MARYLA^-D GEOLOGICAL SURVEY 109 



which carries casts of Pecten liumphveysii and other Miocene fossils. 

 Above this sandstone is the diatomaceons earth proper. This bed, 

 which is abont 20 feet thick, is greenish blue when fresh, but 

 weathers to a brown or a light-buff to white color on long exposure 

 to the atmosphere. In the extensive pits at Lyons Creek wharf in 

 Calvert County, just across the river from this county, where the 

 material is worked commercially, the transition from greenish blue 

 to buff is very conspicuous. 



From Fairhaven the diatomaceons beds cross southern Maryland 

 in a northeast-southwest direction, following the line of strike, and 

 are worked at Lyons Creek on the Patuxent, and at Popes Creek on 

 the Potomac in Charles County. They may 'also be found at numer- 

 ous places l)etween these points, in cuts made by waterways. South- 

 east of this diagonal line they gradually disappear below tide. The 

 Fairhaven diatomaceons earth is further subdivided into three zones 

 that are recognized by the materials and fossils which they contain. 



The Plum Point marls constitute the remainder of the Calvert 

 formation above the Fairhaven diatomaceons earth. At Plum Point, 

 Calvert County, the beds are typically developed, and this fact has 

 suggested the name of this member. It consists of a series of sandy 

 clays and marls in which are embedded large numbers of organic 

 remains, including diatoms. The color of the material is bluish 

 green to grayish brown and l)uft". Fossil remains, although abundant 

 through the entire member, are particularly numerous in two prom- 

 inent beds, from 30 to 35 feet apart, in the Calvert Cliffs. These 

 nun-Is vary in thickness from -il/2 to 13 feet. Along Patuxent River 

 the Plum Point marls are not exposed so extensively as in the Cal- 

 vert Cliffs, but they are visible at intervals from the cliffs below 

 Lower Marllioro southward to Ben Creek, in Calvert County. On 

 the west bank of the river they may be seen here and there from a 

 point opposite Lower Marlboro downstream to a point fl/ii miles 

 below Forest Wharf, in St. Mary's County. 



Along the Potomac Eiver, the banks are usually very low and 

 composed of Columbia sand and gravel. In consequence of this the 

 Plum Point marls are exposed at but few places. (3n the Maryland 

 side of the river they mav be seen in the low cliffs at the mouth of 



