Trimlines, blockfields and the vertical extent of the last ice sheet in southern Ireland

Trimlines separating glacially abraded lower slopes from blockfield‐covered summits on I rish mountains have traditionally been interpreted as representing the upper limit of the last ice sheet during the L ast G lacial M aximum ( LGM ). Cosmogenic 10 B e exposure ages obtained for samples from glacially deposited perched boulders resting on blockfield debris on the summit area of S lievenamon (721 m a.s.l.) in southern I reland demonstrate emplacement by the last I rish I ce S heet ( IIS ), implying preservation of the blockfield under cold‐based ice during the LGM , and supporting the view that trimlines throughout the B ritish I sles represent former englacial thermal regime boundaries between a lower zone of warm‐based sliding ice and an upper zone of cold‐based ice. The youngest exposure age (22.6±1.1 or 21.0±0.9 ka, depending on the 10 B e production rate employed) is statistically indistinguishable from the mean age (23.4±1.2 or 21.8±0.9 ka) obtained for two samples from ice‐abraded bedrock at high ground on B lackstairs M ountain, 51 km to the east, and with published cosmogenic 36 C l ages. Collectively, these ages imply (i) early (24–21 ka) thinning of the last IIS and emergence of high ground in SE I reland; (ii) relatively brief (1–3 ka) glacial occupation of southernmost I reland during the LGM ; (iii) decoupling of the I rish S ea I ce S tream and ice from the I rish midlands within a similar time frame; and (iv) that the southern fringe of I reland was deglaciated before western and northern I reland.