Nonsorted Circle Development in a Colorado Alpine Location

Abstract

The origin, development, and dynamics of nonsorted circles, a variety of patterned ground produced by seasonal frost heaving, are described and analyzed for an alpine location in the Colorado Front Range. Age relations of nonsorted circles and allied features are also discussed.Nonsorted circles favour exposed snow-free locations, particularly the frontal areas of turf-banked lobe and terrace treads. Here, wind scour causes the disruption and removal of the protective mat of the alpine turf. Frost penetration into freshly exposed sites promotes, in the presence of adequate moisture, differential frost heaving with the subsequent doming of circle centres.A buried A horizon observed in a number of pits dug into nonsorted circles occupying a turf-banked lobe, was dated at 10,400 ± 400 radiocarbon years. The material in which the circles subsequently developed became sufficiently stable for soil development by the close of the Pinedale Glaciation (7,500 B.P.). Field evidence suggests they may be a recent phenomena, initiated after the close of the Gannett Peak stade of Neoglaciation.