Desiccation Cracking and Sorted Polygon Development, Jotunheimen, Norway

Abstract

On a till sheet exposed over the last 35 yr by retreat of the glacier Storbreen, there is a spatial transition from simple networks of desiccation cracks on the youngest ground to mature sorted polygons on the oldest. The cracks form 2 to 6 yr after deglaciation. Analysis of the development of sorted polygons from nonsorted desiccation cracks reveals that over 35 yr the width and depth of the cracks, together with network length and complexity, increase in an exponential fashion. Sorting of clasts into cracks proceeds slowly but regularly and is accompanied by an increase in the proportion of fine material coarser than 250 μm within the cracks while adjacent ground becomes relatively rich in particles finer than 250 μm. The migration of coarse sand into the cracks is accompanied by mass-displacement of silt from the cracks into the adjacent ground and depletion of fine sand from both cracks and adjacent ground. The slow rate of sorted polygon development at Storbreen is attributed to the slow development of the crack network, itself possibly related to the depth and granulometry of the till. The appearance of vegetation on the oldest polygons suggests that these may be approaching the end of their development.