Natural Bridge

1852 painting by Frederic Edwin Church Natural Bridge
1852 painting by Frederic Edwin Church vs. 2008 scene
Source: Museum Syndicate

Natural Bridge is a geological rarity, and one of Virginia's earliest tourist attractions. Rockbridge County, created in 1778, was named after the natural bridge.

The first European to document seeing the geologic feature was John Peter Stallings. He was a captive of Native Americans in the Piedmont for six years, and may have seen it while in their custody. He settled in the upper James River valley in 1736, and wrote about the bridge in his journal in 1742.

Thomas Jefferson probably saw the Natural Bridge for the first time on August 23, 1767. He purchased it and 157 acres around it in 1774. That required getting a land warrant in 1773, followed by metes and bounds of the survey ("Beginning at 2 poplars on a line of Hugh Barkeley's land...") completed by James Tremble (not George Washington...). The survey made no reference to the Natural Bridge on the property.

Jefferson paid the equivalent of less than $200 in modern money to obtain the land warrant, get the property surveyed, and file the necessary papers with the Secretary of the Colony in Williamsburg.1

Natural Bridge, as portrayed in the 1870's
Natural Bridge, as portrayed in the 1870's
Source: Picturesque America (p.42)

Jefferson later tried to sell "one of the sublimest curiosities in nature" for development as a tourist resort. However, the property stayed in his family until 1833, and was sold only after he died.2

Thomas Jefferson at Natural Bridge, as portrayed by Caleb Boyle
Thomas Jefferson at Natural Bridge, as portrayed by Caleb Boyle
Source: The Atlanta 100, Did Jefferson Seed National Parks? (June 28, 2016)

Natural Bridge stayed in private ownership after that 1774 purchase. There were numerous proposals since at least the 1940's for the state or Federal government to purchase Natural Bridge and to preserve it in a park, ensuring public access through public ownership.

In 2013, a real estate deal was structured so the bridge was sold to a new owner, but would be transferred ultimately into public ownership and become the 37th unit of the Virginia State Park system. The Natural Bridge State Park was established in 2016, but technically the land still remained private. Before the transfer to public ownership by the Commonwealth of Virginia, the debt for purchase of the property had to be paid.3

Natural Bridge is the remnant of an ancient cave roof, and Natural Bridge Caverns nearby offers public tours of an underground cave
Natural Bridge is the remnant of an ancient cave roof, and Natural Bridge Caverns nearby offers public tours of an underground cave

In 1787, Thomas Jefferson described the bridge in his Notes on the State of Virginia:4

The _Natural bridge_, the most sublime of Nature's works, though not comprehended under the present head, must not be pretermitted. It is on the ascent of a hill, which seems to have been cloven through its length by some great convulsion.

The fissure, just at the bridge, is, by some admeasurements, 270 feet deep, by others only 205. It is about 45 feet wide at the bottom, and 90 feet at the top; this of course determines the length of the bridge, and its height from the water. Its breadth in the middle, is about 60 feet, but more at the ends, and the thickness of the mass at the summit of the arch, about 40 feet.

A part of this thickness is constituted by a coat of earth, which gives growth to many large trees. The residue, with the hill on both sides, is one solid rock of lime-stone. The arch approaches the Semi-elliptical form; but the larger axis of the ellipsis, which would be the cord of the arch, is many times longer than the transverse. Though the sides of this bridge are provided in some parts with a parapet of fixed rocks, yet few men have resolution to walk to them and look over into the abyss. You involuntarily fall on your hands and feet, creep to the parapet and peep over it.

Looking down from this height about a minute, gave me a violent head ach. If the view from the top be painful and intolerable, that from below is delightful in an equal extreme. It is impossible for the emotions arising from the sublime, to be felt beyond what they are here: so beautiful an arch, so elevated, so light, and springing as it were up to heaven, the rapture of the spectator is really indescribable! The fissure continuing narrow, deep, and streight for a considerable distance above and below the bridge, opens a short but very pleasing view of the North mountain on one side, and Blue ridge on the other, at the distance each of them of about five miles.

This bridge is in the county of Rock bridge, to which it has given name, and affords a public and commodious passage over a valley, which cannot be crossed elsewhere for a considerable distance. The stream passing under it is called Cedar creek. It is a water of James river, and sufficient in the driest seasons to turn a grist-mill, though its fountain is not more than two miles above.

The private company that owned Natural Bridge until 2014 developed the site as a commercial tourist attraction, along with Natural Bridge Caverns. The company added additional attractions to draw 160,000 visitors annually. They paid the admission fee and purchased souvenirs at the gift shop, including 65,000 who stayed at the Natural Bridge hotel, Attractions included Professor Cline's Haunted Monster Museum, a wax museum, Dinosaur Kingdom, and a replica of a Monacan village staffed by Native Americans from that nearby tribe.

Natural Bridge stimulated development of tourist hotels on Route 11
Natural Bridge stimulated development of tourist hotels on Route 11
Source: Boston Public Library, Tichnor Brothers Postcard Collection, Sunset Cottages, Luray, Virginia

The interpretation at the site has mixed myth with history. A light and music show called the Drama of Creation was provided at night. Nearby attractions, including a styrofoam replica of Stonehenge nearby called Foamhenge, sought to take advantage of the ability of Natural Bridge to attract tourists.5

Marketing by the private owner included repeating stories designed to magnify the significance of the site, "gilding the lily" in a manner that would not be acceptable for professional interpretation at a state or national park. Though there is no historical documentation to support the claim, the company website advertised:6

Legend holds that young George Washington surveyed the Natural Bridge site for Lord Fairfax. Landmarks remain of the work and on the wall of the bridge where he carved his initials.

the website for Natural Bridge long claimed that George Washington had surveyed the site
the website for Natural Bridge long claimed that George Washington had surveyed the site
Source: Natural Bridge of Virginia
Caveat emptor - the real estate broker trying to sell Natural Bridge in 2013 repeated the George Washington story
Caveat emptor - the real estate broker trying to sell Natural Bridge in 2013 repeated the George Washington story
Source: Woltz & Associates, Inc.

The initials are visible from the trail across Cedar Creek, 23 feet above ground level. Most likely, they were carved into the stone long after George Washington died, perhaps in 1927 when a stone was found with the initials "G.W." plus a surveyor's cross. At one time, there were hundreds of names and initials carved as graffiti on the wall at that spot.7

The claim of a George Washington survey is as mythical as "George Washington slept here" for may sites seeking to manufacture an association with the most historic colonial Virginian. A list of all known surveys by George Washington includes none in the James River watershed. His survey work was concentrated in the Northern Neck, where Lord Fairfax allowed him to work even after he was no longer officially the surveyor for Culpeper County. IF he saw the Natural Bridge, it would have been through

In 1773, Thomas Jefferson paid James Tremble to survey the tract at Natural Bridge, and the land grant to Jefferson was issued on July 5, 1774. As one of Washington's biographers has noted:8

no evidence has ever surfaced showing that Washington surveyed the Natural Bridge or any other land in the area.

Other myths about the site involve ghost sightings, Monacan Indians discovering a magical "Bridge of the Gods" providing a path across Cedar Creek when fleeing from Shawnee warriors, and an arbor vitae tree that was supposedly 1,500 years old when it died in 1980. In 1873, a hoax that the limestone bridge had caught fire and burned was published by newspapers across the country.

In another invocation of George Washington, he is reported to have thrown a dollar over the bridge. That would require tossing a coin 190 feet high, and the idea that Washington threw away good money ignores his character.

The bridge has been a popular tourist attraction since Jefferson's purchase. The nomination of the site to the National Register noted:9

Since the settling of America, Natural Bridge has served as one of the nation's most recognizable icons of the wonders of nature. Its image was popularized by artists throughout the centuries and by a stream of illustrious visitors who waxed eloquent on its inspiring characteristics.

The bridge so captured the attention of Thomas Jefferson that he purchased the site, obtaining a grant from George III in 1774, and later wrote that he considered the bridge a public trust and would not allow it to be injured, defaced, or masked from public view. Indeed, Natural Bridge and Niagara Falls in the late eighteenth and early nineteenth centuries occupied the top tier as the most impressive natural wonders in the New World.

Monacan village exhibit at Natural Bridge reconstructed Monacan house
Monacan village exhibit at Natural Bridge

The size and geological rarity of the bridge makes the site significant, independent of the cultural history and myths about the tourist attraction. There is only one other natural bridge in Virginia, a much-smaller arch located in Lee County.

Natural Bridge in Rockbridge County is a remnant of a cave roof that collapsed. It was not formed by an earthquake that split the land surface, though earthquakes may have cracked portions of the cave roof and triggered rockfalls.

The role of water in forming an underground channel of water and a cave at Natural Bridge, and its common relationship to the formation of Natural Tunnel in Scott County, was recognized even during Jefferson's lifetime.10

The safe answer to the question "how old is Natural Bridge" is that it was formed a long time ago. The rock itself was deposited around 450 million years ago, when the edge of the continental plate was under the Sauk Sea. During the Ordovician period, a thick accumulation of limestone developed on the ocean bottom as marine plankton and larger creatures with calcium-rich shells died.

In later collisions of tectonic plates, the limestone was buried by other sediments and shoved around. Magnesium in the groundwater joined with the calcium to convert the limestone into dolomite, with a chemical composition of CaMg(CO3)2.

Geologists map the bedrock of Natural Bridge in Rockbridge County as part of the Beekmantown Formation. It forms a remnant arch, overlying the Chepultepec Formation through which Cedar Creek flows.11

Natural Bridge is an arch of the Beekmantown Formation, roughly 450 million years old
Natural Bridge is an arch of the Beekmantown Formation, roughly 450 million years old
Source: Virginia Division of Mineral Resources, Geology of the Natural Bridge, Sugarloaf Mountain, Buchanan and Arnold Valley quadrangles, Virginia (by E. W. Spencer, 1968)

The bridge itself is much younger, and may have developed only in the last 500,000-1,000,000 years. As rainwater seeped underground, it absorbed carbon dioxide and became slightly acidic. The CO(sub>2 was added as the water traveled through the organic litter on the surface and the A horizon of the soil, which are filled with decomposing humus and animal life exhaling carbon dioxide. The water became slightly acidic as it trickled down through the topsoil into the dolomite.

The slightly-acidic water dissolved some of the calcium carbonate crystals. The groundwater carried away the calcium and carbonate ions in solution, replacing a crystal of solid rock with an empty space. As water dissolved many crystals over time, pores developed in the rock. Portions became similar to a sponge.

Where there were cracks in the bedrock, even more water could move underground to create channels connecting the pores. After many centuries, enough rock was removed for a cave to form underground. The "hole in the ground" expanded until it stretched west of Cedar Creek and intersected Pogue Run, near modern I-81. The roof of the underground channel that diverted the headwaters of Pogue Run was near the surface. Sinkholes probably formed above the channel, and ultimately "windows" developed where portions of the underground cave were exposed directly to the surface.

Surface water that used to flow down Pogue Run was diverted, and instead flowed through the underground limestone channel to Cedar Creek. This act of "stream piracy" changed the watershed divide on the surface. The length of Pogue Run was truncated, and its former headwaters became part of Cedar Creek.

pirated portion of Cedar Creek (in blue), and current headwaters of Pogue Run (in yellow)
pirated portion of Cedar Creek (in blue), and current headwaters of Pogue Run (in yellow)
(former underground channel was located between I-81 and current Natural Bridge)
Source: US Geological Survey (USGS), Natural Bridge 7.5x7.5 topo map (2011)

Gradually, the dissolving power of the extra water flowing through the underground channel of Cedar Creek etched away at the cave's roof. The layer of rock at the top became thinner and thinner until portions collapsed. More of the cave was exposed to the sky, until only a small portion remains today. Modern Natural Bridge is an arch of dolomite rock that has not collapsed yet.12

As rocks fell from the cave roof and exposed the underground cave channel to the sky, Cedar Creek carried away the limestone debris. The process of collapse may have been gradual, but there could have been dramatic moments. At a similar natural bridge in China, Tianmen Shan:13

the entire opening formed in one cataclysmic event when the back of a huge cave collapsed in 263 AD.

Calcium-saturated water that emerges at the surface is making tufa deposits in the streambed. The deposition process resembles the way cave formations such as stalactites and stalagmites are growing in nearby Natural Bridge Caverns.

calcium is deposited on rocks in the streambed on the walk down to Natural Bridge, creating an above-ground equivalent of cave formations
calcium is deposited on rocks in the streambed on the walk down to Natural Bridge, creating an above-ground equivalent of cave formations

Today Natural Bridge is stable, but it is still collapsing. Inevitably, over geologic time, the remaining portion of the ancient cave roof will collapse into the creek.

On October 23, 1999, a 6'x1' slab of dolomite spalled off the bottom of the arch, along with a shower of smaller rocks. One of the smaller rocks hit near a tourist from Georgia reading a plaque beneath the bridge, and debris from that falling rock killed her.

To prevent a repeat, loose rock was scraped away from the bridge. Holes were drilled from the top to the bottom, steel cables connected to metal plates installed on the bottom of the rock arch, and the cables were pulled tight. The plates on the bottom of Natural Bridge were camouflaged with paint to maintain the natural appearance.

Hard hats are available for visitors to wear, if they are concerned about a repeat rockfall. A hat might not provide any value if a slab landed directly on someone's head, but could reduce harm from rock shrapnel if a piece fell and shattered near a tourist again.

visitors to Natural Bridge now have the option of wearing a hard hat
visitors to Natural Bridge now have the option of wearing a hard hat

a warning sign alerts visitors to Natural Bridge that rockfalls are possible
a warning sign alerts visitors to Natural Bridge that rockfalls are possible

Bolting together the rock layers was intended to reduce the chance of another rock peeling loose and dropping onto the trail below the bridge. The cabling also mitigated the effect of vibrations from traffic on Route 11.

Native American trails crossed Cedar Creek on the arch for thousands of years, and there has been a road across Natural Bridge since 1753. Route 11 was constructed across it in the 1930's, and in 2017 the state estimated 2,000 vehicles/day traveled on top of Natural Bridge. Only 5% were trucks, and since 2000 there has been a 20-ton weight limit on them. However, that limit is waived when traffic must be diverted from I-81 due to an accident.

prior to construction of I-81, most north-south traffic used Route 11 over Natural Bridge
prior to construction of I-81, most north-south traffic used Route 11 over Natural Bridge
Source: Library of Congress, State of Virginia, base map with highways and contours ("Highways corrected to 1956")

The effect of 2,000 vehicles crossing the bridge each day is not clear. The natural structure is so large that engineers have assumed it could handle the weight of vehicles and whatever shaking they might stimulate.

The Virginia Department of Transportation does not consider the natural stone arch to be a "bridge" that requires regular inspections for safety. The state agency also assumes that even if there are voids in the bridge, it could be structurally sound. There are other roads in Virginia's karst country that cross unrecognized sinkholes. Since they retain a portion of their cave roof, those natural bridges are still underground, not exposed to view, and not identified as needing regular inspections.

As described by the Virginia Department of Transportation (VDOT) in a 2019 study proposing realignment of Route 11:14

Route 11 travels over the Natural Bridge geological formation and VDOT is responsible for the maintenance of the Route 11 road surface and roadbed. At this location, Route 11 is not considered by modern standards to be on a bridge. Instead Route 11 sits on a geological formation made of limestone; this type of rock material is common throughout the Shenandoah Valley and results in karst features like sinkholes and caves.<

where Route 11 crosses Natural Bridge, fences block the view of Cedar Creek below
where Route 11 crosses Natural Bridge, fences block the view of Cedar Creek below

In 2017, the Virginia Department of Transportation agreed to use ground penetrating radar to identify if there were any voids in that portion of the bridge underneath Route 11. The chief engineer later recommended finding a way to close the bridge to traffic, but based that proposal on the designation of the site as a state park rather than on any data indicating a safety issue.

The initial conclusion was "the bridge itself is not a danger to the motoring public. It's not likely the bridge is going to collapse." Though reassuring, the initial concern remained that vehicle-induced vibrations could cause future rockfalls. Tourists visiting the bridge are now given the opportunity to wear a hard hat, as a precaution.15

Route 11 used Natural Bridge to cross Cedar Creek Route 11 used Natural Bridge to cross Cedar Creek
Source: Virginia Department of Transportation, Rockbridge County 鈥?Route 11 Natural Bridge State Park

In 2019, the Virginia Department of Transportation and Virginia State Parks agreed that the best strategy was to relocate Route 11. Two preferred alternatives were identified, so Route 11 traffic would no longer have to cross the natural geological feature. 16

two alternatives were identified in 2019 to move Route 11 off Natural Bridge
two alternatives were identified in 2019 to move Route 11 off Natural Bridge
Source: Virginia Department of Transportation, Route 11 Alignment Study at Natural Bridge (January 14, 2019)

Natural bridges and arches can form through various processes. Natural Tunnel in Scott County is an underground channel created by dissolution of limestone. Natural Bridge in Rockbridge County was created by an almost-complete roof collapse when the underground cave/tunnel reached too close to the surface.

Natural Bridge formed because a portion of the cave's roof has not collapsed into Cedar Creek - yet
Natural Bridge formed because a portion of the cave's roof has not collapsed into Cedar Creek - yet
Source: British Museum, The Natural Bridge, Virginia (by W. Roberts, 1808)

The other natural bridge in Virginia is located in Lee County near Jonesville. Like the more-famous bridge in Rockbridge County, it was formed by the almost-complete collapse of a cave roof.

State Route 622 uses the arch as a bridge to cross Batie Creek, which flows through karst topography. The stream emerges at the base of a 25-foot high cliff of the Martin Creek limestone, at the south edge of The Cedars, and flows to the nearby Powell River. There are two sinkholes between the cliff and State Route 622, showing where the cave roof has already collapsed.

the natural bridge in Lee County crosses Batie Creek
the natural bridge in Lee County crosses Batie Creek
Source: US Geological Survey (USGS), 7.5-minute topographical map for Hubbard Springs, VA-KY (2016)

A third sinkhole still has a portion of the cave roof intact, and that natural bridge in Lee County is 49 feet wide. Though the arch is 11 feet high, underneath it Batie Creek fills about half of that height. Batie Creek does not have the power, or has not had the time, to wash away the rock that has fallen from the arch. The upstream end of the stream passing below the arch is clogged with boulders.17

the natural bridge in Lee County is crossed by State Route 662 west of Jonesville
the natural bridge in Lee County is in karst topography
the natural bridge in Lee County is crossed by State Route 662 west of Jonesville
Source: ESRI, ArcGIS Online

The longest natural bridge east of the Mississippi River is in Alabama. Alabama's Natural Bridge crossing New River is 148 feet long, compared to the 90-foot long Natural Bridge in Virginia crossing Cedar Creek. The natural bridge in Alabama is formed from sandstone, with a relatively high iron content that resists erosion.

Natural Bridge in Alabama is a sandstone structure, unlike Virginia's limestone bridge over Cedar Creek
Natural Bridge in Alabama is a sandstone structure, unlike Virginia's limestone bridge over Cedar Creek
Source: Jimmy Emerson, The Natural Bridge

Arches and bridges can be distinguished by various definitions, including:18

A natural bridge is a type of natural arch where a current of water, such as a stream, clearly was a major agent in the formation of the opening (hole).

- At Natural Bridges National Monument in Arizona and Natural Bridge State Park in Kentucky, erosion at the surface carved river channels into the local sandstone. River valleys shifted, as the running water etched downward and meandered to the sides. Occasionally, stream channels looped close to each other on opposite sides of a ridge. Erosion by water, and freezing/thawing ice, sometimes cut through the base of the rock barrier between stream channels and created an arch.19

- At Arches National Park, sandstone layers were cracked by land subsidence, and water on the surface excavated the layers to create "fins" of narrow rock formations on the edges of valleys. Further erosion by water and ice crystals cracked the sandstone rock on the edges of the fin walls, carving off chunks of debris that dropped into the valley below. Occasionally the cracks aligned to cut completely through the fins to create a hole.20

- In Massachusetts, the Natural Bridge at North Adams was carved by glacial meltwater carving into a layer of marble. By chance, the water cut through the marble deposit and left a ridge intact above the eroded hole, creating "the only naturally formed white marble arch" in North America.21

- In Arizona, Tonto Natural Bridge was formed in travertine rock by non-glacial erosion. Travertine was deposited by springs at the edge of limestone bedrock. The groundwater carried dissolved calcium carbonate, then deposited the dissolved rock as travertine when spring water met air at the ground surface. The deposits of travertine created a rock dam, and then a stream re-dissolved a channel through that dam to create the natural bridge.22

- Sea arches are formed when waves cut through rock promontories on the seacoast. On the West Coast of the United States, there are sea arches along the shoreline from California to British Columbia. Holei Sea Arch was carved by waves eroding the base of a volcanic lava flow that reached the edge of the island, at Hawaii Volcanoes National Park.23

Delicate Arch, a sandstone fin eroded by water and ice sea arch formed by wave erosion
Delicate Arch, a sandstone fin eroded by water and ice, and a sea arch formed by wave erosion
Source: National Park Service, Arches National Park; Bureau of Land Management, California Coastal National Monument

Caves and Springs in Virginia

Natural Bridge - From Private Ownership to State Park

Nature-Oriented Tourism

Links

References

1. "Natural Bridge," National Register of Historic Places Registration Form, 1997, Section 8 p.5, https://www.dhr.virginia.gov/historic-registers/081-0415/; Edmund Pendleton Tompkins, Joseph Lee Davis, The Natural Bridge and Its Historical Surroundings, The Natural Bridge of VA, 1939, p.1, p.3, https://www.monticello.org/site/research-and-collections/natural-bridge (last checked September 2, 2019)
2. Thomas Jefferson. "Letter to William Jenkings," July 1, 1809, in Landmarks of American Nature Writing - Natural Bridge, http://www2.lib.virginia.edu/exhibits/nature/bridge.html (last checked November 25, 2011)
3. Curtis Carroll Davis, "The First Climber of the Natural Bridge: A Minor American Epic," The Journal of Southern History, Vol. 16, No. 3 (Aug., 1950), p.279, Washington Post, December 18, 2013, http://www.washingtonpost.com/local/nonprofit-to-buy-natural-bridge-va-landmark-after-239-years-of-individual-ownership/2013/12/18/6b016ebc-6730-11e3-a0b9-249bbb34602c_story.html (last checked November 15, 2014)
4. Thomas Jefferson, "QUERY V Its Cascades and Caverns?" in Notes on the State of Virginia, 1787, http://xroads.virginia.edu/~hyper/jefferson/ch05.html (last checked November 24, 2011)
5. Cape Leisure Corporation website, http://www.capeleisurecorp.com/nbv.html (last checked November 25, 2011)
6. "Natural Bridge History," Natural Bridge, http://www.naturalbridgeva.com/natural-bridge-history.php (last checked November 24, 2011)
7. "Natural Bridge," National Register of Historic Places Registration Form, 1997, Section 7 p.2, https://www.dhr.virginia.gov/historic-registers/081-0415/; Edmund Pendleton Tompkins, Joseph Lee Davis, The Natural Bridge and Its Historical Surroundings, The Natural Bridge of VA, 1939, p.43, https://books.google.com/books?id=mEwSAAAAYAAJ (last checked July 26, 2019)
8. Frank E. Grizzard, George Washington: A Biographical Companion, ABC-CLIO, 2002, p. 235 https://www.monticello.org/site/research-and-collections/natural-bridge (last checked July 26, 2019)
9. "Natural Bridge," National Register of Historic Places Registration Form, 1997, Section 8 p.4, https://www.dhr.virginia.gov/historic-registers/081-0415/; Edmund Pendleton Tompkins, Joseph Lee Davis, The Natural Bridge and Its Historical Surroundings, The Natural Bridge of VA, 1939, p.27, https://books.google.com/books?id=mEwSAAAAYAAJ (last checked July 26, 2019)
10. Francis William Gilmer, "On the Geological Formation of the Natural Bridge of Virginia," Transactions of the American Philosophical Society, New Series, Vol. 1 (1818), pp. 187-192 http://www.jstor.org/stable/1004907 (last checked November 24, 2011)
11. Callan Bentley, "Natural Bridge," Mountain Beltway blog, August 22, 2013, http://blogs.agu.org/mountainbeltway/2013/08/22/natural-bridge-virginia/; E. W. Spencer, "Geology of the Natural Bridge, Sugarloaf Mountain, Buchanan and Arnold Valley quadrangles, Virginia," Virginia Division of Mineral Resources, 1968, https://ngmdb.usgs.gov/Prodesc/proddesc_38868.htm (last checked July 4, 2017)
12. Herbert P. Woodward, "Natural Bridge and Natural Tunnel, Virginia," The Journal of Geology, Vol. 44, No. 5 (July-August, 1936), pp. 607-10, http://www.jstor.org/stable/30067367; Edgar W. Spencer, "Geology of the Natural Bridge, Sugarloaf Mountain, Buchanan, and Arnold Valley Quadrangles, Virginia," Virginia Division of Natural Resources, 1968, pp.4-6, https://www.dmme.virginia.gov/commercedocs/RI_13.pdf (last checked July 4, 2017)
13. "NABSQNO 49R-449620-3213600 - Tianmen Shan," The Natural Arch and Bridge Society, http://www.naturalarches.org/db/arches/china02.htm (last checked November 26, 2011)
14. "Route 11 Alignment Study at Natural Bridge," Virginia Department of Transportation, January 14, 2019, p.1, http://www.virginiadot.org/projects/resources/Staunton/US_11_Natural_Bridge_Final_Full_Report_with_Appendix_01142019.pdf (last checked October 30, 2020)
15. "Canadian Rock Experts Work To Shore Up Natural Bridge," Daily News, November 5, 1999, http://articles.dailypress.com/1999-11-05/news/9911050218_1_shore-up-natural-bridge-bridge-stretches-slab; Ernst H. Kastning, Natural Bridge, Arcadia Publishing, 2014, pp.122-123, The Roanoke Times, July 4, 2017, The Roanoke Times, July 21, 2017, The Roanoke Times, September 9, 2017, The Roanoke Times, January 23, 2018, http://www.roanoke.com/news/virginia/new-tests-at-natural-bridge-look-at-the-highway-that/article_2f664689-a41e-5a12-b7a7-a49eab7ad7a7.html (last checked January 23, 2018)
16. "Route 11 Alignment Study at Natural Bridge," Virginia Department of Transportation, January 14, 2019, pp.21-22, http://www.virginiadot.org/projects/resources/Staunton/US_11_Natural_Bridge_Final_Full_Report_with_Appendix_01142019.pdf (last checked October 30, 2020)
17. "Natural Bridge of Lee County-EarthCache," Geocaching, http://www.geocaching.com/seek/cache_details.aspx?guid=54644efc-f26d-45c0-9d2c-0e76c6c23091; Ralph L. Miller, William P. Brosge, "Geology and Oil Resources of the Jonesville District, Lee County, Virginia," Geological Survey Bulletin 990, US Geological Survey, 1954, pp.163-164, https://pubs.er.usgs.gov/publication/b990 (last checked July 4, 2017)
18. "Frequently Asked Questions," Natural Bridge and Arch Society, https://www.atlasobscura.com/places/natural-bridge (last checked October 25, 2020)
19. "Geology Fieldnotes - Natural Bridges National Monument," National Park Service, http://www.nature.nps.gov/geology/parks/nabr/ (last checked November 26, 2011)
20. "Geologic Formations," Arches National Park, National Park Service, http://www.nps.gov/arch/naturescience/geologicformations.htm (last checked November 26, 2011)
21. "Natural Bridge State Park," Massachusetts Department of Conservation and Recreation, http://www.mass.gov/dcr/parks/western/nbdg.htm (last checked November 26, 2011)
22. "Geology of the Bridge," Tonto Natural Bridge State Park, Arizona State Parks, http://azstateparks.com/Parks/TONA/science.html (last checked November 26, 2011)
23. Andrew Alden, "Geological Outings Around the Bay: Natural Bridges," blog post in Quest KQED, November 24, 2011, http://www.nps.gov/havo/planyourvisit/upload/Explore_SB_0110_web.pdf (last checked November 26, 2011)


Virginia Geology
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