Sunday 23 August 2020

The Stonehenge sarsens -- did they come from Overton Down / West Woods? On this evidence, probably they did.

Discussing:  Origins of the sarsen megaliths at Stonehenge 


Geochemical evidence suggests the large sarsen stones at Stonehenge originate from the vicinity of West Woods, Wiltshire.  

The announcement about the Sarsen stone analysis pointing to West Woods as a probably source of the majority of the Stonehenge stones has mainly been well received but there has been some carping whether the samples from the Stonehenge Core actually match closely enough the samples taken in the field.

It is important of course to note that many more samples should, and I expect will, be taken to look at other areas and also to narrow down potential sources. It is also obvious that Sarsen is a difficult stone to fingerprint because of its formation from pure sandstones and that variations can happen in very small areas.

A secondary conclusion from the paper is about likely routes of the Stonehenge Sarsens. This blog was set up all those years ago to explore exactly that idea and so that the route that was first published here emerges as a favourite is pleasing.  See .

Displaying a West Wood Sarsen and my enthusiasm for it.

Especially because I want a result to be true means I try to check it by looking at it closely. Rather than replicate the paper's analysis I did a "Fruitcake Recipe" analysis. Are the ingredients similar enough in type and ratio to be the same recipe?

To check that the potential source and the core are similar enough the first pass through the data (from ) simply removes any sources that are markedly out of the range of the core for each element (chemical compound). One of the West Woods samples was removed in this process. How tight to constrain the range is open to argument and various methods could be used to determine a significant acceptable variation.

To create a chemical fingerprint I then removed the data of the compounds that convey little information to discriminate between the remaining sources. Again how tightly to draw that net is open to debate but I chose to leave more in than the paper did even though is makes the results less clear to read.

So I am pleased that my simplistic alternative analysis of the data centred on the core show that most of the closest matches come from the West Woods, Lockeridge, Piggledene and Clatford area. The other similar matches are from much further away and Occam's razor would suggest we should accept the source of the Stonehenge Sarsens is indeed from West Woods and adjacent areas.



  1. With just a quick scan I noticed that they said: "50 of the 52 stones examined came from West Woods ..."
    This prompted me to assume they had omitted Stones -54 & -56 from the tally due to those being 'Orange Sarsen'.

    On reading more carefully it says Stone-60 & Lintel-160 are the odd ones out.

    So from what corner of West Woods did the distinctly different orange Stones come? Seems it should be fairly easy to determine.


    1. From Analytical Surveys of Stonehenge and its Environs, 2009–2013: Part 2 – the Stones
      David Field et al

      "Field examination coupled with analysis of the laser
      scan data, however, indicates that at least three different types of sarsen are present, potentially indicating that the stones originate from several sources. It is
      particularly notable that different coloured sarsens
      have been employed (Tilley et al. 2007, 196) and while
      the visual effect has, no doubt, been diminished
      through weathering and has been obscured by lichen
      coverage the different colours are quite clear on an
      overcast day. The majority of the stones are grey, but
      Stones 54, 55, 101, and 156 exhibit an orange hue
      while Stones 53, 56, and 154 are purple-grey. These
      are conspicuously more extensively dressed than the
      majority, potentially indicating that this raw material
      is marginally softer than the ‘grey’ sarsen; in this
      respect it is also notable that Stones 53, 54, and 55
      also contain most of the recent deeply incised graffiti.
      Stones 53, 54, 55, and 56 are also among the largest
      stones at Stonehenge. Circular ferric inclusions measuring c. 10 mm in diameter were also observed in a c.
      0.25 m wide band on one side of Stones 53, 56, and
      possibly 101; no other inclusions were observed. In
      contrast the ‘grey’ uprights of the Sarsen Circle contain only small flint/?quartzite inclusions. These
      observations require confirmation by petrography and
      highlight the need for further research on the source of
      the sarsens."

  2. Others think that different stones are Orange and Purple - I know it was mentioned in the Field et al 2015 paper and has a diagram of his thoughts. I will ask.

  3. David Nash has kindly responded to my query. My paraphrase of his thoughts: The colour of the stones comes from very low levels of minerals such as Fe which are soluble and mobile in the rock, which is why he excluded them from his analysis as they can be post formation contaminants. In the eons of time before the rocks were excavated some of them will have developed a slightly different coloured crust which will colour to a lesser extent the interior. Depending on the colouration and the depth of dressing the finished stone will show a tint or not of the colour. .

  4. Fair enough, and many thanks to Dr Nash.
    But it causes me to wonder how deep the iron actually runs. It must be pretty deep, as -56 was scaled to the nines in getting it that specific shape. So it's not a simple crust thing.

    Iron is also a prominent component in the Slaughter Stone, yet standing water in its numerous criks and hollers is a more normal rust-color.

    As we know, sarsen is a tricky rock and is more than likely to change its 'fingerprint' within only a few feet in the beds.
    It would be interesting to fish around West Woods to see if there's any prominent spikes in ferrite somewhere in the soil.


  5. Thanks for your analysis, Tim - good to see that the results hold up even with relatively simplified plotting. As I said on Twitter, you’re actually looking at more elements than we did. We only included immobile trace elements that we could be sure were not moved during weathering while the sarsens were still underground. We avoided Fe2O3 (iron oxide), for example, as we know iron gets introduced into sarsens during shallow burial. I’d add that using the raw data rather than dividing by an immobile element (e.g. Zr) to normalise them might introduce complications. Sarsens contain variable amounts of SiO2 (silica) cement. If there is more SiO2 cement, there is less “space” for other elements so the raw ppm for other elements will appear lower (higher if less SiO2).

    I was interested to see your comment that there had been some 'carping' about the match between the Phillips' Core samples and those from West Woods. That most likely arises where people don't understand the concept of instrumental error. Just like a set of bathroom scales, even instruments like ICP-MS have a degree of imprecision. The difference between ICP-MS and a set of scales, however, is that we can quantify that imprecision very precisely (if you get what I mean). We do this by analysing 'standards' of known composition and seeing how far 'out' the ICP-MS readings for that standard fall - this is the instrumental error. We use this measure to create the error bars on our analysis of the Phillips' Core (Fig 3 in our paper) and to define the boundaries of the pink zone for our silcrete oucrops. On the figure, even though the line for the Phillips' Core sometimes falls just outside the pink zone, the error bars always overlap. West Woods is the only area where this occurs for all of our element ratios. This means that - within the limits of the instrument - we can say that the sarsen in the Phillips' Core and at West Woods are chemically the same (or, putting it technically, that there is extremely high confidence that they aren't different).

  6. Thanks also for your comments Neil. The issue of colour of a sarsen/silcrete is a tricky one and might be a red herring in terms of stone sourcing. From my work on Sussex sarsens, many stones have silghtly 'orange' surfaces. This is caused by iron oxides (attached to clay minerals) washing into or through the sarsens while they were still buried in the soil but long after they had been cemented by silica. The clays line or fill tiny pore spaces in the rock and give a pale orange colour - and you only need a small proportion to give a strong colour. The staining can be of the order of a few cm thick or can be more pervasive if iron or clay have penetrated deep into cracks in the stone. Staining is often thicker on the original upper surface (i.e. the 'top') of a tabular stone as this would have been the surface that was most affected by clays washing down through the soil. The key thing though is that it doesn't seem to be that predictable.

    I'd need to look at the laser scanning report again to see how much working is thought to have occurred on the stones of the trilithon horseshoe. If the entire outer surface of a stone was removed to more than a few cm depth then any iron staining would also be removed. However, if the staining was thick and the working widespread but fine/shallow you might still see colour all across the stone. Lots to think about here, but I wouldn't get too focussed on colour as it is almost invariably surficial (or at least 'near-surface' only) in the sarsens I have analysed.

  7. If you want to have a read about iron-staining (or 'ferruginization' as we call it) in Sussex sarsens, have a look at this paper by Stewart Ullyott and myself. The relevant section starts on p.398.

  8. Hi Dr Nash. Many thanks for your clarifying remarks.

    You'll find that the East- North- and West Trilithons were the least-worked examples. One need only glance at S-60 to confirm this. Therefore it's likely that only the crust was removed to properly finish them, erasing evidence of color, as you describe.

    The South Trilithon's Stone-53 is obviously worked pretty hard to achieve that remarkably flat face, while even the corner inclusions on S-54 were worked and polished. The rear of -54, though worked, retains enough of the original bulk to showcase the color.

    Clearly the most worked Stone at the complex is fallen and broken Stone-55, which, because of the bulb or heel on the ground end indicates that an enormous amount of its bulk was removed to achieve this, also creating that ultra flat face. Because of this heavy hand, none of the original crust is present.

    Stone-56 was also worked very hard to ensure all six sides were uniform, create the graceful NE 'belly' and the marvelously flat rear face. Though I doubt that as much of its bulk was removed as -55, it still seems as though the idea of a crustal, or shallow depth of color might not apply in this case.
    Perhaps the agent creating the color permeated the stone earlier in its creation, possibly back when it was still 'mushy', (for lack of a better term)?

    Definitely keep us posted on any ongoing research. This aspect of the investigation fascinates me.

    Best wishes,
    Neil Wiseman

  9. David,assessing the significance of colour depends on what model of Stonehenge you are working to, and that's even if you have, or "archaeology" has one, or a range of them. It's an important and possibly critical aspect of interpretation.

    I suppose that the basic model is one of a cemetery, but there are others, depending on which bodies of evidence you apply. "Archaeology" has played down the significance of astronomy, geometry, and geodesy for decades, never mind the issue of metrology to which Stonehenge and it's wider landscape monuments would have had a master plan design. Even if it changed significantly since Phase I, the core design principle would have remained the same - in my opinion as a former profesional landscape practitioner, including the design of many a landscape scheme and advice to decision makers on aspects of policy, strategy and impact of development planning proposals.

    With that broad perspective from working life experience, I think that Neil's query regarding colour is spot on, because I too share it. We are after all, even if only using the model of a cemetery, talking about the Sun. In my written representation regarding the Stonehenge tunnel issue, in as much as one is allowed to as an "interested party", I covered some key aspects of geometry and astronomy. In my oral submission - only allowed five minutes - I broached the issue of both use as a cemetery and of celebrating a new birth in the realm of the "ancestors", albeit probably reserved for the elite. So the ambience therefore possibly even more significant.

    So we are talking the bright white/orange of the rising sun, to the deep red of the setting sun. Tim knows a bit or two about this in connection with his long barrow at All Cannings and I look forward to reading the article in the forthcoming issue of Skyscape Archaeology. But as you know, it is now one of a handful with a potential to "catch on" in the future.

    To finish, I would respectfully say that "...but I wouldn't get too focussed on colour...", could do with a bit of a rethink, even if it applies to only some selected stones within the overall design.


  10. Thanks Neil, Thanks Richard.

    To clarify Richard, when I say "...I wouldn't get too focussed on colour..." I mean strictly with reference to identifying the source of the sarsen stones rather than the overall 'design' of Stonehenge. Having read various studies, including the excellent laser scanning report, I - like you - am convinced that the colour configuration of the sarsens that make up the trilithon horseshoe was not accidental. Rather it was pre-planned and intentional.

  11. Indeed, everything about Stonehenge was intentional!