Multiple Catastrophic Drainage of Glacial Lake Missoula, Montana

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Route

Start point

No route given. This field guide describes two localities: (1) Camas Prairie approximately 65 miles northwest of Missoula, and (2) roadcuts on and near I-90 in the vicinity of the Ninemile Exit, 19 miles west of Missoula.

End point

No route given.

Roads

Montana 382, Montana 28, I-90

Total distance

No route given.

Geology

Summary

This field guide describes two localities related to the history and drainage of Glacial Lake Missoula northwest of Missoula, Montana. The first site highlights spectacular erosional and depositional features that illustrate the effects of extremely large discharge related to the drainage of the lake. In a sense, the area is the world's most extravagant natural flume experiment. Features include bedrock basins (scoured by flood waters), large deltaic bars, and megaripples as large as 35 feet from crest to trough. The second site, a large roadcut along I-90, contains a record of at least 36 fillings of the lake, presumably during Pinedale(?) time, suggesting that drainage occurred many times. Another roadcut exposes a similar, but unstudied record of deposition in a higher terrace that probably formed during Bull Lake(?) time and may indicate an older history of lake formation and drainage.

Key Lithologic Features

  • Precambrian quartzite and argillite (Belt Supergroup)
  • large deltaic bars
  • sedimentary debris (cobbles, sand, and silt) deposited in megaripples
  • alternating couplets of varved siltstone (lacustrine) and sandstone (fluvial) in Glacial Lake Missoula deposits

Structures

  • extremely large-scale sedimentary ripples
  • varves

Landforms

  • bedrock basins
  • deltaic bars
  • megaripples

Other Features

Reference

Alt, D.D., 1987, Multiple catastrophic drainage of Glacial Lake Missoula, Montana, in Beus, S.S., ed., Centennial Field Guide Volume 2: Rocky Mountain Section of the Geological Society of America , p. 33-36.

Availability

Out of print. Check Amazon.com. Digital access to this publication is available via the links in the references section above.