Nesbitt Realty has divided Fairfax County into three subdivisions; Central Fairfax County, Southern Fairfax County, and Northern Fairfax County. This division is made to easily understand the real state market of the area. Central Fairfax County includes the areas of Annandale, Bailey's Crossroads, Burke, Fairfax Station, and Falls Church. Given below is a brief guide about the geology of Central Fairfax County.
The area of Central Fairfax County is part of the coastal plain located just east of the Fall Line separating the coastal plain of Virginia from Piedmont. It is characterized by rolling hills, stream valleys, and heavy red clay soils.
The geological formations found in Central Fairfax County are as follow.
Occoquan Granite (Cambrian-Ordovician)
Occoquan Granite covers 10 % of this area at the surface. It consists of light-gray, medium- to coarse-grained, muscovite-biotite monzogranite and lesser granodiorite and tonalite. In many places, the rock has a strong quartz-rod lineation, and locally, two foliations. The Occoquan has been dated at about 560 Ma (U-Pb zircon; Seiders and others, 1975), 494±14 Ma (Rb-Sr whole-rock; Mose and Nagel, 1982), and 479±9 Ma (U-Pb single-crystal zircon; J.N. Aleinikoff, written communication, 1993).
Pliocene Sand and Gravel (Tertiary)
Pliocene Sand and Gravel covers 4 % of this area at the surface. It consists of interbedded yellowish-orange to reddish-brown gravelly sand, sandy gravel, and fine to coarse sand, poorly to well-sorted, cross-bedded in part, includes lesser amounts of clay and silt in thin to medium beds. Commonly caps drainage divides (altitude 250-170 feet) in the western part of Coastal Plain. The lower part of the unit, showing flaser and lenticular bedding and containing rare to abundant Ophiomorpha nodosa represents deposition in marginal-marine environments and is, in part, a nearshore equivalent of the more downdip, marine facies of the Yorktown Formation. In the northern part of the Coastal Plain, the more poorly sorted and less cleanly washed the upper part of the unit, which lacks fossils, comprises fluvial-deltaic sediments that prograded eastward across the shelf during a regressive phase of the Yorktown. To the south, the upper part of the unit is massively bedded clayey sand in places containing heavy mineral concentrations that average 8 percent or more; the sands are nearshore, beach and dune origin; interstitial clay was derived, in part, from in-situ weathering of feldspar sand. Its thickness is 0 to 50 feet.
Indian Run Formation (Cambrian)
Indian Run Formation covers 2 % of this area at the surface. It has poorly- to well foliated metasedimentary melange consisting of a medium-grained quartz-plagioclase-muscovite-biotite-chlorite-garnet matrix containing quartz "eyes" and a heterogeneous suite of olistoliths. These include foliated Accotink Schist and Lake Barcroft Metasandstone, and foliated ultramafic, meta gabbroic, and felsic and mafic metavolcanic rocks. The Indian Run predates Occoquan Granite.
Falls Church Intrusive Suite (Cambrian-Ordovician)
Falls Church Intrusive Suite covers 2 % of this area at the surface. It consists of tonalite, granodiorite, monzogranite, and trondhjemite. Tonalite, medium-dark-gray, medium- to coarse-grained biotite-hornblende tonalite and biotite tonalite with abundant inclusions of mafic and ultramafic rock. Tonalite is typically well-foliated; in many places, it has a strong quartz-rod lineation. Granodiorite, medium-gray, medium- to coarse-grained, poorly- to moderately-well-foliated, biotite- and biotite-muscovite-bearing. Monzogranite, medium-grained, muscovite-biotite and garnet-muscovite-biotite-bearing. Trondhjemite, light-pink to light-gray, medium-grained, muscovite-bearing; strongly deformed and recrystallized. Tonalite from this suite has been dated at 481±11 Ma (U-Pb single-crystal zircon; J.N. Aleinikoff, written communication, 1993).
Annandale Group - Lake Barcroft Metasandstone (Proterozoic Z-Cambrian)
Lake Barcroft Metasandstone covers 1 % of this area at the surface. Light-greenish-gray to light-gray, medium-grained meta-arenite; and, very-light-gray, fine- to medium-grained metagraywacke. Meta-arenite beds are massive, up to 2 m thick, and probably result from sedimentary amalgamation; metagraywacke beds are regular, sharp, and flat based, containing graded and parallel-laminated intervals; thickness ranges from 10 to 15 cm. The minerals found are (meta-arenite), quartz + epidote + plagioclase + chlorite + muscovite + magnetite; (metagraywacke), quartz + biotite + muscovite + plagioclase + garnet + epidote + magnetite. This unit is about 1200 feet thick; grades downward into Accotick schist.
Annandale Group - Accotink Schist (Proterozoic Z-Cambrian)
Accotink Schist covers 2 % of this area at the surface. It has light-gray, yellowish-gray to moderate-brown to very-pale-orange-weathering schist and interbedded with micaceous metagraywacke. Locally, schist contains randomly scattered, discontinuous beds of meta-arenite. Schist intervals are 20 to 210 cm thick, but individual sedimentation units average about 1 cm. The minerals found are quartz + muscovite + biotite + chlorite + plagioclase; accessory minerals include garnet, magnetite, epidote, apatite, zircon, and pyrite. Micaceous metagraywacke appears to be a more quartzofeldspathic element of a pelitic sedimentary sequence. Unit thickness is unknown because the base is nowhere exposed as the Accotink is intruded by Occoquan Granite.
Popes Head Formation - Station Hills Phyllite Member (Cambrian-Ordovician)
Station Hills Phyllite Member covers 1 % of this area at the surface. It has light-greenish-gray, dusky yellow-weathering phyllite and lesser amounts of very-fine grained metasiltstone. Beds are 2 to 12 cm thick; many have thin basal intervals of graded siltstone. Minerals found in this formation includes muscovite + quartz + biotite + chlorite + plagioclase + magnetite + epidote. The top of the unit is nowhere exposed; the maximum known thickness is 300 m. Some chlorite-rich phyllite is probably mafic metatuff; unit grades down into Old Mill Branch Metasiltstone.
Sykesville Formation (Cambrian)
Sykesville Formation covers 7 % of this area at the surface. The formation consists of 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. These include Mather Gorge Formation migmatite, phyllonite, and metagraywacke; also, ultramafic, metagabbroic, and felsic and mafic metavolcanic rocks, plagiogranite, and quartzite. The Sykesville is intruded by Occoquan Granite.
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