Within‐valley asymmetry and related problems of Neoglacial lateral moraine development at certain Jotunheimen glaciers, southern Norway

Abstract

A difference in the size of Neoglacial lateral moraines on either side of a valley axis (within‐valley asymmetry of lateral moraine development) is described. Analysis of clast roundness has revealed subangular material in latero‐terminal and terminal moraines; lateral moraines, however, exhibit a compositional gradient of increasing angularity with distance from the former glacier snout. Comparisons with clasts of known origin suggest that this ‘roundness gradient’ may be explained with reference to either or both of two hypotheses: (1) a variable proportion of supraglacial (or englacial) to subglacial transported material; and (2) the variable composition of regolith incorporated by a push mechanism from the valley sides. Within‐valley asymmetry is inferred to result where the supply of debris to lateral moraines from these sources is unequal either side of a valley axis. Both interpretations are also consistent with the relatively large size of latero‐terminal sections of end moraines. In order to account for the discrepancy between moraine size and apparent debris supply rates, it is suggested that the largest lateral moraines may have been formed over a longer time scale than the ‘Little Ice Age’, and that reworking of deposits may have occurred. The supply of debris to the north‐facing lateral moraine at Nordre Illåbreen has been so great that it has developed into a rock glacier; this suggests the possibility that subglacial material and valley‐side regolith, as well as supraglacial material, contributes to the formation of ice‐cored rock glaciers.