7. 



APPALACHIAN MOUNTAINS 



MAJOR STRUCTURAL DIVISIONS 



The index map of Fig. 7.1 shows the structural divisions of the east- 

 ern margin of the continent from New York to Alabama. The interior 

 stable region of the continent is represented by the Appalachian plateaus 

 province, where the strata are nearly horizontal and dissected by an 

 elaborate arborescent drainage system. In southern New York, central 

 Pennsylvania, and northern West Virginia, the strata are cast into very 

 gentle folds which are the site of extensive gas and oil fields. The folding 

 is so gentle that the drainage is little affected, and the arborescent plateau 

 type exists, scarcely distinguishable from the region farther west. 



The folded and thrust-faulted province represents the Appalachian 

 Mountains proper. It is the well-known region of flat-topped, parallel, or 

 subparallel ridges and valleys that are carved out of anticlines, synclines, 

 and thrust sheets. The drainage pattern is rectangular (trellis), and stands 

 conspicuously apart from the arborescent pattern of the Appalachian 

 plateaus. The strata are of Paleozoic age in both provinces but thicken 

 from the shelf along the western margin of the plateaus to the geosyn- 

 cline in the eastern part of the plateaus and in the folded and thrust- 

 faulted belt. See Fig. 7.2. 



The Blue Ridge province is made up of Cambrian and Late and prob- 

 ably Early Precambrian metamorphic and igneous rocks, which are older 

 than those of the Appalachians to the west, and are more or less meta- 

 morphosed. It is widest in the south, and highest in the Great Smoky 

 Mountains of Tennessee and North Carolina (Fig. 7.1). It dies out in 

 southern Pennsylvania only to take up again in eastern Pennsylvania, 

 New Jersey, and New York. The Blue Ridge province is generally one of 

 conspicuous relief east of the Great Valley of the folded Appalachians 

 and west of the crystalline Piedmont. The Piedmont province is broad and 

 generally of low relief. Its rocks are not well exposed and, as yet, 

 thoroughly known in only a few places. They are chiefly metamorphosed 

 Precambrian and Paleozoic sediments and volcanics, and Paleozoic 

 plutons, a number of which are of batholithic proportions. 



Several long, narrow basins of Triassic sediments rest unconformably 

 on the older rocks of the Piedmont, and in one place on the Blue Ridge 

 belt. They are down-faulted troughs, all apparently part of a major fault 

 or rift zone. The Triassic sediments are mostly red standstones and shales, 

 and are cut by numerous large dikes and sills of diabase, also of Triassic 

 age. 



The Atlantic Coastal Plain is a continuation of the Gulf Coastal Plain, 

 and is made up of Cretaceous and Tertiary sediments that rest uncon- 

 formably on the older rocks of all the structural systems of the Appala- 

 chian Mountains. They overlap the Triassic deposits slightly in New 

 Jersey. They dip gently seaward and probably extend out under water 



91 



