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One type of ribbed moraine - which we have given the genetical name Niemisel-type ribbed moraine - can be shown to consist of three distinct architectural elements formed due to subglacial folding, thrust stacking and lee-side cavity infill (proximal and distal elements), followed by draping of a deforming-bed till element (Lindén et al. 2008). This type of landform occurs in abundance in valleys in northern Sweden from the areas around the highest shoreline (∼200 m a.s.l.) and towards the present position of the Bothnian Bay coast. Associated with these quite large moraine ridges are the much smaller De Geer moraine ridges, occurring in dense agglomerations and with individual ridges, laying both in the troughs between and superimposed on top of the ribbed moraine ridges. De Geer moraines are suggested to form at the ending of the subglacial 'conveyer belt', i.e. at the grounding line of the glaciers' font, standing in water depths of 100-200 m, and deposited as subaqueous fans consisting of both diamicts and sorted sediments which in their distal part interfinger with glaciolacustrine sediments (Linden & Möller 2005).
Proposed depositional models for these moraine types suggest that Niemisel-type ribbed moraine formed in a transitional zone between deforming bed and rigid bed conditions, the latter propagating up-glacier as a result of falling basal meltwater pressure at the end of the peak season or, alternatively, propagating down-glacier down-glacier at the beginning of the melt season when the basal drainage recharged, gradually increasing basal meltwater pressure. Lee-side cavities, developed behind thrust/fold-induced ridges, formed an integral part of a subglacial linked-cavity network, regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal ice flow velocity. Contrary, De Geer moraines were formed preferentially during the summer season at temporary halts in the grounding-line retreat as a result of subglacial sediment advection to the ice margin, at places producing up to 4-6 ridges each melt season. During ice-marginal retreat, De Geer moraine(s) were occasionally superimposed on previously formed ribbed moraine ridges, demonstrating the proximal-distal and temporal relations between these ridge types and also their linkage, being products at various phases of deforming bed conditions.
References:
Lindén, M., Möller, P. & Adrielsson, L., 2008: Ribbed moraine formed by subglacial folding, thrust stacking and lee-side cavity infill. Boreas 37/1, 102-131.
Lindén, M., & Möller, P., 2005: Marginal formation of De Geer moraines and their implication on the dynamics of grounding-line recession. Journal of Quaternary Science 20, 113-133.
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