584 E. W. SHAAY AGES OF APPALACHIAN PENEPLAINS 



and to give some indication of the form and seaward extent of the sub- 

 merged portion. They are based on a large number of observations made 

 along the outcrops of the various beds and in the drilling of wells. They 

 show a generalh" uniform seaward slope of the floor under the Cretaceous 

 of 25 to 30 feet to the mile. Data on the slope of the base of the Tertiary 

 are not so abundant, but apparently this slope is more like that at the 

 base of the Cretaceous than like the present surface, the average general 

 seaward slope of which is less than four feet to the mile. 



The extent of the submerged portion of the Coastal Plain deposits can 

 only be conjectured, still from a study of the fairly definite information 

 concerning the land portion, the shape of the continental shelf, 'and the 

 known Cretaceous and Cenozoic history of the Atlantic coast some infer- 

 ences can be drawn which carry a rather heavy weight of probability. 

 On such a basis, involving seaward extrapolation of the base of the de- 

 posits, the following table has been compiled : 



Average Cross-section Area of Atlantic Coastal Plain Deposits 



Land portion 760 million square feet (estimated) 



Between coast and 50-fathom contour. 840 million square feet (conjectural) 

 Between 50-fatliom and 400-fatliom 



contours 400 million square feet (conjectural) 



Between 400- fathom and 1,500-fatliom 



contours 200 million square feet (conjectural) 



Total 2,200 million square feet (conjectural) 



Even in the Gulf embayment, where the deposits extend much farther 

 inland and the base slopes not toward the sea, but more nearly toward the 

 axis of the embayment, the rate of decline is still the same, being gen- 

 erally from 27 to 30 feet to the mile. A dip section along a curving line 

 from the northeast corner of Mississippi southwest and south would show 

 the altitude of the inner border of the Coastal Plain about 850 feet, the 

 distance to the present coast 440 miles, to the — 300-foot contour 500 

 miles, and the — 2,400-foot contour 530 miles, figures more than twice as 

 great as those for corresponding features given in the above table. But 

 the slope of the peneplain on which the Coastal Plain sediments r^st is 

 certainly about 30 feet to the mile for 100 or 200 miles, and several deep 

 wells recently drilled near Vicksburg and Jackson, together with other 

 wells in Louisiana that although thousands of feet deep do not reach 

 strata within thousands of feet of the base of the Cretaceous, indicate that 

 this slope continues nearly, if not quite, to the GwlL The Coastal Plain 

 deposits at Vicksburg are probably 4,000 to 4,500 feet thick and at the 



