Landform transitions from pronival ramparts to moraines and rock glaciers: a case study from the Smørbotn cirque, Romsdalsalpane, southern Norway

Abstract

ABSTRACTLandform transitions are defined as intermediate forms that represent transient developmental stages between conventional landform types. This study evaluates possible cases of landform transitions from pronival (protalus) ramparts to moraine ridges, and from pronival ramparts to lobate rock glaciers (protalus lobes) at the foot of the headwall of Smørbotn cirque in southern Norway. The five landforms had been previously classified as pronival ramparts. We conclude that only two (Smørbotn 2 and 3) are undisputed, active pronival ramparts, which developed under the seasonal-freezing regime of the Holocene. It is inferred that a third (Smørbotn 1) represents the transition to a moraine ridge formed during the ‘Little Ice Age’ of the last few centuries as a semi-permanent snowbed grew into a small temperate glacier. The two others (Smørbotn 7 and 8) appear to be relict embryonic rock glaciers that developed between the Last Glacial Maximum and the Younger Dryas Stadial under a permafrost regime and benefited from enhanced debris supply as a result of rock-slope instability affected by glacier debuttressing and permafrost degradation. Variable landscape settings and distinctive environmental histories contribute to the differences in the morphology of these landforms. We highlight continuing controversies over the modes of formation and diagnostic characteristics of pronival ramparts by positioning them, together with push/dump moraines, ice-cored moraines and rock glaciers, in a conceptual model of the periglacial–glacial landform continuum. The model links snow, ice and debris fluxes under seasonal-freezing and/or permafrost climatic regimes to the process thresholds between landform types.