PARTICLE SIZE DISTRIBUTIONS WITHIN NONSORTED STONY EARTH CIRCLES, COLORADO

Abstract

While stony earth circles, like many forms of patterned ground, result from a variety of processes, a common a priori assumption seems to be that a single process dominates circle formation in a particular region. This study evaluates two models (frost heave and deflation) for the formation of nonsorted stony earth circles in two populations of circles on Niwot Ridge, Colorado, by examination of the coarse particle and silt distributions within the circles using Principal Components, Cluster, and Discriminant Analyses. Samples were collected at 2.5 cm depth increments from two populations (Sites A and B). Circles from the two sites differ in particle size distributions and possibly in dominant mechanism of formation. At Site A, circles form in materials with higher silt content (i.e., greater frost susceptibility) than adjoining vegetated surfaces, and with moderate amounts of coarse material. Although frost heave may be active within the circles, the coarse particle distribution suggests deflation creates the coarse circle surface. Site B circles also contain moderate amounts of coarse particles but do not contain more silt than abutting vegetated surfaces. Particle distributions with depth support the conclusion that frost heave does not dominate the formation of Site B circles. Two of the circles show slight coarsening at the surface, likely the result of deflation. Data generated in this study do not support the idea that circles within a small region (<10 km2) necessarily result from the same process. [Key words: periglacial geomorphology, patterned ground, Rocky Mountains.]