Last British–Irish Ice Sheet · Celtic Sea and Bristol Channel · 31–15 ka

Where the ice stopped: the Bristol Channel at the Last Glacial Maximum

Five published reconstructions of the Irish Sea Ice Stream, drawn from their authors' own GIS files and measured against the modern north coast of Cornwall, Devon and Somerset. One margin is drawn along that coast. Every other margin ends at sea.

BRITICE-CHRONO model reconstruction, 31–15 ka (Clark et al. 2022). Thousand-year slices are interpolated. Flow lines follow the modelled surface slope and velocity. Land and sea are from the same study's palaeotopography; the thin line is the modern coast. Go to the map
Move over the map for position and ice cover. Select a ringed place for its distances.
Relief: ETOPO 2022 · Coast: GSHHG 2.3.7
Shoreline at the time: BRITICE-CHRONO palaeotopography
Transverse Mercator, WGS84, origin 51°N 5°W

Distance from the north coast to the nearest ice

The coast is followed for 316 km from Land's End to Brean Down and sampled every 2 km. Each line gives the distance from that point of the coast to the nearest ice in one reconstruction. Zero is ice at the shore; values below zero are ice on land. The chart follows the time setting and the margins selected above.

Ice front on land or in water

For the part of each ice front that now lies under the sea, south of 52.2°N, the BRITICE-CHRONO palaeotopography gives ground level relative to sea level at the time. The bar shows the share of that front standing on dry land. In the maximum-reach view each margin is tested at the time-slice of its greatest extent.

Distance to ice at named places

Kilometres from each place to the nearest ice. A filled cell means the place lies under ice, with its distance inside the margin. Amber marks ice within 5 km.

What the reconstructions show

Measured results, all time-slices combined

    Reading the margins

    • Three source families, not five independent tests. PaleoMIST 1.0 takes its Eurasian margins from the DATED-1 most-credible lines, with intermediate slices interpolated. Batchelor et al. is a hemispheric compilation of published outlines. BRITICE-CHRONO supplies both an empirical reconstruction and an ice-sheet model nudged towards it.
    • Bounds are not predictions. BRITICE-CHRONO and DATED-1 each publish a minimum, a preferred and a maximum extent. The minimum and maximum are the authors' uncertainty bounds on the preferred line. In this sector the BRITICE-CHRONO maximum follows the present shoreline.
    • Chronologies differ. DATED-1 has a census date of 1 January 2013 and places the Celtic Sea lobe in its 23 ka slice. BRITICE-CHRONO, with its later dating programme, places the greatest extent in this map area at 26 ka.
    • Ice on Dartmoor and Exmoor in the model is local. The modelled ice on the South-West peninsula forms as separate upland ice and is not connected to the Irish Sea Ice Stream in any time-slice.
    • Distances are to the modern coast by design. The modern shoreline is where the onshore evidence begins and is the line the maximum bound follows. It was not the shoreline of the time.
    • The shoreline of the time moved with the ice load. In the BRITICE-CHRONO glacio-isostatic palaeotopography the shoreline lies a median 47 km from the modern north coast at 31 ka. Crustal depression under the ice brings it to a median 19 km at 25 ka, and it withdraws to 52 km by 19 ka. At no date does it come within 5 km of the coast measured here. A uniform lowering of sea level by 120 m would not reproduce this.

    Method

    Ice-extent polygons were read from each publication's deposited GIS data, reprojected to a transverse Mercator projection on WGS84 (origin 51°N 5°W, scale factor 1) and clipped to the map area. The BRITICE-CHRONO model margin is the outline of grounded-ice cells in the deposited mask grids, which have a cell size of about 2.5 km, and is drawn unsmoothed. Other margins are simplified with a 250 m tolerance for display; all distances were computed before simplification.

    The coast is the GSHHG full-resolution shoreline. For the profile, the mainland outline was closed across inlets narrower than 3 km and generalised to 400 m, so that the Taw–Torridge, Camel and Hayle estuaries do not register as coast. Distances are planar, from a point to the nearest part of an ice polygon; a point inside a polygon is given its distance to the polygon edge as a negative value. Ice bodies lying wholly on the peninsula, which occur only in the model (Dartmoor and Exmoor), are left out of the coast profile so that it measures ice arriving from the sea; they are included in the distances at named places. Maximum reach is the union of every time-slice of a given margin. The shoreline of the time is the zero contour of the BRITICE-CHRONO palaeotopography grids (500 m cells, lightly smoothed), one per thousand years; shading marks present sea floor that stood above sea level. It is an output of the glacio-isostatic model tied to the BRITICE-CHRONO ice history, so beside the DATED-1, Batchelor and PaleoMIST margins it is indicative only. The land-or-water test samples each ice front every kilometre. Land area under ice is measured on mainland Cornwall, Devon and west Somerset ( km²).

    Line-work in regional and hemispheric compilations is drawn at small scale. Differences of a few kilometres between margins, and between a margin and the coast, are within drafting precision and should not be read as geological statements.

    Sources

    1. MarginsClark, C.D., Ely, J.C., Hindmarsh, R.C.A., Bradley, S., Ignéczi, A., Fabel, D., Ó Cofaigh, C., Chiverrell, R.C., Scourse, J. et al. (2022). Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE-CHRONO reconstruction. Boreas 51(4), 699–758. doi:10.1111/bor.12594. GIS data: Clark, C.D., Ely, J.C., Fabel, D., Bradley, S.L. (2022), PANGAEA, doi:10.1594/PANGAEA.945729 (CC BY 4.0). Empirical minimum, optimum and maximum extents, model masks, and palaeotopography grids, 31–15 ka.
    2. MarginsHughes, A.L.C., Gyllencreutz, R., Lohne, Ø.S., Mangerud, J., Svendsen, J.I. (2016). The last Eurasian ice sheets – a chronological database and time-slice reconstruction, DATED-1. Boreas 45(1), 1–45. doi:10.1111/bor.12142. GIS data: PANGAEA, doi:10.1594/PANGAEA.848117 (CC BY 3.0). Minimum, most-credible and maximum extents, 25–15 ka; single extents for 27, 29–28 and 32–30 ka.
    3. MarginsBatchelor, C.L., Margold, M., Krapp, M., Murton, D.K., Dalton, A.S., Gibbard, P.L., Stokes, C.R., Murton, J.B., Manica, A. (2019). The configuration of Northern Hemisphere ice sheets through the Quaternary. Nature Communications 10, 3713. doi:10.1038/s41467-019-11601-2. Shapefiles: osf.io/7jen3. LGM best estimate.
    4. MarginsGowan, E.J., Zhang, X., Khosravi, S., Rovere, A., Stocchi, P., Hughes, A.L.C., Gyllencreutz, R., Mangerud, J., Svendsen, J.I., Lohmann, G. (2021). A new global ice sheet reconstruction for the past 80 000 years. Nature Communications 12, 1199. doi:10.1038/s41467-021-21469-w. Data: PANGAEA, doi:10.1594/PANGAEA.905800 (CC BY 4.0). Eurasian margins at 2.5 ka steps, 30–17.5 ka.
    5. ContextScourse, J.D., Chiverrell, R.C., Smedley, R.K., Small, D., Burke, M.J., Saher, M. et al. (2021). Maximum extent and readvance dynamics of the Irish Sea Ice Stream and Irish Sea Glacier since the Last Glacial Maximum. Journal of Quaternary Science 36(5), 780–804. doi:10.1002/jqs.3313.
    6. ContextSmedley, R.K., Scourse, J.D., Small, D., Hiemstra, J.F., Duller, G.A.T., Bateman, M.D., Burke, M.J., Chiverrell, R.C., Clark, C.D., Davies, S.M., Fabel, D., Gheorghiu, D.M., McCarroll, D., Medialdea, A., Xu, S. (2017). New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly. Journal of Quaternary Science 32(1), 48–62. doi:10.1002/jqs.2922.
    7. ContextRolfe, C.J., Hughes, P.D., Fenton, C.R., Schnabel, C., Xu, S., Brown, A.G. (2012). Paired 26Al and 10Be exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles. Quaternary Science Reviews 43, 61–73. doi:10.1016/j.quascirev.2012.04.003.
    8. ContextCarr, S.J., Hiemstra, J.F., Owen, G. (2017). Landscape evolution of Lundy Island: challenging the proposed MIS 3 glaciation of SW Britain. Proceedings of the Geologists' Association 128(5–6), 722–741. doi:10.1016/j.pgeola.2017.06.005.
    9. ContextFink, D., Hughes, P., Rolfe, C., Bateman, M., Brown, A., Simon, K. (2026). Constraining decay of the British-Irish Ice Sheet on Lundy, Celtic Sea, during the Last Glacial Cycle. EGU General Assembly 2026, abstract EGU26-11276.
    10. Base mapNOAA National Centers for Environmental Information (2022). ETOPO 2022 15 Arc-Second Global Relief Model, 30 arc-second bedrock grid.
    11. Base mapWessel, P., Smith, W.H.F. (1996). A global, self-consistent, hierarchical, high-resolution shoreline database. Journal of Geophysical Research 101(B4), 8741–8743. GSHHG version 2.3.7, full resolution.