RECONSTRUCTING THE TRANSPORT HISTORY OF GLACIOGENIC SEDIMENTS - A NEWAPPROACH BASED ON THE CO-VARIANCE OF CLAST FORM INDEXES

Two indices are used to describe the aggregate shape and roundness cha racteristics of glacially transported clasts: the C40 index (the perce ntage of clasts with c/a ratios less-than-or-equal-to 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 indic es provides a powerful tool for discriminating actively- and passively -transported clasts in glacigenic deposits. Use of these indices is il lustrated in an analysis of the form characteristics of clasts from la teral and frontal moraines at Storbreen, Jotunheimen. These fall betwe en those of actively transported clasts in basal till (low C40 and RA values) and those of unmodified frost-weathered clasts (high C40 and R A values). Both indices decline down-moraine. Within-sample analyses o f clast shape demonstrates that these aggregate clast form gradients r eflect changes in the relative proportions of actively and passively t ransported debris on the moraines rather than progressive modification of actively transported clasts. These analyses also indicate that act ively transported clasts form the dominant component of the moraines, particularly near the former glacier terminus. Further applications of the approach developed here are suggested.