Choptank Formation

The Choptank Formation is part of the Chesapeake Group. The formation receives it's name from the Choptank River, Maryland. The Choptank Formation is confined to the Southeastern portion of Prince George's County. The Choptank Formation lies uncon- formably upon the Calvert formation. The unconformity is in the nature of an overlap, but is not easily discernible even where the contact is visible. The best place to observe the unconformity is in the portion of the Calvert Cliffs just below the mouth of Parker Creek. Even here it can not be seen from the beach, but is visible from a boat a short distance from the shore. This unconformity is also proved by the fact that at the above-mentioned locality the fossil- iferous bed which lies lowest in the Choptank formation rests upon the Calvert, while at Mount Harmony, in Calvert County, and farther north the upper fossiliferous hed of the Choptank rests upon the Calvert formation. 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 permit any conclusion to be drawn from them. Above the Choptank is the St. Mary's formation, which is not represented in this county.

Character of materials
The formation is interbedded brown to yellow with very fine-grained to fine-grained sand and gray to dark bluish-green argillaceous silt. It is locally indurated to calcareous sandstone, has prominent shell beds, and has a varying thickness of 0 to 50 feet.

Fossils
Although the Choptank Formation is filled with fossils, they are for the most part confined to two well-developed beds which seem to be distributed very extensively through the areas of the deposit, though not represented in Prince George's County. The fossils of this formation have recently been fully described and illustrated in the two volumes on the Miocene published by the Maryland Geological Survey.