From push moraine to single-crested dump moraine during a sustained glacier advance

Abstract

In the 1980s, the NE part of the principal outlet glacier Kötlujökull on the east flank of the temperate ice cap Mýrdalsjökull was advancing; the maximum average rate of annual advance of 32 m was recorded during the period 1982-84. Along the advancing glacier margin a frontal push moraine was currently being produced by continuous mixing of proglacially thrust and folded outwash sediments with material avalanched down the steep front of the debris-covered glacier snout. During periods of fast glacier advance, the pushed ridge was mainly a combination of slabs of outwash deposits, but during periods of slow glacier advance, the pushed ridge chiefly consisted of front cliff-fall debris. By the end of the 1980s the glacier advance had stopped, and during the 1990s the most terminal part of the debris-mantled glacier developed into a 300-500 m wide zone of dead ice with a complex morphology of steep-sided mounds, ridges, collapsing edges and ice walls. At present the total lowering of the ice-cored terrain amounts to 25-30 m. The glacier has left a single-crested ridge, 4-7 m high, without evidence of thrusting; the ridge consists exclusively of front cliff-fall debris and can be defined as a dump moraine. Without having observed the active mechanism of ridge formation throughout the 1980s, we could mistakenly have interpreted this ridge as a moraine ridge associated with a pause in a general ice recession. However, the dump-moraine ridge marks the maximum extent of a sustained glacier advance, and during this advance the basic mechanisms involved in the frontal moraine formation were pushing and thrusting.