Editing Great Falls, Montana (section)

=== Geology ===
The city of Great Falls lies atop the [[Great Falls Tectonic Zone]], an intracontinental [[Shear (geology)|shear zone]] between two [[geologic province]]s of [[Basement (geology)|basement rock]] of the [[Archean]] period which form part of the North American [[continent]].<ref>Boerner, D.E.; Craven, J.A.; Kurtz, R.D.; Ross, G.M.; and Jones, F.W. "The Great Falls Tectonic Zone: Suture or Intracontinental Shear Zone?" ''[[Canadian Journal of Earth Sciences]].'' 35:2 (1998); O'Neill, J. Michael and Lo, David A. "Character and Regional Significance of Great Falls Tectonic Zone, East-Central Idaho and West-Central Montana". ''AAPG Bulletin.'' 69 (1985); Mueller, Paul A.; Heatherington, Ann L.; Kelly, Dawn M.; Wooden, Joseph L.; and Mogk, David W. "Paleoproterozoic Crust Within the Great Falls Tectonic Zone: Implications for the Assembly of Southern Laurentia". ''Geology.'' 30:2 (February 2002); Harms, Tekla A.; Brady, John B.; Burger, H. Robert; and Cheney, John T. ‘Advances in the Geology of the Tobacco Root Mountains, Montana, and Their Implications for the History of the Northern Wyoming Province.’ ''Precambrian geology of the Tobacco Root Mountains, Montana.'' Special Papers, Volume 377. John B. Brady, H. Robert Burger, John T. Cheney, and Tekla A. Harms, eds. Boulder, Colo.: Geological Society of America, 2004. {{ISBN|0-8137-2377-9}}</ref> The city lies at the southern reach of the [[Laurentide Ice Sheet]], a vast glacial sheet of ice which covered much of North America during the [[last glacial period]]. Approximately 1.5&nbsp;million years ago, the Missouri River flowed northward into a [[Endorheic basin|terminal lake]].<ref name="Clawson">Clawson, Roger and Shandera, Katherine A. ''Billings: The City and the People.'' Helena, Mont.: Farcountry Press, 1998. {{ISBN|1-56037-037-8}}</ref><ref>McRae, W.C. and Jewell, Judy. ''Moon Montana.'' 7th ed. Cambridge, Massachusetts: PublicAffairs, 2009. {{ISBN|1-59880-014-0}}</ref> The Laurentide Ice Sheet pushed the river southward.<ref name="Clawson" /><ref name="Montagne">Montagne J.L. 'Quaternary System, Wisconsin Glaciation.' ''Geologic Atlas of the Rocky Mountain Region.'' Denver: Rocky Mountain Association of Geologists, 1972.</ref> Between 15,000 [[Common Era|BCE]] and 11,000 BCE, the Laurentide Ice Sheet blocked the Missouri River and created [[Glacial Lake Great Falls]].<ref name="Feathers">[http://gsa.confex.com/gsa/inqu/finalprogram/abstract_53968.htm Feathers, James K. and Hill, Christopher L. "Luminescence Dating of Glacial Lake Great Falls, Montana, U.S.A."] {{Webarchive|url=https://web.archive.org/web/20141129125733/https://gsa.confex.com/gsa/inqu/finalprogram/abstract_53968.htm |date=November 29, 2014 }} ''XVI International Quaternary Association Congress.'' Stratigraphy and Geochronology Session. International Quaternary Association, Reno, 2003.</ref><ref name="Montagne" /><ref>Hill, Christopher L. and Valppu, Seppo H. "Geomorphic Relationships and Paleoenvironmental Context of Glaciers, Fluvial Deposits, and Glacial Lake Great Falls, Montana". ''Current Research in the Pleistocene.'' 14 (1997); Hill, Christopher L. "Pleistocene Lakes Along the Southwest Margin of the Laurentide Ice Sheet". ''Current Research in the Pleistocene.'' 17 (2000); Hill, Christopher L. and Feathers, James K. "Glacial Lake Great Falls and the Late-Wisconsin-Episode Laurentide Ice Margin". ''Current Research in the Pleistocene.'' 19 (2002); [http://pubs.usgs.gov/sim/2005/2860/ Reynolds, Mitchell W. and Brandt, Theodore R. ''Geologic Map of the Canyon Ferry Dam 30' x 60' Quadrangle, West-Central Montana: U.S. Geological Survey Scientific Investigations Map 2860, scale 1:100,000.''] Scientific Investigations Map 2860. Washington, D.C.: U.S. Geologic Survey, 2005.</ref> About 13,000 BCE, as the glacier retreated, Glacial Lake Great Falls emptied catastrophically in a [[glacial lake outburst flood]].<ref name="Feathers" /> The current course of the Missouri River essentially marks the southern boundary of the Laurentide Ice Sheet.<ref>"Agriculture". In ''Encyclopedia of the Great Plains.'' David J. Wishart, ed. Lincoln, Neb.: University of Nebraska Press, 2004. {{ISBN|0-8032-4787-7}}</ref> The Missouri River flowed eastward around the glacial mass, settling into its present course.<ref name="Clawson" /> As the ice retreated, meltwater from Glacial Lake Great Falls poured through the [[Highwood Mountains]] and eroded the mile-long, {{convert|500|ft|m|adj=mid|-deep}} [[Shonkin Sag]], one of the most famous prehistoric meltwater channels in the world.<ref>Axline, Jon and Bradshaw, Glenda Clay. ''Montana's Historical Highway Markers.'' Rev. ed. Helena, Mont.: Montana Historical Society, 2008. {{ISBN|0-9759196-4-4}}; Bowman, Isaiah. "Forest Physiography: Physiography of the United States and Principles of Soils in Relation to Forestry". ''American Environmental Studies.'' Reprint ed. Charles Gregg, ed. New York: Arno Press, 1970. {{ISBN|0-405-02659-5}}</ref>