Anastomoses-defined pendants (Endless Caverns, Virginia, USA) 4 : 無料・フリー素材/写真
Anastomoses-defined pendants (Endless Caverns, Virginia, USA) 4 / 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 | This photo depicts deeply-incised anastomoses in the ceiling of a cave passage in Endless Caverns, Virginia. Anastomoses are networks of channels formed by dissolution of calcium carbonate at partings and bedding planes in limestones. Dissolution occurred as slightly acidic groundwater seeped or moved horizontally through the bedrock in the phreatic zone (at or below the water table). All groundwater is at least slightly acidic - the acid is usually carbonic acid (H2CO3) and some organic acids are also usually present. Carbonic acid forms by a simple chemical reaction: water + carbon dioxide = carbonic acid (H2O + CO2 --> H2CO3). This happens as raindrops fall through the atmosphere and also occurs as water percolates downward through soil and bedrock. The partial pressure of carbon dioxide gas is higher in soil gas, so even more carbonic acid forms in that environment. In landscapes with limestone bedrock, the slightly acidic groundwater dissolves the calcium carbonate (calcite) in the rock. This dissolutional action can result in caves and cave systems.Once anastomoses are formed, beds below the parting plane or bedding plane are removed by gravitational collapse into a large underlying passage, or by complete dissolution. The result is a cave ceiling with a network of channels.The examples shown here are relatively deep and have resulted in the development of numerous, closely-spaced pendants. In caves, pendants are projections of rock formed by dissolution.The publicly accessible portions of Endless Caverns are developed in structurally tilted carbonates of the New Market Limestone (Middle Ordovician). Other portions of Endless Caverns are in overlying units: the Lincolnshire Limestone and the Edinburgh Formation.Locality: Endless Caverns, south of the town of New Market, eastern Rockingham County, northern Virginia, USA |
| 撮影日 | 2016-05-13 12:16:40 |
| 撮影者 | James St. John |
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