The Five Most Interesting Soils of Fairfax County


A Completely Subjective List of the Five Most Interesting Soils in Fairfax County

By Dan Schwartz, Soil Scientist

How do you separate the stars from the mediocrities, the wheat from the chaff, the clay from the silt?

Fairfax County is lucky to have an unusually wide variety of soils. There are 108 unique soil types listed in the most recent county-wide soil map. Part of the reason for this abundance is the County’s diverse geology: we have areas of igneous, metamorphic and sedimentary bedrock, and the land east of Interstate 95 has no bedrock, but rather deep, variable bands of marine sediment. Rock and sediment are the “parent material” from which soil forms, and each unique parent material weathers and erodes into unique soil. Another reason for the abundance is development. Soils are identified by their unique natural layers. When land is developed, those layers are mixed together and a new – but similar – soil is formed. The county soil maps identify both the natural soils as well as these newly created “human disturbed” soils.

But out of these 108 soil types, how do you separate the stars from the mediocrities, the wheat from the chaff, the clay from the silt? Fear not! Listed below you will find five standout soils, chosen with unrepentant subjectivity, that shine for a variety of reasons. Some are great soils, easy to work and easy on the eye; others are justly infamous, pity the poor developer who finds them on his newly acquired tract; while others are under-appreciated gems that don’t get the respect they deserve. So without further ado, the top 5 soils of Fairfax County!

Sassafras-Marumsco Complex

How much damage can shrinking and swelling clay really cause? Plenty.

Complex is a good word to use in association with this soil, because if you find it on your property, it may give you a mental complex. This mixture of two soil types – the innocuous Sassafras sandy loam and the notorious Marumsco silt loam – is commonly known as Marine Clay. The reason for its notoriety? Marumsco soils contain clay minerals called smectites. Smectites belong to a class of clays that is very sticky; they shrink when they are dry and swell when they are wet. How much damage can shrinking and swelling clay really cause? Plenty. Swelling Marumsco soils can exert thousands of pounds of pressure on basement walls, causing them to bulge and buckle. During the hot and dry summer months, shrinking Marumsco soils beneath the house can cause irregular settling. This settling can warp the house’s frame and foundation, causing cracks to form in walls and floors, jamming windows and doors, and putting stress on utility lines.

The dense smectite clay is also nearly impermeable to water. During heavy rains, groundwater backs up on top of the clay, saturating the sandier soil layers above it. At a certain point, these sandy layers can become a muddy slurry and cause a landslide. A decade ago, a nearly two acre chunk of land slid off a Marine Clay hillside next to Telegraph Road during a particularly wet spring. No one was hurt, but it took VDOT several months to reopen the road’s shoulder and install a retaining wall to prevent further damage.

Sassafras-Marumsco complex is found primarily along hillsides in Southeastern Fairfax County in the Lee and Mount Vernon Supervisor Districts. Particular hotspots include Franconia, Hybla Valley, and Mason Neck.

Glenelg Silt Loam

Glenelg is Fairfax County’s soil beauty: docile, predictable, accommodating.

If Sassafras-Marumsco is the beast, then Glenelg is Fairfax County’s soil beauty. Docile, predictable, accommodating; Glenelg has few problems to note. Glenelg is also the County’s most extensive soil type, covering about 65,000 acres. Glenelg weathers from micaceous schist bedrock, the same 450 million year old metamorphic stone that the Potomac crashes over at Great Falls. This rock type occurs in a wide swath that runs from Great Falls in the northeast to Clifton in the southwest. Glenelg has fairly thin topsoil over a rusty red band of clay – often several feet thick – that starts just a few inches below the surface. In the summer, when this layer becomes dry and hard, it sometimes earns the ire of gardeners and landscapers, who refer to it by the derogatory term “Red Virginia clay.” Its brick-like consistency and color is fitting: brickmakers in the state still make their wares using soils very similar to Glenelg. Below the clay layer, the soil gets looser as the clay content decreases with depth. Shiny mica flakes weathered from the Schist bedrock can be found throughout Glenelg, but they increase with depth. If you ever dig in Glenelg, your hands will look like they are covered in glitter. The mica flakes are loose, light and hard to compact. As a result, the deepest soil layers have an almost fluffy texture.

Glenelg is well drained, has no shrinking-swelling clay, and has good bearing strength. A house built on Glenelg is a happy house indeed. While gardeners may not be thrilled with the natural fertility of Glenelg, it’s certainly no worse than any other soil in the county. A good addition of organic matter or compost should fix most deficiencies.

Fairfax Silt Loam

Fairfax silt loam’s layering tells a strange story about the county’s geologic past.

What’s in a name? When it comes to soil, the rules of the US Department of Agriculture state that a soil must be named after the town where it was first discovered. That makes Fairfax silt loam our homeboy (or girl)! It was first described in 1956 and it remains local, mapped only in Fairfax and a few neighboring counties in Maryland and Virginia. Fairfax silt loam is interesting in that its layering tells a very strange story about the county’s geological past. Fairfax looks almost exactly like Glenelg silt loam covered with a 1-4 feet capping of marine sediments. That in and of itself is not so interesting – plenty of Fairfax County is covered with marine sediments – but Fairfax silt loam is often found on hilltops, including the highest hilltop we have: Tysons Corner (about 500 feet above sea level). So the mystery that this soil presents is how waterborne sediment could have been placed on our highest elevations. Needless to say, Fairfax County is much drier now than it used to be! So how did the sediment get there? One theory states that vertical movement of land on each side of a fault line running through the state (remember that 2011 earthquake?) has caused Fairfax County to slowly rise over many millions of years from near sea level to its current elevation. Whatever the cause, Fairfax silt loam reminds us that the present National Capital region may be very different from its distant past.

Orange Silt Loam

It forms on top of igneous remnants of volcanic activity during the Triassic era.

Like Sassafras-Marumsco, this soil features shrinking and swelling clays that can damage foundations and prevent groundwater percolation. Unlike Sassafras-Marumsco, this lovely soil also features naturally occurring asbestos deposits in its bedrock. Orange occurs in a fairly concentrated band running from western Clifton to Braddock Road, through Fairfax City and up to Route 123 in Oakton, but there are a few small, diffuse spots of the soil found in Reston and Great Falls. It forms on top of igneous bedrock called Greenstone – the remnants of volcanic activity during the Triassic era. Rope-like veins of asbestos can be found in the Greenstone, a great example of which is on display at the USGS visitor center in Reston. Asbestos is not a problem unless it gets airborne, so as long at its kept safely underground, human health impacts will be minimized. Keeping a healthy stand of vegetation – whether it be grasses, shrubs, trees, or flowers – will keep soil stable and prevent dust from kicking up. Caution should always be used during any construction project on Orange silt loam, especially if rock blasting is involved.

Huntington silt loam

Much of the Potomac’s sediment consists of quartzite eroded from the sandstone ridges of the Appalachian Mountains.

Huntington is a fairly rare soil found on only one landform in the county – the banks of the Potomac. Huntington inhabits the low bottom floodplains, and despite the inclusion of “silt” in its name, is often quite sandy. The reason for this is that it is made up of whatever solids the Potomac happens to be pushing along. Much of the Potomac’s sediment consists of quartzite sand eroded from the sandstone ridges of the Appalachian Mountains. When the river levels rise and the Potomac spills its banks, floodwaters spread out over the flood plain, slow down, and drop their sediments on top of the Huntington soils. After a good flood, it is not unusual for Huntington to gain several inches of new material. Despite its limited extent, you can experience Huntington soils first-hand by enjoying the riverbank footpaths in Riverbend Park, Great Falls. If you’ve ever wondered who had been putting down fresh sand on the trail, it was simply the Potomac sprinkling eroded remnants of the ancient mountains to the west!

If you’re curious about the 103 other soil types in the county, or what type of soil you have on your property, check out the Fairfax County soil map.


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