Marine geological and geophysical investigations (42) coupled with numerous 2D seismostratigraphical interpretations have allowed the general thickness of Quaternary deposits, or the depth to bedrock, to be mapped on the continental shelf around NW Scotland (19, 20). These offshore Quaternary thickness maps show both Pleistocene (i.e., predominantly glacial, glaciomarine, and nonglacial marine) and Holocene sediments (exclusively nonglacial, marine) sediments as an amalgamated unit. In this study, we determined the ice-stream bed substrates at the time of the last glaciation (Late Weichselian; MIS2-3). Using existing and newly acquired geophysical sub-bottom profiles (figs. S1 and S2), sediment cores (figs. S2, S8, and S9), and archived marine geological data (fig. S1), we separated the deglacial stratigraphic component (deposited mainly as glaciomarine sediments during ice-stream retreat), and the Holocene component (deposited during nonglacial marine conditions), from the Late Weichselian subglacial and pre-Weichselian sediments (older than MIS3) (fig. S2). Note, for simplicity and brevity in the main manuscript, that these younger sediments are sometimes referred to as simply “deglacial sediments” to distinguish them from sediments deposited at the bed of the ice stream during Late Weichselian times. This classification has enabled a simplified geological substrate map to be generated for the ice-stream bed at the time of maximal Late Weichselian ice sheet glaciation (MIS2-3) (i.e., 22 to 28 ka BP) (36, 43).

We converted the substrate map into a first-order “bed strength map” based on the uniaxial compressive strength of the dominant substrate types present at the Late Weichselian ice-stream bed (table S1). The reference compressive strengths were derived by other workers through geotechnical testing of the same rock type, or equivalent lithostratigraphic units, sampled in NW Scotland and/or elsewhere in the UK (4446). Most unlithified silt and clay/mud-rich Pleistocene sediments on the continental shelf around Scotland are classified as “very weak” or “extremely weak”: with compressive strengths typically between 50 and 500 kPa (<<1 MPa) (19, 20, 46, 47, 48). Certain marine muds and glaciomarine sediment facies within the Pleistocene sequence in the outer Minch may have compressive strengths as low as 10 kPa (0.01 MPa). However, waterlain tills, subaqueous subglacial facies, and other clay-rich matrix-supported diamictons can have strengths of up to 1 MPa, depending on water content, clast density, and compaction. Coarse granular (sand-rich and/or gravelly) facies are difficult to classify in terms of compressive strength as they have very low cohesive and intergranular strength. By contrast, lithified rocks within the ice-stream catchment are predominantly classified as “strong,” “very strong,” or “extremely strong” with compressive strengths typically >100 MPa, with several lithologies [e.g., Lewisian gneiss and Eriboll sandstone (quartzite)] in the >250 MPa range (4548). In general, the weakest lithified rocks present at the ice-stream bed would have been the Jurassic sandstones in the inner Minch (between Skye and the East Shiant Bank), typically ranging between 50 and 100 MPa (4548). However, even these less strong (not weak) rocks have been further hardened in many places by contact metamorphism during the emplacement of numerous Palaeogene (Tertiary) igneous bodies (19), hence our general classification of the bedrock bed as strong to very strong (>100 MPa). It is probable that moderately strong to moderately weak (5 to 50 MPa) bedrock, within the oldest and youngest parts of the Jurassic sequence (Hettangian and Oxfordian/Kimmeridgian Formations), may have locally occurred at the bed of the MnIS during earlier glaciations. However, no seabed or subseabed outcrops of these rocks have been identified in the Minch; with only localized onshore exposures found on the Isle of Skye and southern Raasay (19). It is very probable that any such soft Jurassic rocks (partially lithified clays, weak shales, etc.), if originally present in the path of the MnIS, would have been preferentially removed by earlier Pleistocene glaciations before the Late Weichselian (MIS2-3). Our resulting maps and cross sections highlight the major geological boundary at the former ice-stream bed: with very strong/hard substrata to the south and weak/soft substrata to the north of ~58°10′N (Figs. 1 and 3 and fig. S2).

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