The Fremington Clay Erratics

 

Tim Daw All Cannings Cross, Wiltshire, UK Email: tim.daw@gmail.com

© Tim Daw 2025. This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0). To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

 

A definitive list of all the erratics identified from the Fremington Clays, Devon, with a refreshed interpretation and conclusion.

 

Erratic No./Name/Location	Lithology/Type	Description & Key Features	Original Source Suggestion	Modern Interpretation/Source	Primary Reference(s)
Boulder III / No. 6 / Maw's boulder (quarter mile east of Combrew Farm; moved to Combrew Farm garden; Bickington clay-bed)	Spilite / Vesicular granophyre (pillow-lava type; early ID: basaltic trap)	Dark grey, finely crystalline; small white porphyritic albite felspars (plates/laths); micropegmatite groundmass; vesicular with pleochroic calcite infill; chlorite replaces scarce ferromagnesian minerals; no free quartz; ~40 x 30 x 25 inches; no striae or wedge-shape; longest axis E-W. Recorded as isolated in middle of clay-bed (Unit B); early description as large striated basaltic trap.	North/East Cornwall spilites (altered basalts); possible but uncertain import.	Cornish spilitic pillow lavas (Meneage district); or local SW England volcanics (e.g., Meldon spilitic lavas with chlorite-epidote alteration and granophyric overprint); periglacial/fluvial reworking; glacial dropstone or reworked.	Maw (1864); Dewey (1910); Taylor (1956); Arber (1964); Croot et al. (1996)
Boulder IV / No. 7 (Combrew Farm garden; later Chilcotts Farm gate-post)	Hypersthene andesite / Hyalopilitic andesite	Dark grey-green, glassy, porphyritic, brittle; large pale olive-green acid labradorite felspars (two generations, ~50% rock volume, fresh, twinned on albite/pericline laws, zonal inclusions, RI=1.560 = 50% Ab-An); rhombic pyroxene (hypersthene) prisms only ferromagnesian (no augite/hornblende/olivine); abundant magnetite (rods/feathery/gridiron); ~50% brown glass base with zonal borders; ~16 inches across; well-rounded. Found ~22 ft below surface c. 1870.	West coast Scotland (e.g., Watt Carrick, Dumfries; Loch Craignish, Argyll); tentative, differs by lacking augite.	Local SW dykes (Tamar/Dartmoor); matches hypersthene-phyric andesite lenses in Meldon tuffs; no long-distance glacial transport needed.	Dewey (1910); Taylor (1956); Arber (1964); Croot et al. (1996)
No. 8 (Fishley Pottery clay-pit; now near old pottery gate, approaching Combrew Farm)	Quartz porphyry	Light grey, holocrystalline, granitic texture; altered felspar/quartz phenocrysts (up to 5 mm, avg. 3 mm); little mica; fine pale base; amorphous red matrix; crushed/irregular plagioclase, porphyritic quartz, long apatite prisms; epidote replaces mosaic; lichen-covered; flat top/base; 47 x 19 x 16 inches.	Local source (e.g., porphyritic dyke west of Devon/Cornwall coasts).	Devon–Cornwall intrusions (e.g., rhyolitic dykes/sills near Mary Tavy with devonisation); possible non-local Cornish alternative.	Taylor (1956); Croot et al. (1996)
No. 9 (Brannam's pits, Tews Lane, Bickington; through coarse white clay)	Quartz dolerite	Grey, highly crystalline/compact; fine-grained granite-like texture; soft milky-white kaolinized felspar (lath outlines); trace quartz (primary); reddish fresh augite (slight edge alteration); little magnetite/secondary calcite; long needle apatite prisms with inclusions; ellipsoidal, rounded; lower mean weight; no sub-ophitic texture. Found in middle of brown clay.	Local Devon intrusions; no diagnostic distant features.	Dartmoor dykes (e.g., Meldon dolerite sheets); hydrothermal alteration common; fluvial entrainment via Taw/Okement.	Taylor (1956); Arber (1964); Croot et al. (1996)
No. 10 (Brannam's pits; far side of pit surface, two pieces)	Olivine dolerite	Darker grey, more crystalline than No. 9; soft felspar crystals with calcite (weak acid reaction); micro-pegmatitic ophitic structure with crossed flows; plagioclase tabs enclosed; transverse ilmenite prisms; yellow olivine grains; slight quartz orientation; some felspar extinction; ~300 lb; irregular/angular.	Local Devon; common type.	Devon minor intrusions (e.g., aureole basic sheets); fresh, with diagnostic apatite; local reworking.	Taylor (1956); Arber (1964); Croot et al. (1996)
No. 13 (Brannam's pits, 1962 excavation)	Quartz dolerite	Similar to No. 9; specifics limited; found ~10 ft from top of clay.	Unspecified local.	As per No. 9; unified aureole source.	Vachell (1963); Arber (1964); Taylor (1956, implied)
Unidentified boulder 1 (Brannam's pits, pre-1957)	Unspecified igneous	Smooth, rounded; ~19 inches long; excavated 10 ft below upper surface of clay.	Unspecified.	Likely local mafic/intermediate; embedded in clay heart.	Arber (1964)
Unidentified boulder 2 (Brannam's pits, 1957)	Unspecified igneous	Smooth, rounded; size unspecified; excavated 16 ft below upper surface of clay.	Unspecified.	As above.	Arber (1964)
Olivine-dolerite pebble (Brannam's pits, 1955)	Olivine dolerite	Small pebble; 2–3 ft above clay base; similar to No. 10.	Local Devon.	As per No. 10; pre-depositional wear.	Taylor (1956); Arber (1964)
Carboniferous grit slab (Brannam's pits)	Carboniferous grit (sedimentary)	Waterworn slab; ~5 inches, 1.25 inches thick; flat surfaces with red ferric oxide skin (infiltration along bedding cracks).	Local Carboniferous.	Okement-derived; waterworn pre-inclusion; minor erratic.	Taylor (1956)
Dolerite boulder (Brannam's pits / potter's clay; removed post-discovery)	Dolerite (unspecified)	Large (1–5 tons); erratic in stoneless potter's clay (Unit B/C); similar to Saunton Downend erratics.	Unknown provenance.	Local (e.g., Culm Measures intrusives); dropstone in quiet-water deposit.	Wood (1973/1974); Croot et al. (1996) [N.B.: Arber (1964) cited by Wood for context, but describes distinct smaller clasts, pace Croot et al.]
Granodiorite boulder (Brannam's pits / potter's clay; removed post-discovery)	Granodiorite	Large (1–5 tons); erratic in stoneless potter's clay (Unit B/C); similar to Saunton Downend erratics.	Unknown provenance.	Possible exotic (Irish Sea); but aligns with local granites (e.g., Dartmoor);	Wood (1973/1974); Croot et al. (1996) [N.B.: Arber (1964) cited by Wood for context, but describes distinct smaller clasts, pace Croot et al.]
Small striated alkali micro-dolerite cobble (Higher Gorse pits, 1994 excavation / Brannam's pits, Unit B; 5 m depth)	Alkali micro-dolerite	Small striated boulder/cobble (flat-iron subglacial type with exceptionally well-striated faces); in main clay unit (Unit B); ~50% plagioclase felspar phenocrysts (poorly twinned, skeletal, zoned, sericite-altered); altered ferromagnesian minerals (chlorite pseudomorphs); titaniferous augite laths; numerous unfilled vesicles.	Unspecified; first unequivocal glacial transport.	Local basic intrusive; glacially transported in-situ clast (dropstone from iceberg/glacier); supports glaciolacustrine origin.	Croot et al. (1996); Gilbert (pers. comm.)
Trachy-andesite (Pen Hill, Taw Estuary beach/estuarine sand)	Trachy-andesite	Partially buried boulder; specifics limited.	Unspecified.	Possible western British source; estuarine reworking.	Croot et al. (1996); Gilbert (pers. comm.)
Grey elvan (general Fremington area)	Grey elvan (porphyritic dyke rock)	Quartz-felspar porphyry type; specifics limited; multiple occurrences.	Local dykes (Devon/Cornwall).	As per No. 8; common in aureole.	Taylor (1956); Croot et al. (1996)
Miscellaneous till erratics (Brannam's pits; Units B/D)	Varied (dolerite, granite, gneiss, flint, local Devonian/Carboniferous grits/shales/sandstones/limestones)	Sub-angular/rounded pebbles/cobbles/boulders; in lodgement till; includes striated stones, lignite, shell fragments; derived microfauna (e.g., Nonion labradoricum).	Irish Sea Basin (e.g., flint from Chalk, gneiss/granite exotics).	Mostly local (10 km radius, e.g., Culm gravels); rare exotics via ice-rafting; supports Irish Sea ice oscillations.	Stephens (1966); Wood (1974); Croot et al. (1996)

 

 

Conclusion

The Fremington Clay erratic assemblage is dominated by lithologies that can be sourced within the immediate South-West England bedrock province, particularly the Culm Basin, Dartmoor aureole, and Cornubian igneous suite. The petrography, rounding, and stratigraphic context of the larger clasts indicate derivation and short-distance transport within local fluvial systems and, at times, by locally confined ice or ice-rafted debris. A very small number of rounded pebbles, including rare far-travelled lithologies, may reflect limited marine or estuarine ice-rafting rather than sustained glacial incursion. Crucially, all confirmed erratics occur at elevations of approximately 10 - 20m OD, with no verified examples above this level. This altitude constraint strongly suggests that the Fremington deposits record low-level glacial–marginal or proglacial processes, rather than a substantial high-level Irish Sea ice lobe overriding the North Devon coast. It is essential to distinguish between the erratics of the Fremington Clay Series and those associated with the Saunton raised beach deposits, lest they be conflated in discussions of Quaternary glaciation in North Devon. Saunton's assemblage features larger, more exotic boulders—such as granites and gneisses potentially ice-rafted from the Irish Sea Basin—resting on shore platforms at similar elevations but indicative of marine incursion during sea-level highstands.  The evidence is therefore consistent with a landscape influenced by local fluvial and periglacial dynamics, episodic ice-rafting, and short-distance glacially assisted transport, rather than long-distance inland ice movement.

 

Photographs:

No.6 – Taylor 1956

No. 6 – Tim Daw – Nov. 2025 (rotated compared to Taylor’s photograph)  https://maps.app.goo.gl/8uvFckNu1TB7BFhd6

 

No.7 - Taylor – 1956

No.7 - Tim  Daw – Nov 2025.  https://maps.app.goo.gl/ijcV53LLrDUywAKR8

 

No. 8 – Taylor - 1956

 

 

References

·         Arber, M.A. (1964) Erratic boulders within the Fremington Clay of North Devon. Geological Magazine, 101, 282–3.

·         Croot, D.G., Gilbert, A., Griffiths, J. and Van Der Meer, J.J. (1996) The character, age and depositional environments of the Fremington Clay Series, north Devon. In The Quaternary of Devon and East Cornwall: Field Guide (eds D.J. Charman, R.M. Newnham and D.G. Croot), Quaternary Research Association, London, pp. 14–34.

·         Stephens, N. (1966) Some Pleistocene deposits in North Devon. Biuletyn Peryglacjalny, 15, 103–14.

·         Taylor, C.W. (1956) Erratics of the Saunton and Fremington areas. Report and Transactions of the Devonshire Associaton for the Advancement of Science, Literature and Art, 88, 52–64.

·         Vachell, E.T. (1963) Fifth report on geology. Report and Transactions of the Devonshire Association for the Advancement of Science, Literature and Art, 95, 100–7.

·         Wood, T.R. (1974) Quaternary deposits around Fremington. In Exeter Field Meeting, Easter 1974 (ed. A. Straw), Quaternary Research Association Handbook, Exeter, pp. 30–4.