86 The Lower Cretaceous Deposits of Maryland 



of the subjacent feldspathic rock in the process of its decomposition into 

 the residual kaolin mined at this point. Though the scale on which 

 the folding occurs is small, the phenomena afford a suggestion as to the 

 possible causes of some of the lesser irregularities in the Patuxent beds 

 which lie near the crystalline floor. 



Surface Configuration of Crystalline Floor and its relation to Potomac 



Basin of Deposition 



The basal beds of the Potomac Group rest on a more or less uneven 

 surface of crystalline rocks, in which certain of the more important 

 drainage lines of the present day were already established, as is shown 

 both at the marginal contact and by the well borings near the landward 

 border of the formations. 



The great increase in the dip of the Patuxent and succeeding forma- 

 tions along the Pall-line has already been alluded to, as well as the 

 evidence that it represents in part at least a fault scarp. 



It is significant, however, that there is a marked though less pro- 

 nounced decline in the dip of the strata eastward of the Fall-line all 

 the way to the seaward margin of the Coastal Plain. The evidence for 

 this is furnished by the deep-well borings in Delaware, Maryland, and 

 Virginia, the number of which is not as great as could be desired, al- 

 though they all show, without exception, a progressively lessened dip of 

 the beds as the distance from the landward margin increases. 



The following wells of the middle Atlantic slope reach the crystalline 



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



Distance from t^^^+i, „* „_„„ 

 point where crys- ^tlCne S ^^^te of 

 Location of well. talline rock a.i^fiJrfh^irL descent 



surface reaches ®"«fli'f„^®i ^ per mile, 

 tide level. "'^^ ^®^®^- 



Miles. Feet. Feet. 



Ice works, South Wolf Street, 



Baltimore % 151 200 



Baltimore Copper Works, Balti- 

 more 



Farnhurst, Delaware 



Middletown, Delaware 



Sandy Point, Virginia 



Quantico, Virginia 



North End Point, Virginia 



2 



187 



93.5 



2 



111 



55-1- 



12 



452 



37.7 



2 



270 



135 



2 



210 



105 



72 



1,162 



15.7 



