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The Driftless Area or Paleozoic Plateau is a region in the American Midwest noted mainly for its deeply carved river valleys. While primarily in southwestern Wisconsin, it includes areas of southeastern Minnesota, northeastern Iowa and extreme northwestern Illinois. The region includes elevations ranging from 603 to 1,719 feet (184 to 524 m) and covers an area of . The region's peculiar terrain is the result of its having escaped glaciation in the last glacial period.〔It may be termed a geologic province of North America. Several names for this area are used. ''Driftless Area'' is widely used by both federal and state sites and seems to be the common name. ''Paleozoic Plateau'' shows up in a number of learned and popular articles and may be on its way to being the preferred name. ''Coulee Region'' is primarily a term used in Wisconsin for the greater La Crosse Metropolitan area. ''Little Switzerland'' is encountered for the Iowa region of the Driftless Area, particularly for the southern portion of Dubuque County and Swiss Valley Park while areas west of Madison, Wisconsin are occasionally referred to as ''Little Norway''. ''Driftless Zone'', ''Driftless Region'', ''Driftless Land'' are also encountered〕〔("Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin: Section IV. Driftless Area" ), USGS, Retrieved July 13, 2007; another government site, ("Driftless Area Initiative" ), USDA, retrieved July 15, 2007, gives and 〕 ==Geologic formation== Retreating glaciers leave behind silt, clay, sand, gravel, and boulders called drift. Glacial drift includes unsorted material called till and layers deposited by meltwater streams called outwash〔("The Driftless Area" ), ''Minnesota Conservation Volunteer'', March 2007 (popular article from the Minnesota Department of Natural Resources (MDNR)), Retrieved July 7, 2007〕 While some glacial drift has been discovered, this is said to be of Pre-Illinoian age, about 500,000 years old.〔〔("Yellow River State Forest" ), Iowa Department of Natural Resources (IDNR), Retrieved July 7, 2007〕〔Byron Crowns. "Wisconsin through 5 Billion Years of Change", Wisconsin Earth Science Center, 1976, Wisconsin Rapids, Wisconsin, p. 131, 〕 The region has been subject to the regular catastrophic effects of glacial lake outburst floods involving the cataclysmic collapse of ice dams holding in such bodies as Glacial Lake Agassiz, Glacial Lake Grantsburg, and Glacial Lake Duluth (''see'' Jökulhlaup). The earlier local phases of the Wisconsinan glaciation are poorly understood, but the last phases involved several major lobes: the Des Moines lobe, which flowed down toward Des Moines on the west; the Superior lobe and its sublobes on the north; and the Green Bay lobe and Lake Michigan lobes on the east.〔() 〕 The northern and eastern lobes were in part diverted around the area by the Watersmeet Dome, an ancient uplifted area of Cambrian rock underlain by basalt. The Green Bay and Lake Michigan lobes were also partially blocked by the bedrock of the Door Peninsula, which presently separates Green Bay from Lake Michigan.〔() 〕 In earlier phases of the Wisconsinan, the Driftless Area was totally surrounded by ice, with eastern and western lobes joining together to the south of it. The latest concept of the origin of the Driftless Area is the pre-Illinoian continental glacial ice flowing over the Driftless Area and depositing on it pre-Illinoian till, which is more than 790,000 years old. When the ice retreated and uncovered the area, periglacial erosion removed it: Anticyclonic snow-bearing winds episodically dropped large amounts of snow, which then gradually removed superficial sediment by solifluction and snowmelt overland flow, so-called sheetwash.〔Iannicelli (2010), Evolution of the Driftless Area and contiguous regions of midwestern USA through Pleistocene periglacial processes, The Open Geology Journal, volume 4, pp. 35 - 54〕 In the adjacent glaciated regions, the glacial retreat left behind drift, which buried all former topographical features. Surface water was forced to carve out new stream beds.〔 ("Native American use of the Mississippi River" ), ''Archaeology Education Program'', Volume 22, Number 2, Fall 2004, Mississippi Valley Archaeology Center at the University of Wisconsin–La Crosse, Retrieved July 8, 2007 ( *pdf)〕 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「Driftless Area」の詳細全文を読む スポンサード リンク
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