The Development of Sorted Circles on Recently Deglaciated Terrain, Jotunheimen, Norway

Abstract

The nature and origin of sorted circles that have formed on the glacier foreland of Slettmarkbreen, southern Norway, since ca. A.D. 1750 are examined. The circles range in size from <0.6 m in diameter on a fluted till plain deglaciated within the last 50 yr to >3.0 m on the three oldest moraines of the glacier foreland. Their structural and granulometric characteristics indicate that they are the result of frost sorting. The sizes of Rhizocarpon geographicum agg. on clasts and the pattern of vascular plant colonization indicate that the circles stabilize within 50 yr of exposure of the ground. The distributional characteristics and relatively short life of the circles suggest that they formed through rapid frost-sorting of water-soaked till under the colder and wetter conditions of the ice-marginal zone during periods of slow glacier retreat, and not in response to regional climatic conditions. Restriction of circles > 1.5 m in diameter to the crests of well-developed moraines is considered to reflect exposure of the moraine crests in winter and consequent deeper ground freezing. These findings imply that caution must be exercised in interpreting the paleoenvironmental significance of large-scale sorted circles in formerly glaciated areas, as such circles may have formed at the margin of a slowly retreating glacier and would consequently be unrepresentative of regional conditions.