THE PHYSIOGRAPHY AND GEOLOGY OF THE 

 COASTAL PLAIN PROVINCE OF VIRGINIA 



BY WM. BULLOCK CLARK AND BENJAMIN LE ROY MILLER. 



INTRODUCTION 



Geographers have long recognized three physiographic regions within the 

 Middle Atlantic slope. They are known, beginning on the west, as the 

 Appalachian Mountains, the Piedmont Plateau, and the Coastal Plain. 

 While each one of these regions has its own peculiar character] ?tics, it never- 

 theless passes into that adjoining by insensible gradations. 



The Appalachian Mountain region is composed of flat-topped ridges 

 separated by deep, steep-sided and flat-bottomed valleys which have been 

 carved from folded beds of limestone, sandstone, and shale of Paleozoic age. 



The Piedmont Plateau exhibits a rolling surface which, along its eastern 

 margin, is dissected by deep river gorges. It consists of metamorphosed 

 sediments of pre- Cambrian and early Paleozoic age, into which great 

 masses of igneous rocks have been injected. Overlying these ancient rocks 

 in certain regions are Triassic beds which are in turn cut by eruptive rocks. 

 The structure of the Piedmont Plateau is exceedingly complicated and has 

 only been thoroughly worked out in a few regions. 



The Coastal Plain also has a rolling topography along its western margin, 

 where it blends with the Piedmont Plateau, but throughout most of its 

 eastern half it is flat and featureless. The deposits of the Coastal Plain 

 are much younger than those of the other two regions and consist of uncon- 

 solidated sediments of late Mesozoic and Cenozoic age which have suffered 

 few disturbances since their deposition. 



The boundary of the Atlantic Coastal Plain to the eastward is marked by 

 the well-defined scarp of the continental shelf which, off the Virginia coast, 

 lies from 30 to 50 miles to the eastward of the present shore-line. It forms 

 an escarpment along the western portion of the true oceanic basin with a 

 height of from 5,000 to 10,000 feet or more. By common practice the 

 100-fathom line is regarded as the boundary of the continental shelf 

 although the depth of the water is often nearly twice that amount where the 

 abrupt change occurs. The descent of the slope to the greater ocean depths 

 is rapid ; at Cape Hatteras there is an increase in depth of 9,000 feet in 13 



