110 THE GEOLOGY OF PKINCE GEOKGE S COUNTY 



Chaptico Bay, in St. Mary's County, and on the Virginia side a 

 considerable thickness of the marls is exposed along the entire length 

 of the l^omini Cliffs. When fresh, the Plum Point marls and the 

 Fairhaven diatomaceous earth do not differ much in appearance. 

 The thickness of the marls increases constantly down the dip. This 

 member is subdivided into 12 zones, which are distinguished by the 

 lithologic character of the materials and by characteristic fossils. 



The Clioptank Formation. 



This formation receives its name from Choptank River, Maryland, 

 because of its great development on the northern bank of that estuary 

 a short distance below Dover Bridge. 



Arcal Distribution. — The Choptank formation is confined to the 

 southeastern portion of Prince George's County. It is exposed in 

 the headwaters of the minor streams in the vicinity of Orme and 

 Aquasco. In Calvert County it may be found in a long line of out- 

 crops extending from the hilltops just west of Herring Bay to Patux- 

 ent River, but west of the Patuxent it is almost completely obscured 

 by younger dej)osits. The boundaries of the Choptank formation in 

 Calvert County are better known than in any other portion of south- 

 ern Maryland, but west of the Patuxent have been determined more 

 by calculation than by observation. They are believed, however, to 

 be approximately correct and are fixed as accurately as present 

 knowledge warrants. In its broader relations the formation extends 

 from Virginia northwestward across Maryland and Delaware into 

 ivTew Jersey, where it has an extensive development. 



Character of Materials. — The materials composing the Choptank 

 formation are extremely variable. They consist of fine yellow quartz 

 sand, bluish-green sandy clay, slate-colored clay, and, at some places, 

 ledges of indurated rock. In addition to these materials, abundant 

 fossil remains are disseminated throughout the formation. The 

 sandy phase is characteristic of the formation in this county. 



Paleontologic Character. — Although the Clioj)tank formation is 

 abundantly supplied with fossils, these are for the most part concen- 



