50 MEMOIRS NATIONAL ACADEMY OF SCIENCES. BvoL.xn. 



piracy of a stream which flowed to the Clinch through Big Moccasin Gap. To the west of Clinch 

 Mountain this plane probably moved up the Clinch to about the mouth of Powell River. It is 

 also probable that it was upon this plane that the latest adjustments have been made in the 

 lower courses of the tributaries of the upper Tennessee. The Holston, Nolichucky, and lower 

 French Broad have probably undergone important changes. The afiinities of the lo shells 

 in the lower Nolichucky and those of the lower Holston (near Morristown) are apparently closer 

 than those existing between the French Broad and the Nolichucky, and therefore suggest that 

 the lower Nolichucky may have once flowed to the northwest and joined the Holston, and that 

 later it was captured and diverted by a tributary of the French Broad. This peneplain may 

 provisionally be considered as having been formed during the Pleistocene. 



The 'present cycle of erosion. — The youngest plane in this region, according to Keith ('96, 

 pp. 522-523), is only in its initial stages, and is shown by terraces and bottoms which are formed 

 at an elevation of from 600 to 700 feet in the lower part of the Great Valley. This plane has 

 probably not yet been influential in causing any very important drainage changes. The uplift 

 initiating this latest cycle of erosion will of course hasten erosion in all the streams. 



By way of summary, the position of Keith ('96, p. 523) is well shown in the following 

 quotation : 



Thus in the Tennessee V^alley are seen four distinct groups of peneplains and associated features, marking four 

 periods of stable land and long degradation. The greatest of these is the first, because it extended to the headwaters 

 of the main rivers, and only the most obdurate and remote masses escaped reduction. Each successive period was 

 less important than the preceding as measured by the results accomplished. The forms of any minor period would 

 have been obliterated, however, by a greater subsequent one, so that the record can only be expected to preserve those 

 which were in descending order of magnitude. At the present day the most conspicuous are the 1,600 to 1,800 and the 

 1,000 to 1,100 foot peneplains, which occupy much of the Great Valley. * * * Areas occur in which peneplains are 

 indubitably warped, but they are readily recognized on the ground and are distinctly the exception. In short, erosion 

 has produced in this basin at least four peneplains, each approximately level and each swinging around the heads of 

 the lower plains in successive steps. 



5. GENERAL SUMMARY. 



Let us sum up the history of the Upper Tennessee region and its drainage from the stand- 

 point of lo. The region is a very old land mass, which has, in an orderly manner, been ele- 

 vated, degraded, or base-leveled several times. The process of elevation, although it has not 

 been continuous, has been persistent and cumulative and has perpetuated a condition of un- 

 stable equiUbrium to the rain which fell upon its surface, so that the degradative processes of 

 running water have been continuously active. Furthermore, the variable durability of the 

 rocks, the presence of lime-bearing rocks, the mild chmate, abundant rainfall, and the large 

 streams are physical features which combine to produce the rapid water environment and 

 vital optimum demanded by lo if it persists in a region. 



As the Appalachian Valley belt was elevated and folded in the Permian, the streams flowing 

 westward from Appalachia continued to flow across this bolt as it emerged from the sea.^ 



It is probable that the Permian drainage over this coastal zone was mainly to the west and 

 northwest, plate 57, A, and the New River, even to this day, has persisted, in the main, in this 

 ancient course across the Appalachian Mountains as they arose, showing clearly that its course 

 is older than the mountains themselves. The New Riv.er has thus had not only a very remark- 

 able history, but it has for a correspondingly long period bounded the upper Tennessee drainage 

 area on the northeast and has repeatedly been pirated by the Tennessee system. 



The topography and drainage produced during the Triassic and Jurassic were etched 

 away during the prolonged erosion which developed the Cretaceous peneplain. This relatively 

 perfect peneplain resulted in a relatively balanced condition between the stream which drained 

 from the eastern uplands into the western embayment, and it is probable that at this stage the 

 Appalachian Valley was not differentiated. The three main drainage Unes at this stage are 

 indicated in plate 57, B. 



' These conditions may give us some idea of how the marine ancestors of these shells might have invaded freshwater. These animals, inhabiting 

 a wave-washed, rooky shore, and living in conditions approximating the agitated waters of rapid rivers, might thus have been afforded a favorable 

 opportunity to make a change of habitat. As the zone of rapidly flowing fresh-water migrated inland, on the rising coast with progressive degra- 

 dation, and as the sea migrated to the west with the progress of the uplift, such animals in time would be led far inland. 



