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Fins are narrow, residual walls of hard sedimentary rock (e.g., sandstone, limestone) that remain upright after adjacent rock has been eroded away along systematic parallel joints or fractures on either side. (They are named by analogy with a fish's dorsal fin, which they resemble.) A fin is typically formed when a narrow butte or plateau develops a series of vertical cracks in a parallel pattern (often due to tensional stresses along anticlinal or monoclinal fold axes). Differential water erosion, sometimes as part of a freeze-thaw cycle, opens the cracks wider and wider over time. Where two or more fins are present, a slot canyon may be formed between them. A wall left standing in between two slot canyons may be termed a fin. Alternatively, an entirely isolated single fin may develop where a rock stratum is notably harder (more resistant to weathering) relative to neighboring strata. Fins are considered an intermediary stage in the formation of other striking erosional features including alcoves, windows and arches; these are natural holes that form along cracks and weak spots in the sides of fins. Myriads of fins, and more advanced erosional features, may be seen at Arches National Park in Utah, US. Rock fins and arches are believed to form slowly over long periods of time by physical and chemical weathering. Four steps are proposed: (1) uplift that causes deep vertical, parallel fractures to form; (2) weathering and erosion that enlarge fractures resulting in narrow walls or fins; (3) continuing erosion with some fins breached from below; and (4) continued weathering that enlarges the holes and eventually causes the arch to collapse, forming spires or hoodoos. ==Gallery== File:Garden of the Gods 01.jpg|A cluster of fins at Garden of the Gods, Colorado Springs, Colorado, US. File:Seneca Rocks by Asilverstein Oct 2013 High Dynamic Range Merge from 7 Exposures.jpg|Seneca Rocks, a prominent single fin in eastern West Virginia, US. File:Arches National Park aerial geology.jpg|Aerial view of innumerable rock fins at the Fiery Furnace area of Arches National Park, Utah, US. File:More Devils Garden.jpg|Rock fins at Devil's Garden, Arches National Park. File:Nelson Rock - two fins.jpg|A recreationalist exploits the double rock fins at Nelson Rocks, West Virginia, US. Image:Pia16128-640-MatijevicHill-EnduranceCrater-20120928.jpg|Fins are not restricted to the Earth: Matijevic Hill, Mars 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「Fin (geology)」の詳細全文を読む スポンサード リンク
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