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PLASTER & PAINT PHASE -

17. Ice Age Floods: Oregon's best soils lifted from eastern Washington

Exotic Terranes

Toward the end of the last ice age, 15,000 to 20,000 years ago, the massive ice cap that covered the northern hemisphere repeatedly blocked the Clark Fork of the Columbia River in Montana, forming a huge lake behind the ice dam equal to two of today’s Great Lakes. When the dam broke, some of the largest floods known to have occurred on Earth raged across eastern Washington and down the Columbia River to the ocean. Over 40 of these huge floods, called Missoula, Ice Age, or Bretz floods (for J Harlen Bretz, the geologist who identified the floods), scoured the landscape and left behind thick deposits of sand, silt, and gravel along the Columbia River in Umatilla and Morrow counties and in the Willamette and Tualatin valleys. Boulders embedded in the ice were carried in these massive floods and were dropped along the way as the ice melted. These “glacial erratics” include famous rocks like the Willamette Meteorite and the rock at Erratic Rock State Natural Site near Sheridan. Topsoil in Montana, Idaho, and eastern Washington was scoured away by the floods and deposited here, producing the rich agricultural lands of the Willamette Valley and Columbia Basin.
   About 14,500 years ago a giant pluvial lake (Lake Bonneville) in Utah flooded north along the Snake River, leaving deposits near Ontario.

OGDC units: includes Quaternary Missoula and Bonneville Flood Deposits

Age Range: 23,000 to 17,000 years ago
Rock Types: gravel, silt
Did You Know? The peak flow of the ice age floods through the Columbia Gorge was 350 million cubic feet per second, 350 times greater than the historic 1948 Columbia River flood that destroyed the city of Vanport.

 

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