Interglacial Seas in Somerset: The Burtle Beds and the Demise of Glacial Transport for Stonehenge's Bluestones

Introduction

The transport of Stonehenge’s bluestones—igneous rocks sourced from the Mynydd Preseli region of west Wales—remain one of the most debated questions in British prehistory. Two major explanations have dominated: transport by Pleistocene glaciation or deliberate human movement during the Neolithic. The glacial hypothesis, once influential, suggested that Irish Sea ice carried the stones into southern England during the Devensian glaciation. However, accumulating geomorphological evidence increasingly contradicts this scenario.

A key contribution comes from Kidson et al. (1978), whose investigation of the Burtle Beds in the Somerset Levels provides robust evidence for intact interglacial marine deposits. This finding strongly challenges the idea that Devensian ice reached the lowlands of Somerset and, by extension, the feasibility of glacial delivery of the bluestones.

(Figure 1: Distribution of Burtle Bed sites across the Somerset Levels, - Kidson et al., 1978. Note the confinement to river valleys below 23 m OD, with no upland glacial signatures.)

The Burtle Beds: An Interglacial Marine Record

The Somerset Levels, a low-lying basin between the Mendip and Quantock Hills, preserve a complex sequence of Quaternary sediments. Among them are the Burtle Beds—Pleistocene sands, gravels, silts, and clays forming raised patches (“batches”) in the landscape. Their origin was historically contested, with interpretations ranging from interglacial marine transgression to glacial outwash.

Kidson et al.’s trench investigation at the Greylake No. 2 sandpit provided decisive clarification. By examining the full sedimentary sequence, including faunal assemblages, granulometry, and geomorphological context, they concluded that the beds represent in situ estuarine and nearshore marine environments.

(Figure 2: Schematic cross-section of Burtle Beds at Greylake No. 2, showing marine transgression sequence. After Fig. 2 in Kidson et al., 1978.

Key Findings

• Faunal Assemblages: Abundant molluscs (Hydrobia ulvae, Littorina spp.), foraminifera (Ammonia spp., Elphidium spp.), and ostracods (Cyprideis torosa, Leptocythere spp.) indicate in situ estuarine and near-shore marine deposition. These are life assemblages, not reworked glacial debris, with population structures (e.g., juvenile-to-adult ratios) confirming local habitats from brackish mudflats to fully saline channels. Water temperatures mirrored the modern Bristol Channel, ruling out cold-stage periglacial sorting.
• Stratigraphy and Environment: The sequence records a progressive marine transgression: basal clays (samples 8–15) represent intertidal mudflats at ~15–30‰ salinity, grading into sands (samples 16–24) deposited near low-water mark in a channel-shoal setting. No agglutinating marsh species or glacial tills appear; instead, phytal (algae-attached) forms suggest open-coast influx.
• Age and Elevation: Radiometric and palaeomagnetic assays were inconclusive, but geomorphology and ostracod affinities favour an Ipswichian (Marine Isotope Stage 5e, c. 130,000–115,000 years BP) attribution over Hoxnian (MIS 11). Critically, 25 sites (Table 1) yield elevations from 4.6 m to 22.8 m OD (Ordnance Datum Newlyn), with the authors estimating peak mean sea level at 9–12 m above present, and Mean High Water Spring Tides (MHWST) at 15–18 m OD. Post-depositional erosion accounts for the upper limit; these are not storm ridges but intact transgressive beds.

This marine signature directly rebuts Kellaway's outwash model, as the fauna demand temperate, current-swept accumulation incompatible with meltwater deposition.

 

Site	Elevation (m OD)	Notes
Ponfield Nr Langport	15.2–22.8	Highest; sand at depth
Sedgemoor Hill	18.2	Isolated batch
Greylake No. 2	7.0–7.6	Excavation site
Middlezoy	10.7–12.2	Valley fill

(Table 1 excerpt: Selected Burtle Bed sites and elevations, adapted from Kidson et al., 1978. Full table spans 25 localities, emphasising lowland confinement.)

Implications for Devensian Glaciation

If Devensian ice had advanced into the Somerset Levels, the Burtle Beds would show evidence of disturbance, erosion, or burial beneath glacial deposits. Yet the stratigraphy above them contains only periglacial head deposits and solifluction layers, indicating cold-climate processes without direct ice contact. This supports broader reconstructions placing the southern limit of Irish Sea ice offshore in the Bristol Channel, not onshore in Somerset.

These findings parallel research in Devon and surrounding regions, where low-elevation erratics once attributed to Devensian glaciation have since been reassessed as either pre-Devensian or non-glacial in origin. Together, these data strongly suggest that Devensian ice did not traverse the Somerset lowlands.

Conclusion

The Burtle Beds provide a clear and coherent record of interglacial marine deposition in the Somerset Levels. Their intact state decisively argues against a Last Glacial Maximum incursion in the region, undermining key assumptions of the glacial transport hypothesis for Stonehenge’s bluestones. 

References

BULLEID, A., & JACKSON, J. W. (1937). The Burtle Sand-Beds of Somerset. Proceedings of the Somerset Archaeological and Natural History Society, 83, 171–196. https://sanhs.org/wp-content/uploads/2020/12/16-A-Bulleid.pdf 

KIDSON, C., GILBERTSON, D.D., HAYNES, J.R., HEYWORTH, A., HUGHES, C.E. and WHATLEY, R.C. (1978), Interglacial marine deposits of the Somerset Levels, South West England. Boreas, 7: 215-228. https://doi.org/10.1111/j.1502-3885.1978.tb00280.x