ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN 



147 



lasted during all of the Jurassic. By the beginning of Cretaceous time, an 

 extensive and very subdued surface across the folded and thrust-faulted 

 Appalachians, and across the Blue Ridge, the Triassic basins, and the 

 Piedmont had formed. This is known as the fall zone peneplain. Study 

 diagrams 1 and 2 of Fig. 10.8. The entire area as far westward as the 

 plateaus province, according to Johnson ( 1931 ) , was then invaded by 

 shallow epicontinental seas, and in them Cretaceous sediments were de- 

 posited (diagram 3). From subsurface studies of the Coastal Plain sedi- 

 ments, it has been shown that the Lower Cretaceous is entirely buried by 

 the Upper, and it appears that the extensive overlap that Johnson visual- 

 izes occurred in Upper Cretaceous time. Others admit that the Cretaceous 

 extended farther inland than the present erosional margin but do not be- 

 lieve that it extended beyond the Blue Ridge. Johnson and later Strahler 

 (1945) believe the overlap necessary to explain the stream pattern of the 

 Ridge and Valley province. 



The fall zone peneplain was then arched broadly with the crest in the 

 Ridge and Valley and Blue Ridge provinces and the flanks far westward in 

 the plateaus and far eastward in the site of the present Coastal Plain and 

 continental shelf. Another episode of base-leveling followed, which, like 

 the previous one, established an extensively subdued surface, but lower 

 and younger. This is known as the Schooley peneplain. See diagrams 4 

 and 5 of Fig. 10.9. The only remnant of the fall zone peneplain is that 

 buried beneath the Cretaceous sediments of the Coastal Plain. The 

 Schooley surface is now generally recognized in remnants as the highest 

 flat tops of ridges in the Appalachian region. 



Broad arching again occurred, and the Schooley peneplain was dis- 

 sected in the manner represented in diagrams 6 of Fig. 10.9 and 7 of Fig. 

 10.10. A few master streams persisted across the folds and thrusts, while 

 many subsequent streams etched out the resistant formations to produce 

 the first appearance of flat-topped, subparallel, ridges and valleys. The 

 new base level below the flat-topped ridges is known as the Harrisburg 

 peneplain. See diagram 7 of Fig. 10.10. Still third and fourth stages of 

 arching are recognized in the dissection of the Harrisburg peneplain and 

 the establishment of the lower Somerville surface, and the dissection of the 

 Somerville to the present stream bottoms. See diagrams 8 and 9 of Fig. 



10.10. An extensive literature may be found on the geomoiphology <>i the 

 Appalachians, and most premises and conclusions of the above summary 

 of Johnson's work have been contested. Most authorities recognize the 

 vertical uplift, but some contend that a symmetrical arching did not occur. 

 It may also be argued that the arching was a slow, continuous pro< 

 and not one of four stages with interims of standstill. 



Physiographic Provinces of North Atlantic Floor 



Echo sound tracts of fifty expeditions in the North Atlantic including 

 over 200,000 miles by vessels of the Lamont Geological Observatory with 

 the Luskin precision depth recorder were compiled by Heezen ct at 

 (1959), and a physiographic relief map of the ocean floor was prepared. 

 From it the physiographic provinces are resolved as shown on the map of 

 Fig. 10.11. Profiles to accompany the map are reproduced in Fig. 10.12. 

 There are three major divisions, each with its subdivisions as follows: 



Continental Margin 

 Category I 



Continental Shelf 



Epicontinental Seas 



Marginal Plateaus 

 Category II 



Continental Slope 



Marginal Escarpments 



Landward Slopes of Trenches 

 Category HI 



Continental Rise 



Marginal Trench-Onter Ridge Complex 



Marginal Rasin-Outer Ridge Complex 

 Ocean Rasin Floor 

 Abyssal floor 



Abyssal Plains 



Abyssal Hills 



Abyssal Gaps and Mid-Ocean Canyons 

 Oceanic Rises 

 Seamount Groups 

 Mid-Oceanic Ridge 

 Crest Provinces 



Rift Valley 



Rift Mountains 



High Fractured Plateau 



