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The Three Sisters are three rocky islands in the Potomac River in Washington, D.C., west of the Key Bridge. A notable landmark in colonial times, the islets are less well known as the Three Sisters Islands〔Evelyn, Dickson, and Ackerman, p. 245; Potomac Planning Task Force, p. 64.〕 and Three Sisters Island.〔Flynn and Mason, p. 9; Stanton, p. xiv.〕 Bridges have been proposed for the Three Sisters several times, most recently in the 1960s, when the Three Sisters Bridge proposal led to a decade of protests and ultimately, its cancellation. ==Geographical description== The Three Sisters are part of the fall line, a geologic feature which distinguishes the sedimentary coastal plain of the mid-Atlantic region of the United States from the basement rock of the inland.〔Wahll, p. 128.〕 Part of this basement rock, the Three Sisters are composed of granite.〔Smith, p. 22.〕 Roughly 300,000 years ago,〔(Emory Scott A.; Fanz, Amy K.; Hayes, Daniel; Schopp, Paul W.; and Gill, Christine. ''Combined Phase IB Archaeological Survey and Phase II Archaeological Investigation: SR 1 North Frederica Grade Separated Intersection, Frederica, Kent County, Delaware.'' Volume I. February 2007, p. 4. ) Accessed 2013-03-03.〕 Atlantic Ocean levels were much higher. At that time, the lower Potomac River was submerged beneath the Atlantic Ocean, whose shores reached the Three Sisters.〔McAtee, p. 62.〕 The Potomac River near Washington, D.C., is usually no more than deep, on average.〔Wennerstrom, p. 27.〕 However, there is a deep channel near the Three Sisters which generally is about deep, but can drop to just or less during low tide or periods of little precipitation.〔Mason, p. 5.〕〔 The rock formation generates sand bars on which grass grows, and these are often visible at low tide.〔Office of Conservation, Interpretation, and Use, p. 44, 46.〕 Some sources claim that the Three Sisters represent the farthest navigable point on the Potomac River,〔〔Porterfield, p. 33.〕 although other sources claim that navigability extends farther north to Little Falls Branch.〔Myer, p. 1; Jones, p. 71.〕 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「Three Sisters (District of Columbia)」の詳細全文を読む スポンサード リンク
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