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

Posts tagged “magnetic gradiometry

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.
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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.


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!