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

Five published reconstructions of the last British–Irish Ice Sheet give the position of the ice margin in the Celtic Sea and Bristol Channel between 31,000 and 15,000 years ago. Their deposited GIS data are drawn together and measured in an interactive map: Bristol Channel LGM ice limits. No reconstruction carries Irish Sea ice across the north coast of Cornwall, Devon or Somerset. One margin is drawn along that coast. The others end between 4 and 28 km from it at their closest.

Modelled ice extent and flow, 31–15 ka. BRITICE-CHRONO model reconstruction (Clark et al. 2022), thousand-year slices interpolated; flow lines follow the modelled surface slope and velocity; land and sea from the same study's palaeotopography; the thin line is the modern coast. 

Open the interactive map

The reconstructions

• BRITICE-CHRONO empirical reconstruction (Clark et al. 2022): minimum, optimum and maximum ice extents at thousand-year steps from 31 to 15 ka.
• BRITICE-CHRONO model reconstruction (same paper): an ice-sheet model nudged towards the empirical limits, deposited as grids of about 2.5 km.
• DATED-1 (Hughes et al. 2016): minimum, most-credible and maximum extents for the Eurasian ice sheets, with a census date of 1 January 2013.
• Batchelor et al. 2019 (Nature Communications 10, 3713): a single best-estimate LGM outline within a Northern Hemisphere compilation.
• PaleoMIST 1.0 (Gowan et al. 2021): global margins at 2,500-year steps.

These are three source families, not five independent tests. PaleoMIST takes its Eurasian margins from the DATED-1 most-credible lines, with intermediate slices interpolated. Batchelor et al. compile published outlines. BRITICE-CHRONO supplies both an empirical reconstruction and a model fitted to it. Reconstructions published only as figures, without deposited geometry, are not included.

Distance from the modern north coast

The coast was followed for 316 km from Land's End to Brean Down and sampled every 2 km. For each margin, the distance from each coastal point to the nearest ice was computed, using the union of all time-slices so that each margin is taken at its farthest reach.

Margin	Closest approach	Median distance offshore
BRITICE-CHRONO maximum bound	at the coast	1.6 km
BRITICE-CHRONO model, grounded ice	3.7 km	14.6 km
BRITICE-CHRONO optimum	8.6 km	21.7 km
Batchelor et al. 2019	14.8 km	30.1 km
DATED-1 most credible	18.5 km	42.7 km
PaleoMIST 1.0	23.6 km	75.3 km

The BRITICE-CHRONO maximum bound lies within 5 km of the shore along 89% of the coast measured. It overlaps 53 km² of land, 0.4% of the peninsula, as coastal slivers no more than 1.8 km deep at the sampled points. The optimum of the same reconstruction lies 8.6 to 41.5 km offshore.

The model grows ice on the South-West peninsula, 546 km² summed over all time-slices, but as separate upland ice on Dartmoor and Exmoor. It is not connected to the Irish Sea Ice Stream in any slice.

Lundy, Scilly and Fremington

Lundy separates the margins. The island lies inside the BRITICE-CHRONO maximum bound and inside the model's grounded ice, and 6.0 km outside the optimum. Rolfe et al. (2012) report ¹⁰Be exposure ages of 31.4–48.8 ka from the island and read them as glaciation before the LGM. Carr, Hiemstra and Owen (2017) argue that Lundy was not ice-covered in the Devensian.

The Isles of Scilly sit on the BRITICE-CHRONO empirical margin, within 0.1 km in all three versions. Smedley et al. (2017) date the ice limit on the northern islands to about 26 ka.

Fremington lies outside every margin in every time-slice. The nearest is the BRITICE-CHRONO maximum bound, 12.6 km away.

Sea level

Distances are measured to the modern coast because that is where the onshore evidence begins, and because it is the line the maximum bound follows. It was not the shoreline of the time. The shoreline of each date is taken from the glacio-isostatic palaeotopography published with the BRITICE-CHRONO reconstruction.

In that model 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. A uniform lowering of sea level by 120 m would not reproduce this pattern.

A margin that follows the modern shoreline for most of 316 km therefore follows a line with no physical expression at the time. It is most simply read as the limit of the area lacking accepted onshore evidence of Late Devensian ice, rather than as a reconstructed ice front.

The same palaeotopography shows how much of each ice front that now lies under the sea stood on dry land. At 26 ka the figure is 37% for the BRITICE-CHRONO optimum, with the remainder in water of 48 m median depth, and 47% for the model. For the DATED-1 most-credible margin at 23 ka it is 5%.

Limits

• 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.
• The palaeo-shoreline is an output of a glacio-isostatic model tied to the BRITICE-CHRONO ice history. Beside the DATED-1, Batchelor and PaleoMIST margins it is indicative only.
• The chronologies differ. DATED-1 places the Celtic Sea lobe in its 23 ka slice; BRITICE-CHRONO places the greatest extent in this area at 26 ka.
• In the animation, flow direction is inferred from the modelled ice-surface slope. The deposited data give flow speed but not direction.

Sources

• Clark, C.D. 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. Data: doi:10.1594/PANGAEA.945729.
• Hughes, 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. Data: doi:10.1594/PANGAEA.848117.
• Batchelor, C.L. et al. (2019). The configuration of Northern Hemisphere ice sheets through the Quaternary. Nature Communications 10, 3713. doi:10.1038/s41467-019-11601-2. Data: osf.io/7jen3.
• Gowan, E.J. et al. (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: doi:10.1594/PANGAEA.905800.
• Smedley, R.K. et al. (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.
• Rolfe, C.J., Hughes, P.D., Fenton, C.R., Schnabel, C., Xu, S., Brown, A.G. (2012). Paired ²⁶Al and ¹⁰Be exposure ages from Lundy. Quaternary Science Reviews 43, 61–73. doi:10.1016/j.quascirev.2012.04.003.
• Carr, 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.
• Base map: NOAA ETOPO 2022 relief; GSHHG 2.3.7 shoreline (Wessel and Smith 1996).

Map, measurements and code outputs: github.com/TimDaw37/bristol-channel-lgm-ice. Measurements made on 15 November 2026 from the data as deposited.