MARYLAND GEOLOGICAL SURVEY 77 



in southeastern Calvert and St. Mary's counties. In this region 

 it is very much obscured by a mantle of younger material be- 

 longing to the Columbia Group and is, therefore, seldom seen on the 

 surface. Good exposures, however, are found along the Bay shore and 

 the PatiLxent Eiver and its tributaries. The most extensive exposure 

 is found in Calvert County along the Bay shore from Point of Eocks 

 to Drum Point. In St. Mary's County the surficial cover of loam and 

 gravel has greatly obscured the St. Mar/s 'formation, and consequently 

 there are few places where it can be observed. But from the vicinity 

 of Hollywood southward to St. Jerome Creek the St. Mary's formation 

 is well developed. In the northern portion of its area it occupies the 

 higher portions of stream valleys and is underlaid by the Choptank 

 formation. Further to the south, in the valley of St. Mary's Kiver, 

 the Choptank formation has disappeared and the St. Mary's formation 

 is the only representative of the Miocene. Good exposures of this 

 formation may be seen on the Patuxent Eiver, near Millstone, on the 

 Bay shore at Langleys Bluff, five miles south of Cedar Point, Avhere it 

 forms the base of a section at beach level beneath the overlying fossil- 

 iferous beds of the Talbot formation. It also occurs in the valley of 

 St. Jerome Creek, and is well developed along the banks of St. Mary's 

 Eiver and its tributaries. At Windmill Point on the St. Mary's Eiver, as 

 well as on the Patuxent, half a mile west of Millstone, the St. Mary's 

 formation contains chisters of gvpsum crystals. 



Strike, Dip and Thickness. 



Tlie strike of the St. Mary's formation, like that of the two preceding 

 ones, is from northeast to southwest. On the Western Shore, because 

 of the great diversity in the topography, the outcrop is extremely irregu- 

 lar and departs very widely from the direction of the strike. The 

 St. Mary's formation rests conformably on the underlying Choptank 

 and is overlain unconformably by younger materials. The dip averages 

 about 10 feet to the mile toward the southeast. 



The thickness of the St. Mary's formation in southern Maryland 

 varies from a few to about 280 feet. In this county, where the formation 

 lies between the Choptank beneath and the Sunderland above, it is 



