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.
[Axline, Jon and Bradshaw, Glenda Clay. Montana's Historical Highway Markers. Rev. ed. Helena, Mont.: Montana Historical Society, 2008, p. 91.]
Location and size
Shonkin Sag is located in central Montana. It begins south of Highwood, Montana, (about east of the city of Great Falls) and runs in an easterly direction for about until it reaches the
Judith River.
["Societies and Academies: Geological Society of Washington." Science. May 7, 1895, p. 559-560.] It varies from to in width
and is about deep.
[ Calhoun, Fred H.H. The Montana Lobe of the Keewatin Ice Sheet. Washington, D.C.: Government Printing Office, 1906, p. 40.] Its midpoint is located at approximately . Shallow
may still be found along the now-dry channel.
[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.]
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.[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.]
Formation
Prior to the Quaternary glaciation, the Missouri River drained to the northeast into a
Endorheic basin or
Hudson Bay.
[Howard, A.D. "Drainage Evolution in Northeastern Montana and Northwestern North Dakota." Bulletin of the Geological Society of America. 69 (1958): 575-588.] 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.
[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.]
According to geologist Fred H.H. Calhoun and others, the ice sheet forced the waters of Glacial Lake Great Falls to reach at least above sea level.[Calhoun, F.H.H. The Montana Lobe of the Keewatin Ice Sheet. Washington, D.C.: Government Printing Office, 1906], p. 42.][ 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.] About 15,000 years ago, a glacial lake outburst flood occurred.[ 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.] 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.[Calhoun, F.H.H. The Montana Lobe of the Keewatin Ice Sheet. Washington, D.C.: Government Printing Office, 1906], p. 42-43.]
Laccolith
The Shonkin Sag lends its name to the Shonkin Sag
laccolith, a famous laccolith thick and wide near the western mouth of the Shonkin Sag.
[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.] 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.
[Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.][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.] The Shonkin Sag laccolith is cited by geologists as a classic example of igneous differentiation in a single igneous intrusion.
[Barksdale, J.D. "The Shonkin Sag Laccolith." American Journal of Science. 23 (1937): 321–359.]
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).
[USGS contour maps o47110d7 et al; "Guide to the Milwaukee Road in Montana", McCarter, Montana Historical Society Press, 1992, p. 80]
Notes
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