Ice Rafted Boulder

June 25, 2012


Photographer: Marli Bryant Miller
Summary Author: Marli Bryant Miller

June 2012 Viewer's Choice This 40-ton boulder of Precambrian argillite (slightly metamorphosed shale) lies some 250 ft (76 m) above the valley floor in Oregon's Willamette Valley. The closest bedrock exposures of similar rock lie in northwestern Idaho.

Beginning with research by J. Harlen Bretz back in 1923, the effects of the Missoula Floods have been a point of discovery and controversy for countless geologists. Caused by catastrophic failures of a glacially dammed river in what is now northern Idaho and Montana between 18,000-15,000 years ago, some 40 mega flood events inundated the Columbia Basin. Several of these floods had flow rates that exceeded 230 million cfs (6.5 million cubic m per second) -- about 400 times the average of the Mississippi River. Where the channel became restricted, the floods backflooded tributary valleys; one such restriction exists northeast of Portland, Oregon, causing the entire Willamette Valley to flood all the way to what is now the city of Eugene. Huge glacial erratics like this boulder hitched rides on blocks of ice and came to rest on hillsides at the elevation of the flood stage. Of course, the valley in the background must have been full of water up to the level of the boulder. Photo taken on May 27, 2012. [Revised December 2017]

Photo Details: Camera: Canon EOS 5D Mark II; Lens: EF17-40mm f/4L USM; Focal Length: 20.0mm; Aperture: f/8.0; Exposure Time: 0.0080 s (1/125); ISO equiv: 100; Software: Adobe Photoshop CS Macintosh.