Natural Bridge

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

Thomas Jefferson purchased the bridge and 157 acres around it in 1774, about 40 years after Natural Bridge was first documented by an explorer other than a Native American.¹ Though Jefferson tried to sell "one of the sublimest curiosities in nature" for development as a tourist resort,² the property stayed in his family until 1835 (ten years after his death). The site has been privately owned ever since that 1774 purchase, though there have been proposals since at least the 1940's for Virginia to purchase Natural Bridge for a state park.³

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."

1852 painting by Frederic Edwin Church Source: Wikipedia

Natural Bridge in 2008

The private company that now owns Natural Bridge has developed the site as a commercial tourist attraction, along with Natural Bridge Caverns. To draw 160,000 visitors annually to pay the admission fee and purchase souvenirs at the gift shop, including 65,000 who stay at the Natural Bridge hotel,⁵ the company has added additional attractions - 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. A light and music show called the Drama of Creation is provided at night. There is even a styrofoam replica of Stonehenge nearby, called "Foamhenge."

Marketing by the private owner includes repeating myths designed to magnify the significance of the site, "gilding the lily" in a manner that would not be acceptable for interpretation at a state or national park. Though there is no historical documentation to support the claim, the company website advertises: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 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 "uncovered in 1927 with the initials 'G.W.' and a surveyors cross."⁷ As one of Washington's biographers has noted, "no evidence has ever surfaced showing that Washington surveyed the Natural Bridge or any other land in the area."⁸

Other myths about the site involve ghost sitings, Monacan Indians discovering a magical "Bridge of the Gods" providing a path across Cedar Creek when fleeing from Shawnee warriors, an arbor vitae tree supposedly 1,500 years old when it died in 1980, and George Washington throwing a dollar over the bridge (nearly 200 feet high). Since Jefferson's purchase, however, the bridge has been a popular tourist attraction, as described in the nomination to the National Register: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

The geological rarity of the bridge makes the site significant, independent of the cultural history of the tourist attraction. Natural Bridge 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

Long ago in the past, rainwater seeped underground and absorbed carbon dioxide as the water travelled through the organic litter on the surface and the A horizon of the soil, filled with decomposing humus and animal life exhaling carbon dioxide. The water became slightly acidic as it trickled down through the topsoil to the Beekmantown Formation, a limestone deposited originally in a shallow sea during the Ordivician period 440–505 million years ago. In that limestone, 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 limestone rock and potions 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 near Natural Bridge. 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.

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.¹¹

As Cedar Creek flowed through the partially-underground channel, it carried away the limestone debris every time a portion of the roof fell. At a similar natural bridge in China, Tianmen Shan, "the entire opening formed in one cataclysmic event when the back of a huge cave collapsed in 263 AD."¹²

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)

The bridge is stable, but not unchanging. On October 23, 1999, a tourist was killed by a rockfall from the bridge. To prevent a repeat, metal plates were installed on the bottom of the bridge and attached with steel cables, then camouflaged with paint to maintain the natural appearance. There has been a road across Natural Bridge since 1753, but inevitably that remaining portion of the ancient cave roof will collapse into the creek.¹³

Natural bridges and arches can form through various processes, in addition to creation of underground channels by dissolution of limestone (as occurred at Natural Tunnel in Scott County) or by collapse when the underground cave/tunnel reached too close to the surface (as occurred at Natural Bridge in Rockbridge County). There is a second natural bridge in Virginia, located in Lee County near Jonesville. It spans Beatty Creek, and a state highway (Route 622) crosses it.¹⁴

Arches and bridges can be distinguised by various definitions, including "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)."¹⁵

- 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.¹⁶

- 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.¹⁷

- 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.¹⁸

- 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 cabonate, 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.¹⁹

- 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.²⁰

Delicate Arch, a sandstone fin eroded by water and ice Source: National Park Service, Arches National Park

sea arch formed by wave erosion Source: Bureau of Land Management, California Coastal National Monument

Links

• Competitive Enterprise Institute
• Natural Bridge of Virginia
• Natural Bridge
• Tianmen Shan (Heaven's Gate Mountain in China, where humans recorded the collapse of a cave and creation of a natural bridge)
• University of Virginia
• Landmarks of American Nature Writing - Natural Bridge
• Thomas Jefferson, Notes on the State of Virginia
• Virginia Division of Mines, Mineral, and vergy (DMME)
• Geology of the Natural Bridge, Sugarloaf Mountain, Buchanan, and Arnold Valley Quadrangles, Virginia (1968)
• Natural Bridge and Vicinity (1964)
• Scenic Landforms of Virginia
• The Geology of Natural Tunnel State Park (1990)
• Virginia Register of Historic Landmarks
• Natural Bridge National Register Final Nomination (1997)
• Natural Bridge National Historic Landmark Nomination (1998)

county zoning category for Natural Bridge is B-1, for business (not conservation)
Source: Rockbridge County Zoning Map

References