Dixon Cave Trail (Mammoth Cave National Park, Kentucky, USA) 1 : 無料・フリー素材/写真
Dixon Cave Trail (Mammoth Cave National Park, Kentucky, USA) 1 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Dixon Cave is located a little north of Mammoth Cave's Historic Entrance in Mammoth Cave National Park, Kentucky, USA.This relatively short cave (~1500 feet long) is not part of public tours but an overlook is accessible via a hiking trail from the Historic Entrance. Dixon Cave is a giant canyon passage and an important bat hibernaculum. It is geologically part of the Mammoth Cave System, although it has become erosively separated from it. Dixon Cave is widely considered to be the downstream continuation of Houchins Narrows, a giant canyon passage (nearly sediment-filled) that commences at Mammoth Cave's Historic Entrance.The rocks at the mouth of Dixon Cave, and the walls of the interior, are part of the upper Girkin Limestone (lower Upper Mississippian). Fine-grained limestones and fossiliferous limestones of the upper Girkin Ls. can be closely examined along the trail to Dixon Cave. Obvious macrofossils in these rocks are solitary rugose corals ("horn corals") (Animalia, Cnidaria, Anthozoa, Rugosa).Above the Girkin Limestone are outcrops of quartzose sandstones of the Big Clifty Sandstone (lower Upper Mississippian). This unit is visible upslope along the Dixon Cave trail.The entrance to Dixon Cave is distinctively subovoid in plan view. This mouth of Dixon Cave has been interpreted as a former spring, representing the site where a subterranean river emerged at the land surface and then drained into the paleo-Green River. Elevationally, Dixon Cave occurs at Level B of Palmer (1981). Level B includes some of the most famous Mammoth Cave Ridge passages: Audubon Avenue, Broadway Avenue, Main Cave, and Kentucky Avenue. Level B cave passages at Mammoth Cave formed about 2 to 4 million years ago, during the Pliocene. They have been partially to mostly filled with stream sediments and limestone breakdown rubble, which occurred during the Late Pliocene and Pleistocene. During the Pliocene, the Green River was sitting at a higher elevation than at present. Erosive downcutting by the river and its tributaries has lowered the regional water table. Former mouths of springs, like Dixon Cave, are now on hillsides well above the modern Green River.In the early 1800s, sediments in Dixon Cave were mined for nitrocalcite (= hydrous calcium nitrate - Ca(NO3)2.4H2O), or "false saltpeter". This was later converted to potassium nitrate, a key ingredient of gunpowder.-----------------------References:Palmer (1981) - A Geological Guide to Mammoth Cave National Park. 196 pp.Granger et al. (2001) - Geological Society of America Bulletin 113: 825-836. |
| 撮影日 | 2012-08-16 12:49:04 |
| 撮影者 | James St. John |
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