Ramson Cliff erratic: a second thin section still points Cornubian, not glacial

A second thin section, same answer: still more Cornubian than glacial.

Prof Slack has kindly loaned a second thin section cut from the original hand specimen of the Ramson Cliff (Croyde) epidiorite — the ~700 kg block at ~80 m OD on Baggy Point. Discussion with Dr Mik Markham, who knows Cornubian greenstones and the axe-head groups as well as anyone, prompted a fresh transmitted-light description. It confirms the account in Daw, Ixer & Madgett (Quaternary Newsletter 167, 2026) with only small corrections.

The rock is a highly altered ophitic microgabbro with relict clinopyroxene, no olivine and no quartz. Secondary minerals are dominated by brown-green hornblende and colourless-to-green actinolite, two generations, with actinolite the later, and by minor chlorite, epidote and clinozoisite. Biotite and white mica were not confirmed. That assemblage matches the outer aureole of the Dartmoor Granite as described in the BGS Okehampton memoir, and is consistent with Cornubian contact metamorphism more broadly. It does not match non-aureole Devon or Cornish greenstones, which reach actinolite grade only and carry albitised, sericitised feldspar; nor does it match the Welsh Ordovician metadolerites on the Irish Sea route. A Scottish Dalradian source cannot be excluded on the amphibole evidence alone, but remains less likely on transport grounds; further tests on the relict pyroxene chemistry and plagioclase composition would settle it.

What has not changed is the emplacement argument. The block has been used as evidence that Irish Sea ice reached ~80 m OD on this coast. That reading was already thin: first recorded standing upright in pasture in 1969, absent from earlier maps and photographs, angular and unabraded, and the only claimed erratic on the south Bristol Channel shore above ~30 m OD. A Cornubian source does not prove human movement, but it does make a lone high-level glacial drop even less likely. A second slide from the same specimen does not turn an isolated, poorly documented boulder into proof of high-level ice, nor into a stepping-stone for glacial bluestones on Salisbury Plain.

Full description below.

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Rob Ixer's Analysis

A second thin section from the original hand specimen was generously loaned by Prof Slack and prompted discussions with Dr Mik Markham, an authority on Cornubian greenstone axes and on the petrography of Cornubian altered greenstones.

A petrographical examination of it confirmed the original petrographical description (Daw et al. 2026) albeit with some minor corrections. The un-named green amphibole is identified as actinolite. Trace amounts of white mica/muscovite and mixed muscovite-chlorite were not confirmed (the latter was probably misidentified mixed chlorite–limonite-stained chlorite) but trace amounts of epidote with high interference colours were. The suggestion that the rock could be Cornubian in origin is maintained.

The rock is a highly altered microgabbro, hence locally plagioclase–pyroxene have their characteristic ophitic relationship. It comprises relict primary pyroxenes showing both high and low interference colours, unaltered to highly altered plagioclase feldspar, and skeletal and equant-shaped opaques including probable titanomagnetite. The degree of alteration varies on a small scale and secondary alteration minerals are dominated by brown-green amphibole (‘hornblende’) and green actinolite and minor chlorite, with trace amounts of titanite, epidote and clinozoisite. White mica, if present, is very rare. Amphiboles replace and pseudomorph pyroxene but also occur as discrete and distinctive fine-grained mosaics intergrown with chlorite and enclosing very minor titanite, possible zircon and feldspar (perhaps albite). Texturally there is a strong suggestion of more than one generation of amphibole, with actinolite being the later.

Unzoned polysynthetically twinned plagioclase is variably altered from largely unaltered to highly altered, hence much relict feldspar is present. The main alteration is to fine-grained actinolite crystals, with this alteration initiated along cleavage and twin planes. Very minor amounts of chlorite, epidote with high interference colours and probable clinozoisite with low interference colours accompany actinolite. Plagioclase altering solely to fine-grained clinozoisite is rare; fine-grained white mica could not be positively identified.

Pyroxene displaying both high and low interference colours is the only primary mineral; no olivine or pseudomorphs after olivine are present. Relict pyroxene is enclosed within lower-relief amphibole; both are in optical continuity. Much pyroxene is altered to colourless to grey-brown-green amphibole with a good cleavage and with very fine-grained sphene lying along that cleavage; this amphibole is enclosed within green actinolite rims. Amphibole fringes about pyroxene are absent but total replacement is common. Other pyroxenes are altered to green actinolite along cleavage and fracture planes or to mosaics of fine-grained stubby actinolite. Locally pyroxene with low interference colours is altered to amphibole with unusual yellow interference colours.

Although a positive identification of the opaques is not possible in a normal thin section, their habit (equant and skeletal) and texture (abundant opaque ilmenite laths within a less opaque different phase; titanite replacing magnetite) strongly suggest the presence of altered titanomagnetite. Lobate opaques that would suggest ilmenite are absent. Opaques are replaced/pseudomorphed by actinolite and an opaques–actinolite association is widespread.

Pleochroic colourless to green actinolite (brown-grey-green amphibole is absent from this association) forms mosaics comprising euhedral to subhedral blocky crystals. Although some replace/pseudomorph pyroxene and opaques, most infill spaces between plagioclase laths; all mosaics are fine-grained but they vary in grain size between aggregates. Some just contain actinolite, others actinolite in minor chlorite, and a few are chlorite-rich with actinolite laths, often radiating, growing into the chlorite. Trace amounts of titanite, a high-relief accessory mineral within its pleochroic halo (zircon perhaps) and small twinned feldspar with fluid inclusions (possibly albite) are present but rare.

Colourless to very pale green pleochroic chlorite with blue and very rarely brown interference colours is the second most abundant secondary mineral after amphibole in amount, but is uncommon. It forms thin cross-cutting veinlets or occurs as a very minor secondary mineral in plagioclase. Most chlorite occurs as short stubby crystals surrounding stubby actinolite or as the main phase enclosing actinolite laths and enclosing minor titanite and possible albite. Some chlorite laths are intergrown with a phyllosilicate with high interference colours; although superficially it looks like muscovite, it may be limonite-stained chlorite. Biotite and positively identified muscovite were not recognised.

Trace amounts of titanite may replace primary iron–titanium oxides or lie along cleavage planes in pseudomorphing amphibole. Small euhedral rhombic titanite is present within stubby actinolite–chlorite segregations.

Very minor amounts of high-relief epidote form thin laths along twin planes in plagioclase; other high-relief epidote-group minerals with blue interference colours are visually identified as clinozoisite.

Trace amounts of sulphide, now altered to limonite, are associated with actinolite mosaics.

Possible zircon has a pleochroic halo when enclosed in tremolite.

This new petrographical description continues to allow the possibility that the erratic is Cornubian in origin, namely a microgabbro that has suffered contact metamorphism from the underlying granites. It cannot be matched to any of the main IPG Group Cornubian axe groups (Groups I–IV), but nor can many Cornubian axe-heads.