| Summary: | Periglacial trimlines separating glacially eroded lower slopes from blockfield-covered plateaus on British and Irish mountains have been interpreted either (1) in terms of representing the maximum altitude of the last ice sheet during the Last Glacial Maximum (LGM), or (2) as a thermal boundary separating wet-based ice at pressure melting point from cold-based ice on summit plateaus. We test these competing hypotheses through 10Be exposure dating of high-level erratic boulders above trimlines on five mountains in NW Scotland. Nine out of 14 erratics yielded post-LGM exposure ages ranging from 14.0 ± 0.7 ka to 16.5 ± 0.9 ka or from 14.9 ± 0.9 ka to 17.6 ± 1.1 ka, depending on the 10Be production rate employed in exposure age calculation. These ages refute hypothesis (1) as they imply that the last ice sheet overtopped the mountains. Preservation of apparently intact blockfields on the summits implies cold-based ice cover, supporting hypothesis (2). As altitudinally consistent high-level trimlines extend from our sampled sites across much of NW Scotland and the Hebrides, our conclusions apply to all trimlines in this broader area, and probably to all high-level trimlines elsewhere in the British Isles. Preservation of blockfields under cold-based ice is consistent with blockfield evolution on plateaus throughout much or all of the Quaternary. Averaged exposure ages of ∼15–16 ka for plateau-top erratics implies nunatak emergence from the downwasting ice sheet prior to a regional readvance of the ice margin (the Wester Ross Readvance) and before rapid warming at ∼14.7 ka at the onset of the Lateglacial Interstade, but after the timing of ice-sheet thinning as retrodicted by recent proxy climate-driven thermo-mechanical coupled models. Our findings provide an additional constraint on the future development of such models by implying that high-level trimlines represent the altitude of a former transition zone between ice at pressure-melting point and ice below pressure melting point.
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