Maryland Geological Survey 87 



These records indicate a rapid slope near the Fall-line with a lessen- 

 ing seaward, although actual elevation is suggested in the crystalline 

 floor in the Middletown, Delaware, area, which may represent an exten- 

 sion of an axis from Iron, Chestnut, and Grays hills to the southeastward. 

 They also show an actual thinning of the Potomac deposits to the sea- 

 ward, 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 de- 

 position since Potomac time. The conclusion is readily reached that 

 subsidence took place gradually, and that any barriers which existed 

 along the eastern margin of the Potomac basin were depressed below 

 sea level. 



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

 belief in such a barrier, although not adducing any further concrete e%d- 

 dence of the same than the non-marine character of the Potomac sedi- 

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

 may have been comparable in character and extent to the existing penin- 

 sula of Lower California. Another possible, although perhaps less 

 plausible, interpretation of these phenomena is found in the hypothesis 

 of incipient folding in Potomac time. 



Such interpretations as have been suggested in the foregoing discussion 

 may be understood as but an imperfect and more or less speculative at- 

 tempt to reduce to language a long-continued series of events which in 

 the actual complexities of the interacting factors involved baffles de- 

 scription. 



Surface Configuration of Potomac Deposits and its Possible 

 Interpretation 

 The records of deep artesian well borings to the eastward of the Po- 

 tomac belt indicate some clearly defined irregularities in the rate of 

 decline of the Potomac surface. It will be seen from the following table 

 that only a single record shows a greater decline than 25 feet, while 



