Particle Size and Clay Mineral Distributions within Sorted and Nonsorted Circles and the Surrounding Parent Material, Niwot Ridge, Front Range, Colorado, U.S.A.

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Abstract
Sorted and nonsorted circles are patterned ground forms that develop within silt-rich pockets of alpine tundra that have been stripped of vegetation. Although both circle types are relatively silt-rich compared to the average soil texture beneath the still vegetated areas, sorted circles are richer than nonsorted circles in silt, but nonsorted circles are richer in granule-size (-2 to -1.vphi.) material. An inverse relationship between silt and granule proportions occurs in the 0 to 10 cm depth range within both circle types and nearby vegetated sections; a similar relationship also occurs beneath nonsorted circles and vegetated areas from 10-40 cm, but not beneath sorted circles. Diurnal ground/frost-heave cycles are presumed to produce sorting to .apprx. 10 cm, and seasonal ground/frost-heave cycles to produce the same effect at greater depths. Clay/silt ratios and clay mineral assemblages indicate that weathering is retarded below the circles in comparison to nearby vegetated areas. Eolian infall is accumulating on vegetated areas but deflation dominates the circles surfaces; as a result clay assemblages differ markedly at a microscale. These well-defined mineralogic differences indicate that circle centers are stable for quite long periods. The results obtained from the crest of Niwot Ridge, Front Range, Colorado, USA, in this study tend to corroborate a model for circle development in central Labrador-Ungava, Canada, previously proposed.