Reconstructing the transport history of glacigenic sediments: a new approach based on the co-variance of clast form indices

Abstract

Two indices are used to describe the aggregate shape and roundness characteristics of glacially transported clasts: the C40 index (the percentage of clasts with ca ratios ⩽ 0.4) and the RA index (the percentage of very angular and angular clasts in a sample). Analysis of the between- and within-sample co-variance of these indices provides a powerful tool for discriminating actively- and passively-transported clasts in glacigenic deposits. Use of these indices is illustrated in an analysis of the form characteristics of clasts from lateral and frontal moraines at Storbreen, Jotunheimen. These fall between those of actively transported clasts in basal till (low C40 and RA values) and those of unmodified frost-weathered clasts (high C40 and RA values). Both indices decline down-moraine. Within-sample analyses of clast shape demonstrates that these aggregate clast form gradients reflect changes in the relative proportions of actively and passively transported debris on the moraines rather than progressive modification of actively transported clasts. These analyses also indicate that actively transported clasts form the dominant component of the moraines, particularly near the former glacier terminus. Further applications of the approach developed here are suggested.