Late Quaternary glaciation in the Hebrides sector of the continental shelf : cosmogenic nuclide dating of glacial events on the St Kilda archipelago

The St Kilda archipelago lies similar to 65 km west of the Outer Hebrides and similar to 60 km east of the Atlantic shelf break, and represents a key site for testing the assertion that during the Last Local Glacial Maximum (LLGM; c. 27 ka) the British-Irish Ice Sheet (BIIS) extended to near the shelf edge in all sectors. Two consistent cosmogenic Cl-36 exposure ages averaging (>=) 81.6 +/- 7.8 kafor perched bouldersat 290 maltitudedemonstrate that the last ice sheet failed tooverrun high ground on the largest island, Hirta. Cl-36 and Be-10 exposure ages for glacially emplaced boulders on low ground indicate deposition by small, locally nourished glaciers that last occupied a north-facing valley (Gleann Mor) at c. 30.9 +/- 3.2 ka, prior to extension of the last ice sheet to the outer shelf, and a south-facing valley (Village Bay) at c. 19.2 +/- 2.3 ka, several millennia after the LLGM. Our dating evidence is consistent with previous interpretations of lithostratigraphical, seismostratigraphical and geomorphological evidence and confirms that the last ice sheet failed to encroach on St Kilda. A simple ice-flow model demonstrates that even if thin, low-gradient ice lobes encircled the archipelago during theLLGM, the ice margin can only have reached the outermost moraine banks, similar to 40 kmwest of St Kilda, under extremely low (<2 kPa) driving stresses, implying either surge-like transient streaming behaviour at the ice-sheet margin or that the moraine banks relate to an earlier, more extensive ice sheet. The final glaciation of the Village Bay area at c. 19.2 +/- 2.3 kawas out of phase with the behaviour of the BIIS, which was undergoing net retreat during this period.