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The petrography, geological age and distribution of the Lower Palaeozoic Sandstone debitage from the Stonehenge Landscape

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Abstract

The three major groups of debitage found in the Stonehenge Landscape are dolerites, rhyolitic tuffs (almost exclusivelyfrom Craig Rhosyfelin, now designated as Rhyolite Group A–C) and ‘volcanics with sub-planar texture’ now designatedas Volcanic Group A and Volcanic Group B. The only other significant debitage group, but only accounting for about5% by number, is an indurated sandstone now called the Lower Palaeozoic Sandstone.The Lower Palaeozoic Sandstone is a coherent lithological group with a slight metamorphic fabric and is afine-grained feldspathic sandstone with characteristic dark, mudstone intraclasts. Palynological (acritarch) dating ofthe sandstone suggests that it is Late Ordovician or younger whilst the petrography suggests that it is older and moredeformed than the Devonian (ORS) sandstones exposed in South Wales. Spatially, as with all the major debitage groups, the Lower Palaeozoic Sandstone is widely and randomly distributedthroughout the Stonehenge Landscape; temporally, almost none of the debitage has a secure Neolithic context but somemay have later Roman associations. The debitage cannot be matched to any above-ground Stonehenge orthostat butmay be from one or two buried and, as yet, unsampled stumps. The lithology is believed to be from an unrecognised Ordovician (or less likely Silurian) source to the north or northeast of the Preseli Hills.Although there has been confusion within the archaeological literature between the ‘Devonian’ Altar Stone, Lower Old Red Sandstone (Devonian) Cosheston Group sandstone and the Lower Palaeozoic Sandstone, all three are very different lithologies with separate geographical origins.

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... The detailed petrography for both of the Stonehenge sandstones is now published (Ixer and Turner 2006;Ixer and Bevins 2013;Ixer et al. 2017;, but the Cosheston Group sandstone (Mill Bay Formation) from Mill Bay has not been published and is required. This, together with a critical analysis of the history of the three sandstones in the archaeological literature are given here in full detail so that all earlier confusion can be accounted for, explained and hopefully eliminated. ...
... Ixer and Turner (2006) investigated Altar Stone thin section 277, giving a full petrographical analysis and compared that analysis with earlier descriptions. Further descriptions of believed Altar Stone debitage from Stonehenge were given in Ixer et al. (2017) and yet another set of descriptions of possible Altar Stone debitage and a re-analysis of 277 by Ixer et al. (2019) using automated mineralogy showed that all the samples were derived from the same lithology and probably from the same 'large' piece of rock. The most recent investigations, based in part on initial and on-going work on the sandstone clay mineralogy, support the suggestion that the Altar Stone is from a Senni Formation outcrop probably in the east of southern Wales as noted by Parker Pearson et al. (2019). ...
... A polished thin section (656A) made from the other rock fragment namely Rock #656 (79 FN656 L/2 27.5.79) has been described in detail by Ixer et al. (2017). All are now recognised as Lower Palaeozoic Sandstone derived from somewhere in west or possibly central Wales, west of the Twyi Lineament . ...
... Since 2010, there has been an on-going extensive review of the petrography of the bluestones (Ixer and Bevins, 2010, 2011a,b, 2013, 2016Ixer et al., 2015Ixer et al., , 2017Ixer et al., , 2019Ixer et al., , 2020. Petrographic data have been combined with new geochemical data which has included laser ablation ICP-MS zircon chemistry (Bevins et al., 2011), a re-interpretation of whole rock XRF data for the dacites/rhyolites and the dolerites (Bevins et al., 2012(Bevins et al., , 2014 and application of U-Pb zircon radiometric dating of rhyolitic debris at Stonehenge and from the Mynydd Preseli in west Wales . ...
... Analysis using automated SEM-EDS has provided quantitative data which both supports but also modifies earlier petrographic observations to show that there are two different sandstones in the Stonehenge bluestone assemblage, namely the Lower Palaeozoic Sandstone of Ixer et al. (2017) and the Altar Stone interpreted as derived from the Cosheston Subgroup (Late Silurian-Devonian Old Red Sandstone) (see Ixer et al., 2020). The data reveal key mineralogical differences between the two types of sandstone, in particular the notably higher modal % of calcite along with the presence of kaolinite and barite in the Altar Stone sandstone, the latter being absent in the Lower Palaeozoic Sandstone. ...
Article
A review of the provenance of the Altar Stone from Stonehenge and implications for transport of the bluestones to Stonehenge
... Another source-of "rhyolite with fabric" (Ixer & Bevins 2011: 28)-is Craig Rhos-y-felin, an outcrop in the Brynberian tributary of the River Nevern (Ixer & Bevins 2011;Parker Pearson et al. 2015). The fourth source-of Lower Palaeozoic sandstone-is located in sedimentary beds north of the Preseli hills (Ixer et al. 2017). Other Stonehenge bluestones, notably volcanic tuffs, remain to be sourced, but are also thought to originate in the Preseli area Ixer & Bevins 2016). ...
... Other Stonehenge bluestones, notably volcanic tuffs, remain to be sourced, but are also thought to originate in the Preseli area Ixer & Bevins 2016). Finally, Stonehenge's sandstone 'Altar Stone' is now believed to derive from Lower Old Red Sandstone strata of the Senni Formation (and not from the Cosheston Group around Milford Haven, contra Atkinson (1956: 46)), so it could originate from rocks farther east, at some distance from the bluestone sources, in an area such as the Brecon Beacons (Ixer & Turner 2006;Ixer et al. 2017). ...
Article
Geologists and archaeologists have long known that the bluestones of Stonehenge came from the Preseli Hills of west Wales, 230km away, but only recently have some of their exact geological sources been identified. Two of these quarries—Carn Goedog and Craig Rhos-y-felin—have now been excavated to reveal evidence of megalith quarrying around 3000 BC—the same period as the first stage of the construction of Stonehenge. The authors present evidence for the extraction of the stone pillars and consider how they were transported, including the possibility that they were erected in a temporary monument close to the quarries, before completing their journey to Stonehenge.
... Bevins et al. (2020) elaborated on Ixer et al. (2020), presenting detailed analytical data of six presumed Altar Stone fragments using automated SEM-EDS to show that the fragments all have very similar modal mineralogy and that there is a strong likelihood that they were all derived from a single block of sandstone. They compared the modal mineralogy of these six fragments with those of three Lower Palaeozoic Sandstone fragments from Stonehenge (see Ixer et al., 2017) and three samples from the Cosheston Subgroup in west Wales. The data presented by Bevins et al. (2020) clearly showed that the Lower Palaeozoic Sandstone and the Cosheston Subgroup sandstones were different from each other, and both were different to the presumed Altar Stone fragments. ...
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Article
The Altar Stone at Stonehenge in Wiltshire, UK, is enigmatic in that it differs markedly from the other bluestones. It is a grey–green, micaceous sandstone and has been considered to be derived from the Old Red Sandstone sequences of South Wales. Previous studies, however, have been based on presumed derived fragments (debitage) that have been identified visually as coming from the Altar Stone. Portable X-ray fluorescence (pXRF) analyses were conducted on these fragments ( ex situ ) as well as on the Altar Stone ( in situ ). Light elements ( Z <37) in the Altar Stone analyses, performed after a night of heavy rain, were affected by surface and pore water that attenuate low energy X-rays, however the dry analyses of debitage fragments produced data for a full suite of elements. High Z elements, including Zr, Nb, Sr, Pb, Th and U, all occupy the same compositional space in the Altar Stone and debitage fragments, and are statistically indistinguishable, indicating the fragments are derived from the Altar Stone. Barium compares very closely between the debitage and Altar Stone, with differences being related to variable baryte distribution in the Altar Stone, limited accessibility of its surface for analysis, and probably to surface weathering. A notable feature of the Altar Stone sandstone is the presence of baryte (up to 0.8 modal%), manifest as relatively high Ba in both the debitage and the Altar Stone. These high Ba contents are in marked contrast with those in a small set of Old Red Sandstone field samples, analysed alongside the Altar Stone and debitage fragments, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales. This high Ba ‘fingerprint’, related to the presence of baryte, may provide a rapid test using pXRF in the search for the source of the Stonehenge Altar Stone.
... Bluestones are actually a variety of rock types: spotted dolerite, unspotted dolerite, rhyolite, volcanics and sandstone. All derive from on and around the Preseli hills except for the sandstone Altar Stone, which comes from an as yet unidentified source (Ixer and Bevins 2011;Bevins et al. 2013Bevins et al. , 2020Ixer et al. 2017). ...
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Article
Stonehenge is one of the world’s most famous prehistoric monuments, built 4,500–5,000 years ago during the Neolithic in a time long before written history. The recent dramatic discovery of a dismantled stone circle near the sources of some of Stonehenge’s stones in southwest Wales raises the fascinating possibility that an ancient story about Stonehenge’s origin, written down 900 years ago and subsequently dismissed as pure invention, might contain a grain of truth. This article explores the pros and cons of comparing the legend with the archaeological evidence.
... Geologists have recently identified several of the sources of bluestones through geochemistry and petrography (Ixer and Turner 2006;Ixer et al. 2017). The major source of the spotted dolerite is a small outcrop called Carn Goedog on the north flank of the Preseli hills (Bevins, Ixer and Pearce 2013). ...
... Geologists have recently identified several of the sources of bluestones through geochemistry and petrography (Ixer and Turner 2006;Ixer et al. 2017). The major source of the spotted dolerite is a small outcrop called Carn Goedog on the north flank of the Preseli hills (Bevins, Ixer and Pearce 2013). ...
Article
Excavations at two of the sources of Stonehenge’s bluestones in Mynydd Preseli, west Wales, have led to the discovery of stone tools associated with megalith quarrying in the final centuries of the fourth millennium BC, shortly before the suspected date of the bluestones’ erection at Stonehenge, 240 km away. Among the most plentiful of these tools are stone wedges, three of which were found in situ at the rhyolite bluestone quarry of Craig Rhos-y-felin. Two of these were positioned in the joints of a rhyolite pillar adjacent to a recess left by a removed pillar. Geochemical analysis reveals that these and the third wedge are of compositions different to the rock on either side of the cracks into which they had been driven, confirming their identification as quarrying tools. This research sheds new light on the methods used to extract the stones for Stonehenge.
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Geologists and archaeologists have long known that the bluestones of Stonehenge came from the Preseli Hills of west Wales, 230km away, but only recently have some of their exact geological sources been identified. Two of these quarries—Carn Goedog and Craig Rhos-y-felin—have now been excavated to reveal evidence of megalith quarrying around 3000 BC—the same period as the first stage of the construction of Stonehenge. The authors present evidence for the extraction of the stone pillars and consider how they were transported, including the possibility that they were erected in a temporary monument close to the quarries, before completing their journey to Stonehenge.
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Article
Stonehenge on Salisbury Plain is one of the most impressive British prehistoric (c. 3000–1500 BC) monuments. It is dominated by large upright sarsen stones, some of which are joined by lintels. While these stones are of relatively local derivation, some of the stone settings, termed bluestones, are composed of igneous and minor sedimentary rocks which are foreign to the solid geology of Salisbury Plain and must have been transported to their present location. Following the proposal of an origin in south-west Wales, debate has focused on hypotheses of natural transport by glacial processes, or transport by human agency. This paper reports the results of a programme of sampling and chemical analysis of Stonehenge bluestones and proposed source outcrops in Wales . Analysis by X-ray-fluorescence of fifteen monolith samples and twenty-two excavated fragments from Stonehenge indicate that the dolerites originated at three sources in a small area in the eastern Preseli Hills, and that the rhyolite monoliths derive from four sources including northern Preseli and other (unidentified) locations in Pembrokeshire, perhaps on the north Pembrokeshire coast. Rhyolite fragments derive from four outcrops (including only one of the monolith sources) over a distance of at least 10 km within Preseli. The Altar Stone and a sandstone fragment (excavated at Stonehenge) are from two sources within the Palaeozoic of south-west Wales. This variety of source suggests that the monoliths were taken from a glacially-mixed deposit, not carefully selected from an in situ source. We then consider whether prehistoric man collected the bluestones from such a deposit in south Wales or whether glacial action could have transported bluestone boulders onto Salisbury Plain. Glacial erratics deposited in south Dyfed (dolerites chemically identical to Stonehenge dolerite monoliths), near Cardiff, on Flatholm and near Bristol indicate glacial action at least as far as the Avon area. There is an apparent absence of erratics east of here, with the possible exception of the Boles Barrow boulder, which may predate the Stonehenge bluestones by as much as 1000 years, and which derived from the same Preseli source as two of the Stonehenge monoliths. However, 18th-century geological accounts describe intensive agricultural clearance of glacial boulders, including igneous rocks, on Salisbury Plain, and contemporary practice was of burial of such boulders in pits. Such erratics could have been transported as ‘free boulders’ from ‘nunataks’ on the top of an extensive, perhaps Anglian or earlier, glacier some 400,000 years ago or more, leaving no trace of fine glacial material in present river gravels. Erratics may be deposited at the margins of ice-sheets in small groups at irregular intervals and with gaps of several kilometres between individual boulders . ‘Bluestone’ fragments are frequently reported on and near Salisbury Plain in archaeological literature, and include a wide range of rock types from monuments of widely differing types and dates, and pieces not directly associated with archaeological structures. Examination of prehistoric stone monuments in south Wales shows no preference for bluestones in this area. The monoliths at Stonehenge include some structurally poor rock types, now completely eroded above ground. We conclude that the builders of the bluestone structures at Stonehenge utilized a heterogeneous deposit of glacial boulders readily available on Salisbury Plain. Remaining erratics are now seen as small fragments sometimes incorporated in a variety of archaeological sites, while others were destroyed and removed in the 18th century. The bluestones were transported to Salisbury Plain from varied sources in south Wales by a glacier rather than human activity.
Article
The Lower Devonian (Lochkovian-Emsian) Cosheston Group of south Pembrokeshire is one of the most enigmatic units of the Old Red Sandstone of Wales. It consists of a predominantly green, exceptionally thick succession (up to 1.8 km) within the red c. 3 km-thick fill of the Anglo-Welsh Basin, but occupies a very small area (27 km2). Four formations—Llanstadwell (LLF), Mill Bay (MBF), Lawrenny Cliff (LCF) and New Shipping (NSF)—group into lower (LLF + MBF) and upper (LCF + NSF) units on stratigraphical and sedimentological criteria. Two palynostratigraphic associations (Hobbs Point and Burton Cliff) are recognised in the LLF. Overall, the Cosheston succession comprises a fluvial, coarsening-upward megasequence, mostly arranged in fining-upward rhythms. It is interpreted as the fill of an east-west graben bounded by faults to the north and south of the Benton and Ritec faults, respectively. Both ‘lower Cosheston’ formations were deposited by east-flowing, axial river systems draining a southern Irish Sea landmass. Drainage reversal, early in the deposition of the LCF, resulted in ‘upper Cosheston’ lateral, SW-flowing rivers which carried predominantly second- and multi-cycle detritus. The ‘lower Cosheston’ is characterized by an abundance of soft-sediment deformation structures, probably seismically triggered by movements along the graben's northern bounding fault. A minimum average (≥ mesoseismic) earthquake recurrence interval of c. 4000 yr is estimated for the MBF. This and the correlative Senni Formation of south-central Wales form a regionally extensive green-bed development that represents a pluvial climatic interval. Copyright © 2006 John Wiley & Sons, Ltd.
Article
The Early Palaeozoic phytoplankton (acritarch) radiation paralleled a long-term increase in sea level between the Early Cambrian and the Late Ordovician. In the Late Cambrian, after the SPICE δ13C carb excursion, acritarchs underwent a major change in morphological disparity and their taxonomical diversity increased to reach highest values during the Middle Ordovician (Darriwilian). This highest phytoplankton diversity of the Palaeozoic was possibly the result of palaeogeography (greatest continental dispersal) and major orogenic and volcanic activity, which provided maximum ecospace and large amounts of nutrients. With its warm climate and high atmospheric CO2 levels, the Ordovician was similar to the Cretaceous: a period when phytoplankton diversity was at its maximum during the Mesozoic. With increased phytoplankton availability in the Late Cambrian and Ordovician a radiation of zooplanktonic organisms took place at the same time as a major diversification of suspension feeders. In addition, planktotrophy originated in invertebrate larvae during the Late Cambrian-Early Ordovician. These important changes in the trophic chain can be considered as a major palaeoecological revolution (part of the rise of the Palaeozoic Evolutionary Fauna of Sepkoski). There is now sufficient evidence that this trophic chain revolution was related to the diversification of the phytoplankton, of which the organic-walled fraction is partly preserved. © 2008 The Authors, Journal compilation
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