B. Roberts, J. A. Evans, R. J. Meniman & M. Smith write: In their recent, stimulating paper o... more B. Roberts, J. A. Evans, R. J. Meniman & M. Smith write: In their recent, stimulating paper on low grade metamorphism of the Lower Palaeozoic succession of the Welsh Basin, Bevins & Robinson (1988) claim that, following an initial early period of high heat flow, temperatures remained high in the sedimentary/volcanic rock pile until the end-Silurian to Middle Devonian deformation. The claim rests on three principal lines of evidence: (1) the recognition in North Wales by Roberts and Merriman (1985) of a pre-Arenig burial metamorphic event; (2) the observation that the patterns of distribution of metamorphic grade in the central and southern parts of the basin show a close relation to the basin form, lowest grades being found along the basin margins with grade increasing towards the centre where the succession is thicker and older; (3) on certain ‘critical accounts’ of pre-cleavage, low greenschist facies assemblages developed in the Wenlock of central Wales and the Lower Ordovician of Snowdonia. It is agreed that the Lower Palaeozoic succession now consists of assemblages belonging to the zeolite, prehnite-actinolite, prehnite-pumpellyite and greenschist facies and that the rocks comprise a low pressure facies series, as demonstrated by Robinson & Bevins (1986). What is disputed is the claim for a pre-cleavage ‘diastathermal’ metamorphism (Robinson 1987) with temperatures up to low greenschist facies conditions and, further, that temperatures remained at or about this level in the sedimentary/volcanic rock pile across the basin throughout the Ordovician and Silurian. Relevance of the pre-Arenig metamorphic event in North wales.
Any X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an ... more Any X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an exact equivalent R0 smectite-corrensite or R0 chlorite-corrensite. The identification of a mineral as chlorite (0.8)-smectite R1 is, therefore, not evidence that the fundamental layers in such an interstratified structure are actually chlorite and smectite. Furthermore, there are no obvious differences between XRD patterns of R0 and R1 chlorite-smectite at high or low smectite contents. Therefore the frequently supposed identification of R0 chlorite-smectite with high smectite contents is also questionable. It follows that the identification of R1 chlorite (0.8)-smectite by Robinson & Bevins (1994) and of R0 chlorite (≈0.2)-smectite in an earlier publication (Robinson et al.,1993) cannot be used as evidence regarding the debate over the nature of the component layers in interstratified minerals intermediate between trioctahedral smectite and chlorite.Diffraction patterns of natural mixed...
A Neolithic stone circle at Waun Mawn, in the Mynydd Preseli, west Wales, has been proposed as th... more A Neolithic stone circle at Waun Mawn, in the Mynydd Preseli, west Wales, has been proposed as the original location of some dolerite megaliths at Stonehenge, including one known as Stone 62. To investigate this hypothesis, in-situ analyses, using a portable XRF, have been obtained for four extant non-spotted doleritic monoliths at Waun Mawn, along with two weathered doleritic fragments from a stonehole (number 91). The data obtained have been compared to data from spotted and non-spotted dolerite outcrops across the Mynydd Preseli, an area known to be the source of some Stonehenge doleritic bluestones, as well as data from in-situ analysis of Stone 62 (non-spotted dolerite) and ex-situ analysis of a core taken from Stone 62 in the late 1980's. Recently, Stone 62 has been identified as coming from Garn Ddu Fach, an outcrop some 6.79 km to the east-southeast of Waun Mawn. None of the four dolerite monoliths at Waun Mawn have compositions which match Stonehenge Stone 62, and neither do the weathered fragments from stonehole 91. Rather the data show that the Waun Mawn monoliths, and most probably the weathered stonehole fragments, can be sourced to Cerrig Lladron, 2.37 km southwest of Waun Mawn, suggesting that a very local stone source was used in construction of the Waun Mawn stone circle. It is noted that there is evidence that at least eight stones had been erected and subsequently removed from the Waun Mawn circle but probability analysis suggests strongly that the missing stones were also derived, at least largely, from Cerrig Lladron.
The Stonehenge bluestones were first sourced to outcrops in the high parts of the eastern Mynydd ... more The Stonehenge bluestones were first sourced to outcrops in the high parts of the eastern Mynydd Preseli in SW Wales by H.H. Thomas in the early 1920s. He recognised the distinctive ‘spotted dolerite’ from his fieldwork in that area and suggested that the tors of Carn Meini (also known as Carn Menyn) and Cerrigmarchogion were the most likely sources. In the early 1990s, in a major contribution to our understanding of the Stonehenge bluestones, the geochemistry of a set of samples from Stonehenge monoliths and debitage was determined and compared against the geochemistry of dolerites from the eastern Mynydd Preseli by a team from the Open University led by R.S. Thorpe. They argued that the majority of the Stonehenge dolerites could be sourced from outcrops in the Carn Meini-Carn Gyfrwy area, based on the concentrations of the so-called ‘immobile’ elements (elements which are not affected by rock alteration processes), in particular TiO2, Y, and Zr. However, these elements are incompatible during crystallization of mineral phases in basaltic systems (that is they do not enter into the mineral phases which are crystallizing but are concentrated in the residual liquid) which severely hampers their use in discriminating between different pulses of an evolving magma (as is the case of the doleritic sills emplaced high in the crust and now exposed in the Mynydd Preseli). An alternative strategy in this study re-examines the data set of Thorpe's team but investigates the concentration of elements which are compatible in such basaltic systems (that is elements which do enter into the crystallizing mineral phases), namely MgO, Ni, Cr and Fe2O3. On the basis of the abundances of these elements on bivariate plots and also by using Principal Component Analysis on the dataset available and various sub-sets we identify three compositional groupings for the Stonehenge doleritic monolith and debitage samples and conclude that the majority of them (Group 1 of this paper) can be sourced to the prominent outcrop in the eastern Mynydd Preseli known as Carn Goedog. We also offer potential sources (with one exception) for those Stonehenge dolerites which appear not to relate to Carn Goedog.
The source of the Stonehenge bluestones was first determined in the early 1920s by H.H. Thomas wh... more The source of the Stonehenge bluestones was first determined in the early 1920s by H.H. Thomas who was an officer with the Geological Survey of England and Wales. He determined that the so-called 'spotted dolerites' could be petrographically matched to a small number of outcrops in the Mynydd Preseli district in southwest Wales. The bluestones, however, comprise a number of additional lithologies, including rhyolite and 'calcareous ash', as well as various sandstones. Thomas was convinced that the volcanic lithologies in the bluestone assemblage were all sourced from a small area at the eastern end of the Mynydd Preseli, with the rhyolites originating from the prominent outcrop known as Carn Alw. Recently, provenancing of these rhyolites to Carn Alw has been questioned on the evidence of wholerock geochemistry. This raised concerns over the original petrographical attribution. Accordingly a reinvestigation was undertaken of the rhyolite petrography by reexamining the original specimens used by Thomas. Three of the original four thin sections studied by Thomas were reexamined , along with a newly made thin section from the fourth of Thomas' rock samples as the original thin section could not be located. The new petrographical evidence demonstrates convincingly that the two pairs of thin sections from the Preseli and Stonehenge as examined by Thomas do not match despite his contention and argues strongly that Carn Alw is not the source of the Stonehenge rhyolites which Thomas described. This reinforces the geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is an important source of rhyolitic debris in the Stonehenge Landscape. Nevertheless, there remain uncertainties over the provenance of other Stonehenge rhyolites (and dacites), including four of the orthostats themselves.
E Ex xc ca av va at ti io on n o of f A Au ub br re ey y H Ho ol le e 7 7 a at t S St to on ne eh... more E Ex xc ca av va at ti io on n o of f A Au ub br re ey y H Ho ol le e 7 7 a at t S St to on ne eh he en ng ge e i in n 2 20 00 08 8.. P Ph ho ot to og gr ra ap ph h: : A Ad da am m S St ta an nf fo or rd d o of f A Ae er ri ia al l-C Ca am m / // / E Ex xc ca av va ac ci ió ón n d de el l H Ho oy yo o A Au ud dr re ey y 7 7 e en n S St to on ne eh he en ng ge e e en n 2 20 00 08 8.. F Fo ot to og gr ra af fí ía a: : A Ad da am m S St ta an nf fo or rd d, , d de e A Ae er ri ia al l C Ca am m.
B. Roberts, J. A. Evans, R. J. Meniman & M. Smith write: In their recent, stimulating paper o... more B. Roberts, J. A. Evans, R. J. Meniman & M. Smith write: In their recent, stimulating paper on low grade metamorphism of the Lower Palaeozoic succession of the Welsh Basin, Bevins & Robinson (1988) claim that, following an initial early period of high heat flow, temperatures remained high in the sedimentary/volcanic rock pile until the end-Silurian to Middle Devonian deformation. The claim rests on three principal lines of evidence: (1) the recognition in North Wales by Roberts and Merriman (1985) of a pre-Arenig burial metamorphic event; (2) the observation that the patterns of distribution of metamorphic grade in the central and southern parts of the basin show a close relation to the basin form, lowest grades being found along the basin margins with grade increasing towards the centre where the succession is thicker and older; (3) on certain ‘critical accounts’ of pre-cleavage, low greenschist facies assemblages developed in the Wenlock of central Wales and the Lower Ordovician of Snowdonia. It is agreed that the Lower Palaeozoic succession now consists of assemblages belonging to the zeolite, prehnite-actinolite, prehnite-pumpellyite and greenschist facies and that the rocks comprise a low pressure facies series, as demonstrated by Robinson & Bevins (1986). What is disputed is the claim for a pre-cleavage ‘diastathermal’ metamorphism (Robinson 1987) with temperatures up to low greenschist facies conditions and, further, that temperatures remained at or about this level in the sedimentary/volcanic rock pile across the basin throughout the Ordovician and Silurian. Relevance of the pre-Arenig metamorphic event in North wales.
Any X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an ... more Any X-ray diffraction (XRD) pattern of mixed-layered maximum R1 ordered chlorite-smectite has an exact equivalent R0 smectite-corrensite or R0 chlorite-corrensite. The identification of a mineral as chlorite (0.8)-smectite R1 is, therefore, not evidence that the fundamental layers in such an interstratified structure are actually chlorite and smectite. Furthermore, there are no obvious differences between XRD patterns of R0 and R1 chlorite-smectite at high or low smectite contents. Therefore the frequently supposed identification of R0 chlorite-smectite with high smectite contents is also questionable. It follows that the identification of R1 chlorite (0.8)-smectite by Robinson & Bevins (1994) and of R0 chlorite (≈0.2)-smectite in an earlier publication (Robinson et al.,1993) cannot be used as evidence regarding the debate over the nature of the component layers in interstratified minerals intermediate between trioctahedral smectite and chlorite.Diffraction patterns of natural mixed...
A Neolithic stone circle at Waun Mawn, in the Mynydd Preseli, west Wales, has been proposed as th... more A Neolithic stone circle at Waun Mawn, in the Mynydd Preseli, west Wales, has been proposed as the original location of some dolerite megaliths at Stonehenge, including one known as Stone 62. To investigate this hypothesis, in-situ analyses, using a portable XRF, have been obtained for four extant non-spotted doleritic monoliths at Waun Mawn, along with two weathered doleritic fragments from a stonehole (number 91). The data obtained have been compared to data from spotted and non-spotted dolerite outcrops across the Mynydd Preseli, an area known to be the source of some Stonehenge doleritic bluestones, as well as data from in-situ analysis of Stone 62 (non-spotted dolerite) and ex-situ analysis of a core taken from Stone 62 in the late 1980's. Recently, Stone 62 has been identified as coming from Garn Ddu Fach, an outcrop some 6.79 km to the east-southeast of Waun Mawn. None of the four dolerite monoliths at Waun Mawn have compositions which match Stonehenge Stone 62, and neither do the weathered fragments from stonehole 91. Rather the data show that the Waun Mawn monoliths, and most probably the weathered stonehole fragments, can be sourced to Cerrig Lladron, 2.37 km southwest of Waun Mawn, suggesting that a very local stone source was used in construction of the Waun Mawn stone circle. It is noted that there is evidence that at least eight stones had been erected and subsequently removed from the Waun Mawn circle but probability analysis suggests strongly that the missing stones were also derived, at least largely, from Cerrig Lladron.
The Stonehenge bluestones were first sourced to outcrops in the high parts of the eastern Mynydd ... more The Stonehenge bluestones were first sourced to outcrops in the high parts of the eastern Mynydd Preseli in SW Wales by H.H. Thomas in the early 1920s. He recognised the distinctive ‘spotted dolerite’ from his fieldwork in that area and suggested that the tors of Carn Meini (also known as Carn Menyn) and Cerrigmarchogion were the most likely sources. In the early 1990s, in a major contribution to our understanding of the Stonehenge bluestones, the geochemistry of a set of samples from Stonehenge monoliths and debitage was determined and compared against the geochemistry of dolerites from the eastern Mynydd Preseli by a team from the Open University led by R.S. Thorpe. They argued that the majority of the Stonehenge dolerites could be sourced from outcrops in the Carn Meini-Carn Gyfrwy area, based on the concentrations of the so-called ‘immobile’ elements (elements which are not affected by rock alteration processes), in particular TiO2, Y, and Zr. However, these elements are incompatible during crystallization of mineral phases in basaltic systems (that is they do not enter into the mineral phases which are crystallizing but are concentrated in the residual liquid) which severely hampers their use in discriminating between different pulses of an evolving magma (as is the case of the doleritic sills emplaced high in the crust and now exposed in the Mynydd Preseli). An alternative strategy in this study re-examines the data set of Thorpe's team but investigates the concentration of elements which are compatible in such basaltic systems (that is elements which do enter into the crystallizing mineral phases), namely MgO, Ni, Cr and Fe2O3. On the basis of the abundances of these elements on bivariate plots and also by using Principal Component Analysis on the dataset available and various sub-sets we identify three compositional groupings for the Stonehenge doleritic monolith and debitage samples and conclude that the majority of them (Group 1 of this paper) can be sourced to the prominent outcrop in the eastern Mynydd Preseli known as Carn Goedog. We also offer potential sources (with one exception) for those Stonehenge dolerites which appear not to relate to Carn Goedog.
The source of the Stonehenge bluestones was first determined in the early 1920s by H.H. Thomas wh... more The source of the Stonehenge bluestones was first determined in the early 1920s by H.H. Thomas who was an officer with the Geological Survey of England and Wales. He determined that the so-called 'spotted dolerites' could be petrographically matched to a small number of outcrops in the Mynydd Preseli district in southwest Wales. The bluestones, however, comprise a number of additional lithologies, including rhyolite and 'calcareous ash', as well as various sandstones. Thomas was convinced that the volcanic lithologies in the bluestone assemblage were all sourced from a small area at the eastern end of the Mynydd Preseli, with the rhyolites originating from the prominent outcrop known as Carn Alw. Recently, provenancing of these rhyolites to Carn Alw has been questioned on the evidence of wholerock geochemistry. This raised concerns over the original petrographical attribution. Accordingly a reinvestigation was undertaken of the rhyolite petrography by reexamining the original specimens used by Thomas. Three of the original four thin sections studied by Thomas were reexamined , along with a newly made thin section from the fourth of Thomas' rock samples as the original thin section could not be located. The new petrographical evidence demonstrates convincingly that the two pairs of thin sections from the Preseli and Stonehenge as examined by Thomas do not match despite his contention and argues strongly that Carn Alw is not the source of the Stonehenge rhyolites which Thomas described. This reinforces the geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is an important source of rhyolitic debris in the Stonehenge Landscape. Nevertheless, there remain uncertainties over the provenance of other Stonehenge rhyolites (and dacites), including four of the orthostats themselves.
E Ex xc ca av va at ti io on n o of f A Au ub br re ey y H Ho ol le e 7 7 a at t S St to on ne eh... more E Ex xc ca av va at ti io on n o of f A Au ub br re ey y H Ho ol le e 7 7 a at t S St to on ne eh he en ng ge e i in n 2 20 00 08 8.. P Ph ho ot to og gr ra ap ph h: : A Ad da am m S St ta an nf fo or rd d o of f A Ae er ri ia al l-C Ca am m / // / E Ex xc ca av va ac ci ió ón n d de el l H Ho oy yo o A Au ud dr re ey y 7 7 e en n S St to on ne eh he en ng ge e e en n 2 20 00 08 8.. F Fo ot to og gr ra af fí ía a: : A Ad da am m S St ta an nf fo or rd d, , d de e A Ae er ri ia al l C Ca am m.
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