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Mackinac Falls is a submerged 100-foot (30 m)-high waterfall formation under the waters of the Straits of Mackinac. The formation, which lies approximately 1 mile (1.5 km) east of Arch Rock on Mackinac Island, was the former outflow point for water flowing eastward from Lake Chippewa (proto-Lake Michigan) into Lake Stanley (proto-Lake Huron). The formation was discovered on August 16, 2007. ==Postglacial Straits of Mackinac== During the period that immediately followed the end of the Wisconsin glaciation, the geographic features that would become North America's Great Lakes significantly changed in shape and size. Glacial melt, catastrophic erosion events as dammed-up waters found new outlets, and the post-glacial rebound of this section of the Earth's crust, caused various freshwater lakes in this area to form, drain away, and re-form. During one of these periods, dated 10,000 years before the present, what is now the upper Great Lakes drained towards the Atlantic Ocean through a deep channel that passed eastward through Georgian Bay and what is now the small city of North Bay, Ontario. This channel was very efficient at draining water; the Great Lakes drainage basin was as big then as it is now, and collected a considerable amount of rainfall, but the upper Great Lakes were physically smaller. In particular, the overall level of present-day Lake Michigan was much lower than its current level of 581 feet (177 m) above sea level. This shallow lake, called Lake Chippewa, collected water from streams throughout its watershed and then discharged it through a gorge at its east end, towards a similarly small and shallow precursor to Lake Huron, called Lake Stanley. The sole outflow for the water in Lake Chippewa was the ''Mackinac Channel'', with a water depth of as much as 70 meters (230 feet), which generally followed the pathway of the current Straits of Mackinac. However, just east of present-day Mackinaw City, the flowing water that carved the gorge encountered an impasse; the limestone breccia underneath what is now Mackinac Island and Round Island partially dammed the gorge. In response, the water formed a horseshoe curve detour to the north around the area.〔http://www.ngdc.noaa.gov/mgg/greatlakes/lakemich_cdrom/html/geomorph.htm〕 Just east of Mackinac Island were the shores of Lake Stanley. Lake Stanley's level was approximately lower than that of Lake Chippewa, and when the Mackinac Channel gorge found its way around the obstruction, it discharged its water over Mackinac Falls. Later, the North Bay channel was blocked by further post-glacial rebound, forcing both Lake Huron and Lake Michigan, along with Lake Superior, to drain southward into the Lake Erie Basin. The levels of both Lake Huron and Lake Michigan rose towards their current level, and the two lakes merged hydrologically with each other. Mackinac Falls was completely submerged.〔(【引用サイトリンク】 publisher = Mackinac Island Town Crier )〕 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「Mackinac Falls」の詳細全文を読む スポンサード リンク
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