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Natural Bridge (Virginia)
Natural Bridge in Rockbridge County, Virginia, is a geological formation in which Cedar Creek (a small tributary of the James River) has carved out a gorge in the mountainous limestone terrain, forming a natural arch 215 ft (65.532 m) high with a span of 90 ft (27.432 m). It consists of horizontal limestone strata, and is the remains of the roof of a cave or tunnel through which the creek once flowed. Natural Bridge has been designated a Virginia Historic Landmark and a National Historic Landmark.
==Geology==
Rocks in the area are limestones and dolomites from the Ordovician Period, which implies deposition about 470 million years ago. The carbonates of the Natural Bridge occur within the Chepultepec Formation, making the buttress of the bridge, and the Beekmantown Formation, deposited conformably over the Chepultepec and comprising the span and the upper part of the bridge.〔[ Spencer, Edgard W. ,1968, Geology of the Natural Bridge, Sugarloaf Mountain, Buchanan, and Arnold Valley quadrangles, Virginia: Report of Investigations 13, Viginia Division of Mineral Resources, Charlottesville, VA, 55 p. and 4 plates. https://www.dmme.virginia.gov/commercedocs/RI_13.pdf.〕 Strata at Natural Bridge are close to horizontal, in contrast with dipping and folded beds as close as 300 ft away along Highway 11, which points to the fact that structural geology the rocks making the Natural Bridge are the bottom part of a syncline.
The process of formation of the bridge is less certain, but the ideas proposed by different individuals have in common the importance of three elements: the southeast dipping of the strata, the capability of water to slowly dissolve carbonates and the existence of numerous fractures. Spencer 〔Spencer, Edgard W. ,1964, Natural Bridge and vicinity. Virginia Minerals, v. 10, no. 2, p. 1−6.〕 has provided a summary of the most respected interpretations and produced the presently most widely accepted hypothesis. According to him, the Natural Bridge would be the remnant of an underground river tunnel that diverted water to the south. Initially, the underground river started capturing water from the Poague Run about two miles to the north, contributing to enlarge the incipient Cedar Creek drainage basin. Over time, through weathering and erosion, the underground river collapsed and disappeared almost completely, the Natural Bridge being the only surviving relic of the roof of the ancient underground river. The creek’s larger flow contributed to form the present day gorge, significantly increasing the distance from the bottom of the water flow to the roof of the underground channel. Today, the “Lost River”, about half a mile upstream from the Natural Bridge, is a vivid example supporting the hypothesis of the underground river. Water from the side of the gorge flows into the Cedar Creek, similarly as the ancestral Cedar Creek underground river allegedly flew into the Cascade Creek, presently entering the Cedar Creek approximately 500 ft below the Natural Bridge.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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