Geologists long recognized that something was odd about the landscape of eastern Washington state. The landscape is marked by dry canyons and deeply eroded basalt flows—the channeled scablands. In other areas, parallel ridges of undulating low hills cover the bedrock.
In the 1920s geologist J Harlen Bretz interpreted these features as the result of erosion and deposition from floods originating with the breach of glacially dammed lakes—the ripple-shaped hills (shown in the aerial view photo) were exactly that—giant ripples deposited by enormous outpouring of water from Glacial Lake Missoula.
The channeled scablands formed around 15,000 years ago after an ice dam holding back the waters of glacial Lake Missoula failed, sending 2,000 cubic kilometers of water rushing through the breach and reshaping the landscape in a matter of days.
Further info: http://www.nps.gov/history/history/online_books/geology/publications/inf/72-2/intro.htm
Photo credit: http://hypography.com/forums/earth-science/12606-missoula-floods-channeled-scablands-drumheller-channels.html
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