Wave-cut strandlines cut into the slope at left in photo. These cuts record former high-water lines, or shorelines. Gullies above the highway are the result of modern-day erosion. (NPS
Sediment deposited by the lake with a hammer for scale.
Lake Missoula was a prehistoric proglacial lake in western Montana that existed periodically at the end of the last ice age between 15,000 and 13,000 years ago. The lake measured about 7,770 square kilometres (3,000 sq mi) and contained about 2,100 cubic kilometres (500 cu mi) of water, half the volume of Lake Michigan.
The Glacial Lake Missoula National Natural Landmark is located about 68 miles northwest of Missoula, Montana at the north end of the Camas Prairie Valley, just east of Montana Highway 382 and Macfarlane Ranch. It was designated as a National Natural Landmark in 1966 because it contains the great ripples, (often measuring 25 to 50 feet (7.6 to 15 m) high and 300 feet (91 m) long), that served as a strong supporting element for J Harlen Bretz's contention that Washington State's Channeled Scablands were formed by repeated cataclysmic floods over only about 2,000 years, rather than through the millions of years of erosion that had been previously assumed.
The lake was the result of an ice dam on the Clark Fork caused by the southern encroachment of a finger of the Cordilleran Ice Sheet into the Idaho Panhandle (at the present day location of Clark Fork, Idaho at the west end of Lake Pend Oreille). The height of the ice dam typically approached 610 metres (2,000 ft), flooding the valleys of western Montana approximately 320 kilometres (200 mi) eastward. It was the largest ice-dammed lake known to have occurred.
The periodic rupturing of the ice dam resulted in the Missoula Floods – cataclysmic floods that swept across Eastern Washington and down the Columbia River Gorge approximately 40 times during a 2,000 year period. The cumulative effect of the floods was to excavate 210 cubic kilometres (50 cu mi) of loess, sediment and basalt from the channeled scablands of eastern Washington and to transport it downstream. These floods are noteworthy for producing canyons and other large geologic features through cataclysms rather than through more typical gradual processes.
See also 
Cordilleran Ice Sheet
maximum extent of Glacial Lake Missoula (eastern) and Glacial Lake Columbia (western)
areas swept by Missoula and Columbia floods
Cited references 
- ^ Bjornstad, Bruce N. (c2006). On the trail of the Ice Age floods : a geological field guide to the mid-Columbia basin / Bruce Bjornstad. Sandpoint, Idaho: Keokee Books. p. 4. ISBN 978-1-879628-27-4.
- ^ Soennichsen, John (c2008). Bretz's Flood : The Remarkable Story of a Rebel Geologist and the World's Greatest Flood / John Soennichsen. Seattle, Washington: Sasquatch Books. pp. 215–248. ISBN 978-1-57061-631-0.
- ^ Alt, David (c2001). Glacial Lake Missoula and Its Humongous Floods / David Alt. Missoula, Montana: Mountain Press. pp. 27–28. ISBN 0-87842-415-6.
- ^ Allen, John Eliot; Burns, Marjorie and Sargent, Sam C. (c1986). Cataclysms on the Columbia : a layman's guide to the features produced by the catastrophic Bretz floods in the Pacific Northwest. Portland, OR: Timber Press. p. 104. ISBN 0-88192-067-3.
External links