The Shonkin Sag is a prehistoric fluvioglacial landform located along the northern edge of the

Highwood Mountains The Highwood Mountains are an island range (sub-range of the Rockies entirely surrounded by prairie) which cover approximately 4,659 km2 (1,799 sq mi) of the Central Montana Alkalic Province in north central Montana in the U.S. They are in ...

in the state of

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. The Sag is a river channel formed by the

Missouri River The Missouri River is a river in the Central United States, Central and Mountain states, Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Moun ...

and glacial

meltwater Meltwater (or melt water) is water released by the melting of snow or ice, including glaciers, glacial ice, tabular icebergs and ice shelf, ice shelves over oceans. Meltwater is often found during early spring (season), spring when snow packs a ...

pouring from Glacial Lake Great Falls. It is one of the most famous prehistoric

meltwater channel A meltwater channel (or sometimes a glacial meltwater channel) is a channel cut into ice, bedrock or unconsolidated deposits by the flow of water derived from the melting of a glacier or ice-sheet. The channel may form on the surface of, within, ...

s in the world.

Location and size

Shonkin Sag is located in central Montana. It begins south of

Highwood, Montana Highwood is an unincorporated community and census-designated place (CDP) in Chouteau County, Montana, Chouteau County, Montana, United States. The population was 165 at the 2020 Dicennial Census. History Highwood first had a post office in 1881, ...

, (about east of the city of Great Falls) and runs in an easterly direction for about until it reaches the

Judith River The Judith River is a tributary of the Missouri River, approximately 124 mi (200 km) long, running through central Montana in the United States. It rises in the Little Belt Mountains and flows northeast past Utica and Hobson. It is ...

."Societies and Academies: Geological Society of Washington." ''

Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...

.'' 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

glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10,0 ...

s may still be found along the now-dry channel. The sag is named for the town of Shonkin, Montana, settled in the 1870s. The name "Shonkin" is allegedly the

Blackfeet The Blackfeet Nation (, ), officially named the Blackfeet Tribe of the Blackfeet Indian Reservation of Montana, is a federally recognized tribe of Siksikaitsitapi people with an Indian reservation in Montana. Tribal members primarily belong ...

word for the Highwood Mountains, or an adulteration of one of the town's early settlers, John Shonk.

Formation

Prior to the

Quaternary glaciation The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial period, glacial and interglacial, interglacial periods during the Quaternary period that began 2.58 Year#SI prefix multipliers, Ma (million ...

, the Missouri River drained to the northeast into a terminal lake or

Hudson Bay Hudson Bay, sometimes called Hudson's Bay (usually historically), is a large body of Saline water, saltwater in northeastern Canada with a surface area of . It is located north of Ontario, west of Quebec, northeast of Manitoba, and southeast o ...

. 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 Lake Great Falls was a prehistoric proglacial lake which existed in what is now central Montana in the United States between 15,000 BCE and 11,000 BCE.Hill, Christopher L. and Valppu, Seppo H. "Geomorphic Relationships and Paleoenvironmental Con ...

. 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.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 A glacial lake outburst flood (GLOF) is a type of outburst flood caused by the failure of a dam containing a glacial lake. An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a j� ...

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. 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.

Laccolith

The Shonkin Sag lends its name to the Shonkin Sag

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apart ...

, a famous laccolith thick and wide near the western mouth of the Shonkin Sag. A laccolith is an

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

injected between two layers of

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

. Significant amounts of

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). It is considered a shonkinite Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite. It makes up much of the hard dark grey rock comprising certain mountains ...

can be found in the laccolith. The Shonkin Sag laccolith is cited by geologists as a classic example of

igneous differentiation In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of ...

in a single igneous intrusion.

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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External links

"The Shonkin Sag: An In-Depth Excursion Along the East Side of the Highwood Mountains." Gerald T. Davidson, PhD
- Personal Web site conducting a geological trip along the Shonkin Sag by a noted physicist Geology of Montana Glacial landforms Former rivers