Cavityfill drumlins

The nature of the flows that formed drumlins in the form of infilled cavities (Fig. 4.3) can be determined only by deduction based on shape and composition of the drumlins themselves. The initial work in this regard was on the so-called Livingstone Lake drumlin field, northern Saskatchewan (Figs 4.1 & 4.2; Shaw, 1983; Shaw & Kvill, 1984). In the first instance, the drumlins in this field, when inverted, were seen to be identical in form to sets of ero-sional marks in areas of bedrock erosion by wind and water and, in particular, where turbidity currents scoured the ocean floor, generating sole marks (Fig. 4.1). Cavity-fill drumlins are positive forms on the landscape and are considered to be infills of cavities (inverted erosional marks) eroded upwards into the ice bed by broad sheets of meltwater (Fig. 4.3). The dominance of sorted sediment within the cavity fill drumlins and the predominance of

Figure 4.4 NASA Shuttle Radar Topography Mission (SRTM) hill shades. (a) The southwestern portion of the Laurentide Ice Sheet together with the extraglacial area. Arrows, indicating a synchronous flow of meltwater with strong channelling towards the west, mark flow paths. The northern part of the Livingstone Lake 'flood path' is a narrow filament in a broad flow. (b) Close-up of part of the flow with strong differential erosion demarking the Missouri Coteau and the Coteau du Prairie. The bedforms used for mapping flow directions in Fig. 4.4a are clearly visible in 'Mars like' channels. The Pas Moraine stands out, yet other tracts show no moraines over long stretches (Fig. 4.4b). In these stretches there is no overprint of glacial retreat. Rather, confluent flows eroded meltwater tracts which are preserved in the landscape.

Figure 4.4 NASA Shuttle Radar Topography Mission (SRTM) hill shades. (a) The southwestern portion of the Laurentide Ice Sheet together with the extraglacial area. Arrows, indicating a synchronous flow of meltwater with strong channelling towards the west, mark flow paths. The northern part of the Livingstone Lake 'flood path' is a narrow filament in a broad flow. (b) Close-up of part of the flow with strong differential erosion demarking the Missouri Coteau and the Coteau du Prairie. The bedforms used for mapping flow directions in Fig. 4.4a are clearly visible in 'Mars like' channels. The Pas Moraine stands out, yet other tracts show no moraines over long stretches (Fig. 4.4b). In these stretches there is no overprint of glacial retreat. Rather, confluent flows eroded meltwater tracts which are preserved in the landscape.

subangular clasts with over 80% local bedrock lithologies indicate that the cavities were very efficient sediment traps. The sediment had been transported by water, but not very far (Bouchard, 1989; Shaw, 1989; Shaw et al., 1989; Fisher & Shaw, 1992).

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