Callanish Geophysics

Callanish (Re-visited)

At the end of last year we were invited to do some further recognisance work in the Outer Hebrides at the iconic Callanish (Calanais) standing stone sites.  We had visited these back in 2016 as part of surveys for the Ness Historical Society in northern Lewis and were not only impressed by the stones and their setting but a little surprised about the lack of any other significant historical sites in the surrounding landscape.  On return from this trip I started to read more about the area and it turns out that relatively few excavations have taken place here over the last 50yrs or so.  For those who have not visited the sites then I can only urge you to do so. 

So what was our job here?

We have two primary objectives for studies at Callanish.

• First we need to map the old landscape to enable us to understand where and how people might have used it.
• Second we need to investigate potential occupation sites, in other words, find the people in the landscape!

The stones are very different to those in the Orkney Isles that we have been working on for the last few years.  The primary rock at Callanish is the beautiful, banded metamorphic Lewisian Gneiss.  At a little over 2.7 billion yrs old these are some of the oldest rocks in the UK – their age is a testament to their surviving power.  Deformed under numerous tectonic events they are hard and resistant to erosion.  Their beauty, with swirling patterns of dark amphibole minerals coalescing into augens (literally eyes), gives an additional sense of mystery to the standing stones.  The rock’s resistance to erosion is another feature of the landscape that the Neolithic people would have had to deal with.  Unlike in the Orkney, where stone is readily prised from the bedrock, Lewisian Gneiss does not break off with the same ease.  Rather, substantial effort is required.  This, along with the soft curves of the standing stones suggests that this material was not quarried in substantial quantities from the bedrock, rather at best it was prised from the surface or picked up from where it had fallen.  (for more on the creation of the Lewis landscape as a function of successive glaciations see Dawson et al. 2016).

Palaeo-environmetal work in Lewis and Harris have shown that prior to the re-peopling of the islands there was open woodland cover but with increasing human activity around 3900yrs before present there was an expansion of heather moor and the spread of grassland.  The latter possibly occurred as a result of fire and grazing. (Birks and Madsen).  The changes in vegetation and the deterioration of weather further encouraged the growth of the peat that is such a dominant part of the landscape today. In fact the peat growth is so prolific that when the main standing stone site was first investigated in the 19^th C over a meter of peat removal was necessary to expose the old landscape. Electromagnetic surveying around site III indicates that this technique could well be used to map peat thickness.

This result of the removal is manifest by the weathering marks seen on old photographs of the site.  The prolific peat growth is likely one of the reasons that we do not know a lot more about this old landscape as it remains deeply buried today.  There is another part of the landscape however and that is the one that has been lost to sealevel change.  As with many places around the Scottish coastline the sealevels have risen significantly since last glacial maximum.  The story in the Orkney Isles is fairly simple with a steady rise to about 4000 years ago when it reached  near enough present levels.  In the Outer Hebrides it is a little more complex with a sharp rise followed by slightly higher sealevel than today and then a fall and rise again.  Such a complex signature is a result of the interplay between global sealevel and local tectonics (that is the compensation for the pushing down affect of the ice on mainland Scotland).

The field work was undertaken over three trips earlier in the year.  Initially I visited with Mel Chocholek and James Killingbeck in order to survey some of the lochs surrounding the main site.  At this time we also undertook more photography for structure from motion 3D models of the upstanding stones sites and tested electromagnetic ground conductivity geophysics across some peat areas near the main stone circle.  In January I returned to the site with Vince Gaffney and Chris Gaffney from Bradford, James and Shiobhan Killingbeck, Tim Raub from St Andrews and Fanny Bessard from Bristol.  During this trip we concentrated on one site, Callanish XI in order to test a range of geophysical methods for intense survey.   I will report on this site later.  On Callanish X we scanned all the stones and this will shortly be rendered as a full 3D reconstruction of the site. 

In late January I then returned with my brother and Mel with the Swordsman to conduct further offshore surveys and to acquire core data through some of the sediment units.

https://youtu.be/LtpAJ2CvVbk

This shows some of the sub-bottom profile data that indicates the area will have sediment that we can target for environmental information. 

Geophysics at the Gorgan Wall, Iran

Hunting the Red Snake – part II

Back in Iran, this time with my brother, Martin, to continue the hunt for the remainder of the Gorgan Wall.  Last year I was here with marine geophysical equipment to try and find the remains of both the Gorgan Wall and the Tammisheh Wall as they enter the Caspian Sea as part of a project with the University of Edinburgh and the National Museum, Iran.  From a previous blog (GorganWall I) I showed the results for the Tammisheh Wall extending well over 2km from the southern shores of the Caspian into a large lagoon behind the Miyankale Peninsula.  The sidescan sonar survey and sub-bottom profiling showed the remains of the brick wall with its accompanying ditch/canal.  Unfortunately we so no manifestation of the Gorgan Wall extension – so back to try again.

The Gorgan wall projection to the west towards the Caspian Sea

The Gorgan Wall (the Red Snake) extends from the Alborz Mountains in the east across the Gorgan Plains and was built from the 4^th C AD as a defensive structure to keep out the marauding Turks from the north.  The wall was constructed of bricks fired from local clay in kilns spaced 40-80m apart along the length of the wall. It was at least 2m wide and tall expanding to a more substantial feature at numerous towers and where defensive forts occurred.  In front, to the north, of the wall was a 10-30m wide, up to 3m deep, canal.  The remains of the wall and canal are relatively well documented to the east. H,owever to the west, evidence of it remains elusive with historical writings describing the wall splitting into different parts and extending down to long-lost towns.

Despite being of similar age to the Tammisheh wall and being relatively close to each other the geomorphological setting for both is somewhat different.  The Tammisheh wall extends into the Caspian at a place where the sea is relatively sheltered with fairly benign sedimentation gently silting up the almost closed lagoon.  The projection of the Gorgan Wall however is across an area of coast where there have been highly dynamic changes due to the interplay between the Gorgan Ricer discharge and longshore drift bringing sediment onshore from the Caspian.  The present day shoreline demonstrates these active processes where the waves can reach impressive magnitudes due to a fetch stretching the length of the Caspian.

CMD Mini-explorer and Explorer ground conductivity meters

This year we have returned to attack the challenge of finding the remaining western part of the Gorgan wall as it progresses towards the Caspian Sea. We come armed with electromagnetic instruments, the GFInstruments CMD Explorer and Min-explorer.  Both are frequency domain ground conductivity meters, each with three coil spacings, in order to look to a range of depths.  We have a Leica dGPS for positioning control and for ground truth work a more traditional shovel and trowel.

The western limit of the wall is manifest by a series of “robber” pits that were used by locals to mine the decaying bricks for use in other projects.  The wall and remains of the canal that ran along the north side of the wall run in a straight course here for at least 20km.  Our first geophysical grids were laid over the robber pits and known wall/ditch locations in order to characterise their geophysical signature. The figure below shows this mapped on the satellite images.  Note that the background satellite images clearly define the agricultural system of 200x200m fields and also parts of the old natural landscape with its river channels and back barrier bars with sloughs.

EM Conductivity over robbed-out portion of wall

We continued to map the wall to the west and after approximately 1km a large modern drainage ditch was encountered and gave us the opportunity to ground truth the geophysics.  A 2D interpretation of the geophysics together with the surface mapping suggested that wall and ditch might exist here.  Excavation proved this to be true and samples have now been taken for analysis.

Using this characteristic signature mapping was continued a further 5km to the west with a complex pattern developing of linear features mixed with sinuous features.  Our current thinking has an interpretation based on a landscape evolution of shorelines behind which the old Gorgan River fights its way to the sea and through which the wall and canal/ditch is cut.  It is likely that the dynamic natural landscape was always an issue for maintaining the wall and canal here and a much larger geophysical and ground truth investigation is going to be necessary before a clearer picture of the history is revealed.

New geophysical results showing extension to the west from last known position

So, more geophysics to come hopefully in 2017. 

During the trip we also tried making maps with and AUV operated by Georgian colleagues.  While the purpose was to make maps along the wall projection we also took time to investigate some of the more interesting geological features.

Dormant mud volcano,Gorgan, Iran

This was way too short a trip as usual but a highly enjoyable one.  Our biggest issue this time was getting the equipment through customs – lessons for the next trip.  Our biggest hazard encountered (apart from the driving)  ........... watch out for those camels!

  

Some of Last year’s projects - The Beginning of Time?

The Begging of Time 

Last summer, just about the time that the midges come out in full force a team of archaeologists and geophysicists from the Universities of Birmingham, St Andrews and Bradford got together in a field in Aberdeenshire near Crathes Castle.  The outing was prompted by the keen-eyed and insightful thinking of Prof Vince Gaffney who had spotted a curious arrangement of Mesolithic pits form air photos and a previous archaeological site report.  

Air photo and site investigation of sinuous pit alignment

The alignment was unlike most other early monuments in the UK where obvious orientations align with solar and lunar events.  This new site showed a sinuous arrangement to the pits.  While most of us would see no significance in this, Vince’s curiosity was piqued and his brain went into overdrive.  The end result, a geophysical survey, a new interpretation and the theory presented that at 10,000yrs old the feature could be the world’s oldest calendar!

The geophysics: This included electromagnetics, (Geonics EM31 and EM38), magnetics (Foerster Mag) electrical (Geoscan) and ground penetrating radar (PulseEkko).

The end result was modeled to examine what the exact solar and lunar tracts would have looked like 10,000yrs ago.  This showed the site mimics the phases of the moon in order to track lunar months over the course of the year. But that is not all, it also contains elements for alignment with the midwinter sunrise thus providing an annual astronomic correction in order to maintain the link between the passage of time indicated by the moon, the asynchronous solar year and the associated seasons.  Pretty smart of those old Scots running around the glens hunting the deer and fishing for the salmon!

For more information see Time and a Place: A luni-solar ‘time reckoner’ from 8^th millennium BC Scotland. Internet Archaeology, July 15 2013 (http://dx.doi.org/10.11141/ia.34.1)

 

Inphase results using EM31 (high resolution over site, lower for rest of field)