1HARD ‘VolcAnics witH sub-plAnAR textuRe’ in tHe stoneHenge lAnDscApe

The three major groups of debitage found in the Stonehenge landscape are dolerites, rhyolitic tuffs and ‘volcanics with sub-planar texture’. Whilst the first two groups are now described and established in the literature, the third has been largely overlooked. These volcanics comprise two groups, namely Volcanic Group A, friable rocks with abundant white mica and a strong metamorphic fabric, and Volcanic Group B, hard rocks that are partially characterised by an unusual mineralogy including two forms of graphitising carbon. Only twelve Volcanic Group B samples have been recognised from the Stonehenge debitage but they share the same petrography as orthostat SH38. Spatially, as with the debitage from the Altar Stone and orthostat SH48, they are widely, if thinly and randomly, distributed throughout the Stonehenge Landscape. Temporally, almost none of the debitage from all three bluestone orthostats, has a secure Neolithic context.

Hard ‘Volcanics with sub-planar texture’ in the Stonehenge Landscapeby Rob A. Ixer,1 Richard E. Bevins2 and Andy P. Giże3

Wiltshire Archaeological & Natural History Magazine, vol. 108 (2015), pp. 1–14

1 University College London Institute of Archaeology, 31–4 Gordon Square, London WC1H 0PY. email: r.ixer@btinternet.com; 2 National Museum of Wales, Cathays Park, Cardiff CF10 3NP email: richard.bevins@museumwales.ac.uk; 3 Lucid Microscopy. Kingsway, Frodsham, Cheshire, WA6 6RU, UK. email: andy_gize@msn.com

Introduction The Stonehenge bluestone ‘debitage’ (loose material believed to be associated with the Stonehenge orthostats (Ixer and Bevins 2013b)) mainly comprises spotted dolerites, the so-called preselites, that originate from the tops of the Preseli Hills notably Carn Goedog (Bevins et al. 2014) but perhaps Carn Menyn (Ixer 1997; Darvill et al. 2009) and elsewhere, rhyolite and rhyolitic tuffs almost exclusively from rocks found at Craig Rhos-y-felin in the low ground to the north of the Preseli Hills (Ixer and Bevins 2010; Bevins et al. 2011; 2012; Ixer and Bevins 2011b; 2013a) and a third major class informally called ‘rhyolite with sub-planar fabric’ (Ixer and Bevins 2010) but now renamed ‘volcanics with sub-planar texture’. The first two major classes of debitage have been described and discussed in the literature (see references in Bevins et al. 2014 and Ixer and Bevins 2013b) but the third has only

briefly been described (Howard in Pitts 1982; Ixer and Bevins 2010).

Macroscopically ‘volcanics with sub-planar texture’ are a distinctive class of material and much occurs as small, often sub-rounded, rather than angular flaked, fragments throughout Stonehenge and its environs (Ixer 2008 unpublished; Ixer and Bevins 2013b). Although examples have been described briefly in Ixer and Bevins (2010; 2011a) there have been few descriptions of them since Judd (1902) other than those of Howard in Pitts (1982). They have been designated as altered basic tuffs in the literature (Pitts 1982; Ixer and Bevins 2010) but are now recognised as including two distinct sub-groups.

A) The most common numerically is the informally called ‘muscovite-no carbon’ argillaceous tuffs and characterised by abundant, fine-grained white mica; commonly they have a strong

THE WILTSHIRE ARCHAEOLOGICAL AND NATURAL HISTORY MAGAZINE2

metamorphic fabric including strain slip cleavage.

B) A smaller group called ‘graphitising carbon-altered ilmenite-no muscovite’. It was noted that members of this group shared an unusual mineralogy with orthostat SH38 (Ixer and Bevins 2011, 9) but this has not been pursued until now.

Neither group has been described in any detail, indeed Volcanic Group A, the ‘muscovite-no carbon’ argillaceous tuffs, has not been discussed at all this century.

In a recent paper Ixer and Bevins (2013b) were able to plot the distribution of debitage within the Stonehenge Landscape that probably came from two named orthostats (Altar Stone and SH48), their so-called ‘knock-offs’ and suggested that this might be possible for other named orthostats. This paper intends to provide the first detailed descriptions of Volcanic Group B, especially its defining characteristic namely graphitising carbon, to show its affinity and suggested relationship with Stonehenge orthostat 38 (Figure 1) and to discuss its distribution in the Stonehenge Landscape in

0 10 30 4020 50

metres

April 2008Excavation

Aubrey Hole 7

Trench 44

Trench 45

Orthostat 48

Altar Stone

N

The Avenue

Heelstone

A344

Orthostat 38

Fig. 1 Map of Stonehenge showing recent excavations, plus the three main orthostats named in the text.

3HARD ‘VolcAnics witH sub-plAnAR textuRe’ in tHe stoneHenge lAnDscApe

order to complement data for the two earlier named bluestones.

Current MethodologyMacroscopical re-examination (May 2013) of all the bluestones examined since 2009, plus all new material showed that almost exactly 1900 debitage fragments weighing between 0.1 and 1120g (the mean weight is <10g) were ‘volcanics with sub-planar texture’. The majority are from the April 2008 Darvill and Wainwright excavation within the Stonehenge stone circle (1848 fragments mainly from the Stonehenge Layer and Roman contexts) but also including lithics from the Heelstone Ditch

excavations (11) (Pitts 1982; Ixer and Bevins 2013a), the Stonehenge Avenue including Trenches 44 (3) and 45 (19) (Ixer and Bevins in press), Aubrey Hole 7 (14) (Ixer and Bevins in press), and from surface finds and test pits in the area close to the western end of the Stonehenge Greater Cursus (3) (Ixer and Bevins 2010). The locations of the find spots are shown in Figures 1 and 2. Almost all, just fewer than 99.5% by number, of these belong to Volcanic Group A and have no petrographical match with any above ground orthostat.

Over 40 standard thin sections and polished thin sections were made of representative volcanics with sub-planar texture recovered from all of the find sites. Amongst the 1900 lithic samples are twelve lithics representing Howard’s class of ‘hard basic tuff ’ (as detailed below), now designated Volcanic

River Avon0 0.5 1

kilometre

DurringtonWalls

extent of World Heritage site

Cursus Field

Greater Cursus

A344

A303

STONEHENGE

The Avenue

Ames

bury

Durrington

N

Fig. 2 A broader scale map of the ‘Stonehenge Landscape’ showing the Stonehenge Greater Cursus and the full extent of the Avenue and Stonehenge.

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Group B. These are SH08 Context 1 196 (three separate samples); Context 3 B/2 EN49; Context 3 V/1; Context 12/2 FN518 and Context 12 East 8 FN267, collected during the Stonehenge April 2008 excavation (Darvill and Wainwright 2009); The Avenue SAV Trench 45 context 19 1074; Aubrey Hole 7 SH08 (308) and SH79+64, SH79+212 and SH80+1142 from the Heelstone excavation (Pitts 1982). Fuller details of these lithics are presented in table 1.

Detailed ‘total petrography’ as defined by Ixer (1994) and Ixer et al. (2004) was undertaken on the polished thin sections using both transmitted and reflected light. Thin section mineralogical identifications in transmitted light were made following Kerr (1959) and Deer, Howie and Zussman (1992) and in reflected light following Ixer (1990), and petrographical and lithological descriptions with reference to MacKenzie et al. (1982), Howells et al. (1991) and the British Geological Survey Rock Classification Scheme.

Pyrobitumen reflectance measurements (Ro%) were made on graphitising carbon found in sample SH08 Context 3 V/1 using a x40 oil immersion lens. The measuring wavelength was set to 545±10nm, with the reflectance spot size of 7.63µm. Calibration used the following standards: leucosapphire (Ro = 0.506%), gadolinium-gallium garnet (Ro = 1.893%) and zirconia (Ro = 3.159%). Bireflectance was measured at specific points of interest. In addition to recording the maximum and minimum reflectance, the average (mean) reflectance was calculated. All measurements were greater than that of the zirconia standard and were therefore extrapolated. Limited numbers of whole rock XRD analyses were used to confirm the identity of the feldspars and phyllosilicate minerals.

Previous studies of Volcanic Group BJudd (1902, 110) recognised a class of debitage that he called “highly altered basic tuffs and agglomerates”. He noted that they were fissile and “consist of chlorite and leucoxene, with various classes of colourless minerals (zoisite, albite etc.), the products of the alteration of minerals found in basic lavas” and that “in many cases the rock is made up of more less (sic) angular fragments and not infrequently these fragments are vesicular, the vesicles infilled with crystalline calcite”. He recognised that the rocks were volcanic, noted their extreme alteration and that many were now “more or less foliated”.

Judd suggested that the rock fragments were so common that “probably a number of stones of this kind were used in the building of Stonehenge” and that these might include the stump discovered by Cunnington in 1881 and noted on his plan as S28 (Cunnington 1884, 142); its present Stonehenge number is 32c. He described this “rock stump’ as ‘a soft calciferous chloritic schist foreign to the county of Wiltshire and differing entirely from any other of the stones of Stonehenge” (Judd 1902, 108, 111).

Thomas (1923, 251) recognised that fragments of a “dark grey compact calcareous rock” were pyroclastic in nature and composed “of small fragments of a much altered vesicular lava of moderately basic composition. The vesicles are filled variously with calcite or chlorite and the whole rock has been subjected to considerable shearing stress that has imposed on it a semi-schistose structure”. He suggested that the origin was the “north side of the Prescelly range” suggesting outcrops “a little

Locality Find spot Context Weight in grms

Thin or Polished

DetailedDescription

Stonehenge Orthostat SH38 SH38 Tonnes PTS, PB I and B 2012aStonehenge Aubrey Hole 7 AH08 308 62.4 TS, PTS I and B in pressStonehenge April 2008 excavation Context 1 Δ196 15.0 PTS x3 In this textStonehenge April 2008 excavation Context 1 Δ196 22.9 TS, PTS UnpublishedStonehenge April 2008 excavation Context 1 Δ196 10.8 TS UnpublishedStonehenge April 2008 excavation Context 3 B/2 ◊49 15.0 PTS UnpublishedStonehenge April 2008 excavation Context 3 V/1 ◊38 20.6 TS, PTS In this textStonehenge April 2008 excavation Context 12/2 Δ518 24.5 PTS UnpublishedStonehenge April 2008 excavation Context 12/E8 Δ267 299.5 PTS UnpublishedStonehenge The Avenue Trench 45 Context 19 Δ1074 47.8 PTS I and B in pressStonehenge Heelstone SH79+64 F/L3 6.6 cut PTS UnpublishedStonehenge Heelstone SH79+212 D/L2 36.8 cut PTS UnpublishedStonehenge Heelstone SH80+1142 122/L13/4 44.2 cut PTS Unpublished

TS thin section, PTS polished thin section, PB polished block. I and B is Ixer and Bevins

Table 1: List and location of Volcanic Group B lithics.

5HARD ‘VolcAnics witH sub-plAnAR textuRe’ in tHe stoneHenge lAnDscApe

north of Foel Trigarn” as a possible origin for some of the debitage (Thomas 1923, 262).

Almost 60 years later Howard in Pitts (1982) carried out a major petrographical description of the non-preselite bluestones from close to the Heelstone using large numbers of samples backed up by 21 thin sections. She examined “less than 1 kg of weathered basic tuffs” weighing “ generally about 10g with rare examples to 100g … heavily abraded amorphous fragments varying in hardness, colour and texture”. All were vesicular and many effervesced with acid. Howard recognised two main varieties of ‘basic tuffs’; most were light blue-grey to green-grey, extremely soft, laminated rocks crumbling under light pressure (in this paper redesignated as Volcanic Group A and not further discussed) but others were harder, dark grey-green, sometimes schistose lithics that fractured with difficulty (in this paper redesignated as Volcanic Group B). She chose eight lithics for thin sectioning including four hard, schistose samples (SH79 64, SH79 212, SH80 1142 and SH80 1451) and tabulated the macroscopical and microscopical characteristics of these eight “samples representing most variations within the rock types” giving their hardness, colour, fracture, ‘groundmass structure’ and their main mineralogy (Table 2, Pitts 1982, 118). Howard suggested the soft tuffs (Volcanic Group A) may have come from orthostats, stating “the crumbly light-coloured type may have been brought to Stonehenge to be erected as standing stones” but also that “the hardness and durability of the dark variety” (Volcanic Group B) “would have been eminently suited to the manufacture of pecked or ground tools”. She further opined correctly that “a reassessment of all examples of basic tuffs found at Stonehenge may help to shed light on this interesting problem” (Pitts 1982, 117).

Thorpe et al. (1991) in their major paper on the bluestones of Stonehenge noted the presence of non-dolerite debitage in passing but no samples of Volcanic Group A or B were included in their geochemical analyses.

A thin section labelled as ‘Stonehenge Altar Stone’ held in the Hunterian Museum, University of Glasgow collection (Cat No. 148302) is described as a volcanic breccia. Sadly the hand specimen associated with this thin section is missing and so a new section cannot be prepared, but it probably belongs to the ‘volcanics with sub-planar texture’ according to Ixer and Bevins (2013b). Macroscopically the thin section resembles Volcanic Group B sample SH08 196 and microscopically it displays many of the features required for membership of Volcanic

Group B including acicular brown amphibole in a large microtonalite clast, titanite-rich vesicular lava and a fine-grained siliceous matrix. As a polished thin section cannot be prepared it is not possible to determine if the local occurrence of very fine-grained opaque areas includes graphitising carbon, critical to this study.

Ixer and Bevins (2010, 8–9) described in detail two ‘altered basaltic tuffs’ from the vicinity of the Stonehenge Greater Cursus. The relationship between orthostat SH38 and debitage from Stonehenge was tentatively discussed in Ixer and Bevins (2011a) where they first suggested that rare debitage found in the Heelstone area and Darvill and Wainwright’s April 2008 excavations shared characteristic petrographical features with that orthostat. They stated that “SH38 differs from all the other sampled orthostats and from almost all of the analysed Stonehenge debitage in having trace amounts of graphitising carbon (a highly unusual phase in igneous or indeed unmineralised rocks) associated with altered pyrrhotite. This, plus the presence of lath-shaped, altered ilmenite, gives a characteristic mineralogical fingerprint to the lithology and shows that SH38 is comparable to a rare, but recognisable, lithic sub-class found within the Stonehenge ‘debitage’ known informally as ‘graphitising carbon-altered ilmenite-no muscovite’. This sub-class belongs, in turn, to a major class of Stonehenge ‘debitage’ known as ‘rhyolite with sub-planar fabric’” (Ixer and Bevins 2010, 5). Petrographical examination of more material showed that although early examined samples were ‘rhyolite’-rich more recently examined lithics are more basic vesicular lava-rich and so ‘rhyolite with sub-planar fabric’ has been renamed as the more accurate and inclusive ‘volcanics with sub-planar texture’.

However, Ixer and Bevins (2013b, 15) cautiously stated “To date no piece of debitage has been recognised as coming from [SH38] … this may be partly through inadequate sampling, as, to date, thin sectioning and subsequent microscopical analysis of the debitage has been biased towards the rhyolitic debitage rather than the ‘volcanics with sub-planar texture’. This in turn is because detailed descriptions and classification of the ‘volcanics with sub-planar texture’ classes have yet to be published”.

This paper is the beginning of the resolution of that problem; it also records in detail an unusual occurrence of graphitising carbon and comments on its relevance and potential in Stonehenge provenance studies and will show that SH38 debitage is present

THE WILTSHIRE ARCHAEOLOGICAL AND NATURAL HISTORY MAGAZINE6

within the Stonehenge Landscape. It also gives a partial answer to Howard’s “interesting problem”.

Results

General petrographical characteristics of Volcanic Group B

Unlike Rhyolite Group E and its parent orthostat SH48 where the degree of petrographical similarity between debitage and standing stone is remarkably high (Ixer and Bevins 2013b), there is a greater petrographical variation between the Volcanic Group B samples. However, they do form a sufficiently petrographically coherent group that the following composite petrography, collated solely from the twelve debitage samples, is both valid and useful.

Macroscopically Volcanic Group B lithics are quite distinctive in being a light to dark grey, indurated (termed ‘hard’ by Howard in Pitts (1980)) fragmental rock with a hackly fracture and a sub-planar fabric. They lack the sheen and good cleavage due to the presence of abundant phyllosilicates that is characteristic of Volcanic Group A lithics. Different lithic clasts, up to 10mm in diameter, are present and include vesicular lavas and very fine-grained ‘basalt’ (microtonalite in thin section) within a light-coloured siliceous matrix. In addition many examples carry thin, planar, olive grey to dark grey ‘mudstone fragments’ (Ixer and Bevins 2011a). Surfaces of some examples are pitted due to the dissolution of carbonate. Before thin sectioning the lithics ranged in weight from 10 to 300g with an estimated (some lithics had been sectioned before weighing) mean of 50g. The dark colour and presence of 1–2mm long, twinned feldspar laths and ‘mudstone’ fragments are defining macroscopical characteristics of this highly indurated rock.

Microscopically the lithic is a vitric-lithic-crystal tuff with distinctive mineralogy and textures; rock clasts are dominated by vesicular lava; these are present in all the samples, whilst microtonalite and ‘mudstone’ clasts are present in about half. The clasts are held within a fine-grained quartz-feldspar-rich siliceous matrix.

Vesicular lava clasts (Figure 3) vary in amount from being the overwhelmingly dominant clast to being quite minor. The clasts, which show grain-size variation, comprise intergrown albite-titanite-chlorite±calcite. They show different sized vesicles which are often oval rather than spherical and are

infilled by chlorite, albite or titanite and more locally by calcite and quartz. Although most vesicular lava clasts are albite-dominated some are titanite- or chlorite-rich. The lava carries rare, large, euhedral, simply twinned feldspar or plagioclase phenocrysts often showing an internal sieve-like texture. Some untwinned feldspars are probably potassium feldspar as suggested by X-ray diffraction data for SH80 1142 that shows approximately 10% potassium feldspar to be present. Locally brown acicular amphibole is found in some clasts.

Half of the lithics carry minor amounts of very fine-grained to fine-grained microtonalite, comprising radiating feldspar-chlorite-titanite plus an acicular high relief phase optically identified as amphibole. Microtonalite clasts, up to ten millimetres in size, are discrete with sharp edges and often fractured and re-cemented by the fine-grained quartz-feldspar matrix. Smaller clasts are lensoidal and more diffuse (SH79 212) and similar to those seen in the Craig Rhos-y-felin rhyolites.

Five lithics have dark-coloured ‘mudstone’ clasts. These are AH08 308, SH80 1142, SH08 267, SH08 V/1 and SH08 196. They are variable in composition comprising a very fine-grained high relief phase (probably amphibole)-titanite-albite/quartz-chlorite intergrowths; graphitising carbon is intergrown with titanite in these clasts. Other ‘mudstones’ are more ‘clastic’ with much fine-grained quartz and have voids infilled with calcite and chlorite. The clasts are enclosed within

Fig. 3 An aggregate of four feldspar crystals, partially surrounded by amphibole (centre) are set within a fine-grained

quartz/feldspar matrix. A large coarser grained vesicular lava clast (bottom right corner) shows a sharp contact against

the main matrix. Sample SH08 196 4b.Crossed polarised transmitted light.

7HARD ‘VolcAnics witH sub-plAnAR textuRe’ in tHe stoneHenge lAnDscApe

a fine-grained siliceous matrix and this is the main component in some samples, notably SH79 212 and SH08 B2/49; locally it has a sub-spheroidal texture or includes recrystallised tube pumice clasts (Figure 4) both textures suggesting a vitric component. The matrix comprises fine-grained quartz/albite mosaics intergrown with lesser amounts of chlorite and minor titanite. Locally some layers carry a pale brown wispy, high relief mineral, optically identified as amphibole, aligned along the sub-planar fabric (Figure 5). White mica laths and unzoned zircon are present in trace amounts. The siliceous matrix carries large, euhedral, untwinned or simply twinned feldspar plus sodic plagioclase phenocrysts; these commonly show an internal sieve-like texture, or show pull-apart textures infilled by chlorite and quartz. In SH08 196 and AH08 308 plagioclase phenocrysts/aggregates are intergrown with ‘large’ subhedral amphibole crystals (Figure 5). Locally siliceous-rich areas appear to be enclosed within thin titanite and amphibole rims.

Feldspar phenocrysts are one of the defining features of Volcanic Group B and rounded but mainly subhedral to euhedral feldspar phenocrysts occur in both the siliceous matrix and vesicular lava clasts. Some untwinned or simply twinned feldspar may be potassium feldspar but many phenocrysts are sodic plagioclase. Feldspars enclose apatite laths, have cloudy, pale brown cores within clear margins and many have a characteristic sieve-like with chlorite or the siliceous matrix (Figures 5 and

6). Shattering and pull-apart textures are common and the feldspars are re-cemented by the siliceous matrix or by quartz, albite, chlorite, amphibole or, very rarely, by calcite. Few feldspars show any alteration but some are replaced by calcite (SH08 V/1), quartz or quartz plus secondary albite. Many are surrounded by thin rims of chlorite, titanite,

Fig. 4 Two plagioclase phenocrysts (pale grey, bottom right) are enclosed within a fine-grained quartz/feldspar-chlorite matrix showing a poorly defined planar fabric. A vesicular lava clast (top right) is darker and titanite-rich. A small,

angular, recrystallised tube pumice clast (top middle) shows alternating clear quartz and darker chlorite bands. Sample

SH08 196 4b. Plane polarised transmitted light.

Fig. 5 Two euhedral plagioclase phenocrysts are set within a fine-grained quartz/feldspar matrix. Dark, east-west orientated

amphibole flakes are common and partly define the poorly defined fabric. The left hand plagioclase crystal shows the

characteristic sieve-like texture. Sample SH08 196 4b. Crossed polarised transmitted light.

Fig. 6 Three subhedral, untwinned or simply twinned feldspars within a fine-grained quartz/feldspar matrix. The top phenocryst shows a characteristic sieve-like texture. The

feldspar aggregate is surrounded by acicular amphibole (middle left) and by a rhombic amphibole (texture dark, middle

of aggregate). A vesicular lava clast is present (bottom right corner). Sample SH08 196 4b. Crossed polarised

transmitted light.

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acicular amphibole and perhaps trace amounts of muscovite. Secondary calcite is present in five of the lithics and a further two have pits and voids suggesting its former presence. Opaque and accessory minerals are important in defining this group and so a detailed description is warranted. Rounded to euhedral zircon is rare and is 20–60 but up to 120µm in diameter. A single occurrence of high relief, olive-brown tabular grains associated with plagioclase may be aeschynite-(Y) (SH08 196 4b). Graphitising carbon (perhaps the most important defining characteristic of the group) is rare to widespread (but present in all twelve samples) and typically forms highly anisotropic, 1–60µm diameter droplets (Figure 7) or highly irregular shapes up to 120µm (Figure 8); it encloses 2–5µm diameter pyrrhotite grains and to a lesser extent is associated with altered ilmenite; locally it surrounds rock clasts or is found within the ‘mudstone’ clasts. It is of significance that no coal macerals, including resistant fusinite, were recognised in any of the samples.

Ilmenite laths 20–120µm in length are variously altered to blocky intergrowths of white or pale yellow TiO2 minerals–relict ilmenite (SH08 B/2, SH08 518, SH08 267, SH79 212) or are enclosed within a thin 2–5µm wide TiO2 rim all within titanite (AH08 308, Tr45 1074, SH08 196 (1–3) or are only present in trace amounts (SH79 64 and SH80 1142).

Titanite is the most abundant semi-opaque phase, occurring as 10–60µm long discrete rhombic

crystals, as pseudomorphs after ilmenite but mainly as 20–80 but up to 150µm diameter, rounded aggregates within vesicular lava clasts. Hexagonal pyrrhotite 5–100µm in diameter and extensively altered to limonite is more abundant than 5–40µm diameter pyrite and very rare 10µm diameter chalcopyrite (SH79 64, SH08 196).

Based upon its petrography, defining characteristics for Volcanic Group B can be outlined. These include the presence of plagioclase phenocrysts with a sieve-like texture, graphitising carbon, characteristic ilmenite alteration (every lithic carries these three characteristics although altered ilmenite is very rare in two samples) plus the presence of acicular brown amphibole, ‘mudstone’ clasts and the absence of muscovite. Other notable features, especially the presence of large amphibole crystals (seen in SH08 196 and AH08 308) and later void-infilling calcite are not present in enough samples to be considered a defining feature.

Graphitising carbon, which is rarely seen in unmineralised rocks, is considered to be the single most important defining characteristic of the Volcanic Group B samples. It has not been recognised in any other Stonehenge-related rocks other than in a rhyolitic crystal-lithic tuff, namely sample Cursus Field 142/1947.25 collected by Stone in 1947 (Ixer and Bevins 2010, 6). There are, however, significant differences between Volcanic Group B samples and the Cursus Field sample as discussed below.

Fig. 7 Diagnostic of the Volcanic Group B lithics are two types of pyrobitumen particles. To the left, an almost spherical

particle shows the start of deformation, as shown by the anisotropy, which is enhanced in the particle to the right with the loss of sphericity. Sample SH08 Context 3 V/1. Image width 200µm, oil immersion, reflected white light,

crossed polars.

Fig. 8 The most common texture in the Volcanic Group B lithics is graphitizing carbon showing continuous

strong anisotropy (>1µm). The anisotropy is aligned with planar silicate orientation, indicating shearing as the deformation process. Sample SH08 Context 3 V/1. Image width 200µm, oil immersion, reflected white light, crossed

polars.

9HARD ‘VolcAnics witH sub-plAnAR textuRe’ in tHe stoneHenge lAnDscApe

Two detailed lithic descriptions are given below, to show the full petrographical range within the tuff.

SH08 Context 3. Stonehenge Layer. ◊38 V/1. Tuff. Polished thin section ◊38 (3) V/1. This is an example of a vesicular lava-rich portion of the tuff and is very similar to the petrography of SH38. The tuff has a sub-planar texture together with a poor lensoidal fabric. It carries a number of different lithic clasts, with a very wide size range, but is dominated by vesicular lava but also includes minor amounts of mica-rich argillite within a fine-grained siliceous matrix. The majority of the rock comprises chlorite-albite-titanite vesicular lava with rounded vesicles infilled with titanite and chlorite. Later generations of coarse-grained sparite are associated with this lava. The more acid-rich and titanite-poor lithology comprises fine-grained mosaics of quartz/feldspar-chlorite with only minor amounts of titanite. It carries feldspar phenocrysts including plagioclase and euhedral, untwinned or simply twinned feldspar. Plagioclase with a sieve-like texture encloses chlorite or the fine-grained quartz/feldspar matrix. Some plagioclase is zoned and displays cloudy cores within clear margins. Thin calcite veinlets lie along the main sub-planar fabric and very locally wispy possible muscovite is present in tabular argillite clasts. Elsewhere secondary albite clusters within chlorite rims are present. Rare, unaltered pyrrhotite up to 20µm in diameter is present in titanite as is altered pyrrhotite. Limonite, 10–100µm in diameter, has replaced pyrrhotite along its (0001) basal planes. Pale-coloured TiO2 is rare but occurs as 5–10µm diameter crystals in titanite as well as 100 x 100µm size, colourless grains replacing ilmenite. Pale-coloured titanite 20–40µm in diameter forms spheres and hemispheres within the vesicular lava clasts; elsewhere euhedral, rhombic crystals are 10µm in length.

Graphitizing carbon occurs in two morphologies. In the rarer morphology, the pyrobitumen occurs as approximately spherical bodies, with a diameter as large as 50µm. These have no orientation controlled by the silicates. The rims of these spheres have higher reflectances than the cores, for example, one had a rim mean bitumen reflectance of 8.9% and a core of 4.3%. The more common morphology, lacking a higher reflectance rim, comprises smaller irregular bodies, up to 100µm across, with strong anisotropy aligned with the silicates. For example, one irregular particle had a bitumen bireflectance of 1.6%, compared to 0.9% in a spherical particle. Shearing occurred during formation of this morphology,

with fine pyrobitumen trails extending from larger bodies. The anisotropy reflects two processes. Most important is the chemical composition, which is suggested to be mixing of variably thermally altered bitumen to form schlieren. Superimposed is an anisotropy due to stress imparted by the host rocks from later geological events, which is minor in the studied sample.

There is no evidence of late pyrolysates having been derived from the pyrobitumen (Gize and Rimmer 1983), or of pyrobitumen shrinkage or the reorientation of the anisotropy perpendicular to the bitumen-water contact.

SH08 Context 1. Modern overburden Δ196. 15.0g tuff. Polished thin section 196 4a.This example of the tuff is dominated by its fine-grained siliceous matrix. The tuff carries a number of different lithic clasts, with a very wide size range, including vesicular lava and a dark, fine-grained microtonalite. A microtonalite clast comprising radiating feldspar-chlorite-titanite and rare plagioclase phenocrysts has sharp edges but is broken and re-cemented by the siliceous matrix. Titanite and acicular amphibole are concentrated at the edges of the clast.

Fine-grained quartz/feldspar-chlorite-minor titanite-acicular amphibole carries euhedral feldspar phenocrysts commonly sodic plagioclase with a sieve-like texture; some show pull-apart textures that are re-cemented by chlorite. Other feldspars may be potassium feldspar. Locally a sub-spheroidal texture is present and small tabular clasts comprising aligned albite-chlorite intergrowths and with variable grain sizes have the texture of recrystallised tube pumice. Some ‘layers’ are acicular amphibole-rich, with needles 40–200 x 5µm in size. Feldspar-chlorite-titanite vesicular lava carries rounded vesicles infilled by titanite and chlorite; minor amounts of acicular amphibole are present in the lava.

The high relief acicular phase is faintly pleochroic from colourless to pale green-brown, has second order interference colours and a small extinction angle and has the optical properties of amphibole. The needles are concentrated at the edges of rock clasts. Subhedral amphibole with pale green to pale brown pleochroism locally comprises two phases with different reliefs, one has low relief and yellow first order interference colours and the other has high relief and very low first order interference colours. Amphibole is associated and intergrown with plagioclase phenocrysts.

Large, single feldspar crystals, with cloudy cores

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and clear rims enclose apatite crystals, and aggregates of unaltered sodic plagioclase are shattered or show pull-apart textures now infilled with chlorite. Plagioclase with a sieve-like texture has the fine-grained siliceous matrix infilling the ‘voids’. A very large plagioclase with the sieve-like texture has intergrown, euhedral iron titanium oxide minerals, now titanite, on its margin.

Small, 60µm diameter, rounded to euhedral, unzoned zircon crystals are present in the fine-grained siliceous matrix.

Graphitising carbon occurs as 10µm diameter, irregular areas and as up to 30µm diameter droplets. Very fine grained graphitising carbon surrounds some rock clasts.

Ilmenite as 5– 40µm diameter relicts surrounded by rare, 2–10µm diameter TiO2 occurs in pale-coloured titanite up to 200µm in diameter. Some ilmenite up to 150µm in length is slightly altered to 10µm wide titanite rims. Pale-coloured titanite has a number of textural generations, 20–30 but up to 60µm diameter titanite occurs within the vesicular lava clasts and in the microtonalite clast but much occurs as up to 200µm long pseudomorphs after ilmenite. Limonite has replaced 10–20µm diameter pyrite.

DiscussionGraphitising Carbon A key characteristic of the Volcanic Group B lithics is the presence of thermally altered organic residues (pyrobitumen). The observed carbonaceous material has been termed “graphitizing” carbon, in contrast to “non-graphitizing” (Franklin 1951). In a graphitizing carbon, the starting material is O-poor and H-rich, and upon heating develops long distance graphitic layers with a low porosity.

The organic petrography shows the absence of any coal macerals but that graphitising carbon occurs as spherical ‘droplets’ as shown in Figure 7 with higher reflectance rims and which is often associated with pyrrhotite, or as irregular shaped bodies with no systematic reflectance variations and without higher reflectance margins.

The textures are related, as it is the shearing of the initially globular pyrobitumen (with its higher reflectance rim) whilst still plastic that forms the later irregular pyrobitumen, with its non-systematic reflectance variations, rather like the mixing of oil paints of two colours, as shown in Figure 8.

The increase in reflectance from core to rim within the spherical ‘droplets’ could be caused by either oxidation or by heat (namely thermal contact metamorphism). However, the presence of pyrrhotite in contact with the pyrobitumens shows its local environment was reducing so that oxidation as a mechanism for an increased rim reflectance is unlikely. The increase in reflectance on the rims of the pyrobitumen globules is therefore heat induced.

The lack of late pyrolysates, any sign of pyrobitumen shrinkage or the re-orientation of anisotropy domains perpendicular to the bitumen-water contact (features that are seen in other geological contexts including some hydrothermal mineral deposits) is consistent with rapid quenching of the pyrobitumen.

Comparisons between Volcanic Group B volcanics, orthostat SH38 and Rhyolite Group DOrthostat SH38 has been described in detail by Ixer and Bevins (2011a) and its geochemistry discussed in Bevins et al. 2012. It was recognised that it had a very unusual and recognisable petrography, including the presence of dark ‘mudstone’ clasts that partially comprised an unidentified, high relief brown ‘phyllosilicate’, blocky altered ilmenite and graphitising carbon (so forming the group known as ‘graphitising carbon-altered ilmenite-no muscovite’). They also showed that SH79+ 212 D/L2, SH79+64 F/L3 and SH80+1142 from the Heelstone area and SH08 Context 12/8 FN267 (Stonehenge April 2008 excavation, Darvill and Wainwright 2009) also were members of the ‘graphitising carbon-altered ilmenite-no muscovite’ group of lithics. Lithics SH79+ 212 D/L2 and SH08 Context 12/8 FN267 shared many features in common with SH38; the latter especially was almost an exact match petrographically.

Comparing the detailed description of SH38 given in Ixer and Bevins (2011a) with the generalised description of Volcanic Group B, as described above, shows that SH38 easily falls within the general description and is therefore an example of the vesicular lava-rich rather than ‘rhyolite’/quartz-albite-rich portion of the tuff. The tabular ‘mudclast’ in SH38 is unusually large, but this too can be expected from the random distribution of clasts found within a tuff. What is important is that the internal composition of the clasts, namely their mineralogy, grain size and textures, shows uniformity between SH38 and examples of Volcanic

11HARD ‘VolcAnics witH sub-plAnAR textuRe’ in tHe stoneHenge lAnDscApe

Group B. This strongly suggest that the clasts, whatever their relative amounts and size within an individual sample, are from the same rocks/source, in turn suggesting that all the samples and SH38 are from the same tuff. The simplest archaeological explanation for this is that Volcanic Group B represents debitage from SH38.

Rhyolite Group D (Ixer and Bevins 2010; 2013a) is a very small group of seven lithics with a distinctive petrography restricted to surface finds close to the western end of the Stonehenge Greater Cursus. They are very similar to each other and may have been removed from the same parent rock. One lithic, namely Cursus Field 142/1947.25, includes graphitising carbon but the key petrographical feature of Rhyolite Group D is a characteristic late-stage titanite-euhedral albite-chlorite-quartz intergrowth as shown in Plates 8 and 9, Ixer and Bevins (2010). This intergrowth with its distinctive texture was not recognised from any member of the Volcanic Group B lithics or SH38 (and indeed all other examined Stonehenge lithics). It is difficult to see where the petrographically-coherent Rhyolite Group D belongs, but it may not be associated with any orthostat including SH38.

Geographical Provenance of SH38 and its debitageThorpe et al. (1991) suggested that orthostat SH38 might have come from Carn Clust-y-ci, based solely on geochemical similarities. However, Bevins et al. (2012, 1017) refuted this showing that whilst SH38 is a dacitic ash-flow tuff the geochemically ‘matched’ Carn Clust-y-ci rock is an intrusive microtonalite and so petrographically very different.

After eliminating a number of potential outcrops Bevins et al. (2012) and Bevins and Ixer (2013) suggested that the presence of a mudstone intraclast, graphitising carbon, tube pumice and feldspar phenocrysts in orthostat SH38 “is consistent with emplacement of a hot ash-flow tuff in a submarine environment, such as described by Lowman and Bloxam (1981) and Bevins and Roach (1982) from the Fishguard Volcanic Group in the Fishguard-Newport area of north Pembrokeshire” and further, they suggested that “ the possibility still exists that all of the dacitic and rhyolitic bluestone lithologies (including orthostats SH38, 40, 46 and 48) can be sourced in outcrops of the Fishguard Volcanic Group, in the north Pembrokeshire area , in the tract of country between Fishguard and Crymych” (Bevins et al. 2012, 1018). This suggestion of a ‘hot

ash-flow tuff in a submarine environment’ is strongly supported by the complex organic petrography reported here.

Potential organic sources for the organic material found within Volcanic Group B lithics range from O-poor, H-rich algal material to O-rich, H-poor coals. The latter suggestion can be eliminated as no coal macerals have been recognised within any of the samples, only graphitising carbon, and, as Franklin (1951) established, graphitizing carbon needs an O-poor, H-rich starting material. Hence the organic petrography suggests the following events. As algal material trapped within buried sediments was heated geothermally (and moved into the ‘oil window’) it matured and migrated to produce a seepage of oil droplets into the overlying unconsolidated sediments and ocean. During episodes of submarine volcanic activity these droplets came into contact with hot ash, which thermally metamorphosed the droplets. The droplets were then quenched and suffered syndepositional shearing as they were incorporated into the final ash-flow tuff. The time between the initial contact of the droplets and their incorporation into the tuff was nearly instantaneous.

Whilst a geographical match has yet to be found, expanding the petrographical data on SH38 strongly reinforces the provenance suggested by Bevins et al. (2012), so increasing the chance of finding an in situ match and hence its geographical origin/provenance.

Temporal and spatial distribution of Volcanic Group B debitage within the Stonehenge Landscape Table 1 lists named Volcanic Group B debitage samples and orthostat SH38 while Figures 1 and 2 show the distribution of debitage find spots and potential parent orthostat. Numbers of samples are very small and so can support little interpretation.

In terms of their spatial distribution within the Stonehenge Landscape, debitage from SH38 and SH48 share some similarities. However, unlike SH48, undoubted SH38 debitage is not known from close to the Stonehenge Greater Cursus but is present in the Heelstone area. There is no concentration of SH38 samples in any find spot, unlike SH48 that has a distinct concentration of its debitage within a disturbed context in the excavation of Trench 45 in The Avenue, or the marked concentration of Altar Stone debris in the Stonehenge Layer and modern overburden within the Stonehenge April 2008 excavation.

Only one SH38 lithic has a secure prehistoric

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context, namely SH80+1142, from the Heelstone area, which is from “the stone floor and the stone was well sealed in that context” (M. Pitts, pers. comm.). Context 3 B/2 and V/1 are from the Stonehenge Layer from Darvill and Wainwright’s April 2008 excavation; this is not a sealed context “but one that starts in prehistoric times but certainly continues accumulating into relatively modern times” (T. Darvill, pers. comm.). Two have Roman contexts (Context 12 12/2 518 and 12/8E 267), again from Darvill and Wainwright’s excavation whilst SH79+64 F/L3 and SH79+212 D/L2 from the Heelstone area are Roman or post Roman (Pitts 1982). The lithic from Trench 45 is “relatively late in the sequence and later than the main period of use of Stonehenge but probably pre medieval or (pre) post-medieval in date” (B. Chan, pers. comm.). All the others are from more modern or disturbed contexts including back fill from earlier excavations by Hawley or Atkinson.

This age distribution is similar to that for the SH48 debitage given in Table 1 (Ixer and Bevins 2013b, 18), showing a lithic from the Stonehenge Layer and from a Roman context in the Darvill and Wainwright excavation and uncertain contexts from Aubrey Holes 7 and 11, but that most debitage came from modern and disturbed contexts within the Stonehenge Landscape. It may be of note that thin section petrography has confirmed that pieces of the Altar Stone and SH38 were found very close to each other within the Stonehenge Layer Context 3 V/1.

The 21 pieces of Altar Stone debitage appear to be different; they have a very restricted spatial distribution, as they have only been recognised from debris within the Darvill and Wainwright excavation. Half of the debris was found within the Stonehenge Layer with only three in Roman and one in medieval or in post-medieval contexts, the rest being in disturbed contexts.

Although the weight range of SH38 and SH48 debris is almost the same (14–276g and 10–300g respectively) the mean weight of the SH38 lithics at 50g is almost exactly half that of SH48 at 101g. However, both are larger than the Altar Stone lithics. These are very small with weights ranging from 1 to 18g with a mean of 6.6g and mode of 4.6g. There appears to be no systematic change in size distribution with context/time for any of the debitage.

Whilst the lithology of SH48 and to a much lesser extent SH38 are suitable for axe head manufacture or in the case of SH38 mauls, as suggested by Howard (Pitts 1982), the present debitage numbers do not

support either idea. Indeed, the very limited data from the recognised debitage taken from SH38, SH48 and the Altar Stone do not suggest any systematic concentration in either time or space as would be expected with lithic working (orthostat dressing or axe manufacture) but rather might suggest a long established, but minor, tradition of souvenir hunting.

Conclusions

Orthostat SH38 and twelve pieces of debitage that constitute the new Volcanic Group B class of debitage are sufficiently uniform in terms of their mineralogy, grain size and textures that it seems probable that they are all from the same rock rather than just from the same outcrop. Although this debitage is numerically rare it has a wide spatial distribution in the Stonehenge landscape notably within the Darvill and Wainwright April 2008 excavation and Heelstone Ditch but also including within Trench 45 in The Avenue and Aubrey Hole 7 in Stonehenge. Although a lithic with graphitising carbon was found from close to the Stonehenge Greater Cursus no SH38 debitage has been recognised from there with any certainty. The SH38 debitage distribution is similar to that found for orthostat SH48 but is more extensive than that for the Altar Stone.

The temporal distribution of the SH38 debitage is very similar to that for SH48 in that most pieces are found from post-Neolithic contexts but are less ‘bunched’ than that from the Altar Stone.

The newly reported SH38 debitage has extended the range of petrographical features beyond those seen in orthostat SH38, notably to include the presence of large zircons, rare earth-bearing minerals, tube pumice and a significant fine-grained siliceous component. This in turn suggests that were the single geochemical analysis for SH38 (Thorpe et al. 1991) and taken from a very small sample, to be augmented by new analyses from the present samples, a geochemistry that was closer to the bulk geochemistry for SH38 could be achieved. An enhanced petrography plus a more representative geochemistry would help to narrow the possible geographical sources for the orthostat. On present knowledge this is still expected to be found within the Ordovician volcanic sequences, in the north Pembrokeshire area but the net is tightening.

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AcknowledgementsWe are grateful to Professors Tim Darvill and Geoff Wainwright for permission to work on material from the Stonehenge April 2008 excavation and for discussions. Salisbury and South Wiltshire Museum freely provided access to material from the Heelstone excavations. Professor Mike Parker Pearson and his team are thanked for access to samples collected during the Stonehenge Riverside Project and for providing permission to describe them. Dr Ben Chan and Mike Pitts are thanked for useful clarifications. Christian Baars, Amanda Valentine-Baars and Dr Helen Kerbey (National Museum of Wales) are thanked for assistance with photography and polished thin section production respectively. Reluctant financial assistance from the Constantine Palaeologos Research Fund was forthcoming.

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