188 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



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Characteristic local section. 203 



Economic products , 204 



Interpretation of the Potomac deposits 204 



Surface configuration of crystalline floor and its relation to Potomac basin of 



deposition 209 



Surface configuration of Potomac group and its possible interpretation 211 



Age of the Potomac deposits , 212 



Study of scientific and economic collections 214 



Introduction 



Along the eastern border of the Piedmont plateau, lying for the most 

 part directly on the flanks of its ancient crystallines and constituting the 

 basal element of the Atlantic Coastal plain, is a series of mostly uncon- 

 solidated, arenaceous, argillaceous, and often ferruginous sediments of 

 highly varied character. The outcrop constitutes a relatively narrow 

 belt, extending from cape Cod to the Mississippi basin and ranging from 

 a few to 20 miles in width, its landward boundary lying somewhat west- 

 ward of the so-called Atlantic Fall line. 



The general strike of the beds along the Atlantic border is northeast- 

 southwest, the belt being divisible into three districts — northern, middle, 

 and southern — whose strike is progressively more and more southward 

 in passing from the northern district to the southern. The normal dip 

 of the beds within the area of outcrop ranges from 30 to 60 feet per 

 mile, a well marked increase occurring toward the landward margin and 

 a decrease to the seaward. The thickness of the deposits, at the point 

 where they pass beneath tide, ranges from 100 to 1,000 feet, dependent 

 largely on whether the full sequence of formations is present or not. 

 The main body of the deposits lies below tidelevel, although the strata 

 occasionally reach 500 feet above sealevel along the western margin of 

 the Coastal plain. 



The belt crosses all of the principal waterways of the Atlantic slope, 

 by which it is divided into a series of broad, low watersheds. The higher 

 elevations have relatively steep and often irregular slopes, the region be- 

 ing one of comparative youth. The streams, when not tidal, are active, 

 but the belt is frequently cut to its western margin by tidal estuaries. 



The flora of these deposits includes thallophytes, liverworts, equiseta, 

 ferns, cycads, conifers, monocotyledons, and dicotyledons. A conspicu- 

 ous feature of the earlier and middle floras was the cycadaceous, one of 

 the most important organic facts, and one which points as well to the 

 prevalence of a subtropical climate as to the Mesozoic age of the deposits. 



