nearly all limestone exposed at the surface in Virginia today was deposited in the Iapetus Ocean, before the addition of the land between the Blue Ridge and Atlantic Ocean
Source: ESRI, Ecological Tapestry of the World
Roughly 500 million years ago, what today is the Shenandoah Valley of Virginia was located at the edge of the supercontinent Rodinia and the edge of the Iapetus Ocean. At the time, "Virginia" was south of the Equator, with coral reefs on that edge. The death of zooplankton and larger forms of life protected by shells gradually built up deposits of limestone (and magnesium-rich dolomite) on the Iapetus Ocean floor.
Today, those deposits are exposed within the Valley and Ridge province west of the Blue Ridge. In addition to the Shenandoah Valley, limestone and dolomite are the common bedrock near Roanoke, in the New River Valley, and in the valleys of the Tennessee River's upper tributaries (Powell, Clinch, and Holston rivers).
Only a few deposits of limestone are exposed on the surface east of the Blue Ridge. That land was accreted to the edge of the continent, primarily through the Taconic, Acadian, and Appalachian (or Alleghanian) orogenies. In the process, the limestone bedrock on the floor of the Iapetus Ocean was compressed, shoved around, and cracked and stacked on top of other layers. During the orogenies massive amounts of other sediments eroded off the new land being added to the east. The eroding pebbles, sand and mud settled on top of the old ocean floor, but did not create enough heat/pressure to metamorphose the buried limestone layers into marble.
the limestone deposits in Loudoun County were mined for "marble" prior to the Civil War
Source: Library of Congress, A map of the state of Virginia, constructed in conformity to law from the late surveys authorized by the legislature and other original and authentic documents (1859)