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Wissahickon Formation; Boulder Gneiss

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 (?)
Primary rock type gneiss
Secondary rock type schist
Other rock types
Lithologic constituents
Major
Metamorphic > Gneiss > Muscovite-gneiss
Minor
Metamorphic > Metasedimentary > Metaclastic > Pelitic-schist
Map references
Cleaves, E.T., Edwards, J., Jr., Glaser, J.D., 1968, Geologic Map of Maryland: Maryland Geological Survey, Baltimore, Maryland, scale 1:250,000.
Unit references
Cleaves, E.T., Edwards, J., Jr., Glaser, J.D., 1968, Geologic Map of Maryland: Maryland Geological Survey, Baltimore, Maryland, scale 1:250,000.
Unpublished digital version of the Maryland state geologic map that was digitized by the USGS Water Resources Office in Towson, MD
Counties Baltimore - Carroll - Cecil - Harford - Howard - Montgomery - Prince George's