Alluvium Shown only along major streams. 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.
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. 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.