210 



CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



The following wells of the middle Atlantic slope reach the crystalline 

 rocks and show the following rates of descent of the crystalline floor : 



Location of well. 





l-> £ 





om poin 

 Igonkiai 

 reache 

 1. 



& o 



O 



0) 



ft 



-1-3 



s 



bJO a>_; 



£ © 



«<1 o £ 



<|o» 



0J^2 



8 u£® 



CHrH 



«4H " 



istan 

 whe 

 s ui- 

 tide 



epth 

 sur 

 tide 



o 



Q 



Q 



cd 



Miles. 



Feet. 



Feet. 



3 



4 



150 



200 



2 



111 



55+ 



2 



187 



93.5 



2 



270 



135 



2 



210 



105 



5* 







12 



452 



37.7 



72 



1,162 



15.7 



1 



o 





Ice works, South Wolf street, Balti 



more 



Farnhurst, Delaware , 



Baltimore copper works 



Sandy point, Virginia 



Quantico, Virginia . . . . 



Indian Head, Maryland 



Middletown, Delaware 



North End point, Virginia 



Feet. 



*122 



*211 

 Unknown 



*170 



*210 

 421.5+ 

 302=h 

 252 



These records indicate a rapid decline near the Fall line in all the 

 landward wells and a marked prominence in the crystalline floor in the 

 Middletown, Delaware, area, which may represent an extension of an 

 axis from Iron, Chestnut, and Grays hills to the southeastward. They 

 also show an actual thinning of the Potomac deposits to the seaward, 

 as shown by the well at North End point, where the thickness of the 

 Potomac beds is only one-half the normal thickness at the outcrop. 



The record of the well borings becomes of the highest significance 

 when it is remembered that this crystalline surface has been receiving 

 along its seaward margin progressively greater and greater loading- 

 through deposition since Potomac time. The conclusion is readily 

 reached that subsidence gradually took place, and that the land barrier 

 along the eastern margin of the Potomac basin was depressed below sea- 

 level. 



Marsh and McGee, as well as most other writers, have expressed their 

 belief in such a barrier, although without adducing any further concrete 

 evidence of the same than the estuarine character of the Potomac sedi- 

 ments. McGee has suggested, as above stated, that the Potomac barrier 

 may have been comparable in character and extent to the existing 

 peninsula of Lower California. 



Another possible, although perhaps less plausible, interpretation of 



*The full sequence of Potomac deposits is not penetrated in the Baltimore, Farnhurst, Sandy 

 Point, and Quantico wells. 



