70 THE GEOLOGY OF ST. MARY's COUNTY 



tionship is nowhere visible in St. Mary's County, for the Eocene beds 

 clip below the surface of the Patuxent Eiver several miles to the north 

 of the borders of this county, but there is every reason to believe that 

 the same relations hold here as in contiguous regions. The exposures 

 of the Calvert formation in this county are neither typical or extensive, 

 but uncovered areas are occasionally to be met with in the valleys of 

 Indian, Trent Hall, Persimmon, Horse Landing, Cat, and Eoslyn creeks 

 in the Patuxent Eiver valle}', and Chaptico and St. Clement creeks in 

 the valley of the Potomac Eiver. 



Strike, Dip and Thickness. 



The strike of the Calvert formation is in general from northeast 

 to southwest, but the outcrop frequently becomes very sinuous, because 

 of erosion and changes in topography. Thus in the northern portion 

 of the county streams have carved out deep valleys, producing a most 

 irregular outcrop, which departs widely from the direction of strike. 



The dip is, as a whole, about 11 feet to the mile toward the south- 

 east. Apart from the exposures in the Calvert Cliffs of this state 

 and the Nomini Cliffs of Virginia, there are no good localities for 

 determining the dip, and as it must be calculated as a whole over 

 extensive regions, many of them beyond the borders of the county, 

 slight changes which may occur are not often brought to light. The 

 Calvert formation is not typically developed in St. Mary's County and, 

 in fact, has almost disappeared beneath the level of tide when it enters 

 the northern borders of the region. It is, therefore, not to be found 

 lying high in the stream valleys, as in Calvert County, but occupies 

 their lower portions and disappears entirely from view in St. Clement 

 and Eoslyn creeks. There is, therefore, little opportunity for studying 

 the dip of the Calvert formation from its exposures in this county. 



The full thickness of the Calvert formation is likewise not developed 

 within the borders of this county and indeed has nowhere been actually 

 observed. The formation has been diagonally truncated above by the 

 Choptank and younger formations under which it lies unconformably. 

 This relation, which is well shown in Calvert, is very much obscured 



