74 THE GEOLOGY OF ST. MARy's COUNTY 



Character of Materials. 

 The materials composing the Choptank formation are somewhat vari- 

 able. They consist of fine, vellow, quartz sand, greenish-blue sandy clay, 

 slate-colored clay, and at times ledges of indurated rock. In addition 

 to these materials there are abundant fossil remains throughout the 

 formation. The sandy phase is well shown in Drum Cliff, on the Pa- 

 tuxent, where the indurated layers mentioned above are also visible. 

 The clayey phases are not well exposed in this county, but may be seen 

 in Calvert County along the Calvert Cliffs, near Point of Eocks, and 

 southward. Fossil remains are abundant in Drum Cliff and along the 

 branches of Cuckold Creek. 



StratigrapJi ic Relations. 

 The Choptank formation lies unconformably on the Calvert forma- 

 tion. This unconformity is in the nature of an over-lap but is not 

 easily discernible even where the contact is visible. The best place to 

 observe it is in that portion of the Calvert Cliffs just below the mouth 

 of Parker Creek. Even here, the unconformity cannot be seen while 

 standing on the beach but may be observed from a boat a short distance 

 from the shore. There are also certain differences between the fauna 

 of the Calvert and that of the Choptank. How far this unconformity 

 continues down the dip after the beds disappear from view is not known, 

 as the data from well records are too meager to draw any conclusion 

 regarding this question. Above, the Choptank formation lies conform- 

 ably beneath the St. Mary's formation. 



Sub-Divisions. 



The Choptank formation has been subdivided into five zones which 

 may be characterized as follows : 



Zone 16. — This zone varies in composition from yellowish sand to 

 bluish or greenish sandy clay. It is about 10 feet thick and may be 

 found exposed along the Calvert Cliffs from near Parker Creek south- 

 ward to a point a little north of Flag Pond, where it disappears be- 

 neath the beach. Wliere the Choptank first makes its appearance in 



