We studied low prairie (Mima) mounds and ridges with sorted stone borders separated by broad rubbly soil intermounds in areas near Mount Shasta, northern California. An earlier study ascribed a purely physical origin for these soil features based on a four-stage conceptual model. Mounds were interpreted as periglacially produced clay domes formed in polygonal ground and stone perimeters as loose gravity accumulations in unexplained shallow trenches at dome peripheries. The model was widely cited to account for similar stone-bordered prairie mounds and rubbly soil intermounds in the Pacific Northwest. Our observations and measurements indicate, however, that these mounded landscapes are...