106 THE GEOLOGY OF PRINCE GEORGE S COUNTY 



The Woodstock member is further subdivided into two zones dis- 

 tinguished by characteristic fossils. These zones are described in the 

 above-mentioned report on the Maryland Eocene, which also contains 

 a full list of the fossils which characterize the member. 



The Miocene Formations, 

 the chesapeake group. 

 The Calvert Formation. 



This formation receives its name from Calvert County, Md., 

 where, in the well-known Calvert Cliffs bordering Chesapeake Bay, 

 its typical characters are well shown. 



Areal Distribution. — The Calvert is by far the most extensive 

 formation in Prince George's County. Although it is largely cov- 

 ered with Lafayette and Columbia gravels, yet stream erosion has 

 cut down to it in so many places that its distribution is very well 

 known. It outcrops in nearly every stream-cutting throughout the 

 southern half of the county and is represented by outliers well up 

 on the divides over a large portion of the northern half. In its 

 larger distribution it extends from Virginia northeastward across 

 Maryland and Delaware into New Jersey. It has by far the most 

 extensive development of all of the Cretaceous and Tertiary forma- 

 tions in this region. 



Character of Materials. — The materials constituting the Calvert 

 formation consist of blue, drab, and yellow clay, yellow to gray sand, 

 gray to white diatomaceous earth, and calcareous marl. Between 

 these all gradations exist. The diatomaceous earth gradually passes 

 into fine sand by the increase of arenaceous material, or into a clay 

 by the addition of argillaceous matter. In a similar way a sand 

 deposit with little or no clay grades over into a deposit of clay in 

 which the presence of sand can not be detected. ISTotwithstanding 

 this variety of materials a certain sequence of deposits is commonly 

 observed ; the basal portions of the formation consist largely of 

 diatomaceous earth, while the upper portions are composed chiefly 

 of sand, clavs, and marls. This difference in materials has led to 



