|
Crowley's Ridge (also Crowleys Ridge) is an unusual geological formation that rises 250 to above the alluvial plain of the Mississippi embayment in a line from southeastern Missouri to the Mississippi River near Helena, Arkansas. It is the most prominent feature in the Mississippi Alluvial Plain between Cape Girardeau, Missouri,and the Gulf of Mexico. This narrow rolling hill region rising above the flat plain is the sixth, and smallest, natural division of the state of Arkansas. Most of the major cities of the Arkansas Delta region lie along Crowley's Ridge. It was named after Benjamin Crowley, known as the first European-American settler to reach the area, sometime around 1820. The Civil War Battle of Chalk Bluff was fought on Crowley's Ridge on May 1–2, 1863. ==Composition and origin== The ridge is primarily composed of the windblown glacially derived sediment known as loess. It contrasts greatly with the flat table land around it and with the black soil that makes up the delta. The ridge varies from half a mile to wide and reaches an elevation of near its northern extremity. The formation is generally thought to have originally been an island between the Mississippi River and Ohio River that was isolated as a long low hilly ridge after the rivers changed course millions of years ago.〔(National Scenic Byways - Scenic Byways and Highways - Arkansas Scenic Routes )〕 Recent research, however, questions the fluvial origin.〔 Loess deposits are found along both sides of the Mississippi River Alluvial Valley. These loess deposits are a classic example of periglacial loess.〔Bettis, E.A., III, D.R. Muhs, H.M. Roberts, and A.G. Wintle, 2003, ("Last glacial loess in the conterminous U.S.A." ), ''Quaternary Science Reviews'', vol. 22, pp. 1907-1946.〕〔Muhs, D.R., and E.A. Bettis, III, 2003, (''Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes.'' ) Geological Society of America Special Paper no. 370, pp. 53-74.〕 Crowley's Ridge is a natural loess accumulation point. A comparable example of this type of deposit is the Loess Hills in northwestern Missouri and southwestern Iowa. There is evidence that the area's elevation has increased over the years, suggesting that uplift is still taking place. This alternative explanation posits a link between the ridge and the nearby New Madrid Seismic Zone.〔Roy B. Van Arsdale, Robert A. Williams, Eugene S. Schweig, Kaye M. Shedlock, Jack K. Odum and Kenneth W.King, "The origin of Crowley's Ridge, northeastern Arkansas: Erosional remnant or tectonic uplift?" ''Bulletin of the Seismological Society of America''; August 1995; v. 85; no. 4; p. 963-985. (Abstract )〕 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「Crowley's Ridge」の詳細全文を読む スポンサード リンク
|