Observations on terminal moraine-ridge formation during recent advances of southern Norwegian glaciers

Abstract

Abstract The formation of terminal moraines dating from glacier advances during the A.D. 1990s was investigated at six temperate glaciers in the Jostedalsbreen and Jotunheimen regions of southern Norway. Morphological and sedimentological information was analysed and combined with detailed observations, photographic evidence, and length variation measurements. Bulldozing (pushing) of proglacial sediments was the predominant process at the low-lying glacier termini of the debris-free outlets of the Jostedalsbreen ice cap. Evidence for bulldozing includes the relocation and rotation of boulders and the presence of lichen-covered boulders in the distal slopes of moraines. Differences in moraine sedimentology between the individual sites are attributed to variations in the pre-existing material pushed up, rather than to specific processes. In Jotunheimen, several modes of moraine-ridge formation were active: dumping of supraglacial debris and the marginal freezing-on and subsequent melting-out of subglacial sediment slabs were more important than bulldozing at some sites and in some years. Low air temperatures in winter at the higher altitude glacier termini enabled freezing-on to occur. However, the forelands are below the lower limit of discontinuous permafrost and evidence of thrusting is absent. Whereas discrete terminal moraine ridges at Jostedalsbreen are related to (seasonal) advances, the less dynamic glaciers of Jotunheimen allow the accretion of successive ridges during consecutive years. No single regional mode of moraine formation is characteristic for southern Norway: several processes are involved depending on individual glacier morphology and dynamics, the glaciological regime, and environmental conditions (especially climate) on the glacier forelands. The importance of specific processes varies within and between glacier forelands. Thus, spatial and temporal variation and local individuality has to be taken into account in the interpretation of moraine-ridge formation. Similar variability is likely to have been characteristic of terminal moraines deposited during the “Little Ice Age” and earlier in the Quaternary.