Sorted circles,relative-age dating and palaeoenvironmental reconstruction in an alpine periglacial environment,eastern Jotunheimen,Norway: lichenometric and weathering-based approaches

Abstract

Lichenometric and weathering-based techniques are employed in an investigation of the chronology of large-scale sorted circles in Jotunheimen, southern Norway. Comparison of diameter measurements of Rhizocarpon geographicum agg. with local growth curves, and size-frequency analysis of lichen population data, indicate that all the sorted circles originated prior to the mid-eighteenth century 'Little Ice Age' maximum. Parts of the circle borders appear to have been stable since before the twelfth century, although minor disturbances to border clasts were widespread between about AD 1640 and 1740. Large proportions of the smaller surface clasts in circle centres have stabilized only since the 'Little Ice Age' maximum. The thickness of surface weathering rinds and measurements of rock surface hardness (R-values obtained using a Schmidt hammer) indicate long periods of clast exposure in circle borders (at least 1500 years at some sites) and shorter exposure times for central clasts. Despite difficulties in inferring the nature of rind thickness/age and R-value/age curves, the evidence based on weathering criteria strongly points to early Holocene origins. These results demonstrate the potential for using relative-age dating techniques in chronological and palaeoenvironmental investigations of sorted patterned ground, and suggest that a similar approach may be applicable in studies of other stony periglacial features.