306 GEOLOGY. 



tract skirting the Appalachians is supposed to have been flanked on 

 the seaward side by a peneplain near sea-level, and on the other side by 

 broad, open valleys of low gradient. It is assumed that the upward 

 bowing was felt first in a relatively narrow belt along the predeter- 

 mined axis, that the rise was gradual, and that the rising arch increased 

 its breadth as it rose. Tin 4 first bowing along the axis rejuvenated 

 the head waters of the streams which reached it, and the surface, deeply 

 mantled with residuum accumulated during the peneplaining stage, 

 readily furnished load to the streams in flood stages. When the 

 streams reached that portion of the peneplain not yet affected by 

 the bowing, they found themselves loaded beyond their competency, 

 and gave up part of their load. Thus arose a zone of deposition along 

 the bowed tract, as illustrated in Fig. 461. With continued rise, the 



Fig. 461. — 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 f , b", b" r , successive zones of deposition corresponding to stages of arching a'\ 

 a" , a"' '. In the stage of arching represented by a", the right hand portion of the 

 previous zone 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'" . 



mountainward border of the depositional zone is supposed to have 

 been shifted seaward, and the previous border elevated and subjected 

 to erosion, while the material removed was re-deposited in a new zone 

 farther from the axis of rise. 



Thus the process is presumed to have continued till the border of 

 the lifted tract passed beyond the present sea-coast, after which the 

 whole mantle was subject to erosion, which had reached a notable 

 degree of advancement before the first known glacio-fluvial deposits 

 were laid down. 



The hypothesis requires that the aggradation in each depositional 

 zone, when at its maximum, should develop a plexus of streams com- 

 petent to fill the shallow valleys and spread rather generally over 

 the low divides of the coastal peneplain, where relief was slight. In 

 the region of more pronounced valleys, such as the Tennessee, the 

 valleys were only partially filled. It has generally been assumed 

 that the formation was once continuous in the areas where patches 

 only now remain; but it may be that the higher divides, especially 



