A day-by-day account of the activities of the Rossnaree Archaeological Project 2010-11

Posts tagged “Electrical resistance tomography

Geophysical Survey 2010

Following the surprise development in the excavation last year where it was conclusively established that the enclosures on the site are likely to be early medieval in date (and probably between 7th-11th century AD), it was decided to carry out further geophysical survey over the core area of the site on the top of the knoll within the tillage field. This was clearly a focus of activity in prehistoric times because of the extensive lithic (flint) scatter found there. It also seems to have been a focus during the early medieval period as this is the area central to all of the enclosures.

Because of the lithic scatter, this area remained the best candidate area for identifying prehistoric activity. However, because of the apparent intensity of activity in the early medieval period, there is a chance that any prehistoric features that may once have existed are completely destroyed and disturbed.

With these problems in mind, a second programme of geophysical survey was carried out in August 2010 at a higher resolution than the original geophysical surveys. The hope was to identify more clearly any features that might be earlier than the early medieval enclosures. Three high resloution surveys were carried out: magnetic gradiometry (Figure 1), earth resistance (Figure 2) and magnetic susceptibility (Figure 3). See the following figures for the results of each of these.

Figure 1: Detailed magnetic gradiometry plot with topography, August 2010

The magnetic gradiometry has shown up more detail than was apparent on the original survey. For example, the second enclosure from the outside clearly deviates from its curvilinear path as we move northwards towards the terrace slope. This may be a modification of an earlier version of the enclosure as there are hints of an earlier, less definite enclosure continuing northwards. The other feature of interest is the relationship between the inner complete oval enclosure and the D-shaped enclosure beneath it. The oval enclosure looks like it cuts through the other enclosure and thus is later in date. Furthermore, there is a difference in the quality of the magnetic response between the two features suggesting that they are constructed differently. The oval enclosure is more similar in the character of its response to the other, outer, enclosures. There seems to be a contrast between the innermost D-shaped enclosure and the rest of the enclosures, possibly suggesting different construction techniques which may be due to their being constructed at different dates. Clearly, the innermost D-shaped enclosure is earlier than the oval one. The question is how much earlier. Are we looking at a possible prehistoric enclosure?

Figure 2: Detailed earth resistance plot with topography, August 2010

The contrast between the different enclosures noticed in the magnetic gradiometry is replicated to some degree in the earth resistance plot in Figure 2 so what we are seeing is likely to be real and reflect the real character of the features themselves. The innermost D-shaped enclosure is almost invisible on this plot, especially along its western side. At the southern end of this inner enclosure feature there is an area of high resistance with a sharp delineation between the high-resistance (light coloured in the plot above) inner area and the low resistance area immediately outside (dark coloured). The junction between the two areas appears to be particularly sharp and suggests a possible stone facing/revetment feature.

Figure 3: Detailed magnetic susceptibility plot with topography, August 2010

The magnetic susceptibility plot shows an area where the soils have been strongly altered by activities like burning and the disposal of organic waste. It shows a clear and distinct focus of activity which seems to be contained to the west by the third enclosure from the edge. It extends a lot further to the south than the line of this ditch and continues on outside the survey area. There is a band of low values running in a north-south direction through the middle of the zone of enhancement which may be due to the way the space was utilised in the past, or it may be due to sampling or post-depositional processes. There seems to be a slightly separate elongated area of enhancement running east-west along the northern boundary of the field which may be a secondary activity focus. It may be significant that this is focused on the southern side of the highest point of the topographic knoll.

Advertisements

Day 19: Best laid plans…

Rossnaree Excavation

Baler in operation

Our excitement today was watching the straw from an entire field being baled in a matter of a few hours by one guy on his own. I had never seen this machinery in operation up close. Mind you, he hadn’t ever seen an archaeological excavation up close either. A first for all concerned.

Rossnaree Excavation Trench 3

Kevin and Matt discuss the results from Trench 3

.

.

.

Kevin the geophysicist visited the site again today to take one last look at the remaining open trenches to see how well the excavated features match up with those recorded by the specialist geophysical equipment. There will be a very interesting comparison done at a later date and there may even be a few papers in geophysics journals reporting on the results of the test.

Rossnaree excavation Trench 1

Hard day at the office for Louise

.

.

.

On site today the atmosphere was a little different to previous days. Everybody, I think, was conscious of the clock ticking and time running out. Most of the work of the day was directed towards recording and sampling, and there was very little actual digging done as most of this had been finished yesterday. There was still a little bit to do cleaning out Trench 1 and the small feature to the west of the big ditch.

Rossnaree excavation

To tea through the fields

We had thought for a while that this was a pit but as more excavation was done, it was clear that it was as predicted in the geophysical survey: an irregular linear ditch-like feature running downslope at a slight angle. We are still in the dark as to when this feature was created as there is no stratigraphic link between it and the other features in Trench 1. There was some flint out of it, but as we found out yesterday, this does not guarantee a Stone Age date – many items can be residual and redeposited in later times.

Rossnaree excavation Trench 1

Trench 1 fully excavated

.

After this, the section drawings had to be completed in Trenches 1 and 3 and a plan had to be drawn in Trench 3. At the same time, we had to check that written descriptions of each of the features and layers (contexts) had been fully completed, and that samples had been taken. Lunch time came and went and despite being reminded of the time, everyone worked until each particular task was completed before walking up the hill to the cabin. Although hungry, everyone was absorbed in the work and was racing to get everything completed. When we did head up, we were able to walk through the barley field which is now stubble. We felt like the journey had been halved by our new route.

Rossnaree excavation Trench 3

The lower courses of the revetment in Trench 3

In Trench 1 Matt and Kieran completed their section drawing, Kieran assisted by Eimear. Matt took a closer look at the revetment feature at the east end of the trench and realised that it has some considerable depth. The stones are several courses deep (and continuing) and are set at an 80 degree angle and act as a facing to the steep bank, keeping it in place. Someone went to a lot of trouble to make this structure, and the clues we have suggest that there were many other such structures all over the slope.

I thought we would be in a position to make a start on backfilling one or other of these trenches today, but I had miscalculated the amount of work to be done. Tomorrow, we need to complete the sampling, much of which was done today, and add levels to the plan of Trench 1 before backfilling begins. Nobody is particularly looking forward to this – both spoil heaps are positioned downslope from the trenches meaning that there will be a lot of uphill barrowing to be done tomorrow. It looks like it will be a long day…


Day 15: Friday fun day

Rossnaree Excavation: Stonehenge

Stonhenge, 2008

There has been a bit of coverage of a recent discovery of a low-visibility monument very close to Stonehenge. This was discovered during an intense geophysical survey of the area by English Heritage and is part of a survey that is planned to last three years. The site is interesting and, as they said themselves, it bodes very well for the rest of their work to have found a site so quickly.

You can view some of the BBC’s reports at this link: Stonehenge.

Reports of this discovery were carried on most of the UK national TV news programmes, and although this is the silly season, I can’t help wondering whether there is a difference in the attitudes to new archaeological discoveries in Britain and Ireland. Geophysical survey work like this has been going on over the past three years here in the Brú na Bóinne WHS, albeit on a smaller scale, which has received scant attention. By contrast, there has been a considerable amount of media interest in the excavation at Rossnaree, at least at a local level. It seems that in the public (or media) consciousness here, the work of archaeologists is perceived as digging holes in the ground and finding stuff. As followers of this blog (and others) will know, there is a lot more work involved in archaeology than just excavation.

We have sites just as good as what was discovered at Stonehenge the other day. Just by way of comparison, the new Stonehenge site is about 25m in diameter. The Rossnaree site is about ten times larger, measuring about 250m east-west and 200m north south. The archaeology of Brú na Bóinne and many other parts of our country are in no way lower quality or less interesting than that of Stonehenge or elsewhere and we (archaeologists and media) should be doing more to celebrate this rich and diverse aspect of our heritage. There is definitely scope for a Stonehenge-type geophysical survey project, similarly structured and resourced,  in the Brú na Bóinne WHS. This is work that really needs to be done for us to more fully understand how this very important landscape was used by past generations and also to more properly take stock of the archaeological resource of the area to inform its effective presentation and management.

Rossnaree Excavation Trench 4

The level is going down...

Anyway, back to the business of the Rossnaree excavation. We are really trying to schedule the remaining jobs to be done around the site at the moment as we look forward to our last week on site. The same work of the last few days continued in each trench. In Trench 1 Kieran, Eimear, Darren and Deirdre continued to take down and sample the levels in the ditch fill, sieving as they went. The extra manpower in this trench payed big dividends and a lot of progress was made. Just two more fills to be removed before the bottom is reached. Next week, to finish off the trench and shut it down, we will move into the western end to see how deep the stony surface is above natural. We suspect (and hope) that it is not too deep because we really don’t have time to get bogged down.

Rossnaree excavation Trench 1

Kevin and Deirdre speculate on the results

The ERT section suggests that there’s not too much depth in this area but we’ll just have to wait and see what turns up. We had a visit from Kevin the geophysicist again today who had a few jobs to do around the site. He is dropping back on a regular basis to see how the excavation progresses and also to see how well the excavation evidence matches up with the results of his earlier geophysical surveys. In Trenches 1 & 3 he is particularly ineterested in the match because the ERT line runs straight through the trenches we dug and should match up exactly. It is relatively rare that geophysical surveyors get to match up the results of their surveys with ‘reality’, i.e., the excavated layers themselves. These are the opportunities afforded by research excavation – the chance to experiment a bit and learn new ways of doing things.

Rossnaree Excavation ERT section

The ERT section with trench positions indicated

Rossnaree excavation Trench 4

Kevin takes samples from the Trench 4 section

Kevin also decided to take another series of soil samples from Trench 4 of very closely spaced intervals. Again, this is part of a research approach and may help to determine whether this technique is worth using in similar situations on other sites. I hope all the hard work pays off!

Fridays tend to be a day when we have a nicer than normal tea break. Some of the family came by to join in and we had a nice gathering in the cabin before getting back to work down by the river. We were spoilt for choice with home-made scones, which always go down well, and Deirdre also brought some extra goodies as well. They always did say that an army marches on its stomach and it is very important to keep morale up. We’ll certainly need it next week.

Rossnaree excavation

Friday tea-time


Week-End 1: No rest for the wicked!

Electrical resistance tomography Rossnaree excavation

ERT section over Trench 4, Rossnaree

No, I’m not telling you how much washing I had to do after the first week or how long I slept for. In fact, although the team had both days off over the week-end, Kevin and Mark of Landscape & Geophysical Survices came down to the site with me to do some geophysical surveying on Saturday (in spite of the very persistent, and sometimes heavy, rain). There were a number of questions to be followed up on and pieces of work to be done. Firstly, we needed to carry out an electrical resistance tomography (ERT) survey over the line of Trench 4 before it is excavated. This had been done previously along the bank where Trenches 1 and 3 are located. This is a little used but very useful technique because, unlike other more conventional geophysical survey methods like magnetic gradiometry or earth resistance which produce map-like plots, and ERT survey produces a section along a line, so it takes a vertical slice downwards across the line chosen. If you are planning to excavate a series of substantial cut features like ditches, this is an ideal technique as it lets you know exactly where the feature is located and it also gives an indication of its depth. Very useful information if you are trying to budget out how long a job might take. An electrical signal is sent down into the soil which is reflected back upwards. The signal is distorted depending on the depth and the nature of the obstacles encountered.

For those of you interested in the technical details of the technique, ERT uses a series of electrodes to pass an electric current into the ground and measure a resulting voltage. Using the values of current and voltage a resistance can be calculated using Ohm’s Law. The depth of penetration of the current is a function of the spacing of the electrodes, the wider the spacing the deeper the current flow. By selectively increasing the electrode separation the resistance of the sediments existing from the ground surface to greater depths can be measured. In order to compensate for the increasing electrode separation each resistance value is multiplied by a factor based on the electrode spacing and it becomes a resistivity. The resistivities are then plotted on a pseudosection which represents the distribution of resistivity with depth. In order to relate the overall resistivity distribution to the individual resistivities of host sediments and possible buried archaeological features such as ditches and walls the resistivity values in pseudosection are subjected to a computer modeling process. This process results in a more realistic model or section of the surface which then can be interpreted in terms of the possible archaeology intersected by the section.

Rossnaree excavation

The ERT survey results.

In the case of Trench 4 we are trying to locate the outer ditch seen in the magnetic gradiometry data. The ditch can be seen as a lower resistivity feature about 9m along the section. The lower resistivity represents the fill of the ditch. Underneath the ditch is even lower resistivity and we are wondering if this is due to clays laid down by the River Boyne and/or water saturated sediments. The excavation may reveal the answer to this question.

magnetic gradiometry survey Rossnaree Excavation

Mark carrying out the magnetic gradiometry survey

The next job we had to do was to trace the full length of the outer enclosure. Earlier survey work had established that this feature continued from the tillage field into the pasture field to the east (where Trench 4 is located). However, we had yet to fully map its entire length. Mark did this using a magnetic gradiometer, the same kind of instrument that had been used previously. This detects minute distortions in the earth’s magnetic field caused by the properties of the soils close to ground surface. Archaeological features like fireplaces and the fills of pits and ditches often contain soils and materials that will affect the earth’s magnetic field and these very sensitive instruments are capable of detecting these changes. This survey was successful and, although there had been a lot of disturbance of archaeological features by later cultivation, the line of the ditch was detected traveling eastwards to meet the riverbank where we guessed it would.

ground penetrating radar survey Rossnaree Excavation

Kevin carries out the GPR survey

We also took the opportunity to carry out a survey with another technique: ground penetrating radar (GPR). This technique works like an ERT survey. A signal (in this case radar, a type of radio wave) is sent downwards into the ground and is reflected back to the surface by different materials. The nature of the reflected signal received on the surface by the unit can tell about what it is (hard, soft, void) and its depth. Thus a vertical slice, like with ERT, is obtained. However, when a series of parallel lines is surveyed, and the data are downloaded and processed with the appropriate software, we end up with a 3-D model of the sub-surface archaeology of the survey area.

We look forward to seeing the results of this one and depending on the success of the survey and the quality of the survey, I’ll give you a look too. Watch this space!