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Rock units on the USGS National Center site
Alluvium (Quaternary) — layered stream deposits of sand, gravel, silt, and clay.
Diabase (Jurassic) — about 195-million-year-old, dark-colored, intrusive igneous rock composed primarily of plagioclase feldspar and pyroxene.
Hornfels (Jurassic) — about 195-million-year-old, gray to mauve metamorphic rock. Diabase intrusion changed the original Triassic shale and siltstone from soft sediments to hard, brittle hornfels.
Sandstone (Triassic) — about 220-million-year-old, red-brown to gray, feldspar and mica-bearing sandstone interbedded with siltstone and shale (member of Manassas Sandstone).
Conglomerate (Triassic) — crudely bedded quartz and schist pebbles in a sandstone and shale matrix (member of Manassas Sandstone).
Schist (Late Proterozoic) — about 550- to 650-million-year- shiny dark green to gray, foliated metamorphic rock containing mica, chlorite, feldspar, and quartz; commonly cut by quartz veins (Peters Creek Schist).