Fremington Clay Erratics - Dewey and Taylor descriptions

 From Dewey, H. (1910). Notes on some igneous rocks from North Devon. Proceedings of the Geologists' Association, 21(4), 429–434.

 

…boulders (III. and IV.) occur in the neighbourhood of Bickington, one (No. IV.) in front of Combrew Farm, and the other (No. III.) on a green about a quarter of a mile to the east of this locality.

The boulder No. III. (green quarter of a mile east of Combrew, 6-inch Ordnance Map, Devon, xiii, N.W.; 1-inch Sheet 293 Barnstaple) is a dark grey, finely crystalline rock with small white porphyritic felspars in the groundmass. Under the microscope it is seen to be rich in albite felspar, which occurs in the form of small plates and laths (Pl. XXIX, Fig. 3). These felspars lie in a groundmass of micropegmatite, but no free quartz is visible. In parts of the slide the granophyric groundmass appears to fill vesicles, and in other cases vesicles are filled with pleochroic calcite. If the rock ever contained a ferromagnesian constituent it could not have been plentiful, and is now completely replaced by chlorite.

In many respects the rock resembles the spilites of North and East Cornwall. The granophyric structure is not seen in the spilites, and in this rock it is mostly of secondary origin filling the vesicles. The other characters, however, resemble the spilites very closely, namely, the small albite felspars, the scarcity of ferromagnesian minerals and their replacement by chlorite; the absence of free quartz and the vesicular groundmass. It is possible that this boulder was derived from North Cornwall, but it is not safe to infer from the data available that it was probably derived from that area.

(click to enlarge)

By far the most interesting rock of those collected is the one found in front of Combrew Farm, boulder No. IV (6-inch Ordnance Map, Devon, xiii, N. W.; 1-inch Sheet 293, Barnstaple). It has been placed on the wall bounding a flower garden in front of the house. The rock is dark grey-green, with large pale olive-green felspars which are glassy and easily chipped from the rock. Microscopic examination reveals its glassy and porphyritic nature (Pl. XXIX., Fig. 4). It possesses felspars of two generations, but both are acid labradorite. The larger ones form a quarter of the rock, and the smaller occur in about equal quantities with the glassy base. The ferromagnesian constituent is a rhombic pyroxene which occurs to the entire exclusion of all other ferromagnesian minerals, for there is no augite, hornblende, or olivine. It occurs as small prisms, nearly the same size as the small felspars of the ground mass, with good cleavages, strong pleochroism, and straight extinction, and the pleochroism ranges from green to reddish yellow. Of the felspathic constituents the larger crystals are perfectly fresh and twinned on the albite and pericline laws, have high angles of extinction in symmetrical twins on each side of the plane of twinning, high refractive indices (the refractive index of this felspar is 1560, and the felspar is thus composed of 50% Ab. and 50% An., or in other words, is an acid labradorite), and inclusions arranged in well-marked zones. The small felspars are twinned on the albite law, but do not differ from the larger ones in their composition and freshness. There is no quartz present in the slide, but magnetite is abundant, and occurs as rods and feathery masses, and also as fine thin lines in the glassy base forming a network or gridiron structure. The crystalline constituents are embedded in a brown glass which constitutes about half the rock. In places this glass passes into the large felspar crystals by insensible gradations, there being no sign of a crystalline boundary between the two; but there is a well-marked zonal border in the brown glass surrounding the felspar.

The rock may be described as a hypersthene andesite. In many respects it resembles the Tholeite of Watt Carrick, Dumfries (1-inch Sheet 10, Scotland), and the hypersthene rocks of Curachan, Loch Craignish, Argyll (1-inch Sheet 36, Scotland), but all of these rocks contain considerable quantities of augite, whereas this rock is free from augite.

 

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From: Taylor, C. W., 1956. Erratics of the Saunton and Fremington areas. Rep. Trans. Devon. Ass. Advmt Sci., 88, 52—64.

 Fremington Area

There are two boulders from this district which are mentioned in the literature, and both of them were found without any difficulty, as they are preserved, and it would appear that enquiries for them are not infrequent. The present owner of Combrew Farm, Mr. Tucker, is aware of their interest, and unnecessary "hammering" is discouraged. The third boulder, which, though mentioned, has not so far been described or figured, came originally from the clay-pit of the old Fishley Pottery, now long disused, while as regards recent finds, two further masses have been obtained from the brown clay of Brannam's pit and are noted below. Although the literature makes no mention of the fact, the boulders Nos. 6 and 7 following are understood from local information to have been found also in the clay of the Fishley pit.

No. 6. A boulder is mentioned by Dewey¹¹ as at Bickington, about a quarter mile east of Combrew Farm, but owing to development, it has recently been moved to the garden on the right of the gateway to the main yard of the Farm. In case any ultra-zealous Glacialist should fit certain scars on portions of this rock, the author hastens to mention that it was dragged in its present position instead of being ruthlessly broken up on site, a consideration which is highly appreciated.

The original measurements and photograph have been given by Dewey, which need not be repeated here; it may be added, however, that the mass shows no distinct striae or wedge-shape, and the boulder measures roughly 40 x 30 x 25 inches high, the longest axis pointing almost E-W. It may be a Crinan spilite, of the pillow-lava type of igneous rock, which occurs in the Menaige district and elsewhere in Cornwall.

No. 7. The next erratic of this group is the hyalopilitic andesite, also previously described with No. 6 above. It is now situated on the right of the gated portion of the driveway to Combrew Farm, and is a glassy, brittle andesite, quite different from any of the foregoing rocks. Well rounded and about sixteen inches across, it contains no augite, but otherwise resembles similar rocks of Dumfries and Loch Craignish, Argyllshire.

No. 8. The rock from the Fishley Pottery clay-pit has hitherto merely been mentioned as "an igneous boulder" which may now be found on the right-hand, inside the gate of the first building (the old disused pottery) on approaching Combrew Farm from the main road. It is well covered with lichen, with flat top and base, and measures 47 x 19 x 16 inches high. Of a light grey colour, it is holocrystalline, with some quartz and much felspar, which appears altered; little mica could be observed. The texture is rather granitic, with a fine, pale coloured base containing the larger phenocrysts of quartz and felspar up to 5 mm. in size, but averaging 3 mm.

A section of this specimen under the microscope shews it to be a much altered quartz porphyry, with crushed and irregular crystals of plagioclase, porphyritic quartz and long prisms of apatite, with a matrix nearly amorphous and red; epidote appears to have replaced part of the mosaic. This is regarded as a highly peculiar textural type, which may be derived from a fairly local source, such as the porphyritic dyke, west of the coasts of Devon and Cornwall.

No. 9. Across the fields in a south easterly direction from Combrew Farm, and through the coarse white clay in the adjacent Brannam's pits (to be distinguished from the Fishley Pottery) there were found two masses of boulders. The better-shaped boulder of the pair showed considerable rounding and exhibits a regular ellipsoidal form, but with lower mean weight, which was found in the clay. It is a highly crystalline and compact igneous rock of a grey colour, the texture resembling a very fine grained granite; a soft milky white crystal of felspar can be seen with a lens, while weak and indicates the presence of a little calcite. Also it includes highly kaolinized felspar, but still revealing the original lath outline, with a small proportion of quartz. Augite is reddish fresh, altering only slightly around the edges, while a little magnetite occurs, and a small amount of secondary calcite. Long needle prisms of apatite are present, with longitudinal inclusions and penetrating other minerals. The quartz, to judge from the textural arrangement, is probably of primary formation.

This rock appears to be a quartz dolerite, with none but a trace of a sub-ophitic texture, and has no characteristic which would enable a source to be named. Its main interest, as is the case with the other boulders here, lies in being included amongst this deposit of clay, the origin of which has given rise to intermittent discussion since the 1860's.

No. 10. A further rock from the same clay-pit as No. 9 may be found, in two pieces, by the surface at the far side of the pit, also of about three hundredweight, and of irregular, angular shape. It is of somewhat similar appearance to the foregoing, but of a darker grey and more crystalline. Again soft felspar crystals may be observed in the interior and with some calcite, indicated by weak acid. Examination of a thin section shows micro-pegmatitic ophitic structure with crossed flows, with tabs of plagioclase enclosed, and prisms of ilmenite are oriented transverse to the cracks. Grains of olivine under the thin section appear yellow. The quartz is light in colour but the texture shows slight orientation. Some extinction occurs here and there in the felspars. This boulder, an olivine dolerite, is quite fresh, but like the foregoing example, offers no means of determining its source; as mentioned under 5b, it is not an uncommon type in Devon.

Through the kind advice of Miss M. A. Arber, the Brannam clay-pit was later again visited to inspect further smaller finds which had been obtained at a depth of 17 feet. One of these proved to be a two-inch smoothed and rounded pebble of olivine dolerite, similar to No. 10 above, while the other comprised a five-inch slab of waterworn Carboniferous grit an inch and a quarter thick. This fragment has its two flat surfaces still partly covered with a smooth, red skin which proved to be ferric oxide, probably deposited by infiltration in situ along bedding cracks. These pebbles call for no further comment, save perhaps that the skin suggests that the grit was waterworn mainly before inclusion in the clay, as otherwise the skin might be expected to be completely removed.

Thus, so far as can be traced from the literature at the author's disposal, all the principal erratics mentioned from time to time as being in this area are still in existence, while a few additional blocks of local or foreign sources may be found at odd points. The older literature also mentioned casually by J. Prestwich, "a smaller block of fine grained white granite, and another of quartz grit, on the beach, S. of the cliff." Neither of these is known to the present writer, while there are several blocks of igneous rock to be found seen on the river or covered by sand. Again in his 1879 Memoir of the North Devon Athenæum there is a specimen labelled "Granite Boulder, Raised Beach, Baggy", but no other note is available.

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 Summary of Fremington Erratics: Dewey (1910) and Taylor (1956)

Original Descriptions

Dewey (1910)

Two principal igneous boulders were described near Combrew Farm, Bickington:

1. Boulder III (east of Combrew Farm)
• Fine-grained dark grey rock with small white felspars (albite).
• Groundmass of micropegmatite, vesicular with calcite fillings.
• No quartz, little or no ferromagnesian minerals (replaced by chlorite).
• Compared to spilites (altered basalts) of North and East Cornwall, though not identical.
• Possible but uncertain source: North Cornwall.
2. Boulder IV (Combrew Farm garden)
• Dark grey-green, glassy and porphyritic, with large acid labradorite felspars.
• Contains hypersthene as only ferromagnesian mineral (no augite or olivine).
• Abundant magnetite; about half of rock is brown glass.
• Identified as a hypersthene andesite.
• Comparison: Similar to rocks from Watt Carrick (Dumfries) and Loch Craignish (Argyll), but differing by lack of augite.
• Suggested source: Uncertain—Scotland suggested by comparison, but not confirmed.

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Taylor (1956)

Taylor relocated and re-described the same boulders and several others from the Fremington and Saunton clays.

1. No. 6 – Crinan spilite
• The same as Dewey’s Boulder III.
• Suggested similarity to Crinan spilitic pillow lavas (Cornwall).
• Probable Cornish source.
2. No. 7 – Hyalopilitic andesite
• Dewey’s Boulder IV.
• Glassy andesite, well-rounded, no augite.
• Similar to Dumfries and Loch Craignish rocks, but not identical.
• Possible Scottish or western British source.
3. No. 8 – Quartz porphyry (Fishley Pottery pit)
• Light grey, granitic texture, much-altered quartz and felspar.
• Possible local source, perhaps a Devon or Cornwall porphyritic dyke.
4. Nos. 9 & 10 – Quartz dolerite and olivine dolerite (Brannam’s clay pits)
• Compact igneous rocks, rounded or angular, unstratified in the Fremington Clay.
• No diagnostic features for distant transport.
• Suggested local origin, possibly Devon intrusions.

Original Provenance Views

• Dewey: Cautious; local/Cornish likely, Scottish comparisons tentative.
• Taylor: Strongly local (Devon–Cornwall intrusions/dykes); erratics support periglacial/fluvial reworking, not distant ice.

Modern Interpretation (Post-1956)

1. Nature of the Fremington Clays

Modern sedimentological and palaeoclimatic work (Croot et al., 1996; Bennett et al., 2024); interprets the Fremington Clays as:

• Late Pleistocene slope and periglacial deposits, possibly including solifluction and fluvial components.
• Not definitively glacial till — no evidence for direct ice-sheet deposition in North Devon.
• Erratics were likely incorporated into the clay by local mass-movement or river reworking, not transported by ice.

2. Source and Petrology

Later petrographic and geochemical comparisons confirm:

• The spilitic and doleritic types closely match Devonian–Carboniferous volcanic rocks from North Cornwall (Mullion–Lizard to Bude).
• The andesitic and doleritic types also resemble Tamar Valley and Dartmoor margin intrusives.
• No evidence supports Scottish or northern origins.
• A few quartz porphyries could derive from local dykes in the Bideford–Instow–Appledore district.

 

Boulder	Lithology	Key Features	Original Source Suggestion	Modern Consensus
III/No. 6	Spilite	Vesicular albite, micropegmatite, chlorite	Cornwall spilites	SW England volcanics (Cornwall/Dartmoor)
IV/No. 7	Hypersthene andesite	Glassy labradorite, hypersthene, no augite	Scottish (tentative)	Local SW dykes (Tamar/Dartmoor)
No. 8	Quartz porphyry	Altered phenocrysts, epidote	Local dyke	Devon–Cornwall intrusions
No. 9	Quartz dolerite	Ophitic augite, apatite	Local Devon	Dartmoor dykes (Meldon)
No. 10	Olivine dolerite	Yellow olivine, ilmenite	Local Devon	Devon minor intrusions

 

Conclusion: Erratics are local, fitting periglacial/fluvial processes; no glacial long-distance transport needed.

No modern studies have found evidence of northern or foreign erratics in the Fremington Clays. Their presence aligns with local volcanic and intrusive lithologies of Devon and Cornwall, reworked into periglacial or fluvial clays during Late Pleistocene climatic episodes.