Ratings of Soils for Urban Development in Fairfax County
Go directly to the soil ratings table.
The original Soil Survey of Fairfax County was published in May 1963 by the USDA Soil Conservation Service (now the Natural Resources Conservation Service) in cooperation with the Virginia Agricultural Experiment Station (Virginia Tech) and Fairfax County. The survey was based on field work that was completed in 1955. The soil maps were published at a scale of 1"=1,667' (1:20000). This original survey, which is now out of print, contained black and white aerial photographs overlain with the soil maps. Approximately 60 percent of the county was completed at that time. The survey discusses the general soil series as well as characteristics of each individual soil. Although the survey placed a lot of emphasis on agricultural uses, the engineering properties of the soils also were covered.
The county opened a soil science office sometime around 1966. The soil science office transferred the original soil maps to county tax maps which are displayed at a scale of 1"=500'. Until the closure of the soil science office in 1996, that office mapped many previously unmapped tracts of land for rezonings, building permits and special studies. In updating the soil maps, some areas were mapped in the field at a true 500 foot scale or better. The maps were maintained by the soil science office. The most recent soil map was published in 1990 and contained work up to that time.
At the present time, it is estimated that about 40,000 acres of land are unmapped. The county is presently in the process of updating the soil maps for ultimate availability on the county's GIS. Most of the unmapped areas are in the eastern part of the county in Dranesville, Providence, Hunter Mill, Mason, Braddock, Lee and Mount Vernon districts. The largest unmapped parcels are in Lee and Mount Vernon Districts. These two districts also contain the most problematic soils.
Unlike the 1963 soil survey maps, which utilized alphabetical symbols (e.g., Gc for Glenelg), the Fairfax County published maps utilized numbers and alphabetical symbols for the soil units (e.g., 55B2). The soil number is 55; the slope class is B; and the erosion symbol is 2. A more detailed description is provided in this guide.
The soil science office also identified 20 additional soil types that were not covered in the original survey. The soil types are:
- 120-Altavista
- 90,110,112-Augusta
- 26-Bertie
- 23-Captina
- 44-Caroline
- 3-Congaree
- 47-Dragston
- 84-Fallsington
- 6-Hyattsville
- 56-Kempsville
- 51-Keyport
- 86-Klej
- 27,29-Legore
- 35-Manteo
- 118-Marine Clay
- 33-Melvin
- 15-Muck
- 39-Othello
- 68-Roanoke
- 70-State
This guide is to be used in conjunction with the Fairfax County published maps for general estimates of properties only. Site specific investigations are recommended for more precise information. The complete 1990 soil map at a scale of 1"=500' may be obtained from the county's Maps and Publication Center at the Government Center, Suite 156, telephone: 703-324-2974 , TTY 711. The individual sheets may be obtained at the GIS/Mapping Services Branch at the Government Center, Suite 117, telephone: 703-324-2712, TTY 711 . The original (1963) soil survey, which is out of print, may be viewed at the Northern Virginia Soil and Water Conservation District, 12055 Government Center Parkway, Suite 905, telephone: 703-324-1460, TTY 711 .
Questions or comments pertaining to the use of this guide may be directed to:
Environmental and Site Review Division
Department of Public Works and Environmental Services
12055 Government Center Parkway, Suite 535
Fairfax, Virginia 22035
Telephone: 703-324-1720, TTY 711
GEOLOGIC SETTING
The soils in Fairfax County can be divided into three major regions based on geology and physiography. The eastern part of the county is underlain by unconsolidated sediments of the Coastal Plain Province. The central part of the county is underlain by crystalline metamorphic and igneous rocks of the Piedmont Province. The western part is underlain by sedimentary and crystalline rocks of the Triassic Lowland Province.
Most of the area of the Piedmont and Triassic Lowland Provinces are covered by soil and weathered rock which formed from the in-place weathering of the underlying bedrock. The weathering products have physical and engineering properties which generally result from unweathered rock from which they are derived. The coastal plain sediments were transported to the area by water and have not been lithified.
The three distinct regions illustrated in the following map are further described below.
Coastal Plain
The Coastal Plain Physiographic Province occupies approximately 26
percent of Fairfax County, predominantly east of Shirley Memorial Highway
(I-95). The province consists of unconsolidated sand, silt, clay and
gravel strata deposited by ancient oceans and freshwater rivers. The
overall drainage is to the southeast. Drainage patterns are well
developed in the western portion of the province. Broad, nearly level
areas are found in the central (Hybla Valley) and southern (Gunston,
Mason Neck) portions.
Except for the Mason Neck/Gunston area, the Coastal Plain was not included in the 1963 Soil Survey of Fairfax County. Additional areas were mapped by the county. Many of the soils in the Coastal Plain have moderately slow to slow permeability. Drainage restrictions create shallow seasonal high water tables in large areas. High shrink-swell clays are often exposed in areas mapped as marine clay.
Piedmont Upland
The Piedmont Upland Physiographic Province occupies approximately 56
percent of Fairfax County. It occurs in the central portion of the
county, west of the Coastal Plain. The province is underlain by
metamorphic rocks, predominantly schist, granite, gneiss and greenstone.
Remnants of the Coastal Plain terrace may be found on high, broad ridge
tops in the eastern half of the province. A well-dissected, dendritic
drainage pattern occurs throughout the province. The hilltops are
typically fairly wide and rolling, except in places along the lower
tributaries of large streams. Here, V-shaped valleys with steep slopes
and narrow ridge tops occur.
Triassic (Culpeper) Basin
The Triassic, or Culpeper Basin is actually a subprovince of the
Piedmont Upland. It occurs in the west of the county, occupying
approximately 18 percent of the area. The geology consists largely of red
sedimentary (sandstone, siltstone, shale and conglomerate) rocks. Two
horseshoe-shaped intrusions of igneous diabase, diorite and syenite rocks
occur in the vicinity of Herndon and Centreville. The drainage is
somewhat dendritic, but not as well developed as in the Piedmont Upland.
The hilltops are wide and gently rolling, with long gently sloping side
slopes and nearly level areas.
The soils over the red sedimentary rocks are often shallow (2 to 10 feet deep). Large flat areas are often slowly permeable and poorly drained. Soils forming over the igneous bedrock have a distinct plastic clay layer.