Wissahickon Formation; Boulder Gneiss

(Formerly mapped as Sykesville and Laurel Formations.) Thick-bedded to massive, pebble- and boulder-bearing, arenaceous to pelitic metamorphic rock; typically a medium-grained, garnet-oligoclase-mica-quartz gneiss; locally an intensely foliated gneiss or schist; apparent thickness 15,000 feet.
State Maryland
Name Wissahickon Formation; Boulder Gneiss
Geologic age Late Precambrian (?)
Lithologic constituents
Major
Metamorphic > Gneiss > Muscovite-gneiss
Minor
Metamorphic > Schist > Pelitic-schist
Stratigraphic units Wissahickon Formation; Glenarm Series
References

Cleaves, E.T., Edwards, J., Jr., and Glaser, J.D., 1968, Geologic Map of Maryland: Maryland Geological Survey, Baltimore, Maryland, scale 1:250,000.

In 1995 the Water Resources Division of the U.S. Geological Survey digitized the geologic map of Maryland from the 1968 paper map (Reference MD001). Information about products from the Maryland Geological Survey can be found at http://www.mgs.md.gov/.

NGMDB product
Counties Baltimore - Carroll - Cecil - Harford - Howard - Montgomery - Prince George's