Appalachian Geosyncline. 251 



Mesozoic; and the rivers during later time have carried 

 to the seaboard great quantities of sediments. Wells penetrat- 

 ing the water-bearing strata show that a progressive tilting has 

 gone forward since the close of the Jurassic, depressing under 

 the Coastal Plain the erosion surface of Jurassic age and 

 elevating the Appalachians. The information is most complete 

 in Virginia on an east-west section, shown in fig. 5, passing 

 through Richmond and Fortress Monroe. These two localities 

 are on the two sides of the emerged portion of the Coastal 

 Plain and 72 miles apart. At Fortress Monroe a deep well 

 entered the crystalline floor below the sediments of the Potomac 

 group at 2246 feet below sea level. The fresh-water beds of 

 the Potomac are given there as 1300 feet in thickness, much 

 thicker than at the outcrop. Numerous wells near the margin 

 show that the crystalline basement has erosional irregularities 

 amounting to several hundred feet. Their influence is, however, 

 practically eliminated on this section because of the great depth 

 of the well and its distance from the margin of the Coastal 

 Plain. No warping apart from the minor irregularities is 

 indicated between Richmond and Fortress Monroe as shown on 

 the structure section recently published by the Virginia Geolog- 

 ical Survey. * The Potomac formations, because of their 

 fresh-water nature, have been held by many geologists to have 

 originated in an estuary. Of a seaward barrier there is, however, 

 no evidence. They are seen to thicken as far east as there is 

 any information and the exposed sediments are best interpreted 

 as the landward side of a delta plain skirting the rising 

 Appalachians and built from their waste, f The erosion from 

 the rising side and the deposition on that which was sinking 

 would tend to promote the tilting. The building outward of 

 the continental terrace would, from the standpoint of isostatic 

 adjustment produce the greatest sinking of the floor under 

 the outer part of the terrace, for there the departure from 

 isostatic compensation is greatest. Such a curve to the Potomac 

 floor is furthermore shown by fig. 5 to be necessary in order to 

 carry the floor beneath the fo reset slope of sediments. The 

 adjustments, however, might be by distributive faulting instead 

 of the smooth curve which is shown. It might further be 

 largely due to internal movements which work in the direction 

 of enlarging the ocean basin, and the sediments in that case 

 may keep the surface far above isostatic compensation. 



From these several lines of argument it is to be concluded 

 that the Atlantic continental shelf is a subaqueous terrace built 



* Samuel Sanford : The Underground Water Kesources of the Coastal 

 Plain Province of Virginia, Bull. No. 5. fig. 6. 1913. 



f For a fuller discussion of this view see Barrel!, Ciiteria for the Recognition 

 of Ancient Delta Deposits, Bull. Geol. Soc. Am., xxiii, 405-411, 1912. 



