We present an 8000‐year history spanning 650 km of ice margin retreat for the largest marine‐terminating ice stream draining the former British–Irish Ice Sheet. Bayesian modelling of the geochronological data shows the ISIS expanded 34.0–25.3 ka, accelerating into the Celtic Sea to reach maximum limits 25.3–24.5 ka before a collapse with rapid marginal retreat to the northern Irish Sea Basin (ISB). This retreat was rapid and driven by climatic warming, sea‐level rise, mega‐tidal amplitudes and reactivation of meridional circulation in the North Atlantic. The retreat, though rapid, is uneven, with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry. Initially, wide calving margins and adverse slopes encouraged rapid retreat (∼550 m a−1) that slowed (∼100 m a−1) at the topographic constriction and bathymetric high between southern Ireland and Wales before rates increased (∼200 m a−1) across adverse bed slopes and wider and deeper basin configuration in the northern ISB. These data point to the importance of the ice bed slope and lateral extent in predicting the longer‐term (>1000 a) patterns and rates of ice‐marginal retreat during phases of rapid collapse, which has implications for the modelling of projected rapid retreat of present‐day ice streams. Copyright © 2013 John Wiley & Sons, Ltd.

The Last Glacial Maximum (LGM) of Marine Isotope Stage (MIS) 2 is defined here as the period either of maximum global ice volume or in which major ice sheets reached their maximum extent. Both definitions lack clarity as there is no reason to suppose that maximum volumes or limits were reached simultaneously at either all ice sheets or around the margins of individual ice sheets. Here we review the traditional terrestrial limits of the BIIS (British-Irish Ice Sheet), examine developments in understanding the offshore record, and attempt a redefinition of the extent and timing of the MIS 2 maximum. As a working chronological model the build-up of the BIIS is assigned to after 35-32 ka, the maximum to the period 27-21 ka and a more limited readvance after 19 ka. As marginal oscillations, driven by ice sheet dynamics, appear to have characterised much of the advance, and especially the retreat of the BIIS, caution should be exercised in assigning climatic significance to the many identified ice limits or still-stands. While much of the terrestrial limit in Britain remains relatively unaltered, significant revision is required offshore around Britain and Ireland, to portray an extensive ice advance to the shelf edge and a period when the BIIS was conjoined with the adjacent Fenno-Scandinavian Ice Sheet. Although there appears to be a broad synchrony between the BIIS and other major ice sheets, local controls have produced some variation in the timing of advance to and retreat from maximum limits for particular ice streams or lobes, but much of the offshore limit remains very poorly age constrained. Consequently, a key objective in future research should be to improve and refine the chronological control for all portions of the maximum margin of the BIIS and its subsequent retreat stages.

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Rates of ice-stream retreat over decades can be determined from repeated satellite surveys and over millennia by paleoenvironmental reconstructions. Centennial time scales are an important temporal gap in geological observations of value in process understanding and evaluation of numerical models. We address this temporal gap by developing a 3 ka and 123 km retreat time series for the Irish Sea ice stream (ISIS), a major outlet draining the last British-Irish ice sheet. The Llŷn Peninsula (northwest Wales, UK) contains numerous ice-marginal indicators from which we reconstructed a robust chronological framework of margin oscillations. The landscape documents the retreat of a former marine-terminating ice stream through a topographic constriction, across a reverse bed slope, and across variations in calving margin width. New dating constraints for this sequence were integrated in a Bayesian sequence model to develop a high-resolution ice-retreat chronology. Our results show that retreat of the ISIS during the period 24-20 ka displayed centennial-scale oscillatory behavior of the margin despite relatively stable climatic, oceanic, and relative sea-level forcing mechanisms. Faster retreat rates coincided with greater axial trough depths as the ice passed over a reverse bed slope and the calving margin widened (from 65 to 139 km). The geological observations presented here over a 123-km-long ice-retreat sequence are consistent with theory that marine-based ice can be inherently unstable when passing over a reverse bed slope, but also that wider calving margins lead to much faster ice retreat.