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

Posts tagged “magnetic susceptibility

Day 13 – Putting our best foot forward.

Work continued from yesterday with Eimear planning in Area 1 (Cutting 6), Matt excavating features in Area 2 (Cutting 7), Niamh extending the cutting to expose the full extent of the grave cut, and the rest of the team taking down the rest of the base of ploughsoil material in Cutting 8. Kevin, our geophysical surveyor, was also on site again to monitor progress, compare the excavated features to the anomalies identified in the initial surveys and take some additional readings.

Eimear adjusts her planning frame.
The plan in progress.

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In Area 1, Cutting 6, Eimear’s plan is nearing completion. The process is taking some time because of the size of the cutting and the complexity of the features and soils there. However, she is nearly finished and we will be taking levels on the surface tomorrow morning and assigning numbers to each of the separate fills and features. We even made a start this afternoon on one spread of material that seems to be occupation/refuse spread associated with the occupation of the oval enclosure. This appears to overlie the lower north-south ditch fill so we set Igor up to put a section across it to assess its depth. By tomorrow we will hopefully know what its exact relationship is to the other features and fills around it.

Matt examines the burnt stone.

Matt continued to deal with other features in Cutting 7 while Niamh and Sarah excavated the 1m x 0.5m extension to the cutting to fully open up the grave cut. His feature today, numbered F.705, appeared on the surface as a spread of charcoal flecked soil extending over an area c.1m in extent and disappearing beneath the northern baulk. As Matt excavated the feature revealed itself to be small furnace with a number of heat-shattered stones around and in it. The fill is mainly charcoal-rich soil, which we sampled. There was also a stone with a distinct concretion suggestive of some sort of slag, waste material from an industrial process, possibly metalworking. The base of the feature was reddened, baked soil, indicating intense heat. The proximity of this feature to the grave is interesting as it is not unusual to have such industrial features close to burials on non-ecclesiastical sites. Niamh, Sarah and Ciara gradually removed all of the ploughsoil from the extension and revealed the line of the grave cut retrieving a fragment of cranium from the ploughsoil Matt then added this detail to the previously drawn plan and proceeded to tidy up the area and continue to excavate it.

A beautifully trowelled surface!
Lisa and the post hole.

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In Cutting 8 the team there continued to remove, using trowels, the base of ploughsoil/interface layer. At the southern end of the cutting Igor had recovered a number of pieces of human bone and a tooth which indicates that there were probably other graves in that area which are now destroyed by ploughing. There is also a small gully-like feature here which appears to be relatively late in date and may be associated with later agricultural activity. Also at the southern end of the cutting, Lisa found her first feature. It looks at this stage like a very nice post-hole – an area of loose darker soil surrounded by a ring of packing stones. We will take a closer look at it in due course.

Kevin takes MS readings.
The metal detector in action.
Setting up the total station.

Kevin was on hand again to look at how work was progressing. He took additional magnetic susceptibility readings from the excavated surfaces of each of the cuttings and recorded interesting results. The values were all significantly higher than those recorded at the surface prior to excavation. There will be a very useful dataset gathered by the end of the excavation to carry out detailed comparisons between pre-excavation surface readings and post excavation readings as well as surface soil samples and ‘top of archaeology’ soil samples. Kevin also took the opportunity to scan the spoil heaps with a metal detector to ensure that we hadn’t missed anything (I don’t think we have but you never know..!) and he also scanned the unexcavated surfaces around the cuttings. In the afternoon, Kevin used a total station to survey in some of the major features identified on the site so far like the grave cut and the edges of the ditches in Area 1 Cutting 6.

In the afternoon we had a bit of diversion when the group of students currently working a the Blackfriary site in Trim paid us a visit. They are part of the Irish Archaeological Field School and on their way they visited the Bective site. I think the students enjoyed their visit and they told us that it was quite different to the other two sites.

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Day 9 – Time to start recording

The Area 1 team.

Today promised to be another fantastic day weatherwise and it was. However, there was a little bit of cloud and a bit of a breeze which made it a bit more bearable. The team in Area 1 continued to clear off the last layer of ploughsoil and expose the surfaces of the features underneath. They have now reached the topmost rows in the trench and the whole surface will have been cleared by tomorrow.

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Ciara with a mattock.
Jamie’s blue glass bead.

It is looking very well and, as outlined yesterday, major features are visible. It will be very interesting to record these and then begin to dig them one-by-one. Some nice finds came up again today – several pieces of flint and a fragment of another blue glass bead, probably dating to the early medieval phase of occupation. The bead was not complete like the previous one and Jamie, who found it,  did very well to spot it. The fragment is so small it wouldn’t have been picked up in the sieve either. It seems to have been more delicate and thin than the one from last week.

‘It could be a feature…’
A flint scraper from Area 2.

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In Area 2 the same questions dogged the team there – have we got base of ploughsoil or is there a spread of occupation material? It seems clear now that there is, in fact, a spread of material below the base of ploughsoil which has frequent large pieces of animal bone – a good indicator that it is undisturbed by ploughing as this would cause a relatively delicate material like bone to break down into smaller pieces. We took the decision to expose this layer as fully as possible and then record it by taking photographs the surface and then drawing a scale plan of it.

Spot the post hole!
Matt starts the plan.

While this final clean-back was being completed, a couple of possible features became apparent. These are small circular or sub-circular areas of slightly darker soil with charcoal flecking, and may be something like post holes. When the planning is finished, we will take a closer look at them. Matt set up to draw the plan using a long tape to orient himself relative to the site grid and a planning frame to help draw the detail on the surface, metre square by metre square. You don’t have to be an artist to do this – it is a technical rather than an artistic process – but, like so much in archaeology, it requires time, patience and a thorough meticulous approach.

Area 2 cleaned surface.

Cutting 8 soil sampling.

The others in the Area 2 team were now redundant – as Matt was busy planning, they couldn’t dig in that cutting or they would be in the way. Also, as the trowelers began to finish their strips in Area 1 they came across to Area 2 to join the diggers there. They started work on a third cutting which will ultimately join with Cutting 7. This is a 2m x 7m trench extending southwards and will investigate further anomalies identified in the magnetic gradiometry there. As with the previous trenches, we used long tapes set out between the site grid pegs to position the trench relative to the grid and set out a line to define the edges. This time we cleared off the loose stalks from the surface to help with the digging and sieveing process. The first step, however, before the digging could begin, was to take a series of magnetic susceptibility soil samples, to keep a consistent record. By late afternoon the first square had been almost completely dug and sieved. More of the same tomorrow; hopefully the weather will hold up, although the forecast is not good. We’ll take it as it comes.


Day 7 – Those wascally wabbits

Rabbit hole 1.
The neighbours.

Our nocturnal visitors had been back again to the site and this time they seem to have tried to set up residence in the Area 1 spoil heap. When we arrived down to the site this morning there were several burrows dug into the heap with a series of cute little footprints all around. They seem to have given up, however, as the holes weren’t too deep and were apparently unoccupied. As long as they confine themselves to the spoil heap I don’t mind too much – on some other sites I have dug on rabbit burrows had done a lot of damage to the archaeology. For some reason they don’t seem to be interested in the Area 2 spoil heaps.

Sophie and Sarah taking soil samples in Area 1.
Troweling down in Area 1.

On the work front the last of the ploughsoil was taken down – two half squares and one full square. This was easily achieved before the morning break and there was a certain air of proud satisfaction at the final moment. 30 square metres of ploughsoil removed and sieved since the start of the dig. After the break we took a second series of magnetic susceptibility samples from the base of ploughsoil using plastic implements in order to reduce the possibility of contamination. One sample was taken from each metre square and duly brought up to the cabin at lunch time. Once this was done we were ready to start troweling the surface to remove the last skim of ploughsoil and start identifying the features. Working from east to west in column I squares 1-5, we uncovered a band of hard, compact, yellowish soil which gradually gave way to darker slightly less compact, more fill-like soil with frequent flecks and larger pieces of charcoal and animal bone, also with occasional pieces of flint. This looks like the top of the fill of the north-south ditch identified in the geophysics. We will continue tomorrow and see what emerges.

The sondage in Area 2.
Matt and Eimear in Area 2.

In Area 2 Matt, Tomás and Ciara continued to take down the last skim of ploughsoil using a combination of troweling and mattocking. As described yesterday, a number of possible features had been identified yesterday along the northern side of the cutting. However, there were no similar features visible elsewhere in the cutting that might correspond the the geophysical anomalies. Matt carried out a sondage, or test area, in the extreme southwest corner of the trench to determine exactly where subsoil was located and it emerged that there was another 10 to 15cm to be taken down. Thus, although the northern edge was relatively clear, there seems to be a shallow wedge of ploughsoil thickening towards the southern side of the cutting. There is usually an occasionally disturbed zone at the very base of the ploughsoil which is transitional between fully disturbed ploughsoil and never disturbed natural/subsoil with archaeological features. When ploughing takes place in a field, it doesnt necessarily plough to exactly the same depth as previous years with the result that there is no clear ‘line’ or interface between ploughsoil and ‘unploughed’ soil. Sometimes, depending on local soils, it is difficult to easily distinguish between regularly ploughed ploughsoil, occasionally ploughed base of ploughsoil and subsoil and it takes a bit of deliberation to establish where each ends and the next begins. Tomorrow they will remove this and trowel the surface back and hopefully we will see some more features.


Day 6 – Moving up a gear.

Magnetic gradiometry, Area 1.

We all arrived down to the site hut revitalised, reinvigorated and ready for the new week. We were close to being completely free of the monotony of mattocking, shoveling and sieving through ploughsoil, although the finds we are coming across are nice. In Area 1 at the start of the day there were still 10 square metres to be dug away and throughout the day the team there got through seven and two half squares. Tomorrow, there are only two half-squares and one full one to go. Once they are gone, we will be nearly ready to begin to search for the features identified in the geophysical surveys carried out by Kevin before the dig started up. In the plot the cutting itself is made up of columns G, H and I and extends from row 1 to row 10. This takes in two very distinct features: one, a possible ditch running from north to south (from previous geophysical surveys this seems to be the innermost enclosure defining the knoll of high ground) and the second, at a higher level and cutting the other feature curving in from the middle of the right side to the bottom of the panel (this seems to be a separate oval enclosure superimposed on the other ditch. See the original plot HERE). This cutting will investigate the exact relationship between these two separate features and hopefully will yield material for dating.

Lisa, Mags and Sarah.
Sophie on the sieve.
Darren gets to trowel.
Magnetic gradiometry, Area 2.

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In Area 2, they started the day with just two metre squares to go and these were quickly removed and sieved. Once this was completed, we took another series of soil samples for lab magnetic susceptibility measurements using the same grid as before. Thus we will be able to compare values taken from the field surface to those at the base of ploughsoil. Once this was done the cutting was ready to be troweled back. There was still a layer of ploughsoil over the cutting – we deliberately didn’t remove everything when we were mattocking off the ploughsoil partly to protect the site from being open too long and from the impact of people crossing over and back as they work. Trowelling, with the help of some light mattocking where necessary, is the next step taken to remove any remaining layer of disturbed ploughsoil and begin to identify the features underneath. We are lucky in that we have a very detailed set of geophysical plots to guide us as to what to expect. However, the picture here is less clear with a series of features that are unfortunately less identifiable than those in Area 1. The excavation here is to explore the nature of these possible features – our hunch is that they may be houses because of their location within the innermost enclosure on the highest point in the site – but we will be scratching our heads if we don’t end up finding the features indicated on the geophysical plots. The cutting itself measures 5m x 4m and is located between squares Q10-13 and U10-13 although this will probably be extended southwards at a later stage.

Eimear searches for the feature in square U13.
All hard at work in Area 2.

The plan for 2011

Based on the geophysics carried out in August 2010 immediately after last summer’s excavations, I put in an application to the Royal Irish Academy for further funding to return to the site and excavate in the core area on the top of the knoll. This appears to have been the focus of all the activity on the site. The lithic scatter is clearly centered on this feature and the later enclosures all encircle it. Standing on top of the knoll, it is not hard to see why it would have been an attractive spot for a settlement, whatever the period, dominating the land to the south and with a good view over the river while being out of the reach of the winter floods. Happily, the application to the RIA was successful and work will start again on the site on 4th July.

Two areas of potential were identified. This was a difficult process firstly because of the huge overall size of the site. The time or the resources are not available to dig everything or even a large sample of the site so positioning cuttings is like playing ‘pin the tail on the donkey’. However, the geophysics are a big help. Choosing areas to focus was also difficult because it is almost impossible to identify features in the geophysics that are more likely to be early (ideally prehistoric and preferably neolithic). You normally get very little indication of the date of features from geophysical plots unless they are very recognisable. The chance that all such early features are long gone, destroyed by over zealous and unwitting early medieval remodelling, is constantly at the back of my mind.

Figure 1: Areas 1 and 2 superimposed on the detailed magnetic gradiometry plot.

Area 1

This is located over the junction between the oval enclosure and the innermost D-shaped enclosure lying beneath it. See the last post for a discussion of these features. The innermost D-shaped enclosure  feature appears to define the top of the knoll or topographic high and appears to be quite different in character to the other ditches. The character of the feature on the magnetometry plot shows it to be narrower and less magnetic than the other enclosures. The magnetometry plot also shows that it is clearly cut by the oval enclosure suggesting a relative chronological relationship between the features. The earth resistance plot (Figure 2, last post) shows that the inner enclosure feature encloses an area of high resistance with a sharp delineation between the high-resistance inner area and the low resistance area immediately outside. The junction between the two areas appears to be particularly sharp and suggests a possible stone facing/revetment feature. This again suggests that the method used to construct the inner enclosure is different to that of the other enclosures. The area chosen lies well within the area defined by the lithic scatter. It is proposed to open a trench over the junction between the two ditches on the western side of the oval enclosure in order to examine the relationship between these features. Hopefully, it will be possible to retrieve material suitable for dating from the earlier enclosure. Given the apparent relationship between the oval enclosure and this inner enclosure, it is possible that the inner enclosure is early in date and possibly prehistoric.

Area 2

The second area of potential is also in the core area close to the highest point on the knoll. It shows up in the magnetic data as a circular cut feature of slightly positive magnetic gradient, c. 6m in diameter and possible with a centrally placed posthole and there are other possible pits or postholes immediately outside it. There is an area of lower resistance in the same location on the earth resistance plot (Figure 2, last post) and the magnetic susceptibility plot also indicates an almost discrete area of enhanced magnetic susceptibility to the north of the main area of enhancement (Figure 2, below). The feature lies at the north-eastern corner of the area defined by the lithic scatter. Given that this feature appears to be relatively well preserved and that it is potentially prehistoric in date based on its position and morphology, it is proposed to excavate a cutting over this feature to establish its date.

Figure 2: Magnetic susceptibility plot with superimposed features from magnetic gradiometry survey


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.


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