Nesbitt Realty has divided the Fairfax County in Central, Southern, and Northern Fairfax County, so the clients can easily understand the real estate market. The Northern Fairfax County includes the areas of Centreville, Chantilly, Fair Lakes, Great Falls, Mclean, Oakton, Reston, and Vienna. The geological formations found in Northern Fairfax County are as follows.
Alluvium (Quaternary)
Alluvium covers 1 % of this area at the surface. It consists of fine to coarse gravelly sand and sandy gravel, silt, and clay, light- to medium-gray and yellowish-gray. It was deposited mainly in the channel, point-bar, and flood-plain environments and includes sandy deposits of narrow estuarine beaches, and mud, muddy sand, and peat in swamps and in fresh- and brackish-water marshes bordering tidewater rivers. This alluvium is as much as 80 feet thick along major streams.
Bear Island Granodiorite (Ordovician)
Bear Island Granodiorite covers < 0.1 % of this area at the surface. The formation consists of leucocratic, fine-grained, muscovite-biotite granodiorite and related quartz-albite-microcline pegmatite. The unit comprises of small- to moderate-sized sheets and crosscutting bodies. Muscovite from pegmatite has been dated at 469±20 Ma interpreted as a time of cooling below 5000 C.
Popes Head Formation - Station Hills Phyllite Member (Cambrian-Ordovician)
Station Hills Phyllite Member of Popes Head Formation covers 1 % of this area at the surface. The formation is made up of light-greenish-gray, dusky yellow-weathering phyllite and lesser amounts of very-fine grained metasiltstone. Beds are 2 to 12 cm thick and many have thin basal intervals of graded siltstone. The minerals found in the formation are muscovite + quartz + biotite + chlorite + plagioclase + magnetite + epidote. The top of the unit is nowhere exposed and the maximum known thickness is 300 m. Some chlorite-rich phyllite is probably mafic metatuff. The unit grades down into Old Mill Branch Metasiltstone.
Popes Head Formation - Old Mill Branch Metasiltstone Member (Cambrian-Ordovician)
Old Mill Branch Metasiltstone Member of Popes Head Formation covers 6 % of this area at the surface. It has light-greenish-gray, pale-greenish-yellow- or yellowish-gray-weathering, medium to fine-grained graded micaceous metasiltstone and lesser fine-grained micaceous metasandstone. The minerals found in the formation are quartz + muscovite + biotite + plagioclase + chlorite + magnetite + epidote. The unit contains felsic metatuff (mineral assemblage quartz + plagioclase + epidote + muscovite + biotite + chlorite + green amphibole + magnetite) and mafic metatuff (mineral assemblage blue-green amphibole + plagioclase + titanite) layers up to 180 cm thick. The maximum thickness of the unit is about 2300 feet.
Piney Branch Complex (Proterozoic Z-Cambrian)
Piney Branch Complex covers 2 % of this area at the surface. The formation is a heterogeneous assemblage of metamorphosed peridotite, pyroxenite, and gabbro; dominant rock-types include serpentinite, soapstone, and actinolite schist. The unit contains dikes and sheets of plagiogranite. It unconformably underlies the Popes Head Formation. Movement of the Piney Branch began in the Late Proterozoic after the metamorphism of the Peters Creek and ended during the Taconic Orogeny.
Yorkshire Formation (Cambrian)
Yorkshire Formation covers 0.2 % of this area at the surface. The formation is a polygenetic melange consisting of a dark quartz-plagioclase chlorite matrix that contains chips, fragments, and small blocks of quartz, ultramafic rock, metagabbro, plagiogranite, mafic volcanic rock, and other exotic blocks. It is characterized by abundant light-colored feldspar grains that contrast with the dark-colored phyllosilicate component. The Yorkshire forms thin, discontinuous lenses at the base of the Piney Branch Complex.
Sykesville Formation (Cambrian)
Sykesville Formation covers 0.1 % of this area at the surface. The formation is light- to medium-gray, medium-grained metasedimentary melange consisting of a quartzofeldspathic matrix that contains quartz "eyes" and a heterogeneous suite of pebble- to boulder and larger-size olistoliths.
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