Limestone in Virginia

the limestone deposits in Loudoun County were mined primarily for agricultural lime prior to the Civil War
the limestone deposits in Loudoun County were mined primarily for agricultural lime 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)

Virginia has different types of rocks rich in calcium carbonate, including limestone, dolomite, marl, and travertine. Various types of cave formations, primarily stalactites and stalagmites, evolved as calcium carbonate dissolved in groundwater has crystalized.

Virginia's oldest limestone accumulated as much as 750 million years ago at a time when Virginia was part of the supercontinent Rodinia. During or after the failed rifting event, when volcanic eruptions and eroding sediments created the formations around Mount Rogers, the sediments were metamorphosed into marble. Local tradition holds that a block of stone from a quarry in Grayson County near Troutdale was used in the construction of Grant's Tomb.1

Around 600 million years ago, carbonates accumulated in shallow freshwater lakes and/or in marine bays in what is now Loudoun County. Those sediments were gradually buried below the sands and cobbles deposited by freshwater rivers, or in saltwater embayments.2

Rodinia split up, massive basaltic lava flows erupted, and the sandy and lime-rich sediments were buried. Later tectonic forces in the Taconic, Neo-Acadian, and Alleghanian orogenies also generated heat and pressure. The sediments were compressed into rocks ("lithified") known today as the Swift Run Formation, and the beds of limestone metamorphosed into marble.

By the time Thomas Jefferson was elected president, the 50' thick layer of marble near Goose Creek in Loudoun County was being quarried. The marble was crushed, burned to produce agricultural lime, and spread on the fields of Oatlands and other plantations to enhance productivity of the soil. The Virginia Marble Company operated the quarries after the Civil War, and it became one of the largest employers of African-American men in the county.

tectonic pressures metamorphosed Precambrian deposits of limestone into marble (circled in blue) which was mined near Goose Creek between 1798-1949
tectonic pressures metamorphosed Precambrian deposits of limestone into marble (circled in blue) which was mined near Goose Creek between 1798-1949
Source: US Geological Survey (USGS), Geologic Map of Loudoun County, Virginia

Its marble was used to produce the Confederate memorial erected in Little Washington, Rappahannock County and the 1898 Confederate Heroes Monument in Page County. (A second marble monument installed later in downtown Luray, and two other Confederate soldier memorials in Lunenburg and Spottsylvania counties, were sculpted in Georgia.) The Virginia Marble Company produced terrazzo (marble chips) until 1949 at Goose Creek.3

The largest Virginia deposits of limestone and dolomite are in the Valley and Ridge physiographic province. Limestone outcrops are readily visible along Interstate 81, which crosses that province from the Shenandoah Valley in the north to the Tennessee border.

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
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, the region was located at the edge of the supercontinent Rodinia. It was underwater, near the shoreline of a part of the Iapetus Ocean known as the Sauk Sea. At the time, "Virginia" was located south of the Equator, and coral reefs grew in the warm waters. Zooplankton and larger forms of life protected by shells thrived in the Sauk Sea. When they died and sank to the bottom, deposits of limestone (and magnesium-rich dolomite) gradually accumulated.

Tectonic forces moved chunks of continental crust and closed the Iapetus Ocean. Terranes from island arcs were pushed against the edge of Virginia in the Taconic, Acadian, and Alleghanian orogenies. The island arcs widened the crust over the last 400 million years to create the Piedmont and Coastal Plain physiographic provinces, but the orogenies also narrowed the old continent. The sediments on the floor of the Sauk Sea were cracked, thrust slid up ramps of softer shales, and stacked on top of other layers like pancakes.

tectonic forces during orogenies cracked sedimentary layers at weak zones and slid layers of limestone on top on one another, like a stack of pancakes
tectonic forces during orogenies cracked sedimentary layers at weak zones and slid layers of limestone on top on one another, like a stack of pancakes
Source: US Geological Survey (USGS), Lateral Ramps in the Folded Appalachians and in Overthrust Belts Worldwide - A Fundamental Element of Thrust-Belt Architecture (Figure 4A)

A slice of the igneous bedrock, the granites and gneisses deep underneath the Sauk Sea sediments, was a slice broken off the edge of the continent. That crust was shoved 40 miles to the west, emplacing the Blue Ridge at least partially on top of the much younger limestone/dolomite layers. The Blue Ridge fault marks the western edge of the slice of igneous bedrock that was moved westward.

Carbonate layers that were once at the bottom of the Sauk Sea, on the edge of Rodinia, are now west of the Blue Ridge and far from the modern shoreline. Those limestone/dolomite sediments are the common bedrock in the Shenandoah Valley, in the Roanoke Valley, in the New River Valley, and in the valleys of the Tennessee River's upper tributaries (Holston, Clinch, and Powell rivers).

Some of large limestone quarries that excavate the limestone crush it to produce agricultural lime, but most quarries sell the stone for construction. The railroads haul limestone gravel east through the Blue Ridge to support the beds of the tracks. East of Thoroughfare Gap, railroad tracks are placed on top of distinctly-colored grey rocks that appear out of place in the iron-rich bedrock of the Culpeper Basin.

As far back as the mid-1700's, Shenandoah Valley residents used limestone blocks to build fortified houses that offered protection against Shawnee, Seneca, and Cherokee raids. The standard architecture at Virginia Tech relies upon quarries near Blacksburg that supply "Hokie Stone" from sediments deposited 500 million years ago. Courthouses and other public buildings west of the Blue Ridge are often made of limestone.

One major deposit of limestone is exposed on the surface near Leesburg, east of the Blue Ridge in Loudoun County. Limestone conglomerate at Raspberry Falls/Leesburg ("Calico marble" or "Potomac marble") is the Leesburg Member of the Balls Bluff Siltstone. The limestone cobbles in the conglomerate was deposited originally in the Sauk Sea, in sediments now exposed in Frederic County Maryland as the Frederick Limestone and Tomstown Formation. During the Triassic Period, carbonate clasts were eroded in storms, varried downstream in flooded river channels, and deposited in debris-flow deposits on alluvial fans in the Culpeper Basin.4

The conglomerate was used to make columns for the original Senate Chamber in the US Capitol. It was also processed into agricultural lime, and one quarry near Leesburg is now a pond in Isaak Walton Park. The only known natural caves in Virginia are located east of the Blue Ridge, in the Leesburg Member north of Leesburg. There are also sinkholes in Loudoun County east of Route 15, and the local geology is reflected in the name of the stream "Limestone Branch."

the only natural caves in Virginia east of the Blue Ridge are in the conglomerate known as the Leesburg Member (circled in red), which also produced sinkholes at Temple Hall Regional Park
the only natural caves in Virginia east of the Blue Ridge are in the conglomerate known as the Leesburg Member (circled in red), which also produced sinkholes at Temple Hall Regional Park
Source: US Geological Survey (USGS), Geologic Map of Loudoun County, Virginia

Caves and Springs in Virginia

The Orogeny Zones and Virginia Geology

Shenandoah Valley Region

Links

References

1. Harry W. Webb, Palmer C. Sweet, "Interesting Uses Of Stone In Virginia - Part I,", Virginia Minerals, Virginia Department of Mines, Minerals and Energy, Volume 38 Number 4 (November 1992), p.31, https://www.dmme.virginia.gov/commercedocs/VAMIN_VOL38_NO04.PDF (last checked May 12, 2017)
2. Scott Southworth, William C. Burton, J. Stephen Schindler, Albert J. Froelich, "Geologic Map of Loudoun County, Virginia" pamphlet to accompany Geologic Investigations Series Map I–2553, US Geological Survey, 2006, p.22, https://pubs.usgs.gov/imap/2553/ (last checked May 12, 2017)
3. Thomas R. Seabrook, Tributes to the Past, Present, and Future: Confederate Memorialization in Virginia, 1914-1919, master's degree at Virginia Tech, April 30, 2015, p.29, https://vtechworks.lib.vt.edu/bitstream/handle/10919/52895/Seabrook_TR_T_2015.pdf; Harry W. Webb, Palmer C. Sweet, "Interesting Uses Of Stone In Virginia - Part I,", Virginia Minerals, Virginia Department of Mines, Minerals and Energy, Volume 38 Number 4 (November 1992), p.31, https://www.dmme.virginia.gov/commercedocs/VAMIN_VOL38_NO04.PDF; "Confederate Heroes Monument," Historical Marker Database, November 2, 2008, http://www.hmdb.org/marker.asp?marker=16457; Deborah A. Lee, "An Introduction to Loudoun County’s African American Communities," Friends of the Thomas Balch Library, 2004, p.8, https://static1.squarespace.com/static/559ec31fe4b0550458945194/t/56833f10dc5cb44ad782a414/1451441936950/An+Introduction+to+Louodoun%27s+African+American+Communities.pdf (last checked May 12, 2017)
4. Scott Southworth, William C. Burton, J. Stephen Schindler, Albert J. Froelich, "Geologic Map of Loudoun County, Virginia" pamphlet to accompany Geologic Investigations Series Map I–2553, US Geological Survey, 2006, p.26, https://pubs.usgs.gov/imap/2553/ (last checked May 12, 2017)


Rocks and Ridges - The Geology of Virginia
Virginia Places