Elklick Woodlands Natural Area Preserve is a 226-acre (91 ha) Natural Area Preserve located in Fairfax County, Virginia. Owned by the Fairfax County Park Authority, it is protected with a local conservation easement, and preserves a globally rare natural community known as a "northern hardpan basic oak-hickory forest". This kind of forest occurs on diabase soil with an underlay of dense plastic clay; such terrain is called "shrink-swell soil" due to extreme variations in moisture availability exhibited throughout the year. Trees which grow in such an environment are stunted, and their relatively open canopies encourage the growth of a wide variety of grasses and herbs in the understory. Such forests were once common around northern Virginia, but many have been lost due to increasing suburb