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. 

Irish Sea Cruise

So if you follow what we have been doing on the Lost Landscapes front over the last couple of years you will know that we have a large programme of work on reconstructing “lost”, submerged and buried landscapes around the world’s continetnatl margins.  The Lost Landscapes project is one of the core elements of this programme and is an EU-funded project based around reconstructions in the North Sea across what has been termed Dogger Land. A recent addition to this project has been an extension into the Irish Sea and specifically in Liverpool Bay and Cardigan Bay off the Welsh coast.  Liverpool Bay was the focus of work a few years ago before the large windfarms were constructed and has been written up in a series of reports and books (see Fitch and Gaffney, 2011 for example). I will leave this for a later discussion.

https://www.youtube.com/watch?v=xl3u7ZymGIY

Cardigan Bay has of course always been of interest to me growing up in Aberystwyth - swimming at Clarach, walking the coastline to Aberaeron and New Quay and playing in the dunes of Ynys Las.  More recently I have also been doing some work with my brother and father on the submerged forests at Borth.  Throughout my childhood I remember walks to see the forest of stumps that are periodically exposed after storms rip in from the Atlantic on this stretch of Welsh coast.  Over the last few years they have been exceptionally well exposed as there have been various engineering works going on along the shore for protection measures.  Back in February 2012 while my parents were taking one of their usual “what’s going on?” walks my father noticed blackened and shattered stones amongst the stumps which my brother later identified as burnt cooking stones.  This led to a series of projects investigating the site that yielded not only some well-preserved animal foot prints but the most impressive set of ancient red deer antlers (dated to 2940+/-30 B.P.)  that I have ever seen!  We did some geophysics (resistivity imaging and electromagnetics) on the beach which was published in 2016 (Bates and Bates, 2016). The forest here is dated to Bronze age but the deposits extend under the beach and offshore.  It is these earlier deposits that we were attempting to target with the current survey.  Specifically we hoped to be able to map the Mesolithic landscape and acquire material from that time period for environmental analysis including DNA sequencing. 

Borth fossil forest - photo D. Bates

To survey kilometres offshore and get core material through potentially metres of modern marine muds requires large survey vessels.  Thankfully a collaboration with colleagues at UCC (Dr Ben Geary) and IT Sligo (Dr James Bonsall) gave us access to a week on the Celtic Explorer.  This vessel is operated by the Marine Institute, Ireland and is equipped with all the tools of the trade including multibeam sonar, sub-bottom sonar, sidescan sonar, and both vibro-corer and gravity corer!  Lots of TOYS!!!  (For a link to these jump to the end of this blog)

After some initial set-backs with logistics the Voyager crossed the Irish Sea up to Liverpool Bay.  A night of geophysics yielded a number of target spots for the coring.  Here the previous work greatly aided us in picking target sites but the overlying sand proved too much of a challenge for the 3m vibrocorer.  So on to Cardigan Bay.

Background work for Cardigan Bay is a lot more sketchy than up north with most of the previous surveys collected back in the late 70s for speculative oil and gas exploration.  Our first task was once again to gather information from the geophysics.  The subbottom profiler on the Voyager is a two by two pot pinger.  Usually I would be a little sceptical of these for giving any great penetration of the seafloor however this setup has completely made me re-evaluate that.  From the first we started to see small channels buried a few metres beneath the present day seafloor that cross cut what appeared to be a glacial till horizon. Till is the material that is left as a result of ice retreat and so tells us about conditions during these cold periods of history.  The last cold (glaciation) period had its maximum about 27,000yrs ago (Clark et al., 2009) with ice reaching far down the Irish Sea and covering the Welsh landscape.  Draining the ice sheets would have been rivers that flowed across a barren plain in front of the sheets.  Because of the ice global sea level was much lower than today by up to 120m and so this part of Cardigan Bay would have been land following glacial retreat.  Where the rivers coalesced large channels formed which cut deep into the till as the climate became warmer.  By the Mesolithic, it was these landscapes criss-crossed by the rivers that people would have migrated back into before they became covered by the rising sea. 

Some modelling work that I had done a few years ago was used to test where the rivers had once flowed and it proved remarkably insightful.  The extent of the deep erosion of rivers however was something that totally blew us away!  Channels 20-30m deep contain multiple fill events suggesting a complex drainage history to the Welsh landscape.  There is clearly a stunning story to be pulled out of this area that will need a lot more geophysics to understand.  We will also need a lot more coring as our attempts at this proved not as successful as the geophysics due to drift of the boat in the currents and wind.  The cores that were taken will now go back with my brother to his lab for analysis and we hope to return to the site next summer for more geophysics and coring.  Ultimately these core will be examined for a whole range of environmental signatures including some DNA analysis that could potentially reveal what the landscape was like and what or who might have inhabited it!

Channel offshore Aberaeron

The Celtic Voyager and willing crew made the cruise a real pleasure, though I am going to have to go on a serious diet – how the crew do this month after month is beyond me or would be beyond my will power when it came to meal times!

This project is part of the Lost Frontiers EU research grant led by Prof Vince Gaffney at Bradford University.

Multibeam sonar -  Konsberg EM2040 
Pinger - SES500 Geopulse
Sidescan sonar - Edgetech 4200

Bates, C. R. and Bates, M. R. 2016. Palaeogeographic Reconstruction in the Transition Zone: The Role of Geophysical Forward Modelling in Ground Investigation Surveys. Archaeological Prospection, v. 23, pp. 311-323

Clark, et al., 2009. The Last Glacial Maximum. Science, v325. 710-4. 10.1126/science.1172873

Fitch, S and Gaffney, S. 2011. West Coast Palaeolandscape Survey