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}} The Russell Cave National Monument is a U.S. National Monument in northeastern Alabama, United States, close to the town of Bridgeport. The Monument was established on May 11, 1961, when 310 acres (1.3 km2) of land were donated by the National Geographic Society to the American people. It is now administered and maintained by the National Park Service. The National Monument was listed on the National Register of Historic Places on October 15, 1966. With a mapped length of 7.2 miles (11.6 km), Russell Cave is the third longest mapped cave in Alabama, is ranked 90th on the United States Long Cave List, and is currently listed as number 314 on the World Long Cave List. Its exceptionally large main entrance was used as a shelter by prehistoric Indians from the earliest known human settlement in the southeastern United States, through to European colonization. The surrounding forest provided food, tools, and wood fuel for fires. ==Geology== The rock from which Russell Cave was carved was formed over 300 million years ago at the bottom of an inland sea covering the region. Due to continental drift, the area that is now northeast Alabama was located very close to the Equator at the time this limestone was forming. This area is now located in a temperate climate, but 300 million years ago it was a shallow, tropical sea. Carbonaceous deposits of skeletons and shells were slowly transformed into limestone. Rainwater, mildly acidic from atmospheric carbon dioxide, slowly dissolved a portion of the limestone rock resulting in the formation of the cave. About 9,000 to 12,000 years ago, the collapse of a cavern roof beneath a hillside near Doran's Cove created a sinkhole and exposed Russell Cave. Until shortly before the first occupation by Native Americans, the cavern was uninhabitable due to the presence of a large stream which occupied the entirety of the cavern. A rockfall from the cavern roof diverted the stream to one side, however, exposing a portion of the cave floor. The first occupants dwelled on this irregular floor of rock slabs. Debris from occupants and deposits falling from the ceiling slowly raised the floor. The floor ultimately was raised an additional seven to eight feet (2.0 to 2.5 m) and up to 30 feet (9.0 m) near the upper entrance.〔 In the 1960s, the United States Bureau of Mines installed bolts with a length of 15 feet (4.5 m) to prevent additional collapses of limestone rock.〔 The cave mouth faces to the east preventing the ingress of a cold north and northwest winds and allowing in the morning sun. According to a published cave map, the cave has five entrances in addition to the Main Entrance. Three of the entrances are referred to as Canoe Entrances while the other two are named the Picnic Entrance and Pig Entrance. A natural spring flows into the cave and travels underground for 1.5 miles (2.5 km) before joining Widow's Creek and, ultimately, the Tennessee River. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Russell Cave National Monument」の詳細全文を読む スポンサード リンク
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