slopes have been reenforced by uplift 

 and crumpling; an abundant ramfall; a great diversity ui rock structure, including resistant 

 raetamorphic rocks and sandstones and less resistant limestones and shales; vast periods of 

 time; and periods of tension followed by those of adjustment and of relative equilibrium. 



At no time has the region been completely reduced to base level, so that there has persisted 

 the condition of unstable equilibrium or tension which conditioned the persistent denudation of 

 the land surface. The nearest approach to an equilibrium for which there is abundant evidence 

 is the extensive development of the Cretaceous peneplain. 



2. FROM THE ORIGIN OF THE APPALACmAN LAND HABITAT TO THE FORMATION OF THE CRETACEOUS 



PENEPLAIN. 



As /o is a river snail, and therefore dependent upon a land area, as contrasted with the sea, 

 most of the early marine history of the southern Appalachian province may be ignored. It is 

 likely, however, that this family of shells descended from marine ancestors, but at what time 

 or place this occurred is not definitely known. There is the possibility that this evolution may 

 have occurred in this general region, as an ancient land area or continent, Appalachia, lay to 

 the eastward of the present Appalachian VaUey. The products of erosion and other debris 

 from this ancient continent encroached to the westward upon the sea and deposited littoral 

 beds of limestone, shales, and sandstones (Willis, '95, p. 196), which at the close of the Carbon- 

 iferous or during the Permian period were elevated and crumpled to form the first installment 

 of the Appalachian Mountains; and thus was produced a large land habitat with lime-bearing 

 surface rocks. "The streams," says Hayes ('95, p. 329), "flowing from the old land into the 

 interior sea before the emergence [of the Appalachian Valley belt] doubtless continued in the 

 same direction, extending their lower courses across the newly added land as successive belts 

 emerged. Since the process of folding was exceedingly slow, they may have held their original 

 courses for a long time in spite of the folds rising across their path. These folds, however, 

 although not directly able to turn the rivers aside, brought bands of soft rocks above base level, 

 and so were able to indirectly accomplish that result. Streams flowing southward parallel 

 with the folds were located entirely upon soft rocks, and so were able to deepen their channels 

 more rapidly than those flowing westward across many hard beds; hence the streams parallel 

 with the folds encroached upon the territory of the transverse streams and successively captured 

 them and led them by southward courses directly to the Gulf.^ When once fairly started the 

 conquest proceeded rapidly toward the northeast, but before it had reached New River the 

 latter had been able to sink its own channel so deeply that the Holston could not cut through 

 its banks and divert it. New River therefore continued northward from its source in the Blue 

 Ridge, across the mountain belt, the great valley, and the Cumberland Plateau. It is the only 

 stream in the entire Appalachian province which retains throughout its entire length approxi- 

 mately its original position." 



An alternative view, that the Appalachian uplift of the Permian may have ponded some 

 of the transverse streams and turned them northward toward the headwaters of the New 

 River or southward toward the headwater of the axial line from Asheville to the Chattahoochie 

 River, seems plausible. This appears to have been an axis of depression in the following Tri- 

 assic period (Campbell, '96, p. 676) and the later movement may have obscured the earlier 

 depression. 



The hypothetical drainage of the Permian is shown in plate 57, A. This is an effort to re- 

 store an intermediate stage in the development of the Tennessee system of drainage. It is 

 assumed that the northwestern slope of the Cumberland Plateau drained into the interior sea 

 and was mainly traversed by through-flowing streams from the abundantly supplied rainfall 

 upon the mountains to the eastward. The transverse drainage of the Appalachian Valley is 

 thus a characteristic featm^e at this stage of development, part of which, the New River and 

 the projected heads of other northwestward draining streams, have persisted through the 

 Cretaceous and Tertiary to the present time. 



' A questionable interpretation, to be discussed later. C. C. A. 



