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The origin and characteristics of soil mounds and patterned ground in North Central Oregon

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https://ir.library.oregonstate.edu/concern/graduate_projects/rf55z832m

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  • Sections of the landscape in north central Oregon are dominated by numerous soil mounds surrounded by stone rings. Mounds vary in size and shape depending on slope and mound density. Internal soil mass is free of stones to the bedrock level and is covered by successive layers of volcanic ash. Stone ring profiles exhibited strong vertical sorting of stones by size and a decrease in ring width with depth. Stone areas were generally void of soil except for a shallow layer on the bottom. Noticeable depressions in the bedrock layer were found beneath the rings. Inter-mound areas appear as large surface drainage swales with a very shallow soil layer containing numerous stones. Formation of stone rings is attributed to the ejection of stones from the soil mass by intense frost action under a former periglacial climate. Increased subsurface drainage, piping, and erosion led to a lowering of the soil surface above the stone rings, thus segregating the high centered mounds. Small scale frost features continue to be formed though the climate is no longer of sufficient intensity to produce stone rings and soil mounds.
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