The areas that are included in Southern Fairfax County are Alexandria, Franconia, Fort Belvoir, Huntington, Lorton, Mason Neck, Mount Vernon and Springfield. The division is made by Nesbitt Realty so the clients can better understand the real estate market of Fairfax County. A primer about the geology of the Southern Fairfax County is given below.
The modern Southern Fairfax County landscape is the product of fluvial depositional and erosional processes that began in the late Tertiary or earlier, and have continued essentially uninterrupted to the present time. Major landforms include a prominent series of river terraces that descend step-wise from the highest parts of the landscape to the lowest. All of the terraces were deposited by the Potomac River (or a precursor stream) and its tributaries. The ages of the terraces cannot be directly determined but their deposition is thought to span a roughly 10 million year interval from the middle or late Miocene to the late Pleistocene.
The highest terraces in Southern Fairfax County are characterized by intense weathering profiles and deep ultisols suggestive of considerable age; their counterparts in adjacent parts of northern Virginia, the District of Columbia and Maryland are generally considered to be of late Miocene through Pliocene age. In contrast, the massive Old Town terrace—one of the lowest and most extensive terraces in the Southern Fairfax County almost certainly was deposited during the late Pleistocene (circa 150,000-15,000 ybp), based on detailed geochronology of presumably correlative deposits in the Hybla Valley just south of the Southern Fairfax County.
A variety of much smaller intermediate terraces fall within this broad age spectrum. All but the lowest terraces are extensively dissected by erosion, a process that was undoubtedly accelerated by repeated sea-level lowering and rapid climate changes during multiple Pleistocene glaciations. Late Pleistocene erosion has completely stripped off the terrace sediments at many places, exposing the underlying early Cretaceous Potomac Formation in many ravines and hillsides, as well as the early Paleozoic bedrock in Holmes Run Gorge.
Rapid stream incision, coupled with locally unstable Potomac Formation sediments, has led to many oversteepened slopes characterized by a host of slope processes and deposits, notably landslides and thick colluvium in a variety of forms. Several types of slope deposits and younger alluvial deposits are also found in Southern Fairfax County. In general, colluvium and related slope deposits dominate or strongly influence the soil profile or slope morphology. Virtually the entire landscape in excess of 2% slope is nearly completely covered by debris fans of one kind or another, as most ravine bottoms are by alluvium and colluvium.
