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Brook, Martin; Hagg, Wilfried; Winkler, Stefan (2017): Contrasting medial moraine development at adjacent temperate, maritime glaciers: Fox and Franz Josef Glaciers, South Westland, New Zealand. In: Geomorphology, Vol. 290: pp. 58-68
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Medial moraines form important pathways for sediment transportation in valley glaciers. Despite the existence of well-defined medial moraines on several glaciers in the New Zealand Southern Alps, medial moraines there have hitherto escaped attention. The evolving morphology and debris content of medial moraines on Franz Josef Glacier and Fox Glacier on the western flank of the Southern Alps is the focus of this study. These temperate maritime glaciers exhibit accumulation zones of multiple basins that feed narrow tongues flowing down steep valleys and terminate similar to 400 m above sea level. The medial moraines at both glaciers become very prominent in the lower ablation zones, where the medial moraines widen, and develop steeper flanks coeval with an,increase in relative relief. Medial moraine growth appears somewhat self-limiting in that relief and slope angle increase eventually lead to transport of debris away from the medial moraine by mass-movement-related processes. Despite similarities in overall morphologies, a key contrast in medial moraine formation exists between the two glaciers. At Fox Glacier, the medial moraine consists of angular rockfall-derived debris, folded to varying degrees along flow-parallel axes throughout the tongue. The debris originates above the ELA, coalesces at flow-unit boundaries, and takes a medium/high level transport pathway before subsequently emerging at point-sources aligned with gently dipping fold hinges near the snout. In contrast at Franz Josef Glacier, the medial moraine emerges farther down-glacier immediately below a prominent rock knob.Clasts show a mix of angular to rounded shapes representing high level transport and subglacially transported materials, the latter facies possibly also elevated by supraglacial routing of subglacial meltwater. Our observations confirm that a variety of different debris sources, transport pathways, and structural glaciological processes can interact to form medial moraines within New Zealand's Southern Alps.