NO. 2.] SNAILS OF THE GEmJS 10— ADAMS. 45 



The rate and characteristic details of the erosive piocesses during the Triassic and Jurassic 

 periods is not now recognized, because, as expressed by Chamberlin and Salisbury ('06, III, 

 p. 60): 



Simce the uplifted and deformed Triassic eystem, along with the Appalachain Mountain Region, was essentially 

 baseleveled before the Cretaceous period was far advanced, the intervening Jurassic period must have been a time 

 of great erosion, so far as the Appalachian belt and the Piedmont Plateau to the east were concerned. 



It is therefore probable that most of the topographic features developed during these periods 

 were effaced during the prolonged erosion which resulted in the most perfect peneplain ever 

 developed in this province, the Cretaceous peneplain. 



A characteristic feature of a weU-matxu'ed peneplain near sea level is the truncation and 

 reduction of both the hard and the soft rocks to nearly the same plane. Such a tendency is un- 

 favorable to the preservation or development of distinct surface relief. It is probable that at 

 this time the Appalachian Valley had not been eroded, or if so, was only a slight topographic 

 featm-e. South of New River, to Chattanooga, a marked characteristic of this valley has been 

 its basin-like character, which has been drained only to the westward by transversely flowing 

 streams over a rim of resistant sandstone; and this has effectually prevented the reduction 

 of the valley at a rate greater than the lowering of the outlets. The relative perfection of the 

 Cretaceous peneplain, as shown by the truncation of both hard and soft rocks, suggests that the 

 balanced character of the drainage toward the embayment was quite probable. The alignment 

 of the drainage from the bases of the mountain favors outlets via Cumberland Gap, Emory 

 River, and Chattanooga. These important relations make it possible to estimate the origin 

 of this valley, and the recent investigations of Johnson ('05, p. 218) show that the Tennessee 

 has persisted in its present course across Walden Ridge at Chattanooga since Cretaceous times. 

 These relations seem to indicate clearly the post-Cretaceous erosion or development of this 

 valley. These interpretations are introduced at this point to emphasize the very strong proba- 

 bility of the transverse drainage across this valley throughout the Cretaceous — a view in har- 

 mony with the opinion expressed by White ('04, p. 38). This view favors the interpretation 

 that the development of axial streams was moderate or slight, and that the critical period of 

 transitional development from the transverse to the axial drainage was post-Cretaceous, and 

 was brought about by the destruction of the Cretaceous equilibrium initiated by the Tertiary 

 uplift. As expressed by Campbell ('96, p. 665): 



If streams are in their old age, the surface of the land will constitute a peneplain, and if in extreme old age, this 

 peneplain will approach very closely to baselevel. At such times the drainage basins are delicately balanced against 

 each other; not alone are the systems so balanced, but each individual stream is pitted against its neighbors in a balance 

 so delicate that the least outside influence may turn the scale, and the favored stream conquer the ground now occupied 

 by its neighbors. It is at such times that crustal movements are accompanied by the most profound results; conse- 

 quently we find that a large majority of the changes in the alignment of the drainage systems of the Appalachian region 

 have occurred after a period of extensive baseleveling; they were caused by the first movement which terminated 

 the quiet of the long period of uninterrupted erosion. 



These general drainage relations are indicated in plate 57, B, which summarizes the con- 

 ditions at the close of the Cretaceous. 



The general topographic relations of the Cretaceous peneplain were somewhat as follows: 

 The unreduced areas upon this peneplain, which are now preserved, lie scattered throughout 

 the present Appalachian Valley, generally as small areas, plate 56 (Hayes and Campbell, '94, 

 pi. 5), but in the southern part of the mountains there was, as to-day, an extensive tinreduced 

 area, where the mountains stood at a height of from 3,000 to 3,600 feet (Hayes and Campbell, 

 1894, p. 78); these were deeply eroded or dissected by numerous valleys. The height of these 

 mountains, their relatively abundant rainfall, and drainage to the northwest suggest transverse 

 streams across the Appalachian Valley and northwest or west over Kentucky, as indicated. 

 That all the drainage was not down the valley, as an axial stream, is an opinion expressed by 

 Campbell ('94, p. 29) as follows: 



In Cretaceous time Powell River was the sole surviving member of the Cumberland River system within the 

 Appalachian Valley. 



