Blue Ridge soils, from the rock upWhat shallow, stony mountain ground means for a tract

Virginia Energy counts five physiographic provinces in the state: the Appalachian Plateau, the Valley and Ridge, the Blue Ridge, the Piedmont and the Coastal Plain. The Virginia Department of Conservation and Recreation puts the consequence of that division plainly: "Each province is unique in topography, soil pH, soil depth, elevation, availability of light, and hydrology."

Two of the five matter most in this corner of Southwest Virginia. Virginia Energy describes the rocks of the Blue Ridge as igneous and metamorphic. According to DCR, "the Blue Ridge widens south of Roanoke Gap into a broad plateau topped by the highest peaks in Virginia". Carroll County's comprehensive plan places the majority of the county in that province and describes it as a mountain plateau of moderate to steeply rolling land, with pre-Cambrian rocks that are igneous and metamorphic. Slightly more than half of Carroll's total land area slopes more than 20 percent.

On the footprint's northwestern side the Valley and Ridge takes over. DCR describes it as long, even-crested, parallel ridges rising above valleys of various size. Carroll's plan places a small part of the county's northwestern corner in that province, and Wythe County's plan says Wythe lies primarily within the Ridge and Valley and borders the Blue Ridge.

Virginia Energy adds a blunt line about both provinces: the mountains and hard rock of the Valley and Ridge and the Blue Ridge are difficult to farm. Which kinds of ground turn up on the Wythe side of that boundary is set out in the kinds of land common in Wythe County. The rest of this guide stays with what the rock does to the soil on top of it.

Carroll County's comprehensive plan says the county's soils have been mapped and delineated into 11 soil associations. The plan describes each one, and two of those descriptions carry most of what this guide is about.

Ramsey-Wiekert-Hazel association: Shallow to moderately deep, excessively drained, very stony and channery, sloping to very steep soils on strongly dissected uplands.

Carroll County Comprehensive Plan (2024)

Manor-Talladega-Watauga Association: Shallow to moderately deep, well-drained to excessively drained, micaceous, sloping to very steep soils on strongly dissected uplands.

Carroll County Comprehensive Plan (2024)

Read against the depth classes above, both descriptions start in the same place: soil that may run out at less than 20 inches and at most reaches 40 before the rock. Both describe ground that is sloping to very steep, on uplands the plan calls strongly dissected. The first adds very stony. None of that names a parcel. It describes a spread of ground across a part of the county, and a single tract inside it can differ from the one-line summary.

The same plan ties the soil to the water under it. Recharge to full storage, it says, "except in aquifers near streams and below the elevation of their beds, is slow because of the steep land surface and low absorption rate of most soils." Steep ground sheds water before much of it soaks in. How a well is drilled, permitted and tested on ground like this is its own subject, covered in private wells explained.

Virginia Cooperative Extension sums up the Blue Ridge in one line: "The overriding soil use constraints in the Blue Ridge are steep slopes, stoniness, and shallow depth over bedrock." Each of the three reaches a building site in a different way.

For septic, the soil is read on the site. The soil evaluation under 12VAC5-610-450 weighs, among other things, the physiographic province, the degree of slope and the soil profile, so the rock story above is part of the evaluation itself; a percolation test, where one is required, is only one of many criteria, and the order of that work is set out in how perc tests work. Under 12VAC5-610-593, subsurface soil absorption trench systems may not go on slopes greater than 50 percent unless terraced. Where the soil falls short, Extension says alternative systems can be designed for less than ideal soil and site conditions; whether an alternative septic system fits a given drainfield site is settled through the local health department.

Extension also describes shrink-swell, or expansive, clays: soils high in them change volume as they wet and dry and "can cause severe damage to foundations and footings of buildings and to sidewalks, roads, and other structures." That is a statewide educational statement, not a finding that these counties have such clays. Extension's advice when they are encountered or expected is an on-site evaluation by a qualified soil scientist.

Slope carries its own set of rules once a house site is being placed. How slope and grade decide where a house, a drive and a septic site can go is worked through step by step in slope, aspect and building on a tract.