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Geology and Hydrology of the USGS Site

Leesburg Conglomerate

Conglomerates are sedimentary rocks that are made up of various fragments of rock interspersed with finer grained material.

Cordierite Hornfels

Hornfels rocks are metamorphic rocks. They usually start out as sedimentary rocks like sandstone or shale, but then volcanic material pushes up.

Cockeysville Marble

Marble is famous metamorphic rock known for its use in sculpture and architecture.

Diopside Crystals

Diopside crystals are made up of Magnesium, Calcium, Silicon, and Oxygen, and are usually found in metamorphic rocks.

The USGS National Center site straddles the boundary of the eastern edge of the Triassic lowland and the margin of the Piedmont crystalline rock province. The Piedmont upland at the east side of the site is underlain by the Peters Creek Schist, a foliated metamorphic rock that is probably 550 to 650 million years old.

The schist is overlain at an erosion unconformity by a layered sequence of conglomerate, sandstone, siltstone, and shale. These sedimentary rocks of Late Triassic age are typically dark red. The sedimentary sequence was intruded by 195-million-year-old diabase, some of which lies 500 meters (1,640 feet) west of the site. The rocks in contact with the diabase intrusion were thermally meta-morphosed (changed by heat) into hornfels, a brittle gray and mauve rock containing abundant green epidote crystals. The layered rocks are locally cut by normal faults and tilted 10 to 30 degrees to the west. The rocks were eroded over time and are weathered near the surface. A veneer of Quaternary alluvial sand, gravel, silt, and clay occupies the two small stream valleys that drain most of the National Center site. 

USGS scientists continuously monitor the groundwater level at several observation wells on the National Center site. Between 1976 and 1980, the wells were drilled or cored to depths of 63 meters (205 feet) to 184 meters (605 feet). An observation well containing a continuous water-level recorder (hydrograph) is northeast of the main building in a small enclosed hut. Display panels explain the hydrograph's operation and provide additional information on the local and regional hydrogeology. The observation well can be reached by taking the Woodland Walk.

Illustration showing rock units on the National Center site.
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Rock units on the USGS National Center siteCenozoic EraAlluvium (Quaternary) — layered stream deposits of sand, gravel, silt, and clay.Mesozoic EraDiabase (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).Precambrian EraSchist (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).

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