No. 2.0 SNAILS OF THE GENUS 10— ADMiS. 49 



Keith ('96, p. 522). Its fragments are here preserved at an altitude of from 1,600 to 1,800 

 feet. This is the best preserved ancient peneplain in the upper part of the Great Valley — along 

 the Powell, Clinch, Holston, and Nolichucky — and into which the present streams are trenched. 

 This peneplain was perhaps formed during the late Eocene or Miocene. 



It was probably late in the formation of this peneplain, or possibly early in the following 

 cycle, that the upper Holston was diverted to the southwest, the North Fork by the upper 

 Holston proper, and the South Fork above the confluence of the Watauga by the lower part 

 of the South Fork. The affinities of some of the spinose lo shells also suggests the possibility 

 that the South Fork and the upper Nolichucky have communicated, and if so, it was probably 

 ended at about this time. 



The southward diversion of the Cumberland-Emory drainage or Loudonensis River thus 

 included the great volume of the westward drainage from the high Appalachians and enables 

 one to see how the course of the lower Tennessee could be maintained through Walden Ridge. 

 At this point Johnson's ('05) paper should be consulted for the detailed evidence favoring 

 the post-Cretaceous persistence of the Tennessee (rather than only a Tertiary cutting) and the 

 formation of the Chattanooga gorge. The character of the meanders across Walden Ridge, 

 and the absence of Tertiary gravels on the Tennessee-Coosa divide, are easily seen to have much 

 weight. At Chattanooga, the Tennessee has been able to lower its channel about 250 feet below 

 the level of the Tertiary peneplain (Hayes and Campbell, '94, p. 91). 



4. THE POST-PLIOCENE AND RECENT CYCLES OF DRAINAGE CHANGES. 



The Lafayette depression along the axis AB, plate 58, must have reduced the rate of erosion 

 to some degree ; but following this, in the late Tertiary, or Pliocene, was an uplift along the axis 

 KL, Hayes and Campbell ('94, p. 94). This uplift was greater to the north and initiated the 

 present cycle of erosion. As expressed by Chamberlin and Salisbury ('06, III, p. 316): 



On the whole, the close of the Pliocene must be looked upon as a time of great crustal movement, a critical period 

 in the history of North America. * * * The Ozarkian epoch, the transition from the Tertiary to the Pleistocene, 

 was, so far as North America is concerned, an epoch of great erosion. 



Toward the close of the formation of the Tertiary peneplain, Cumberland River, according 

 to Hayes and Campbell )'94, p. 108) — 



had cut deeply into the old Cretaceous peneplain and again base-leveled its valley in the soft limestones of the plateau 

 region. It also probably base-leveled a small area of folded rocks in the Appalachian Valley — the present basin of 

 Powell River which then flowed westward through Cumberland Gap. 



Elsewhere Campbell ('94, p. 26) states that the axis KXi had warped the previously formed 

 Tertiary peneplain and diverted the Powell River from the Cumberland drainage. This would 

 place the final diversion of the Powell in the Pliocene or the Ozarkian uplift. These authors 

 do not seem to consider the Clinch and Holston as tributary to the Powell and thus belonging 

 to the Cumberland drainage. 



Another uplift, about the axis AB, tilted the land surface toward the north (Hayes and 

 Campbell, '94, p. 119). This tended to reinforce in part the axis KL, previously mentioned. 

 Early in the present cycle of erosion there was an uplift along the axis MN. This had a tendency 

 to increase the canyon of the French Broad and to hasten the waters of other mountain sections 

 of the Tennessee system. The latest movement, according to Hayes and Campbell ('94, p. 

 95) has been along both the axes KX/ and a reinforcement of OP. 



Whether this uplift was general or local, it hastened the processes of erosion already in 

 operation, caused this plane to begin in the southwest and to migrate progressively to the north-- 

 east, and to destroy the preceding peneplain upon which it was developing. According to 

 Keith ('96, p. 522) this peneplain progressed up the Great Valley and is now a prominent 

 topographic feature near the union of the Nolichucky, Holston, and French Broad Rivers. Here 

 broad bottoms and terraces exist at an altitude between 1,000 and 1,100 feet above the sea. 

 Attenuated portions of this plane appear to reach up the Holston River to Rogersville, Term., 

 and the date of capture of the upper Holston may have taken place at this time, through the 



