New Borehole Analysis Sharply Constrains the Fremington Clay: True Glacial Lake Limited to Low Elevations (~15–28 m OD), with No Lacustrine Deposit South of the Hele Ridge

Borehole Map from https://api.bgs.ac.uk/sobi-scans/v1/borehole/scans/items/703466

A detailed OD-referenced re-examination of the 1972 C.H. Brannam Ltd borehole campaign (BGS records SS53SW62–79) fundamentally refines our understanding of the Fremington Clay Series in North Devon. This 18-hole dataset, drilled for commercial pottery clay prospecting, provides the first systematic elevation control on the deposit. The results demonstrate that the genuine glaciolacustrine "potters clay" — the stone-free, homogeneous lacustrine unit formed in a proglacial lake — is far more restricted in both lateral extent and elevation than shown on BGS Sheet 293. Much of the mapped "Boulder Clay" is actually periglacial head and solifluction deposits. There is no Fremington lacustrine clay confirmed south of the Hele–Bickington ridge, and no lacustrine clay above ~28–29 m OD.

George Maw (1864) originally described a clear two-unit sequence: a lower stone-free potters clay resting on glaciofluvial gravel, overlain by an independent stony gravel/head produced during later erosion and periglacial reworking. Subsequent work by Stephens (1966) and the BGS (Edmonds et al. 1985) collapsed this distinction through a traceable sequence of conflations — Taylor (1956) had already grouped coastal and inland erratics together, inflating the apparent glacial signature; Stephens then treated the entire surface-to-bedrock sequence as a single boulder clay; and Edmonds mapped the valley floor accordingly, drawing an admittedly arbitrary boundary between boulder clay and pebbly drift. Crucially, Edmonds himself noted in the 1985 memoir that gravel was found only in holes that penetrated smooth clay — the diagnostic key to the whole problem — but never followed this observation to its mapping consequences. The 1972 Brannam logs allow us to restore Maw's distinction with quantitative precision.

Of the 18 boreholes, only 8 reached the diagnostic basal glaciofluvial gravel, confirming the full lacustrine stratigraphy (potters clay on gravel on bedrock). Their gravel-top elevations cluster tightly between 15.3 m and 28.0 m OD, with the basin floor around 20–23 m OD and margins rising to ~27–33 m OD. This matches independent data from the A39 Lake cutting (Cattell 2003: clay base 19.9–26 m OD), Maw's Roundswell well (~26.8 m OD clean clay top), and a 1982 A39 bypass borehole (703458) that positively identifies Fremington Clay at ~20.8 m OD — making 9 confirmed lacustrine holes across the full 31-borehole dataset. The ice-dammed lake surface therefore stood no higher than approximately 27–29 m OD.

Three boreholes (9, 10, 11) at surface elevations 41–43 m OD encountered "clay" tops at 38–40 m OD — 12–20 m above the confirmed lacustrine basin. These are unequivocally periglacial head. The remaining non-gravel holes show poor-quality "dirty" clay consistent with head masking thinner lacustrine deposits or basin-margin facies. The pattern is reinforced by a further sample of 13 archival boreholes: 12 are dominated by head throughout, with no trace of stone-free lacustrine clay. None of these additional holes, nor the 1972 set, records Fremington lacustrine clay south of the Hele–Bickington ridge. Notably, C.H. Brannam Ltd — guided by T.R. Wood's prior seismic survey and by generations of direct working knowledge — drilled entirely on the low ground around the existing pits and showed no commercial interest in the ground south of the ridge. A pottery firm whose livelihood depended on finding workable clay would not have overlooked accessible reserves: their silence on the southern ground is itself evidence (Sutton's Law in reverse) that the mapped Fremington Clay extent there reflects head and reworked material rather than genuine lacustrine deposit.

Implications for Irish Sea Ice Incursion and Erratics

This OD-constrained geometry severely limits models of onshore Irish Sea ice advance (likely Anglian/MIS 12). The Fremington potters clay records only a modest proglacial lake ponded in the Taw–Torridge estuary, not a widespread till sheet from grounded ice overriding the Devon landscape to 80–90 m OD. Ice must have been pinned near the Bristol Channel coast. The restriction of lacustrine clay to ~15–28 m OD directly challenges interpretations that rely on high-level glacial material and extensive ice incursion inland.

For producing a rigorous geological map of the glacial clays — loosely mapped and extensively mixed with head deposits — these boreholes supply critical control points. Future work will include a corrected map separating confirmed lacustrine clay from periglacial head, cross-sections (e.g., Lake–Roundswell transects showing the 11° northward basin-margin dip), and targeted investigation south of the Hele–Bickington ridge.

Relevance to Bluestone Transport

A thick, grounded Irish Sea ice sheet capable of glacial rafting or long-distance transport of erratics across Devon is incompatible with this restricted, low-elevation lacustrine basin. The data strengthen the case for predominantly fluvial/periglacial origins for most superficial "glacial" features and constrain maximum ice-surface elevations in the Bristol Channel. This directly informs the viability (or otherwise) of glacial hypotheses for Stonehenge bluestones.

The full April 2026 report (CC-BY-4.0), with tables, maps, and the unified borehole register (31 holes total), is available: 
Constraining the Fremington Glaciolacustrine Clays: and its implications for the Irish Sea Ice extent

and a related paper: A Case for Predominantly Fluvial and Periglacial Origins of the Fremington Clays, Devon