Two bits of "Lower Palaeozoic sandstone" found in Barrow 41, about a mile from Stonehenge. The larger one is a cobble, about 10 cm long.
This is an unpublished catalogue recently placed on Academia -- and therefore not accessible to everybody. It looks as if it is a prelude to a longer and more detailed paper due to be published next year by the Ixer / Bevins duo.
https://www.academia.edu/144337293/Cunningtons_Stonehenge_rocks_an_archive_of_the_thin_section_data
Cunnington's Stonehenge rocks: an archive of the thin section data.By Robert Ixer
referring to: William Cunnington Stonehenge rock thin sections catalogue
Robert Ixer and Richard Bevins
The authors have examined 33 specimens -- thin section slides and hand specimens. They are from a collection of 460 fragments (surface finds) collected in the period 1878-1881from excavations, chance finds in the turf, in waggon ruts etc in the Stonehenge landscape. They are all "foreign rocks" -- so sarsen finds are excluded. (I wonder how many other erratic sandstone finds were excluded too, since in muddy field conditions they are not that easy for a non-specialist to differentiate from sarsen fragments......) Most of the samples are as expected from the "known groups" of rock types represented at Stonehenge, but there were some "odd rocks" as well -- the authors mention a pyrite or marcasite sample which is not analysed here.
Quote:
The first important result is that all the major lithological groups and some minor groups are present and in their relative ‘correct’ (i.e. conforming to other collections) proportions and so this collection is comparable to all others that have been described. This is another confirmation that all collections of bluestone debitage from the Stonehenge Landscape comprise the same limited range of rock types. Rock names given by earlier workers, including Harrison and colleagues, have been equated with Ixer and Bevins’ 21 st century nomenclature, so eliminating incorrect or obsolete rock names. This is important as the plethora of outdated and disused rock names has been used by some to suggest that the range of bluestone rock types is far wider than it really is, in order to promote the belief that the debitage in the Stonehenge Landscape comprises random glacial erratics.
This is the same tired old point from Ixer and Bevins -- in spite of having, over and again, demonstrated in their own papers that in all of the "groups" that they have examined there are ouitliers and exceptions that demonstrate multiple provenances. In addition, they presume to know the makeup of erratic or fereign stone types in the Stonehenge landscape, in spite of the fact that 50% of the area within the circumference of the stone monument has never been excavated. That is very unwise........
The authors claim that seven "rhyolitic tuffs" in the collection are "clearly" from Craig Rhosyfelin, and they persist with this pretence whilst recognising that there are at least three different textures (three of them decribed as Jovian, Snowflake and Zebra textures). Quote: "Despite the range of rhyolitc lithologies suggested by Harrison et al (1979) all are very typical material from Craig Rhosyfelin...." That sounds more like wishful thinking than science, and there is no way that this claim can be accepted.
In Table 1, where all of the samples are listed and allocated to the Ixer/Bevins groups, there is clear evidence of "forcing" lithologically different samples into predetermined groups. Nothing surprising there, then........
There follows a group of Appendices with full descriptions of the samples and hand specimens, accompanied by useful photos. Each numbered sample is allocated a predetermined category, although it is obvious that some are reasonable "fits" and others are not.
Most of the fragments examined are very small, but sample S52 weighs over 74 g. It is difficult to see whether this is a discrete cobble or a knock-off from one or another of the Stonehenge orthostats -- and Ixer and Bevins have traditionally ignored this issue in their long string of analytical lithology papers.
Appendix 4 is interesting, referring to S1 as a lump of "Lower Palaeozoic sandstone" (about 500g in weight) found in Barrow 41, about a mile from Stonehenge. There are five pieces. Sample S69 is given the same label, although it is so broad that it must incorporate many thousands of different lithologies.
Sample S74 is referred to as "volcanic with sub-planar texture" -- covering a multitude of sins. Within Dacite Group B there is a large group and a smaller "sub-group", and it is clear from the cited literature that within the Stonehenge debitage collections there are "related" rock types from a large range of different provenances.
Sample S57 is described as "anomalous" but belonging to Dacite Group D, which is not reporesented in the othostat group of bluestones. There are also two pieces of Greensand in sample 61. These are also deemed to be anomalous and somewhat inconvenient.
Appendix 6 deals with the Altar Stone -- and sample S45 in particular. This is worth repeating:
Sample S45
Canada Balsam has yellowed with age. A uniform, fine-grained (fine sand ≤187 µm grain size),
carbonate-cemented, poorly developed planar laminated sandstone is a dusky yellow (5Y 7/4 on the
Geological Society of America Rock-color chart). Short and thin, 0.1 mm thick, heavy mineral bands are
present and phyllosilicate-rich layers are more limonite stained. The centre of the slide is more
limonite-stained than the edges but this might be a thickness effect. A sinuous ‘stylolitic’ band lies at a
high angle to the laminae. Approximately 50% of the section is cloudy, suggesting the presence of
carbonate.
Microscopical description of the thin section
A fine-grained, well-cemented, calcareous sandstone. The planar fabric is picked out by heavy mineral
and by phyllosilicate (muscovite, biotite, chlorite) bands/laminae, the latter are slightly more limonite-
stained than most of the rock. Clasts display a severely restricted size range and are dominated by
monocrystalline, sub-angular to sub-rounded quartz grains; quartz and feldspars show elongation
within the planar fabric. Smaller heavy mineral grains are rounded, especially the opaques. In addtion
to quartz, plagioclase, untwinned feldspar, muscovite, biotite and chlorite are the main silicates; rock
clasts are common and are internally very fine-grained and appear to be clay-rich. Accessory minerals
include opaques, zircon, tourmaline, apatite, probable rutile and garnet and possible amphibole.
Monocrystalline quartz shows uniform extinction and ‘float’ within the carbonate cement. Where
quartz grains touch contacts are sharp and there is no overgrowth or embayment. Unaltered,
polysynthetically twinned plagioclase is more abundant than slightly altered pale brown plagioclase
(altering to fine-grained white mica). Untwinned feldspar is pale brown and cloudy and may include
potassium feldspar. Microcline was not recognised.
Phyllosilicates are abundant and in order of decreasing abundance are muscovite, biotite and chlorite.
All three form laths lying within the main fabric but also, rarely, occur at high angles to that fabric; all
show kinking about quartz and feldspar grains. Some muscovite laths show splaying at their ends and
some biotite is altered to chlorite. Chlorite also is present as, or within, fine-grained rock clasts.
Chlorite with deep blue-green colours may be pumpellyite.
Heavy mineral bands are quite broad and are dominated by rounded opaques and rounded to
subhedral zircon, murky green-brown tourmaline, rounded to lath-shaped apatite and elongated
brown rutile; rounded garnet and possible amphibole are also present.
Rock clasts, many are rounded, are widespread and all are internally very fine-grained. Many appear to
be phyllosilicate/clay rich or fine-grained micrite but some fine-grained polycrystalline quartz,
including ‘chert’, is present. Feldspar-rich rocks, including graphic granite are very rare.
Minor amounts of kaolinite, some associated with muscovite, is a very local cement. The main cement
is carbonate; some is poikoblastic calcite. Two generations of cement may be present, namely an
earlier high relief, brownish carbonate followed by clear, lower relief calcite.
Earlier descriptions
Cunnington (1884) recognised the presence of “micaceous sandstone” debitage and suggested that it
might be the Altar Stone. Teall (1894) listed S45 within his “Grits and Sandstones”, suggesting that
most “do not seem to be in any way remarkable”, while Judd (1902) noted that un-numbered Altar
Stone was a micaceous sandstone but found with “other more micaceous sandstones”. Harrison et al.
(1979) noted S 45 as “Fine sandstone, feldspathic, (0 06 mm) well-graded, carbonate cement; micas
common, and heavy minerals conspicuous (garnet, tourmaline, zircon)”.
There has been general agreement that S45 is from the Altar Stone, as confirmed by the present study. The Altar Stone has been described in a number of papers, notably Ixer and Turner (2006), Ixer and Bevins (2013a), Ixer et al. (2019; 2020) Bevins et al. (2020a, b; 2022, 2023a, b) and Clarke et al. (2024).
-----------
There is still doubt as to where the mysterious slide called Wilts 277 came from:
Was it really from the Altar Stone? .... and the "garnet problem" is another interesting one for geologists to sort out:
https://brian-mountainman.blogspot.com/2023/11/new-altar-stone-paper-professional.html
All in all, this is a welcome addition to the literature, but the authors are so stuck in their ruling hypothesis of a small number of rock types quarried in selected and sacred places that they fail to see the obvious conclusion coming from their own researches -- namely that there are so many exceptions, anomalies and outliers in the collections of Stonehenge fragments that they are clearly from multiple provenances, probably in the west. Most if not all of these locations are unknown. It follows that the erratics were probably introduced into the Stonehenge landscape by glacier ice.
No comments:
Post a Comment