. THE PLIOCENE PERIOD 833 



At the opening of the Pliocene, the Appalachian tract is supposed 

 to have been affected by broad, flat, intermontane valleys, mantled 

 by a deep residual soil and subsoil. The Piedmont tract to the 

 east is supposed to have been a peneplain near sea-level. It 

 is assumed that the upward bowing was felt first in a relatively 

 narrow belt along the axis of the mountain system, that the rise 

 was gradual, and that the rising arch increased in width as time 

 advanced. The first up-bowing rejuvenated the head waters of 

 the streams from the mountain tract, and the surface, with its 

 heavy mantle of residual earth, readily furnished load to the 

 streams. When the streams reached that portion of the peneplain 

 not yet affected, or less affected, by the bowing, they dropped 



Fig. 555. Illustrating the progressive stages of arching described in the 

 text, and the attendant shifting zones of deposition; s-s, sea-level; 

 a, original peneplaned surface with graded slope to sea-coast; a', a", 

 a'", successive stages of arching; b, b', b", b'", successive sites of depo- 

 sition corresponding to stages of arching a', a", a'". In the stage of 

 arching represented by a", the right hand portion of the previous site 

 of deposition is lifted and becomes a part of the area of erosion. The 

 same process is carried farther in the next stage represented by a"'. 



part of their load (Fig. 555). With continued rise, the zone of 

 deposition is supposed to have been shifted seaward, and the 

 deposits already made were eroded and this material redeposited 

 farther from the mountains and nearer the sea. Thus the process 

 is presumed to have continued till the border of the upraised tract 

 passed beyond the present sea-coast ; after which the whole deposit 

 within the area of the present land was subject to erosion, which 

 had reached a notable degree of advancement before the first 

 known glacio-fluvial deposits were laid down. A similar erosion 

 of deposits already made, with redeposition of the materials 

 nearer sea, would have taken place without the seaward widening 

 of the up-arched tract. Fig. 149 shows the sequence of erosion 

 and deposition in this latter case. 



The preceding hypothesis of the Lafayette formation postulates 

 that aggradation in each depositional zone developed a plexus of 



