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.
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. It varies from 0.25 miles (0.40 km) to 2 miles (3.2 km) in width and is about 500 feet (150 m) deep. Its midpoint is located at approximately 47°35′31″N 110°41′22″W / 47.591948°N 110.68943°W. Shallow glacial lakes 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.
Prior to the Quaternary glaciation, the Missouri River drained to the northeast into a terminal lake or Hudson Bay. 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.
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