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Sailing stones, sliding rocks, rolling stones, and moving rocks are a geological phenomenon where rocks move and inscribe long tracks along a smooth valley floor without human or animal intervention. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms tend to wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone's wake. Trails differ in both direction and length. Rocks that start next to each other may travel parallel for a time, before one abruptly changes direction to the left, right, or even back to the direction from which it came. Trail length also varies – two similarly sized and shaped rocks may travel uniformly, then one could move ahead or stop in its track. Tracks of sliding rocks have been observed and studied in various locations, including Little Bonnie Claire Playa in Nevada, and most notably Racetrack Playa, Death Valley National Park, California, where the number and length of tracks are notable. At Racetrack Playa, these tracks have been studied since the early 1900s, yet the origins of stone movement were not confirmed〔(【引用サイトリンク】url=http://www.businessinsider.com/sailing-stones-in-death-valley-california-2012-12 )〕 and remained the subject of research for which several hypotheses〔(【引用サイトリンク】url=http://skeptoid.com/episodes/4021 )〕 existed. However, as of August 2014, timelapse video footage of rocks moving has been published, showing the rocks moving at low wind speeds within the flow of thin, melting sheets of ice. The scientists have thus identified the cause of the moving stones to be ice shove.〔http://www.scientificamerican.com/article/mystery-of-death-valley-s-wandering-stones-explained-video/ Mystery of Death Valley's "Wandering Stones" Explained ()〕〔(【引用サイトリンク】title=United States: 'Sailing rocks' mystery finally solved )〕 ==Description== The Racetrack’s stones speckle the playa floor, predominately in the southern portion. Historical accounts identify some stones hundreds of feet from shore, yet most of the stones are found relatively close to their respective originating outcrops. Three lithologic types are identified: (1) syenite, found most abundant on the west side of the playa; (2) dolomite, subrounded blue-gray stones with white bands; and (3) black dolomite, the most common type, found almost always in angular joint blocks or slivers.〔Stanley, G. M., Origin of Playa Stone Tracks, Racetrack Playa, Inyo County California, Geological Society of America Bulletin, 66, 1329–1350, 1955〕 This dolomite composes nearly all stones found in the southern half of the playa, and originates at a steep promontory, , paralleling the east shore at the south end of the playa. Intrusive igneous rock originates from adjacent slopes (most of those being tan-colored feldspar-rich syenite). Tracks are often tens to hundreds of feet long, about wide, and typically much less than an inch (2.54 cm) deep. Most moving stones range from about six to 18 inches in diameter. A balance of very specific conditions is thought to be needed for stones to move: *a saturated yet non-flooded surface *a thin layer of clay *very strong gusts as initiating force *strong sustained wind to keep stones going And in some hypotheses: *ice floes 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「Sailing stones」の詳細全文を読む スポンサード リンク
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