312 GEOLOGY. 



eastward, perhaps to the edge of the continental shelf, across which 

 streams may have flowed, cutting valleys in its surface. To this 

 epoch, the notable submerged continuations of the St. Lawrence, the 

 Hudson, the Delaware, the Susquehanna, and the Mississippi are com- 

 monly referred. Some of these valleys have great depth, and it has 

 been assumed that their depth was a measure of the elevation of the 

 land at the time they were excavated. But if the considerations set 

 forth in Chap. XX have force, it is not necessary to postulate such extraor- 

 dinary changes of level by uplift and depression. Continental creep 

 along the steep slope between the continental platforms and the oceanic 

 basins may have depressed the valleys notably while it extended 

 them seaward. The earlier assumption that the land along the Atlantic 

 seaboard must have stood 2000 to 3000 feet, or perhaps even 7000 

 to 12000 feet, 1 above its present level, to allow of the excavation of 

 these valleys, seems therefore unnecessary. 



During the post-Lafayette interval of elevation and erosion along 

 the Atlantic coast, much of the material of the mountain-ward edge 

 of the Lafayette formation was shifted seaward, and redeposited along 

 the lower courses of the streams. 



In the Mississippi basin there was also notable elevation at this 

 time. It seems possible, or perhaps even probable, that the evolu- 

 tion of the principal physiographic features of the interior, so far as 

 due to erosion, began with the Ozarkian epoch, though the study 

 of the evolution of the topography of this region has not advanced 

 so far as to make this conclusion certain. The amount of uplift in 

 this region at this time was probably less than has sometimes been 

 estimated. 



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

 The plateau, region was in process of uplift, periodically, throughout 

 the Tertiary, during which it has been estimated to have undergone 

 an elevation of 20,000 feet (Dutton), and a degradation of 12,000, leav- 

 ing it 8000 feet above sea-level. How much of this is assignable to the 

 Sierran epcKjh is uncertain. It was Dutton's view that the Colorado 

 plateau was so elevated at this time as to rejuvenate the Colorado 

 River, and that the cutting of its inner gorge some 3000 feet (maxi- 

 mum) below the outer (p. 275), was the work of later times. More 



1 LeConte, op. cit., and Spencer, Am. Jour. Sci., Vol. XIX, pp. 1-15, 1905. 



