Shonkin Sag
Encyclopedia
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.
, (about 23 miles (37 km) east of the city of Great Falls
) and runs in an easterly direction for about 100 miles (160.9 km) until it reaches the Judith River
. It varies from 0.25 mile (0.402335 km) to 2 miles (3.2 km) in width and is about 500 feet (152.4 m) deep. Its midpoint is located at approximately 47.591948°N 110.68943°W. Shallow glacial lake
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
word for the Highwood Mountains, or an adulteration of one of the town's early settlers, John Shonk.
, the Missouri River drained to the northeast into a terminal lake. 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.
According to geologist Fred H.H. Calhoun and others, the ice sheet forced the waters of Glacial Lake Great Falls to reach at least 3900 feet (1,188.7 m) above sea level. About 15,000 years ago, a glacial lake outburst flood
occurred. 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. 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.
, a famous laccolith 200 feet (61 m) thick and 1 miles (1.6 km) wide near the western mouth of the Shonkin Sag. 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. The Shonkin Sag laccolith is cited by geologists as a classic example of igneous differentiation
in a single igneous intrusion.
Fluvioglacial landform
Fluvioglacial landforms are landforms molded by glacial meltwater. This discharge of glacial streams, both over the surface and beneath the ice sheet , is higher in the warmer summer months. As subglacial water often flows under pressure, it has a high velocity and is very turbulent...
located along the northern edge of the Highwood Mountains
Highwood Mountains
The Highwood Mountains cover approximately 4,659 km² in north central Montana in the U.S., east of Great Falls and Benton Lake National Wildlife Refuge, at the northern end of the Lewis and Clark National Forest...
in the state of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. The Sag is a river channel formed by the Missouri River
Missouri River
The Missouri River flows through the central United States, and is a tributary of the Mississippi River. It is the longest river in North America and drains the third largest area, though only the thirteenth largest by discharge. The Missouri's watershed encompasses most of the American Great...
and glacial meltwater
Meltwater
Meltwater is the water released by the melting of snow or ice, including glacial ice and ice shelfs over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing...
pouring from Glacial Lake Great Falls
Glacial Lake Great Falls
Glacial 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. Centered on the modern city of Great Falls, Montana, Glacial Lake Great Falls extended as far north as Cut Bank, Montana, and as far south...
. It is one of the most famous prehistoric meltwater channels in the world.
Location and size
Shonkin Sag is located in central Montana. It begins south of Highwood, MontanaHighwood, Montana
Highwood is a census-designated place in Chouteau County, Montana, United States. The population was 189 at the 2000 census. It first had a post office opened in 1881. It was closed and reopened a few times until 1886 after which it has remained open continuously.-Geography:Highwood is located at...
, (about 23 miles (37 km) east of the city of Great Falls
Great Falls, Montana
Great Falls is a city in and the county seat of Cascade County, Montana, United States. The population was 58,505 at the 2010 census. It is the principal city of the Great Falls, Montana Metropolitan Statistical Area, which encompasses all of Cascade County...
) and runs in an easterly direction for about 100 miles (160.9 km) until it reaches the Judith River
Judith River
The Judith River is a tributary of the Missouri River, approximately 124 mi long, running through central Montana and the United States. It rises in the Little Belt Mountains and flows northeast past Utica and Hobson...
. It varies from 0.25 mile (0.402335 km) to 2 miles (3.2 km) in width and is about 500 feet (152.4 m) deep. Its midpoint is located at approximately 47.591948°N 110.68943°W. Shallow glacial lake
Glacial lake
A glacial lake is a lake with origins in a melted glacier. Near the end of the last glacial period, roughly 10,000 years ago, glaciers began to retreat. A retreating glacier often left behind large deposits of ice in hollows between drumlins or hills. As the ice age ended, these melted to create...
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
Blackfeet
The Piegan Blackfeet are a tribe of Native Americans of the Algonquian language family based in Montana, having lived in this area since around 6,500 BC. Many members of the tribe live as part of the Blackfeet Nation in northwestern Montana, with population centered in Browning...
word for the Highwood Mountains, or an adulteration of one of the town's early settlers, John Shonk.
Formation
Prior to the Quaternary glaciationQuaternary glaciation
Quaternary glaciation, also known as the Pleistocene glaciation, the current ice age or simply the ice age, refers to the period of the last few million years in which permanent ice sheets were established in Antarctica and perhaps Greenland, and fluctuating ice sheets have occurred elsewhere...
, the Missouri River drained to the northeast into a terminal lake. During the last glacial period about 17,000 to 13,000 years ago, the Laurentide ice sheet
Laurentide ice sheet
The Laurentide Ice Sheet was a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and a large portion of the northern United States, multiple times during Quaternary glacial epochs. It last covered most of northern North America between c. 95,000 and...
blocked the Missouri River and created Glacial Lake Great Falls.
According to geologist Fred H.H. Calhoun and others, the ice sheet forced the waters of Glacial Lake Great Falls to reach at least 3900 feet (1,188.7 m) above sea level. About 15,000 years ago, a glacial lake outburst flood
Glacial lake outburst flood
A glacial lake outburst flood is a type of outburst flood that occurs when the dam containing a glacial lake fails. The dam can consist of glacier ice or a terminal moraine...
occurred. 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. 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 laccolithLaccolith
A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at relatively...
, a famous laccolith 200 feet (61 m) thick and 1 miles (1.6 km) wide near the western mouth of the Shonkin Sag. A laccolith is an igneous intrusion injected between two layers of sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
. Significant amounts of syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
and shonkinite
Shonkinite
Shonkinite is a mafic foidal syenite, an intrusive, holocrystalline rock which in the restricted sense is composed of potassic feldspar in the form of sanidine, nepheline, augite, biotite, and olivine. Shonkinite is also used for mafic nepheline syenite with aegerine-augite as the pyroxene, and...
can be found in the laccolith. The Shonkin Sag laccolith is cited by geologists as a classic example of igneous differentiation
Igneous differentiation
In geology, igneous 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.-Primary melts:...
in a single igneous intrusion.
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