IXXX GEOLOGICAL AND PALEOXTOLOGICAL RELATIONS 



Cliffs, Virginia, it is present as a 50-foot bed between tlie Calvert forma- 

 tion below and the St. Mary's formation above. This is the thickest 

 exposure which is open to direct observation. In the well section at Cris- 

 field, mentioned above in connection with the Calvert formation, the 

 Choptank formation attains a thickness of about 175 feet. It will thus 

 be seen that the Choptank formation, like the Calvert, thickens as it 

 glasses down the dip. 



Character of Materials. 

 The materials composing the Choptank formation are extremely vari- 

 able. They consist of fine, yellow, quartz sand, bluish-green sandy- 

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

 addition to these materials, there are abundant fossil remains dissemi- 

 nated throughout the formation. The sand phase is well shown in 

 the Calvert Cliffs from Parker Creek southward to Point of Eocks. 

 The sandy-clay and clayey members may be seen in the same cliffs near 

 Point of Eocks and southward. The indurated rock is well shown in 

 Drum Cliff on the Patuxent and at Point of Eocks, and the fossil re- 

 mains are seen typically developed on the Choptank river, at Drum 

 Cliff and at Governor Eun. 



StratigrapMc Relat ions. 



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 the unconformity is in that portion of the Calvert Cliffs just 

 below the mouth of Parker Creek. Even here, the unconformity can- 

 not be seen w^hile standing on the beach but may be observed from a 

 boat a short distance from the shore. The unconformity of the Chop- 

 tank on the Calvert formation is also proved from the fact that at the 

 above-mentioned locality the fossil bed wdiich lies lowest in the 

 Choptank formation rests on the Calvert, while at Mt. Harmony and 

 northward the upper fossil bed of the Choptank rests on the Calvert 

 formation. 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 



