Fremington Clay: boreholes and the lacustrine basin

North Devon · Brannam 1972 + archival / A39 holes · elevations in metres OD

The BGS map (Sheet 293 “boulder clay” / DigMap TILMP) confuses the picture. Borehole clay tops show the Ice Age lake never rose above about 30 m OD (lacustrine gravel/base ~15–28 m OD; lake surface ~27–29 m OD; BH9–11 above ~38 m OD are head on the ridge, not lacustrine). Pins are the Appendix A register with corrected BGS NGRs — see Constraining the Fremington Clays.

Punchline: the mapped “till” is not the lacustrine lake bed. Confirmed gravel/base ~15–28 m OD; lake surface ~27–29 m OD — the lake did not rise above ~30 m OD. BH9–11 above ~38 m OD are head on the ridge, not lacustrine. Brown squares mark historic pits at Combrew (Fishley pottery) and Higher Gorse (Brannam).

Coloured by classification. Elevations in metres OD. Brown/orange squares = historic clay pits & pottery (toggle). Optional BGS GeoIndex grey pins (default off).

OD cross-sections

Lake–Roundswell long axis and N–S across Hele ridge — BH9–11 head above the ≤~30 m OD basin.

3D lacustrine lens

Confirmed clay hull under LiDAR ground, plus Combrew historic-pit extension. Rotate to compare ridge head with the basin.

Select a borehole.

Punchline

Sheet 293 / DigMap TILMP maps “boulder clay” / till across ground the boreholes show was never under a high lake. Lacustrine gravel and clay base ~15–28 m OD; lake surface ~27–29 m OD; the Ice Age lake did not rise above ~30 m OD. BH9–11 (clay tops >~38 m OD) are head on the Hele–Bickington ridge, not lacustrine Potter’s Clay.

Classification key

Lacustrine confirmed — basal gravel reached, or clay labelled Fremington Clay (green). Uncertain / elevated margin — clay logged but base unknown or elevated rim (amber). Head — above basin — clay top >~38 m OD, BH9–11 (red). Outside basin / no clay — no Potter’s Clay / head-only archives (grey).

Clay pits

Combrew — Fishley pottery and clay-pit (HER MDV912, SS 522 324); Muddlebridge workings nearby (MDV64604). Higher Gorse — Brannam’s last commercial pit / GCR type exposure (SS 530 316) with Claypit Coverts (MDV904). Worked out; brown squares on the map.

Map layers

Toggle (top-right): Lacustrine clay (logged) · Lacustrine + uncertain margin · Clay evidence incl. historic pits · Historic clay pits / pottery · BGS till (Sheet 293 / TILMP) · optional BGS 50k Superficial WMS · EA LiDAR hillshade · OD section lines · BGS GeoIndex.

Sources

Licence

Compilation © Tim Daw / sarsen.org 2026 · CC BY-SA 4.0. Underlying logs © BGS / survey owners. Research gazetteer only; no public land access implied.