8. 



SOUTHERN AND 

 CENTRAL APPALACHIANS 



EXTENT AND DIVISIONS 



The southern and central Appalachians extend from Alabama to New 

 York and the Hudson River, and include the area shown on the index 

 map of Fig. 7.1. They will be treated under their three longitudinal divi- 

 sions, the folded and thrust-faulted Appalachian Mountains province, 

 the Blue Ridge Cambrian and Precambrian province, and the Piedmont 

 crystalline province. The use of the words southern and central implies 

 that a northern division is also recognized, but this is referred to as the 

 New England province. New Brunswick and Nova Scotia will be in- 

 cluded in the northern division because of their close geological relation 

 to New England. 



MAJOR ELEMENTS OF STRATIGRAPHY 



Appalachian Geosyncline 



From the time tiiat James Hall contributed voluminously to geologic- 

 literature (1840 to 1860) to about 1920, the following views were widely 

 held regarding the Appalachian geosyncline. It extended from New- 

 foundland to Alabama and beyond, over 3000 miles; subsided most in the 

 site of the present Valley and Ridge province and the eastern side of the 

 Allegheny synclinorium, where more than 30,000 feet of sediments ac- 

 cumulated in places; shallow shelf seas extended inland from the geosyn- 

 cline over the Central Stable Region; and a great borderland, Appala- 

 chia, lay along its southeast side, from which came much of the sediment 

 that filled the subsiding trough. 



Failure to appreciate facies changes and the absence of detailed 

 mapping, especially in the Blue Ridge and Piedmont provinces, militated 

 against a correct understanding of the tectonic development of the region. 

 It appears now that the Blue Ridge province marks approximately the 

 boundary between a west-lying miogeosyncline and an east-lying eugeo- 

 syncline in Cambrian time, but in post-Cambrian Paleozoic time the 

 Blue Ridge and Piedmont were generally emergent. The concept of a 

 borderland that extended beyond the present continental shelf into the 

 Atlantic ocean is discredited. 



Because of the metamorphosed nature of the strata in the Piedmont 

 and the almost complete failure to find fossils in them, the work of 

 unraveling their stratigraphy and structure has been slow. The stratig- 

 raphy of the Valley and Ridge province, however, has received a gnat 

 deal of attention. It will be seen that geosynclinal subsidence in the site 

 of the Appalachians and the plateaus shifted from time to time and 

 place to place so that a strict coincidence of structural divisions and the 

 sedimentary provinces does not exist. In a broad way, however, the 

 western half of the miogeosyncline is undeformed or cast only into very 

 gentle folds — it is structurally the Allegheny svnclinorium and physio- 

 graphically the Plateaus province — whereas the eastern half of the mio- 

 geosyncline is the folded and thrust-fanlted province. 



97 



