|
Loess (, , , or ; from German Löss (:lœs)) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust.〔 It covers about 10% of the Earth's surface.〔 Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometer size range, twenty percent or less clay and the balance equal parts sand and silt〔Donahue, Miller, Shickluna (1977). Soils: An Introduction to Soils and Plant Growth, 4th edition. Prentice Hall〕 that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs. The word ''loess'', with connotations of origin by wind-deposited accumulation, came into English from German ''Löss'' ((:lœs)), which can be traced back to Swiss German and is cognate with the English word ''loose'' and the German word ''los''.〔http://www.etymonline.com/index.php?term=loess〕 It was first applied to Rhine River valley loess about 1821.〔(【引用サイトリンク】title=loess (sedimentary deposit) - Encyclopedia Britannica )〕〔(【引用サイトリンク】title=DWDS | Suchergebnisse )〕 ==Properties== Loess is homogeneous, porous, friable, pale yellow or buff, slightly coherent, typically non-stratified and often calcareous. Loess grains are angular with little polishing or rounding and composed of crystals of quartz, feldspar, mica and other minerals. Loess can be described as a rich, dust-like soil.〔Pearson Prentice Hall - World Studies - Europe and Russia〕 Loess deposits may become very thick; more than a hundred meters in areas of China and tens of meters in parts of the Midwestern United States. It generally occurs as a blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces.〔Neuendorf, K.E.K., J.P. Mehl, Jr., and J.A. Jackson, 2005, ''Glossary of Geology.'' Springer-Verlag, New York, 779 pp., ISBN 3-540-27951-2〕 Because the grains are angular, loess will often stand in banks for many years without slumping. This soil has a characteristic called vertical cleavage which makes it easily excavated to form cave dwellings, a popular method of making human habitations in some parts of China. Loess will erode very readily. In several areas of the world, loess ridges have formed that are aligned with the prevailing winds during the last glacial maximum. These are called paha ridges in America and greda ridges in Europe. The form of these loess dunes has been explained by a combination of wind and tundra conditions. 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「Loess」の詳細全文を読む スポンサード リンク
|