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Lake Monongahela was a proglacial lake in western Pennsylvania, West Virginia, and Ohio. It formed during the Pre-Illinoian ice epoch when the retreat of the ice sheet northwards, blocked the drainage of these valleys to the north. The lake formed south of the ice front continued to rise, until it was able to breach a low divide near, New Martinsville, West Virginia. The overflow was the beginning of the process to create the modern Ohio River valley. ==Preglacial drainage== The Monongahela flowed north from Lewis County to Pittsburgh. Here, the Lower Allegheny joins and they continue westward in the Ohio River valley to Beaver, Pennsylvania. At Beaver, the predecessor of the upper Ohio River joins and together the water flows northwest, up the Beaver River into Ohio. Using a combination of waterways it crossed the divide into the Lake Erie basin to join ancestral St. Lawrence River through Canada.〔Lake Monongahela: Anatomy of an Immense Ice Age Pond; John A. Harper; Bureau of Topographic and Geologic Survey; 1997〕 *The “Upper Allegheny” River began in McKean County, south of Bradford flowing north into New York. Possibly following Conawango Creek to the Lake Erie basin to join the ancestral St. Lawrence River through Canada.〔 *The “Middle Allegheny” River began in Warren County, east of Warren. Following the modern Allegheny River valley, south to Franklin, Venango County it turned to the northwest into the valley of French Creek. Crossing the divide outside of Erie into the Lake Erie Basin, where it joins ancestral St. Lawrence River through Canada.〔 *The “Lower Allegheny” River originated in the Clarion River valleys in Elk, Forest, and Jefferson Counties. Following the Clarion River, it flowed southward to join the Monongahela River at Pittsburgh.〔 *The “Ohio River” was a tributary of the Monongahela River, joining the Monongahela at Beaver.〔 The preglacial Monongahela River drained three fourths of combined drainage of the modern Ohio, Monongahela, and Allegheny Rivers.〔 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「Lake Monongahela」の詳細全文を読む スポンサード リンク
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