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Great Falls Gold District Rocks

The information contained on this page was adapted from Maryland Geological Survey's Geologic Map Montgomery County and District of Columbia (1953). This information reflects geologic interpretations from over 40 years ago and does not represent an accurate interpretation of currently accepted geologic theory. We present this information for historic purposes only. Do not use this information for anything other than illustrative purposes. When a corrected and updated geologic map of Maryland is available you will see a notification on our web site.

For a more recent interpretation of the geology of this area please see Montgomery County Detail 10 and Montgomery County Detail 11 from the 1968 Geologic Maps of Maryland.

The designation "Wissahickon formation" is no longer used in Maryland. The legend presented here was reproduced from the 1953 Geologic Map Montgomery County and District of Columbia but does not reflect current theory. The "Wissahickon' group has been broken up into various divisions by several authors. The nomenclature is somewhat confusing, and is discussed on the SUMMARY OF NOMENCLATURE AND SUBDIVISION OF THE "WISSAHICKON" page. Also, the rocks that were formerly included in the "Wissahickon" are probably Upper Cambrian or Lower Ordovician in age, and not Precambrian.

Alluvium (Quaternery) Alluvium
Shown only along major streams.
Wissahickon formation, oligoclase-mica facies Wissahickon formation, oligoclase-mica facies
Garnetiferous muscovite-quartz schist of variable composition. Banded.
Terrace  gravels(Quaternery) Terrace gravels
Remnants of gravel on terraces bordering Potomac River and larger streams.
Bear Island granodiorite Bear Island granodiorite
Light, discordant granodiorite, mostly unfoliated and massive. Contains plagioclase, quartz, potash feldspar, muscovite, biotite, rutile, zircon and garnet. Composition rather variable. Alteration is common.
Bryn Mawr gravels (Pliocene ?) Bryn Mawr gravels
Ancient alluvial fans sloping gently downward from an altitude of approximately 500 feet above sea level. The Potomac fan includes many pebbles of chert. About 40 feet thick.
Kensington granite gneiss Kensington granitegneiss
Highly foliated, coarse granite gneiss intrusive into the schist complex and basic rocks.
Wissahickon formation Wissahickon formation
Banded or laminated quartz-rich phyllites and schists and magnetite, quartz veins, sandstone and conglomerate beds composed of muscovite, chlorite, albite, quartz. Grading into coarser schist to the east. Includes some Marburg schist. May be the equivalent of Harpers to the west. Calcareous layers are common. Interfingers with Ijamsville phyllite. Ss: sandstone beds.
Basic igneous rocks Basic igneous rocks
t: tonalite with inclusions; m: metadiorites, gabbros, amphibolites; b: undifferentiated basic rocks.
Sykesville formation Sykesville formation
Formerly Sykesville granite. Granitic-looking schistose rock with numerous inclusions, quartz pebbles, garnets, grading into schist east and westward. Probably granitized schist.
Garnite-rich schists

Garnet-rich schists

quartz veins

Updated 10/14/00

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