Correcting the Record on the Ice Rafted Erratics of the Bristol Channel.

Ice Rafting Reconsidered — The Case for Floating Boulders in the Bristol Channel

In a recent blog post, Brian John reiterates his long-held skepticism toward the idea that the giant boulders scattered along the southern coastlines of the Bristol Channel were transported by floating ice. He argues that such a mechanism is “implausible” given glacial sea-level depression, and suggests that invoking isostatic rebound as a counterpoint is "special pleading."

However, three new peer-reviewed studies published in 2024 present compelling evidence that fundamentally contradicts this view and establish a well-supported case for ice-rafted boulder emplacement during the Middle to Late Pleistocene.

🧊 Gibson & Gibbard (2024): Stratigraphic Evidence for Ice-Rafted Boulders

This comprehensive review of Wolstonian Stage glaciation shows that large erratic boulders, including far-travelled lithologies from Scotland and northern England, are found resting on wave-cut rock platforms around the Bristol Channel and south coast. Crucially, these boulders are:

• Stratigraphically overlain by raised Ipswichian beach deposits, dated to 130–90 ka, showing they pre-date the last interglacial highstand.
• Interpreted as being ice-rafted, not dropped by grounded glacier ice — based on their scattered, isolated distribution and lack of associated till.

This directly challenges the notion that ice rafting is geologically implausible at these elevations.

🌊 Scourse (2024): Glacio-Isostatic Adjustment Enabled High Sea Levels

In a recent review of British–Irish Ice Sheet (BIIS) dynamics from MIS 5d to 2, Scourse provides strong evidence for:

• High relative sea levels during MIS 4 and MIS 3, caused by glacial isostatic loading as the BIIS expanded.
• Coincident ice margins calving into marine waters, which allowed icebergs and sea ice to transport and deposit debris onto present-day coastal sites.

This directly refutes Brian John’s claim that isostatic rebound cannot explain the present elevations of erratics — and shows that conditions were suitable for ice rafting in the Bristol Channel.

🪨 Bennett et al. (2024): New Mapping of Devon and Cornwall Boulders

Using LIDAR and field survey, Bennett and colleagues have documented discrete concentrations of far-travelled giant erratics along the coasts of south Devon and Cornwall. Their findings show:

• These boulders sit in intertidal and raised marine settings, not within glacial tills.
• Their distribution and lithologies are consistent with ice-rafting melt-out — not glacial pushing or dumping.

This further weakens the grounded glacier-only hypothesis and supports episodic marine deposition.

🧠 Common Sense Revisited

Brian John invokes the boulder-strewn Baltic coastlines as an example of wave-modified glacial sediments rather than ice-rafted material. But this comparison overlooks key differences:

• The Baltic has no tides and experienced greater isostatic uplift than southern Britain.
• Its glacial history and hydrodynamics differ greatly from the open Atlantic-fed Bristol Channel.

Context matters. The conditions in SW Britain during MIS 6 to 3 allowed for calving margins and floating ice — and the evidence now supports this interpretation.

✅ Conclusion: Floating Ice Delivered Boulders to the Bristol Channel

The 2024 studies converge on a new, evidence-based understanding:

• The southern coasts of Britain experienced periods of high relative sea level and iceberg activity during the Late Middle and Early Late Pleistocene.
• This created conditions suitable for the deposition of ice-rafted erratics.
• The hypothesis is no longer speculative — it's now robustly grounded in stratigraphy, dating, and geomorphology.

It’s time to update the narrative. Rather than dismiss the ice-rafting model, the latest science shows that it played a real role in shaping these iconic coastal landscapes.

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References:

• Gibson, S. M. & Gibbard, P. L. 2024 (October): Late Middle Pleistocene Wolstonian Stage (MIS 6) glaciation in lowland Britain and its North Sea regional equivalents – a review. Boreas, Vol. 53, pp. 543–561. https://doi.org/10.1111/bor.12674. ISSN 0300-9483
Scourse, J.D. (2024), The timing and magnitude of the British–Irish Ice Sheet between Marine Isotope Stages 5d and 2: implications for glacio-isostatic adjustment, high relative sea levels and ‘giant erratic’ emplacement. J. Quaternary Sci., 39: 505-514. https://doi.org/10.1002/jqs.3611
• Bennett, M. R. et al. (2024). Evidence for Middle Pleistocene ice-rafted debris in south-west England: A GIS and field-based reassessment. https://ussher.org.uk/wp-content/uploads/benettetal1584130v2.pdf.