Geologic Maps of Maryland: Great Falls Gold District Rocks

	Geologic Map Legends
*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.

PLEASE NOTE: 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 Shown only along major streams.		Wissahickon formation, oligoclase-mica facies Garnetiferous muscovite-quartz schist of variable composition. Banded.
Terrace gravels Remnants of gravel on terraces bordering Potomac River and larger streams.		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 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 granitegneiss Highly foliated, coarse granite gneiss intrusive into the schist complex and basic rocks.
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 t: tonalite with inclusions; m: metadiorites, gabbros, amphibolites; b: undifferentiated basic rocks.
Sykesville formation Formerly Sykesville granite. Granitic-looking schistose rock with numerous inclusions, quartz pebbles, garnets, grading into schist east and westward. Probably granitized schist.		Garnet-rich schists
	p q ss	pegmatite quartz veins sandstone

Updated 10/14/00

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