Dranesville, Fairfax Co., Virginia, USA

Age: Late Triassic (201.3 - 237 Ma)

Stratigraphic Name: Reston Member

Description: Conglomerate, sandstone, and minor siltstone, interbedded in upward-fining sequences. Conglomerate is mainly dark red to gray containing subangular to rounded boulders, cobbles, and pebbles of quartzite, gneiss, vein quartz, and phyllitic schist embedded in a friable matrix of coarse-grained and arkosic sand and interstitial silt and clay. Sandstone is mainly a pinkish-gray, grayish-red and dark-red arkose, fairly well to poorly sorted, locally with a silt and clay matrix. Sandstone in the lower part of the unit occurs in lensoid alternations, with conglomerate in cut-and-fill structures with scoured bases. In the upper part of the unit dark-red to pinkish-gray arkosic sandstone fines upward to siltstone, red-brown to dusky-red, sandy, and micaceous. Unit is unconformable or in fault contact with Piedmont schist; it intertongues with and is conformably overlain by arkosic sandstone and siltstone of the Poolesville Member of the Manassas Sandstone. The thickness of the Reston Member is very irregular, ranging from a feather edge to about 330 ft (100 m). As no fossils are known in this unit, the age is inferred to be Late Triassic based on its stratigraphic position.

Comments: tan; medium bedded; clasts composed of vein quartz; Dominant bedding folation; basal channel deposit of fluvial sands

Lithology: Conglomerate

Reference: Davis, A.M., C.S. Southworth, J.E. Reddy, J.S. Schindler. Geologic Map Database of the Washington DC Area Featuring Data from Three 30 x 60 Minute Quadrangles: Frederick, Washington West, and Fredericksburg. U.S. Geological Survey Open-File Report 01-227. [69]

Age: Neoproterozoic to Cambrian (485.4 - 1000 Ma)

Lithology: Metasedimentary/metavolcanic schist

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Age: Neoproterozoic to Cambrian (485.4 - 1000 Ma)

Stratigraphic Name: Mather Gorge Formation

Description: Schist, greenish-gray to gray, reddish-brown-weathering, fine- to coarse-grained, lustrous, quartz-rich; and much lesser mica gneiss; contains interbedded metagraywacke and some calc-silicate rock; also contains abundant mafic and ultramafic rock debris. Typical mineral assemblages from west to east and from low to high metamorphic grade are: (1) quartz + muscovite + chlorite + plagioclase + epidote + magnetite-hematite; (2) quartz + muscovite + biotite + garnet + staurolite + plagioclase + magnetite ± andalusite; (3) quartz + muscovite + garnet + kyanite + plagioclase + staurolite + magnetite; and (4) quartz + biotite + plagioclase + sillimanite ± microcline + magnetite. Higher-grade schists are migmatitic, and in many places show effects of a retrograde metamorphic over print.

Comments: Western Piedmont (Northern)

Lithology: Major:{schist}, Incidental:{gneiss, metagraywacke, calc silicate rock}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]