Sea Stacks of Accreted Terranes Along the Southern Oregon Coast

February 08, 2012

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PhotographerMarli Bryant Miller; Marli's Web site
Summary Author: Marli Bryant Miller

The southern Oregon coast hosts thousands of sea stacks, freestanding rocks that rise above the ocean surface. They form because of erosion: as waves gradually wear away a headland, they don't remove everything at once. The erosional remnants are left as sea stacks. A fascinating corollary to this process is that sea stacks represent earlier positions of the coastline. Some lie nearly a mile away from today's shore!

These particular rocks consist of folded layers of chert, a sedimentary rock made of countless silica-producing microscopic floating organisms called radiolarians. In deep marine environments, radiolarians accumulate at the very slow rates of about 30 to 80 ft (10 to 25 m) in a million years. These rocks were added (accreted) to the North American land mass when they were scraped off the subducting seafloor at the continent's edge. They're part of the so-called Franciscan Assemblage, a series of crustal fragments that were accreted during the late Cretaceous Period. Photo taken on December 6, 2011.

Photo details: Camera Model: Canon EOS 5D Mark II; Lens: EF17-40mm f/4L USM; Focal Length: 28.0mm; Aperture: f/11.0; Exposure Time: 0.200 s (1/5); ISO equiv: 50; Orientation: Normal; Color Space: Adobe RGB (1998); Software: Adobe Photoshop CS4 Macintosh.