Sforms

Landforms and landform distributions produced in the hypothetical subglacial environment are indicated in a general model (Fig. 4.4). Like most bedforms, individuals may be transverse or longitudinal, depending on their alignment relative to the flow. Purely erosional landforms are scoured from hard bedrock and are referred to as s-forms, standing for sculpted forms (Kor et al., 1991; Fig. 4.3). Ljungner (1930) conducted the first comprehensive study of s-forms on crystalline bedrock in southwestern Sweden. Hjulstrom (1935) and Dahl (1965) also argued that these forms are products of meltwater erosion. See Kor et al. (1991) for details.

4.4.3.1 Muschelbruche

Muschelbruche are scoop-like depressions in the rock, with sharp, parabolic upstream rims and a steep slope on the upstream side of the depression. In the down-flow direction the floors of depressions are gently sloping and merge imperceptibly with the rock surface. Experimental muschelbruche may be formed in flumes by running slightly acidic water over a plaster of Paris bed (Shaw, 1996). The forms of the flume muschelbruche and those in nature are identical. This experiment counters a common misconception regarding sheet flows and bedforms. After 30h of flow there was no sign of channels forming in the bed, but muschelbruche continued to be created and to evolve beneath sheet or broad flows. There seems little point in raising theoretical objections to bed formation beneath broad flows when observations clearly show such formation.

4.4.3.2 Sichelwannen

Sichelwannen are erosional forms with sharp, crescentic upstream rims, and a crescentic main furrow or trough wrapped around a medial ridge (Ljungner, 1930). Some sichelwannen have lateral furrows alongside the main furrow (Shaw, 1996). The arms of the main furrow narrow downstream. Like muschelbruche, they form offsetting, en echelon, patterns.

Allen's (1982) sketch of the classic flute, an erosional mark formed by turbidity currents and commonly observed at the base of ancient turbidites (Fig. 4.1), is similar to sichelwannen. Identical forms are produced on the surface of glaciers where strong winds enhance ablation. As well, experimentally produced ero-sional marks show the same morphology as flutes and sichelwannen, including secondary furrows (Allen, 1982; Shaw, 1996). The form analogy between sichelwannen and forms generated by turbulent fluids is compelling.

4.4.3.3 Spindle form

Spindle form erosional marks are found in conjunction with sichelwannen and muschelbruche. They are narrow troughs with sharp rims, commonly asymmetrical about the long axis and pointed at the upstream ends (Allen, 1982; Shaw, 1996). Spindle marks are commonly curved and even sinuous in places, appearing to relate to coiled structures in the flow (Shaw, 1988). Spindle forms also have analogues with erosional marks observed in nature and those produced experimentally. They are particularly common as turbidite sole marks (Dzulinski & Walton, 1965) and as aeolian scours on resistant bedrock.

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