Maryland Department of Natural Resources


The Piedmont crystalline rocks at Bear Island, Potomac River, Maryland

1975, Fisher, G.W.

Guidebook 4


The Maryland Piedmont is underlain by a thick sequence of metamorphosed sedimentary and volcanic rocks, the Glenarm Series, mantling an 1100 my-old basement complex, the Baltimore Gneiss (Table 1). The principal stratigraphic unit in the Glenarm Series, the Wissahickon Formation, has been subdivided into five mappable facies on the basis of inferred original lithology (Table 1). The rocks along the Potomac River at Bear Island Provide outstanding exposures of the pelitic schist facies and the metagraywacke facies, which have been studied by numerous authors (Cloos and Anderson, 1950; Fisher, 1963; Reed and Jolly, 1963; Hopson, 1964; Reed and Reed, 1967; Reed, Marvin arid Magnum, 1970; and Fisher, 1970a). Because they have been so intensively studied, these rocks have played a central role in current interpretations of Piedmont geology.

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Guidebook 4 (pdf, 14 MB)