PALEOZOIC HISTORY OF THE REGION. 479 



The folding was accompanied and followed by erosion, which probably 

 reduced the rising land nearly or quite to sealevel. This period of ero- 

 sion extended over the latter part of the Silurian, but earl 3^ in the Devo- 

 nian the region was again invaded by the sea, and across the truncated 

 edges of the folded strata coarse sands were deposited which now form 

 the Frog Mountain sandstones. The region may have been above sea- 

 level again during the latter part of the Devonian, but if elevation oc- 

 curred at that time it was accompanied by so little folding that practi- 

 cally no erosion took place, and the overlap is indicated onl}^ by the 

 absence of Upper Devonian formations and not by any appreciable un- 

 conformability of strata. During early Carboniferous time the region 

 was again covered by the sea and a continuous layer of cherty limestone 

 deposited over its entire extent. 



The orogenic activity which had manifested itself at intervals during 

 most of Paleozoic time culminated in the great Appalachian revolution 

 probably near the close of the Carboniferous. Considering the entire 

 Appalachian province as a unit, the activity may have been continuous, 

 although in any particular region it apparently consisted of several 

 periods of activity separated by intervals of comparative quiet. During 

 the early part of this complex series of movements in the region under 

 consideration the horizontal comjDression found relief in the formation, 

 first, of simple folds, and then of the minor thrusts. Finally, after a 

 comparatively long period of erosion, the strata thus folded and faulted 

 were disi)laced upon nearly horizontal planes, jjroducing tlie major 

 thrust- faults above descri))ed. 



Post-Paleozoic History of the Region. 



The history sketched above is recorded in the rocks of the region and 

 their underground structure. The post-Paleozoic history, on the other 

 hand, is recorded chiefly in its physiography ; in the forms of surface 

 relief and drainage. The accompanying map represents too small an 

 area for the deci[)hering of this later history, which has been made out 

 from a study of the entire Appalachian province.* 



Evidence has elsewhere been found of two prolonged periods of base- 

 leveling which resulted in the formation of extensive peneplains. The 

 first or Cretaceous peneplain was most perfectly developed, but has been 

 to a considerable extent destroyed by subsequent erosion. In the area 

 mapped the few remnants of this j^lain have an altitude of 1,300 feet. 

 They are found in the crests of the subordinate ridges forming Indian 



*The Geomorphology of the Southern Appalachiai)S, bj'^ C. WiUard Hayes and JIaiius R. Camp- 

 bell. Nat. Geog. Mag., vol. vi, 1894. 



