Shonkin Sag

The Shonkin Sag is a prehistoric fluvioglacial landform located along the northern edge of the Highwood Mountains in the state of Montana in the United States. The Sag is a river channel formed by the Missouri River and glacial meltwater pouring from Glacial Lake Great Falls. It is one of the most famous prehistoric meltwater channels in the world.[1]

A portion of the Shonkin Sag near Highwood, Montana, in 1920.

Location and size
Map of a portion of the Shonkin Sag (outlined in red) near Highwood, Montana.

Shonkin Sag is located in central Montana. It begins south of Highwood, Montana, (about 23 miles (37 km) east of the city of Great Falls) and runs in an easterly direction for about 100 miles (160 km) until it reaches the Judith River.[2] It varies from 0.25 miles (0.40 km) to 2 miles (3.2 km) in width[2] and is about 500 feet (150 m) deep.[3][4] Its midpoint is located at approximately 47.591948°N 110.68943°W / 47.591948; -110.68943. Shallow glacial lakes may still be found along the now-dry channel.[5]

The sag is named for the town of Shonkin, Montana, settled in the 1870s. The name "Shonkin" is allegedly the Blackfeet word for the Highwood Mountains, or an adulteration of one of the town's early settlers, John Shonk.[6]

Formation

Prior to the Quaternary glaciation, the Missouri River drained to the northeast into a terminal lake or Hudson Bay.[7] During the last glacial period about 17,000 to 13,000 years ago, the Laurentide Ice Sheet blocked the Missouri River and created Glacial Lake Great Falls.[8][9]

According to geologist Fred H.H. Calhoun and others, the ice sheet forced the waters of Glacial Lake Great Falls to reach at least 3,900 feet (1,200 m) above sea level.[10][4] About 15,000 years ago, a glacial lake outburst flood occurred.[11][9] These waters and their attendant debris then carved the Shonkin Sag at right angles across the existing drainage valleys, with the ice sheet forming the northern edge of the channel.[11][4] Once the ice sheet retreated, the Shonkin Sag continued to provide a channel for water draining from Glacial Lake Great Falls and the Missouri River, albeit at a much slower rate and at a lower level.[11]

Laccolith

The Shonkin Sag lends its name to the Shonkin Sag laccolith, a famous laccolith 200 feet (61 m) thick and 1 mile (1.6 km) wide near the western mouth of the Shonkin Sag.[12] A laccolith is an igneous intrusion injected between two layers of sedimentary rock. Significant amounts of syenite and shonkinite can be found in the laccolith.[13][14] The Shonkin Sag laccolith is cited by geologists as a classic example of igneous differentiation in a single igneous intrusion.[15]

Railroad Grade

Part of the North Montana line of the Chicago, Milwaukee, St. Paul and Pacific Railroad was constructed within Shonkin Sag (see photo above).[16]

Notes
  1. Axline, Jon and Bradshaw, Glenda Clay. Montana's Historical Highway Markers. Rev. ed. Helena, Mont.: Montana Historical Society, 2008, p. 91.
  2. "Societies and Academies: Geological Society of Washington." Science. May 7, 1895, p. 559-560.
  3. Calhoun, Fred H.H. The Montana Lobe of the Keewatin Ice Sheet. Washington, D.C.: Government Printing Office, 1906, p. 40.
  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, p. 413.
  5. McRae, W.C. and Jewell, Judy. Montana. Berkeley, Calif.: Avalon Travel, 2009, p. 398; Spomer, Ron. Big Game Hunter's Guide to Montana. Belgrade, Mont.: Wilderness Adventures Press, 2005, p. 297.
  6. Aarstad, Rich; Arguimbau, Ellen; Baumler, Ellen; Prosild, Charlene L.; and Shovers, Brian. Montana Place Names from Alzada to Zortman. Helena, Mont.: Globe Pequot, 2009, p. 242.
  7. Howard, A.D. "Drainage Evolution in Northeastern Montana and Northwestern North Dakota." Bulletin of the Geological Society of America. 69 (1958): 575-588.
  8. Alden, W.C. Physiography and Glacial Geology of Eastern Montana and Adjacent Areas. U.S. Geological Survey Professional Paper 174. 1958; Montagne J.L. "Quaternary System, Wisconsin Glaciation." Geologic Atlas of the Rocky Mountain Region. Denver: Rocky Mountain Association of Geologists, 1972; 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); 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.
  9. Feathers, James K. and Hill, Christopher L. "Luminescence Dating of Glacial Lake Great Falls, Montana, U.S.A." XVI International Quaternary Association Congress. Stratigraphy and Geochronology Session. International Quaternary Association, Reno, 2003; Clausen, Eric. "Meltwater Flood Origin for Great Plains Drainage Network." Proceedings of the North Dakota Academy of Science. 43 (1989): p. 40.
  10. Calhoun, F.H.H. The Montana Lobe of the Keewatin Ice Sheet. Washington, D.C.: Government Printing Office, 1906], p. 42.
  11. Calhoun, F.H.H. The Montana Lobe of the Keewatin Ice Sheet. Washington, D.C.: Government Printing Office, 1906], p. 42-43.
  12. James, Furman Kemp. A Handbook of Rocks - For Use Without the Petrographic Microscope. 6th ed. Los Angeles: James Press, 2007, p. 26; Beall, Joseph J. "Pseudo-Rhythmic Layering in the Square Butte Alkali-Gabbro Laccolith." American Mineralogist. 57 (1972): 1294-1302.
  13. Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.
  14. Shonkinite is a coarse-grained rock composed of augite and biotite with a small amount of orthoclase. See: Weed, Walter H. and Pirsson, Louis V. Geology of the Little Belt mountains, Montana, With Note on the Mineral Deposits of the Neihart, Barker, Yogo, and Other Districts. Washington, D.C.: Government Printing Office, 1900, p. 319.
  15. Barksdale, J.D. "The Shonkin Sag Laccolith." American Journal of Science. 23 (1937): 321–359.
  16. USGS contour maps o47110d7 et al; "Guide to the Milwaukee Road in Montana", McCarter, Montana Historical Society Press, 1992, p. 80
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