Maryland Department of Natural Resources


Geology of the Linganore Nappe in the Westminster Terrane, Western Piedmont of Maryland

2012, Edwards, J.

Report of Investigations 80


This report is a description of the bedrock formations exposed in the upland area of the western Piedmont of Maryland, which includes most of Carroll County, the western parts of Howard, and Montgomery Counties, and the eastern part of Frederick County. Two separate series of rocks have been identified: one is a succession of sedimentary rocks now metamorphosed to phyllite, the other is an assemblage of volcanic rocks with intermingled lenses or layers of marble.

For the sedimentary rock sequence, the following stratigraphic succession has been established: The Sugarloaf Mountain Quartzite is the oldest rock unit, and is exposed only on Sugarloaf Mountain in southern Frederick County. Successively overlying this unit are the Urbana Formation, the Ijamsville Formation, and the Marburg Formation, all of which consist primarily of phyllitic rocks, but which also contain thin beds or layers of quartzite. The Silver Run Limestone is the youngest formation in the succession, and overlies the Marburg Formation in Carroll County.

The volcanic rock sequence includes the Sams Creek Formation and the Wakefield Marble. The Sams Creek Formation originated as a volcanic island built up as a pile of basaltic lava flows and has been metamorphosed to massive metabasalt and chlorite schist. The Wakefield Marble may have originated as carbonate reef rocks but has been recrystallized to fine-grained marble.

Interpretation of the stratigraphic and structural relations of the places the assemblage of volcanic rocks and marble in a large overthrust sheet called the Linganore nappe, which overlies the sedimentary rock succession. In addition, a number of nearly vertical, presumably late, faults have been mapped across the western Piedmont. These faults strike northeast-southwest from the Pennsylvania State line in eastern Carroll County to western Montgomery County.