This is the biggest chunk of Lower Palaeozoic sandstone yet found at Stonehenge. Photo courtesy Rob Ixer.
In a previous post I looked at the evidence for the provenances of the "Stonehenge sandstone bluestones" (including the Altar Stone) which appear to be of Devonian age. Let's now take a look at the evidence for the provenancing of other sandstone fragments that appear to be much older -- from the Lower Palaeozoic. See this too:
See also the following paper: ‘No provenance is better than wrong provenance’
https://www.academia.edu/41105834/Mill_Bay_Milford_Haven_and_Stonehenge
For reasons that are obscure, this paper has two titles, one formal and the other informal. (It's on Academia, a web site that seem to enjoy spreading confusion......)
https://www.academia.edu/41105834/Mill_Bay_Milford_Haven_and_Stonehenge
For reasons that are obscure, this paper has two titles, one formal and the other informal. (It's on Academia, a web site that seem to enjoy spreading confusion......)
Back to 2017. Re this paper:
"The petrography, geological age and distribution of the Lower Palaeozoic Sandstone debitage from the Stonehenge Landscape."
by Rob Ixer, Peter Turner, Stewart Molyneux, and Richard Bevins.
Wiltshire Archaeological & Natural History Magazine, vol. 110 (2017), pp. 1–16
"The petrography, geological age and distribution of the Lower Palaeozoic Sandstone debitage from the Stonehenge Landscape."
by Rob Ixer, Peter Turner, Stewart Molyneux, and Richard Bevins.
Wiltshire Archaeological & Natural History Magazine, vol. 110 (2017), pp. 1–16
https://www.academia.edu/32048879/LOWER_PALAEOZOIC_SANDSTONE_DEBITAGE_FROM_THE_STONEHENGE_LANDSCAPE_The_petrography_geological_age_and_distribution_of_the_Lower_Palaeozoic_Sandstone_debitage_from_the_Stonehenge_Landscape
Rob Ixer and his colleagues demonstrate that nearly all of the "sandstone debitage" (from excavated contexts) in the Stonehenge area is of Lower Palaeozoic age, not Devonian. The use of palynology as well as detailed petrography (mostly thin section work) showing a "slight metamorphic fabric" is fascinating and quite convincing. This points to an Ordovician age. The inclusion of microfossils and other older debris pushes the age of the sandstones (note the plural) towards the Upper Ordovician.
Quote: The Darvill and Wainwright May 2008 excavation within the Stonehenge circle found a total of 74 samples of Lower Palaeozoic Sandstone with a total weight of 10440.9g, representing 2.00% of the bluestone debitage by number but 24.55% by weight. Three of the heaviest 20 bluestones from all contexts are Lower Palaeozoic Sandstone.
Using palynology on some samples, the authors suggested that the biostratigraphical ranges and preservation of acritarchs suggest that they were derived ultimately from the Upper Ordovician of the Welsh Basin (Caradoc to Ashgill), or, possibly the Silurian if all the acritarchs are reworked. This age was also suggested by the inclusion of other debris derived from lower in the Ordovician sequence.
The authors argue that the samples examined from the Stonehenge collections have come from the northern or north-eastern corner of Pembrokeshire, to the north of the Preseli Hills. The samples are not all identical, and seem to show at least two sandstone types. Quote: Although this might suggest two separate sandstone sources, the petrography of the two lithic samples (and indeed all of the other debitage samples) suggests that they are part of a single sourced lithology and the apparent age discrepancy is a sampling issue. It is hoped that further sampling of the Lower Palaeozoic sandstone will help to determine this.
I'm not sure what is meant here -- if the samples have come from "a single sourced lithology" they could of course have come from widely separated localities or provenances wherever that lithology outcrops -- and how can the "apparent age discrepancy" be a sampling issue? If there is an age difference between one sample and another, does that not mean that one sample is older than the other, given that there are always some statistical / confidence issues? Whatever the truth of the matter, it is clear that either the Devonian or the Lower Palaeozoic debitage needs to be matched with sandstone stumps 40g and 42c -- if those two stones really have provided some of the debitage fragments, more progress will have been made. And if they don't match, things get even more interesting………
From the abstract: "The lithology is believed to be from an unrecognised Ordovician (or less likely Silurian) source to the north or northeast of the Preseli Hills.”
For obvious reasons, the authors are keen to demonstrate that the fragments have all come from one source -- they are after all quarrying enthusiasts, and the more sources there are, the less likely it is that quarrying was involved. They do not make their case at all convincingly, and from my reading of the evidence, it looks as if there are at least two sources for Lower Palaeozoic fragments. Neither am I entirely convinced that the source area is to the north of the Preseli uplands. That assumption is based on the idea that since the igneous / volcanic bluestones at Stonehenge seem to have come from Mynydd Preseli, the sandstones must also have come from somewhere nearby. But if the Altar Stone and the other Devonian sandstone fragments have indeed come from the Senni Beds, maybe more than 100 km from Preseli, the same disconnect may well apply to the older sandstones as well. Ordovician sandstones, shales and mudstones outcrop across a very wide area, and there are many outcrops on the flanks of the Tywi Valley, which was the route followed by one of the largest outlet glaciers from the Welsh Ice Cap. It is perfectly possible that erratics of Upper Ordovician or Lower Silurian sandstone have been carried by Tywi Glacier ice down into Carmarthen Bay, where they were later picked up by the Irish Sea Glacier and transported eastwards towards Somerset and Wiltshire. This possibility has not been considered by Ixer and his colleagues. Another possibility is that the sandstone erratics have come from North Wales, and that they were carried westwards into Cardigan Bay by outlet glaciers from the Welsh ice cap before later incorporation into the southward-flowing Irish Sea Glacier.
Rob Ixer and his colleagues demonstrate that nearly all of the "sandstone debitage" (from excavated contexts) in the Stonehenge area is of Lower Palaeozoic age, not Devonian. The use of palynology as well as detailed petrography (mostly thin section work) showing a "slight metamorphic fabric" is fascinating and quite convincing. This points to an Ordovician age. The inclusion of microfossils and other older debris pushes the age of the sandstones (note the plural) towards the Upper Ordovician.
Quote: The Darvill and Wainwright May 2008 excavation within the Stonehenge circle found a total of 74 samples of Lower Palaeozoic Sandstone with a total weight of 10440.9g, representing 2.00% of the bluestone debitage by number but 24.55% by weight. Three of the heaviest 20 bluestones from all contexts are Lower Palaeozoic Sandstone.
Using palynology on some samples, the authors suggested that the biostratigraphical ranges and preservation of acritarchs suggest that they were derived ultimately from the Upper Ordovician of the Welsh Basin (Caradoc to Ashgill), or, possibly the Silurian if all the acritarchs are reworked. This age was also suggested by the inclusion of other debris derived from lower in the Ordovician sequence.
The authors argue that the samples examined from the Stonehenge collections have come from the northern or north-eastern corner of Pembrokeshire, to the north of the Preseli Hills. The samples are not all identical, and seem to show at least two sandstone types. Quote: Although this might suggest two separate sandstone sources, the petrography of the two lithic samples (and indeed all of the other debitage samples) suggests that they are part of a single sourced lithology and the apparent age discrepancy is a sampling issue. It is hoped that further sampling of the Lower Palaeozoic sandstone will help to determine this.
I'm not sure what is meant here -- if the samples have come from "a single sourced lithology" they could of course have come from widely separated localities or provenances wherever that lithology outcrops -- and how can the "apparent age discrepancy" be a sampling issue? If there is an age difference between one sample and another, does that not mean that one sample is older than the other, given that there are always some statistical / confidence issues? Whatever the truth of the matter, it is clear that either the Devonian or the Lower Palaeozoic debitage needs to be matched with sandstone stumps 40g and 42c -- if those two stones really have provided some of the debitage fragments, more progress will have been made. And if they don't match, things get even more interesting………
From the abstract: "The lithology is believed to be from an unrecognised Ordovician (or less likely Silurian) source to the north or northeast of the Preseli Hills.”
For obvious reasons, the authors are keen to demonstrate that the fragments have all come from one source -- they are after all quarrying enthusiasts, and the more sources there are, the less likely it is that quarrying was involved. They do not make their case at all convincingly, and from my reading of the evidence, it looks as if there are at least two sources for Lower Palaeozoic fragments. Neither am I entirely convinced that the source area is to the north of the Preseli uplands. That assumption is based on the idea that since the igneous / volcanic bluestones at Stonehenge seem to have come from Mynydd Preseli, the sandstones must also have come from somewhere nearby. But if the Altar Stone and the other Devonian sandstone fragments have indeed come from the Senni Beds, maybe more than 100 km from Preseli, the same disconnect may well apply to the older sandstones as well. Ordovician sandstones, shales and mudstones outcrop across a very wide area, and there are many outcrops on the flanks of the Tywi Valley, which was the route followed by one of the largest outlet glaciers from the Welsh Ice Cap. It is perfectly possible that erratics of Upper Ordovician or Lower Silurian sandstone have been carried by Tywi Glacier ice down into Carmarthen Bay, where they were later picked up by the Irish Sea Glacier and transported eastwards towards Somerset and Wiltshire. This possibility has not been considered by Ixer and his colleagues. Another possibility is that the sandstone erratics have come from North Wales, and that they were carried westwards into Cardigan Bay by outlet glaciers from the Welsh ice cap before later incorporation into the southward-flowing Irish Sea Glacier.
Generalised geological map of Wales. The Ordovician outcrops are shown by the light mauve colour -- and by "Ord" on the map. Most of the rocks (sedimentary and igneous) are of Ashgill age, but there are many other outcrops of Caradoc, Llanvirn and Arenig age, particularly in north-central Pembrokeshire to the north of the Trefgarn Gorge. The igneous rocks of the Fishguard Volcanic Group are associated with Upper Llanvirn sediments, including the rocks of the Aber Mawr Formation, around 460 million years old.
My conclusion from looking at the three most relevant papers on the Lower Palaeozoic sandstones is that there are two -- and possibly more -- provenances involved, and that no evidence has been presented that would tie any of the provenances to north Pembrokeshire.
PS. One last point. Could any of the sandstone fragments discussed in these papers have come from the sedimentary rocks in the vicinity of Carn Goedog, Carn Meini, Rhosyfelin or Waun Mawn? It's extremely unlikely. The sediments into which the Fishguard Volcanics are intruded and within which they are interbedded are from the Aber Mawr formation of Middle Ordovician age, attributed to the Llanvirn -- around 464 million years old. If the sampled fragments have been correctly attributed to the Upper Ordovician, and are of Ashgill or Caradoc age, they must be around 450 million years old, or younger than that. That points not to the uplands of Mynydd Preseli, but the the lower land along the North Pembrokeshire coastal strip.
The age of the Fishguard Volcanics in the Preseli area has been confirmed by Bevins et al (2016) as Darriwilian (c 460 - 465 million years) by the U-Pb zircon dating of five rhyolite samples. See the report here:
https://brian-mountainman.blogspot.com/2016/10/new-dates-confirm-mid-ordovician-age.html
One of the rhyolite samples, thought to have come from Rhosyfelin, was shown not to have come from there, but from some other related foliated rhyolite outcrop as yet unidentified. This reinforces the point which I have made frequently -- namely that both the igneous / volcanic monoliths and the sedimentary monoliths at Stonehenge, and the debitage fragments, have not come from carefully selected quarries but from multiple sources across quite a wide area.
https://www.academia.edu/34770710/U_Pb_zircon_age_constraints_for_the_Ordovician_Fishguard_Volcanic_Group_and_further_evidence_for_the_provenance_of_the_Stonehenge_bluestones
PS. One last point. Could any of the sandstone fragments discussed in these papers have come from the sedimentary rocks in the vicinity of Carn Goedog, Carn Meini, Rhosyfelin or Waun Mawn? It's extremely unlikely. The sediments into which the Fishguard Volcanics are intruded and within which they are interbedded are from the Aber Mawr formation of Middle Ordovician age, attributed to the Llanvirn -- around 464 million years old. If the sampled fragments have been correctly attributed to the Upper Ordovician, and are of Ashgill or Caradoc age, they must be around 450 million years old, or younger than that. That points not to the uplands of Mynydd Preseli, but the the lower land along the North Pembrokeshire coastal strip.
The age of the Fishguard Volcanics in the Preseli area has been confirmed by Bevins et al (2016) as Darriwilian (c 460 - 465 million years) by the U-Pb zircon dating of five rhyolite samples. See the report here:
https://brian-mountainman.blogspot.com/2016/10/new-dates-confirm-mid-ordovician-age.html
One of the rhyolite samples, thought to have come from Rhosyfelin, was shown not to have come from there, but from some other related foliated rhyolite outcrop as yet unidentified. This reinforces the point which I have made frequently -- namely that both the igneous / volcanic monoliths and the sedimentary monoliths at Stonehenge, and the debitage fragments, have not come from carefully selected quarries but from multiple sources across quite a wide area.
https://www.academia.edu/34770710/U_Pb_zircon_age_constraints_for_the_Ordovician_Fishguard_Volcanic_Group_and_further_evidence_for_the_provenance_of_the_Stonehenge_bluestones
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