FORMATIONS AND PHYSICAL HISTORY 



605 



great, resulting in increased height of land. The region covered by 

 tin I, a layette formation was elevated relatively, and perhaps some- 

 what deformed. The coast line was probably farther east than now, 

 perhaps at the edge of the con- 

 tinental shelf. To this epoch the 

 submerged continuations of the 

 St. Lawrence, Hudson, Delaware, 

 Susquehanna, and Mississippi 

 valleys are commonly referred. 

 From these submerged valleys it 

 was formerly assumed that the 

 land along the Atlantic seaboard 

 must have stood 2,000 to 3,000 

 feet, or perhaps even 7,000 to 

 12,000 feet 1 above its present 

 level, to allow of their excavation; 

 but it may not be necessary 

 to postulate such extraordinary 

 changes of level. Continental 

 creep (p. 350) along the slope be- 

 .tween the continental platforms 

 and the ocean basins may have 

 lowered the valleys notably as it 

 carried them sea ward, if such creep 

 is a fact. 



In the Mississippi basin also 

 there was notable elevation at the 

 close of the period, though prob- 

 ably less than has sometimes been 

 estimated. It seems possible, or 

 perhaps even probable, that the 

 evolution of the principal physio- 

 graphic features of the interior, so 

 far as due to erosion, is post- 

 Pliocene. 



In the west, too, there were notable closing-Tertiary movements. 

 The plateau region was in process of uplift, periodically, through- 

 out the Tertiary, during which it has been estimated to have under- 

 gone an elevation of 20,000 feet (Dutton), and a degradation of 



1 Spencer, Am. Jour. Sci., Vol. XIX, 1905. 



Fig. 502. Map showing supposed 

 distribution of land and water on the 

 Pacific coast of the United States 

 during the Pliocene period. (Ralph 

 Arnold.) 



