524 PROCEEDINGS OP PHILADELPHIA MEETING. 



on through the Etowah basin, descending to 1,700 and 1,600 feet. In the hitter 

 basin the 1,600-foot penepkiin is shortly cut out by the 1,100 to 1,200-foot plain, 

 which gradually descends to 1,100 and 1,000 feet with the fall of the river, con- 

 tinuing at that height for nearly 100 miles. In this section the abundance of re- 

 siduals has necessarily increased the grades of the streams and produced the sem- 

 blance of warped surfaces, but the abruptness of the breaks from plain to plain and 

 the direction of the slope away from the divides in most cases remove the possi- 

 bility of warping and make it necessary to distinguish the plains as separate. 



Northwest of Knoxville, Tennessee, the four series show the well developed steps 

 within a radius of eight miles, and in innumerable instances groups of three ai)pear 

 in close connection. In each group of forms the sequence from dissected peneplain 

 through the peneplain into the broad valley, bottom and canyon is normal and 

 complete, and the beginnings of each lower plain cut deep into the heart of the 

 plain next al)ove it. In its broad reaches each plain is remarkably constant in 

 level, but in its narrow portions, where grades are raised by debris from neighbor- 

 ing residuals, the slope upstream is invariably found, regardless of the direction of 

 flow. Areas occur in which i)eneplains are indubitably warped, but they arc 

 readily recognized on the ground and are distinctly the exception. In short, ero- 

 sion has produced in this basin at least four peneplains, each approximately level 

 and each swinging around the iieads of the lower plains in successive steps. 



Sl'M.MARV 



Study of other river basins reveals similar series in every case, and without citing 

 the countless details thus far known it is suflicient to state in brief that Appalachian 

 degradation was marked by at least seven periods of approximate reduction. Each 

 of these produced a vast series of peneplains which ai)pear in various forms at the 

 present day ; the oldest lie along the main divides and the youngest along the mar- 

 gin of the sea. In some cases these plains have been warped from their original 

 level, but far the greater portions of them retain nearly their original attitudes. It 

 follows, therefore, that the disturbances which caused the revival of erosion were 

 characterized by broad, uniform ui)lifts with local zones of warping quite subordi- 

 nate in area. This is typified by the i)eneplain in ^Maryland and eastern Virginia, 

 which ranges for 100 miles east and west and many times that distance to the south- 

 west at an altitude of ")00 to 550 feet, but which along the Staunton river in southern 

 Virginia slopes up northwest 400 feet in iJO miles and then remains level for the 

 next 30 miles. 



This view departs considerably from the theories of other authors, who have de- 

 fined the peneplains as dome-shaped or warped surfaces, making the warping the 

 predominant feature of the uplift. Further differences of view exist in the distinction 

 of many peneplains instead of a few. It is agreed that the higher peneplains are 

 associated with the main divides; that fact, however, is as well explained a priori 

 by the usual sequence of erosion as by the theory of warping. The peneplains must 

 have been deformed, otherwise the land never could have risen ; but, on the other 

 hand, the peneplains must rise with the streams and in regions of massive residuals 

 with a considerable slope ; therefore the diflerences of level must be studied in 

 each group and may be referred to one or both causes. The normal peneplain sur- 

 face is slightly sloping, and for each slope to which a different origin is ascribed 

 proof must be furnished to account for the abnormal condition. The present con- 

 fusion has been caused, in part at least, by correlating as parts of the same pene- 



