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Great Unconformity
Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland,〔Rance, H (1999) (''Historical Geology: The Present is the Key to the Past.'' ) QCC Press, New York, N.Y.〕 or that observed by John Wesley Powell in the Grand Canyon in 1869.〔Merten, G (2005) (''Geology in the American Southwest: New Processes, New Theories'' In MF Anderson, ed., A Gathering of Grand Canyon Historians. ) Proceedings of the Inaugural Grand Canyon History Symposium, January 2002. Grand Canyon Association, Grand Canyon, Arizona.〕 Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods of geologic time sufficiently long to raise great mountains and then erode them away.
==Background==
Unconformities in general tend to reflect long-term changes in the pattern of the accumulation of sedimentary or igneous strata in low-lying areas (often ocean basins, such as the Gulf of Mexico or the North Sea, but also Bangladesh and much of Brazil), then being uplifted and eroded (such as the ongoing Himalayan orogeny, the older Laramide orogeny of the Rocky Mountains, or much older Appalachian (Alleghanian) and Ouachita orogenies), then subsequently subsiding, eventually to be buried under younger sediments. The intervening periods of tectonic uplift are generally periods of mountain building, often due to the collision of tectonic plates. The "great" unconformities of regional or continental scale (in both geography and chronology) are associated with either global changes in eustatic sea level or the supercontinent cycle, the periodic merger of all the continents into one approximately every 500 million years.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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