Just a quick post to say we have been awarded the UCL Provost’s Award for Public Engagement 2017 in the established researcher category. Although the award is to me, I really see it as an acknowledgment of the success of the group as a whole. We should all award ourselves a firm pat on the back. Well done everyone, and very many thanks to Ruth Halliwell for nominating us for the prize.
Partly as a training exercise for UCL students, and partly just to extend our surveys at Verulamium, members of CAGG were out in Verulamium Park and Abbey Orchard last Thursday and Friday.
The magnetometer was out in Abbey Orchard on Thursday in the hopes of completing that small area. Unfortunately, I think it is jinxed. On Thursday we had battery problems, and on Friday the odometer refused to work properly. The sum total of a day and a half’s work? Two partial grid squares… Oh well, just chalk that one up to experience. Here is the image of the slightly extended area, such as it is.
Having given-up in the mag, Jim West, Pauline Hey and I decided to use the Earth Resistance meter on Friday afternoon. I had singled out an area where there is a clear building in the mag data, but some ferrous noise masked the western end of the building. The weather was glorious for a mid-February day.
We managed four grid squares at the usual 0.5m spacing, not too bad for one afternoon’s work. As you can see in the next image, the mag survey shows a lovely building as white lines representing low magnetism.
The wide dark line coming from the SW corner is the ‘1955 ditch’, the first century boundary of the town first excavated by Frere in 1955. The two parallel lighter lines which run just to the north of the building are part of a road.
Unfortunately, the Earth Resistance survey showed nothing of the building at all.
It shows the edge of the road beautifully, and a high-resistance feature parallel to the 1955 ditch. Even the ditch itself shows as a wide band of low resistance. Of the building, however, nothing! It may be simply that the soil is so wet at the moment there is no contrast between the building and the surrounding soil matrix. Alternatively, the building may have been robbed out. We will have to run the GPR over it one day.
Unlike last November, the GPR suffered no glitches, and Mike Smith, John Dent and Graeme Spurway completed an area 160m by 40. Added to the same sized area completed in November, we now have a nice block of GPR data 160m by 80m to look at.
The data were sliced using Larry Conyer’s system in 3ns slices. I’ll go through these from the top down. There are three areas of GPR survey shown. The top half of the large block is the latest survey, the bottom half that undertaken last November. The detached block to the west was undertaken by Ralph Potter in 2014. Remember that this is a rather crude “mash-up” in Google Earth so the edges do not match very well. As always the GPR data are deserving of a much more detailed analysis.
Slice 3 (above) mainly shows modern features surviving in the topsoil, especially broad cultivation marks running NNW–SSE across this field. There are some hints of the archaeology just starting to show through.
Slice 4 clearly shows the upper levels of the archaeology. Watling Street, which is running roughly north-south in the eastern half of the main area, has a big hole in it. It has been severely robbed for building stone. There is a minor road running SW-NE with a square building alongside it to the north with a small room on the western wall and what looks like a courtyard on the eastern side.
The main addition in slice 5 is the complex of buildings in the NW corner of the main area. These are probably associated with the pottery kiln which we have just clipped (the feature that looks like Mickey Mouse’s ears in the underlying mag data).
In slice 6, the spur road is clearer, especially near to Watling Street. It looks as though there is a shallow valley running parallel to Watling Street which is now filled with a greater depth of topsoil which means that the archaeology does not show until the deeper slices. It is also noticeable that the centre of Watling Street has fewer reflections than in the upper slices. I guess that we are getting below the surface of the road, and the reflections either side of the road may be the ditches that Wheeler found filled with rubble. The building complex just to the north of the spur road has hints of two more small buildings.
In this final slice we can see the two small buildings north of the spur road in more detail. There is also a long linear feature running N-S between Watling Street and the modern path. It looks like a modern utility to me, but there isn’t one indicated on the map I have been given.
The GPR results are excellent, and it will be worth continuing to expand this area.
I’ve had a busy time speaking to various groups about CAGG’s work recently. One of the lectures was as part of the Society of Antiquaries public engagement lecture series held on a Tuesday lunchtime once a month. They video the talks and put them online, so if you would like to hear me talking about Verulamium once more, here is the link.
As always, many thanks to Ruth Halliwell, Peter Alley, Jim West, Mike Smith, Pauline Hey, John Dent, and Graeme Spurway, as well as my students from UCL, for turning out in mid-February, although we were extremely lucky with the weather,
Those of you who know Bloomsbury probably imagine I have a lovely view across one of the squares from my office window. Until recently, you would be right! UCL are building a new student hub (everything is a ‘hub’ these days) and had to have somewhere to house the workers. The solution? … a two storey stack of portacabins in a tunnel over the pavement.
Just to add to my joy, the staircase between levels is just by my window, so they have covered it in plastic…
It is scheduled to be like this until December 2018. No wonder I prefer being out surveying. Jarrod Burks, who many of you will remember from the course, was out surveying the other day.
Seems a bit too extreme for my liking!
You may wonder what this tale of woe has to do with geophysics and CAGG? UCL have put up a nice set of panels on the hoardings to advertise what the Institute does and to make the place slightly more inviting. I was really pleased to see…
Yay, fame at last! Sadly, the only caption near the image of our survey doesn’t mention CAGG, Verulamium or geophysics.
Oh well, never mind. Perhaps I should find a plastic holder for our CAGG postcards and leave a few there for the curious.
Local Hertfordians maybe interested to see the article on Batford in this week’s Herts Advertiser and compare that to my posting about the survey.
Merry Christmas everybody.
John Dent kindly sent a photograph of his birthday cake beautifully decorated with an icing GPR.
Which reminded me that I should wish everyone a happy festive season.
I doubt we will be doing any more geophysics until the New Year, but I have some older work to catch-up on so there may be the odd post or two.
Best wishes from Kris and Ellen.
A combination of members of CAGG and students from the Institute of Archaeology, UCL, undertook four days of survey in Verulamium Park a couple of weeks ago. It was supposed to be five days, but the Saturday was called off because of rain. We aimed to:
- extend the magnetometry survey into Abbey Orchard, the field between the Park and the Cathedral;
- undertake some more GPR survey in the Park;
- try the six-probe six-depth method of Earth Resistance survey over one of the buildings we have previously surveyed;
- try a resistivity psuedo-section across the line of the town wall near St Germain’s Block.
It was definitely one of “those” survey periods. Every single technique had some problem or other of varying degrees of seriousness. We managed to sort most of them out in the end, but that and the Archaeology in Hertfordshire: Recent Research conference which was held on Saturday 26th have delayed this posting.
I am going to post the results in two parts, starting with the mag and GPR data.
The mag survey
The area being surveyed is not very large, but is quite awkward. It lies on a steep slope, has many trees and a great number of people walking by. It should, however, have evidence of the abbey, and possibly a late Roman cemetery. It also may have an Iron Age enclosure which was seen on an aerial photograph, and maybe the early Saxon town.
Between teaching, equipment issues and rain, we didn’t get as much done as we hoped, but we have at least started on this, our first extramural area at Verulamium.
Very little can be seen in this plot. It is very “noisy”. There is one potential thin feature, and the vague hint of a larger, wider feature (marked with red arrows in Fig. 2). Given where we are working, this is all very disappointing so far. We do need to complete this area if we can, however, so we’ll be back at some point.
The GPR survey
We did have some problems with the GPR as well, but eventually we managed to resolve those and completed an area 160m by 40m on the southern side of the town near the London Gate. The weather and obstacles could prove challenging (Fig. 3).
As usual, I used Larry Conyers and Jeff Lucius’ free software to “time slice” the GPR data. I created 3ns thick slices which seem to work well at Verulamium. The third slice (10.5–13.5ns; Fig. 4) shows features close to the ground surface. The dark smear running NS is the topmost layers of Watling Street. The diagonal lines, which also show in the magnetometry data, must be some sort of drainage.
The next slice (13.5–16.5ns, Fig. 5) shows the archaeology much more clearly. Watling Street is the black north-south feature to the east of the plot. There is, however, a hole in it! I guess this is another example of the extensive robbing of the town for building materials. Slightly to the west, a narrower minor road is running SW–NE across the plot. This road lines up with the light linear feature in the mag data. On the north side of this smaller road is a building with at least two rooms and what appears to be a paved area to the NE.
Slice 5 (16.5–19.5ns; Fig 6) still shows Watling Street and the building. There is a hint of some linear features that are on the same alignment as the rectangular enclosure which shows in the mag data to the south, and a wall in the SE corner.
The next slice (Fig. 7; 19.5–22.5ns) as there are hints of robbed walls showing in the south side of the plot next to Watling Street. These are showing as white lines of reflection free data and do seem to form structures of some sort. These align with the strange enclosure seen in the mag data.
The last slice (Fig. 8, 22.5–25.5ns) is interesting in that the main line of Watling Street is still visible, but does not have such strong reflections as before. There are, however, two bands of strong reflections either side of the road. Perhaps these are the roadside ditches known from excavation filled with rubble from the road surfaces and construction? The signal at this depth has started to attenuate and I will not show the deeper slices.
In Part 2 I will show the results of the two electrical techniques.
Ellen has designed a logo for CAGG. What do you all think?
Just to the west of Peterborough lies the Roman town of Durobrivae. This town is one of the so-called ‘small towns’, i.e., not one of the public towns with an administrative function. It is, however, somewhat larger than some of the public towns such as Caistor-by-Norwich, the civitas-capital of the Iceni. Duriobrivae had a town wall, parch marks from which can be seen on the Google Earth image along with Ermine Street (the straight line through the town from SE to NW) and the irregular street plan (Fig. 1).
Oblique aerial photography over the years has revealed much about the interior of the town, as well as extensive suburbs, prehistoric features, villas and so on. Fig. 2 shows an oblique image of the town.
I was particularly fascinated to see a large group of circular features to the south of the town, some of which appear to be the ditches around round-barrows, but others are far too large and are tentatively suggested to be some form of henge (Figs. 3 and 4).
Ruth Halliwell (WAS), who has worked with CAGG regularly, is working on the town for her dissertation and we arranged to go and team-up with local archaeologists to undertake some survey. It was very much a “proof of concept” trip: which techniques would work best there? Would the surveys add to what could be seen from the air? We undertook three days survey running all three main machines (magnetometry, resistance and GPR), and Peter Alley also used his UAV to take high-level photographs, partly with a view to creating topographic maps.
Despite early problems with the mag, we managed to survey an 80m wide, 360m long strip NS across the town. The overall results can be seen in Fig. 5.
There is a great deal going on in the results. Ermine Street shows clearly running across the NE corner of the survey transect and matches the parch mark beautifully. Either side of Ermine Street are a series of buildings with their gable ends onto the road in the approved Romano-British manner. Other streets can be seen, again matching the parch marks. Not all the buildings are so clear, but there are clearly other walls that can be seen in the data. Towards the south, the pattern is more complex. The results could be cleaned-up a little more. In places we were suffering from some stagger, partly as a result of the fine reddy-brown dust that settled over all the machines and their operators (Fig. 6) which, combined with the lubricant we use on the cogs, turned to a sticky slurry.
We undertook a radar survey using the Mala GPR we have on loan from SEAHA. Pushing the GPR was quite hard work in the long grass, especially as one goes over the agger on which Ermine Street appears to have been constructed (Fig. 7). The team did, however, manage to complete an excellent six blocks of data.
The images on the screen of the GPR shows that we were getting reasonable depth penetration. I created amplitude maps in 3ns thick slices. The third slice map (Fig. 8) clearly shows the surface of Ermine street, but for most of the area surveyed the radar signal is still in the ploughsoil.
In the fourth time slice (13.5–16.5ns, Fig. 9) some of the other roads are starting to show, and odd bits of wall. One very curious feature is the lighter coloured band across the middle of the southern area. Although it would appear to be related to our grid, our survey was conducted NS across that band. The aerial photograph (Fig. 2) does show a band across the field so perhaps this is related to some sort of cultivation pattern?
The fifth slice (16.5–19.5ns) shows more details in the buildings (Fig. 10). In the centre of the lower block is a square feature. This is the Romano-Celtic temple known from aerial photographs. This type of temple, well-known from many sites across the north-western provinces of the Roman Empire and consists of two concentric squares, usually reconstructed as an inner sanctum and an outer ambulatory. The two roads the the north and south of the temple appear to mark the edges of the temenos or sacred precinct. There is a hint of a possibly paved area to the west of the temple, and a solid feature between the internal and external walls to the east. In the northern block there are hints of the walls on either side of the road as seen in the magnetometry data.
In the sixth time slice (19.5–22.5ns) we can start to see some of the buildings along Ermine Street not, as I had expected, as black ‘high amplitude’ features shown in black i.e., stone walls, but as low amplitude features, i.e., areas which have fewer items that would reflect radar waves (Fig. 11). At Verulamium, I have interpreted these as where the stone foundations have been robbed, but here we know less about the construction techniques used. Part of the difficulty is that Ermine Street is on a quite marked bank which means the radar has a greater depth of deposits to penetrate. I undertook a topographic survey of just the northern block area (Fig. 12) and in the future will be able to process the GPR data taking into account the topography.
In the seventh and eighth time slices (22.5–25.5ns, 25.5–28.5ns), the GPR radar waves are starting to attenuate and we are getting quite faint reflections, but some of the deeper foundations show in these lower time slices (Figs. 13–14), For example, some of the buildings along Ermine Street start to show very well in Fig. 13, and the outer wall of the Romano-Celtic temple shows very well in Fig. 14.
There is a great deal more which can be extracted from the GPR data, especially by looking at the radargrams (the vertical slices) and comparing them to the time slices. The results are less “black and white” than at Verulamium and quite complex, but there is a great deal going on in this data which will take a bit of work to tease out all the details.
As well as the magnetometry and radar surveys, we undertook a resistance survey using UCL’s new RM85 meter (Fig. 15). We took readings every 50cm. We managed to survey an area 60x by 80m which had also been surveyed using the GPR and the magnetometer.
The result of this survey was quite surprising (Fig. 16).
The square within a square plan of the Romano-Celtic temple could not have been more obvious. The small room on the eastern side, partially seen in the GPR survey, shows clearly. The temenos is also quite clear. A spectacular result, but one that raises a question. Why is the inside of the temple such low resistance? Normally, low resistance like this is related to water retention. Is the outer wall of the temple causing water to pool within the wall? It is useful to compare the three surveys (Fig. 17).
It will take a bit of work to draw-up a composite interpretation plan.
Peter Alley also undertook some surveys with his UAV (Fig. 18).
As well as taking high-level images of sites, the UAVs pictures can be used to create topographic models using a technique called “Structure from motion”. Fig. 19 shows a topo plan of part of the site derived from the photographs. The actual heights vary from my plan because the UAV’s plan needs to be corrected against some control points, but the relative heights are great. This technique is going to prove very useful in future.
The aim of this three days of fieldwork was simply to see which survey techniques would provide useful information at this site. The answer is: all of them! We already have a huge amount of data to examine in more detail, and a great deal of thinking to do. It certainly seems that a more extensive programme of geophysical survey would add to our knowledge of the town greatly, as well as other archaeological features such as the “henges” to the south.
As always, many thanks indeed to everyone who came to help, especially Mike Smith for transporting and running the GPR, and Jim West for helping to run the mag. This was a great team effort between local group members and CAGG, and exactly what our group exists to do.
I have a number of small surveys which remain unreported that I need to catch-up on, and so here is the first of them.
Earlier in the year, Mike Smith and I assisted Alex Thomas (University of Bristol) in undertaking a Ground Penetrating Radar survey of land lying to the north of the B653 at Batford, Harpenden, Hertfordshire (TL 148150, Fig. 1). The survey was undertaken over the weekend of 2nd/3rd April 2016. Earth resistance and magnetometry surveys had been undertaken in the area previously.
The underlying geology of the site is Lewes Nodular Chalk formation overlain in places by the Kesgrave Catchment Subgroup sand and gravel.
A Mala GPR with a 450mhz antenna was used, identical to the one CAGG borrow from SEAHA. The survey transects were at a 0.5m spacing collected in a zig-zag fashion. The survey started in the NE corner and proceeded east-west. Radar pulses were set for 0.05m intervals with a time-window of 73ns. The newer Mala systems do not allow manual selection of sample numbers which are determined by the machine, in this case 376 samples per trace.
For the amplitude slices presented here, as usual, the software system developed by Jeff Lucius and Larry Conyers was used (http://www.gpr-archaeology.com/software/). This necessitates the conversion of the Mala rd3 files into GSSI dzt files using the companion conversion program.
For this posting, the slices were 3ns in thickness starting at 3.5ns From these, it appears that the second slice, 6.5–9.5ns represents the immediate ground surface. This agrees with the estimate of the first reflection from the individual radargrams at about 8ns as examined using RadExplorer. Beyond slice 7 (>24.5ns) the signal has completely attenuated. This means that all the usable returns lie in the band between c.6.5 and 24.5ns. This is not unusual for Hertfordshire where the clay soils do not allow for the GPR surveys to penetrate particularly deeply.
As with most of the GPR surveys reported in this blog, the numerical output from that software was turned into images using Surfer v.8. Kriging was used to interpolate the values into a 0.1 x 0.1m grid. The resulting images where then imported into Google Earth.
Six amplitude maps or `time slice’ maps were produced and are shown in Figures 2–7. The topmost map (Fig. 2) shows two strong reflections to the north-east and the south. The second map (Fig. 3), which represents the 9.5–12.5ns range, has the clearest set of features. A number of long linear features are visible, two of which I have labelled A and B. There is a odd-looking curved linear feature with two parallel lines, labelled C, into which a pair of parallel lines cuts, labelled D. Further fainter linear features can be seen, such as those at E.
The third map (Fig. 4) has fewer clear features, most of which are probably `echoes’ of the features seen in the previous map. The next three maps (Figs. 5–7) have successively fewer features in them, none of which are especially clear. By the last map, the GPR signal has largely attenuated and little can be seen. At best, we are getting a depth penetration of about a meter, probably somewhat less.
The question arises, therefore, as to what the long linear features may be. If Fig. 3A is a wall, it would be at least 35m long, and Fig. 3B would be at least 55m long. One possibility is that they represent old field boundaries. Looking at the 1898 OS map (Fig. 8), there is nothing to suggest an origin for those features. The 1799, map now in the Westminster Abbey Muniments Room does, however, show a field boundary behind some buildings to the north of the road. A crude overlay of an extract of this map (Fig. 9) on the Google Earth image with the GPR data, shows a remarkably good correlation between the field boundary and the one of the linear features (Fig. 3A).
The origins of the curved and parallel linear features can be seen if one takes into account the location of the machine-dug test trench marked in Figure 10. These parallel lines, only some 1.8m apart, represent areas of soil compression from the machine used in the excavation of the test trench. Examination of one of the radargrams (Fig. 11) would seem to confirm this. The origin of the reflections, marked with blue arrows, occurs at the very surface and is highly suggestive of compression rather than construction.
The survey results appear to be largely connected to (a) earlier agricultural use of the land in the form of hedgerows and so forth or (b) the recent impact of the excavation of the test trench. It appears highly unlikely the GPR results indicate any sort of structure although the golden rule of ‘absence of evidence is not evidence of absence’ must be applied. The lack of pottery or ceramic building materials on the surface makes it unlikely that a building is indicated.
Perhaps I should have saved a more exciting post for #100!