FORMATIONS AND PHYSICAL HISTORY 603 



the mountains and nearer the sea (at b r , Fig. 501). Thus the process 

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

 passed beyond the present sea-coast. The whole deposit within the 

 area of the present land was then eroded, and the erosion had 

 reached a notable degree of advancement before the first known 

 glacio-fluvial deposits were laid down. This hypothesis of the origin 

 of the formation postulates that the shallow valleys of the coastal 

 plain were filled with sediment, and that later the deposits spread 

 rather generally over the low divides between them. In the region 

 of deeper valleys, such as the Tennessee, the valleys were only partly 

 filled. It has been assumed generally that the formation was once 



Fig. 501. 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 deposition corresponding to stages of arching a, 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'". 



continuous east of the mountains where patches only now remain; 

 but it may be that the higher divide's were never covered by the 

 formation. 



The removal and re-deposition of material as suggested by Fig. 

 501 is regarded as an important part of the interpretation of the 

 formation. Erosion and re-deposition of the material did not cease 

 with the Lafayette epoch, but have been in progress ever since, and 

 the derivatives so closely resemble the parent formation in structure 

 and material that their separation is difficult. 



If it shall be shown ultimately that the seaward portions of the 

 Lafayette, now concealed or unstudied, are marine, the preceding 

 hypothesis would need to be modified only by supposing that as 

 the sources of the streams was bowed up, the coastal border of the 

 plain was submerged. In this case, there should have been estuarine 

 formations in the seaward ends of the valleys. 



The chief alternative view relative to the origin of this forma- 

 tion regards it as marine, 1 deposited during a stage of submergence 

 essentially co-extensile with the area of the formation. This 

 hypothesis has been tried out by geologists of wide familiarity with 



1 McGee, i2th Ann. Rept., U. S. Geol. Surv. 



