Colin Kerr Ballantyne

The term Little Ice Age (LIA) refers to the coldest centuries of the past millennium, and particularly the period AD 1550–1900 when decadal summer temperatures frequently fell below 1961–1990 mean ...

Periglacial vegetation‐banked terraces (VBTs) are roughly parallel, step‐like landforms with bare treads and vegetated risers, where tread length (across‐slope) markedly exceeds tread width (downslope). Many follow the contour, but others dip off‐contour at angles of up to 40°. Most VBTs with a low off‐contour dip occupy lee slopes with gradients of 4°–25°, with dip increasing to windward. They are widespread in marginal periglacial environments characterised by strong winds, abundant precipitation, winter snow cover, shallow ground freezing and frequent freeze–thaw cycles, but also occur in alpine environments and on low‐latitude and tropical mountains. They appear to be rare or absent in polar or subpolar permafrost environments. Riser vegetation is often dominated by cushion plants, dwarf shrubs or bunchgrasses. This review (1) outlines the history of research on VBTs; (2) identifies their global distribution and the environmental conditions under which they have developed; (3) summarises data concerning their dimensions and structure, and measurements of current activity; and (4) proposes models of VBT formation. Many VBTs have apparently evolved from prior establishment of alternating bands of bare and vegetated ground across slopes. This may have occurred either through the lateral extension of vegetation bands or through the formation of wind stripes, where alternating bands of vegetated and bare ground have developed normal to the direction of dominant winds through turf exfoliation by needle ice, frost heave and wind scour. Such vegetated bands trap mobile debris to build risers, with consequent lowering of tread gradients. Some terraces are subsequently immobilized by stacking of clasts under risers, but those on frost‐susceptible soils may move en masse downslope through solifluction. Other VBTs represent stripping of vegetation cover from the windswept treads of vegetation‐covered solifluction terraces, and possibly also selective vegetation colonisation of the sheltered risers of stone‐banked solifluction terraces.

The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling.