Bedrock Streamlining in Palaeo-ice streams: a Review of Bedrock Properties, Formation Mechanisms and Glaciological Implications

English Abstract:

Highly elongate bedrock landforms, including bedrock megagrooves, are increasingly recognised at the beds of former ice sheets. In contrast to soft-bedded elongate bedforms they i) are entirely formed by subglacial erosion, ii) may be formed over several glaciations; iii) are strongly influenced by bedrock geology. However, both soft-bedded and hard-bedded elongate bedforms require sustained unidirectional ice flow at the beds of (palaeo-) ice streams. Bedrock megagrooves occur in palaeo-ice streams where topographic steering is strong (Nass River, BC); weak (Ullapool, Tyne Gap, UK) or near-absent (Ontario, Quebec).

Bedrock control and formation mechanisms can be grouped:

1) Megagrooves on dipslopes/bedding planes (Sudbury, South Baymouth, ON; Assynt, UK); or gneiss (Georgian Bay, ON); often small-amplitude and wavelength, with round cross-profile and probably formed by abrasion focussed by debris concentrations;

2) Megagrooves parallel to bedrock strike. Low-angle dips often result in strongly asymmetric, angular cross-sections (Tyne Gap, Ullapool, UK; Isle Royale, Lake Superior); steeper dips give more symmetric grooves (Knapdale/Jura, UK). Lateral plucking is important for their formation, but in heterogeneous strata differential abrasion may also be significant.

3) Megagrooves at high angles to bedrock strike. Where strata dip with ice flow, ‘bullet-shaped’ mega-rock drumlins may develop (Niagara Escarpment, ON; Anticosti island, QC), with focussed erosion possibly guided by fractures and pre-glacial stream valleys.

The glaciological significance of megagrooves is that most grooved terrain is smooth parallel to ice flow: there are few bedrock obstacles, and no need for Weertman/Schoof type sliding. Instead, basal drag is controlled by a simple friction law, with (low) friction coefficients dependent on debris content. It is this smoothing that, once established, means that grooved terrain will steer and enhance fast ice flow during successive glaciations.