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Archive August 2012

To Destroy, or Preserve? That is the Question on the Effects of Volcanism in Archaeology Brigid Gallagher Aug 16

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In front of me a volcano is erupting. 

This is a slight exaggeration…but from my window last Friday, I could see a long low trail of ash from White Island spread above the horizon, and small billows of white changing form where another burst of steam spews forth.

I have been fighting the urge to go and buy a stronger set of binoculars in case I miss something. 

I have always been fascinated by volcanoes.  It started in earnest during 5th form geography when my teacher Sally Brodie (I really should thank her!) picked up the phone in her office and rang eminent geologist Sir Les Kermode.

The conversation went something like this…

“I am with a student who feels that she cannot fulfil the requirements of her project on the Tongariro Volcanic Field because she cannot adequately explain the difference between andesitic and rhyolitic volcanoes”.

Hmmm, maybe I was a little precious?!

And so I was given this lesson over the course of the next minutes, and I went away a well informed school girl who felt able to finish her assignment.

It was an invaluable lesson, and demonstrated early on that asking the right people the right questions was key to my understanding of science.  Especially when understanding physical sciences where I needed, and still do, need to visualise processes.

But back to volcanoes…and leap forward a few years…what on earth is an archaeological bod doing reminiscing about a desire to be a volcanologist?

Volcanoes have had, and continue to have a huge impact on archaeology in term of;

  1.  Site formation,
  2. Site damage and destruction,
  3. Site preservation and,
  4. Site dating.

…And as a conservation specialist, which is all about understanding the condition and decay mechanisms at play between materials and the environment, volcanic activity is also really exciting;

One single volcanic event can result in site formation, damage, destruction and preservation to different degrees, with long term implications and management needs. 

Assessing and understanding the complex behaviour between the negative and positive effects of one natural disaster, or moment in time, is for me is really interesting because the myriad of problems and solutions that are generated.

Archaeological Examples

Pompeii in Italy has probably made its fame by the effects caused on the Roman town by the eruption of Mt Vesuvius in AD79.  Whilst ash and molten rock formed a covering over the town, its quick cooling resulted in a protective layer to be created taking the form of the object, structure or person it covered.  In many cases this created an air tight capsule in which food stuffs, plaster and paint, human skin and bone, bricks and mortar could be preserved in a relatively pristine manner. Without the preserving qualities of the ash layer, Pompeii it could be argued, would have gone the way of so many other Roman towns, and now be another deteriorating archaeological site in Italy that relies on the tourist dollar, and requires huge amounts of money to help it stay in a state of semi restoration.  Though this has been the very subject of some recent media coverage.

…Luckily for the Romans they built in brick and stone.

New Zealand’s equivalent may well be Te Wairoa in Rotorua. However, buildings and structures of New Zealands prehistoric and historic past have been traditionally made of plant and tree materials such as raupo, punga, kauri and pine.  Placed under the pressures of dense volcanic ash layers, and in combination with the low ignition point of these materials, hot ash and molten rock in many instances has meant fire and destruction, not preservation.

…which has meant loss to the archaeological record.

Though in saying that…should you get a chance to visit The Buried Village at Te Wairoa you would be in for a very nice surprise with some whare that have been carefully excavated and restored, following preservation by volcanic ash. 

This a good example of how volcanic activity has both destroyed and served to keep a site in stasis until rediscovery or excavation. 

A form of time capsule was created.

One of New Zealand’s other big preservation stories as far as archaeology goes is the Sunde site on Motutapu Island. Located in the Hauraki Gulf and joined to Rangitoto Island (Auckland’s youngest volcano, dated at c. 1400) by a land bridge at low tide, Motutapu boasts preserved footprints due to the blanketing effect of Rangitoto’s last volcanic eruption and ash cloud.

Human and dog footprints found side by side are an evocative reminder of real people and animals living in New Zealand some 550 years ago.  Dated at c. 1450AD the foot prints show at least 8 people, 3 of which are children walking with their dogs.  Eruptions and ash fall from Rangitoto sealed the footprints, preserving them like the Laetoli footprints found by palaeontologist Mary Leakey’s team in Tanzania in 1976, and dated to 3.6 million years. 

Discovered in 1981 by archaeologist Reg Nicol, at that time from Auckland University, the ash had a preserving quality for the footprints, but archaeologists believe that it was also responsible for the destruction of settlements, gardens and forests on the Island.  Archaeologist Andy Dodd wrote in his Heritage Assessment “Motutapu Archaeological and Historic Landscapes” for the Department of Conservation (2008);

“The eruption smothered Motutapu in ash and caused widespread deforestation, but also produced friable soils suitable for gardening [and therefore aid regeneration]”.

Once again the destructive and preservative qualities of ash has been recorded by archaeological investigation. 

Its a strange balance that exists between materials and the environment. 

Depending on the perspective or the question there is different degrees of formation, damage, destruction and preservation contained within one volcanic event and ash dispersal.  

And the unfortunate aspect of volcanism is that often scientists don’t know what the ratio will be until the top has blown! Including near Pompeii, where a super volcano has recently been reported.

Never the less, next week we are going on long anticipated family holiday to the Central Plateau.  Hopefully there will be a quiet period of sun and snow… and I will only have to imagine the alternative.  But I think I will pick up a good pair of binoculars on the way…just in case!

Refs:

Te Wairoa – The Buried Village

Motutapu archaeological and historic landscapes heritage assessment, DoC, 2008

The Motutapu Restoration Trust – Sunde Site

Pompeii online

 Note – this is a huge subject of which only a small portion has been considered here…and may be returned to in the future… 

 

Ground Truthing Archaeological Evidence at Castle Howard Brigid Gallagher Aug 06

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‘I am looking for archaeological evidence [before I believe it]’ says field archaeologist Phil Harding during a  recently aired Time Team episode.

On a site where archaeological remains are suspected,  archaeologists go equipped with information ranging from maps, plans and sketches, to remote sensing technology , specialist expertise and experience, but none can guarantee the survival, location or form of  human activity until the ground is dug.

Archaeologists get a true and accurate picture of time and place by gathering the evidence in the ground to understand past activities. Deciding which ground and how much of it is a fundamental decision for most archaeological projects due to value, time and cost pressures.

In this case Phil was referring to archaeology being excavated at Castle Howard in Yorkshire.  I remember this site fondly as it was the second shoot that I ever did with TT.  It is memorable equally for the stunning location, the novelty of a film shoot and the archaeology.

Castle Howard

On the surface this was a no-brainer of a site. Still in the ownership of one family, the Howards, who had held the land since the early 1700’s, they were in possession of plans and maps dating back almost that long suggesting any archaeological evidence remaining in the ground should be easily found.

 The Domesday book (1085AD) records the site as a Viking village called ‘Hildreschelf’.  The Time Team were able to access descriptions of the people and the village.  Again, the various elements of the village should have been easy to locate.

But when it came time to put in the trenches that would find the archaeological evidence of the original settlement on the estate, it proves difficult. 

The Techniques

The geophysical survey team used resistivity (measuring differences in ground density) and magnetometry (measuring changes in ground magnetics) to see if they could see evidence of structures and features in their results.   But the results were inconclusive showing lots of noise in areas and multiple targets for investigation in others.

The maps, plans and sketches were drawn at different scales also generating multiple targets on the ground.

The landscape topography didn’t give many clues because it was clear that during the development of Castle Howard the site had been literally levelled removing many tell tale signs of evidence remaining beneath the ground.

Target 1

During a search for the original church the different techniques came up with different ‘best’ targets. And so Phil says….I want to see the ‘physical evidence’.

Geophysical survey offered 4 good looking responses.  One response with 2 straight lines and square angles, and a concentration of noise was chosen as a good choice for a trench.  The map evidence also suggested the church was in this general area, within 20m…which is pretty good for a lawn as big as Castle Howards. 

David Thorne one of the field archaeologists at the time opens the trench…hotly anticipating the results.  Finds dating to the right time period or a structure would have been good.

….But nothing.  Orange-brown sandy loam by the looks of it on our tv.  Not a hint of a cut, fill, masonry, artefacts, or a stratigraphic profile…the layers of time under the ground.

Target 2

In the walled garden, once a place for growing plants and fruits of warm climates, the search continued for the remains of the homes of Hildreschelf villagers.  It was thought the central road to the village extended into the walled garden and homes lined the road.  This meant that any remains of the Viking village would be found below under the layers associated with the post 1700 garden.

Geophysics suggested a wall may survive which acted as the first target, but much of the garden was full of ‘noise’ – a jumble of unreadable responses.  This was to be expected as the ground has been heavily disturbed as a tropical garden.  Heated piping was run under the earth to warm it and hence encourage growth of such fruits as pineapple.

Myself and Kerry Ely started the trench, removing garden soil to find an edge to a road, and the likely position of any remaining houses – and so I decided to extend the trench.

Underneath the geophysical survey noise was the base of a building, a straggly row of wooden stakes and evidence of a door.  All that survived was the very bottom of many features that were only just holding on in the earth by a few centimetres – all the evidence that was left of a family home.  The rest had been destroyed from above. 

The remains were so slight that the responses in the geophysical survey data had masked the remaining archaeological evidence, hence maps and careful excavation were needed to re-discover them, and prevent them from remaining lost.

Reasons for Trenches

Phil was right, there is nothing like excavation to establish once and for all if something exists.  You cannot rely on the documentary evidence or survey evidence alone. 

And this is the thing…

  1. Map scales and sketch plans are notorious for not translating well onto the actual ground.  There are big disparities between historic maps and actuality.  There wasn’t digital technology, satellites and lasers. There is inbuilt error to many historic documents using rulers, compass and pencil.  And the error increases with time as the maps are reused and translated. 
  2. Documents are often simplistic and the information can be biased to the creators perspective.  They may also reference physical elements that no longer exist.
  3. Modifications to the landscape and environment over time will either preserve or not the evidence of the past.  Massive changes to landscape occur with the laying out of large estates such as Castle Howard, taking away ‘lumps and bumps’, removing traces of previous uses and layouts.  As Mick Aston says at the episode end, this change was often used as a large stamp on the landscape to denote power and subjugation of a people.
  4. Geophysical survey has its own inbuilt set of problems.  A main one being:

It is not a magic tool. You cannot wave the equipment over or into the ground and selectively see only archaeology.  The technique is non-selective, in that it picks up built structures, internal features…AND, natural geology and geomorphology for example.  It can also mask evidence that only just remains in the grounds, and requires excavation more careful in nature.

For a large part its reliability as a tool for decision making in archaeology is only as good as the person analysing and interpreting the data being generated.

Belief in the Archaeological Evidence

Phil Harding is not alone in the opinion that it is necessary to see the archaeological evidence in the ground to confirm its existence.  On most sites it is the only sure way of establishing if the assessment, analysis and interpretation is correct. The expertise of a field archaeologist is to bring together all of the information that the ground retains, and produce interpretations and conclusions grounded in science and fact. 

Sometimes it is called ‘Ground Truthing’, and the sentence speaks for itself. 

 

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