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,
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!
You would think I would have had enough. But no… hot on the heels of the end of the Gorhambury season, we headed off to the mysterious east side of the county. The Greenwich meridian seems to exert a powerful influence in Hertfordshire with its citizens seemingly afraid to cross the invisible line.
Back at the start of the project, we planned to do some survey in and around Braughing. We managed just one site. The area is extremely important with multiple late Iron Age and Roman sites including the Roman “small town” on Wickham Hill. We had an opportunity to work on the small town along with members of the Braughing Archaeological Group for a couple of days, mainly to see if magnetometry would show something useful. The field was, however, rather rough and caused the odometer on the cart to over-run by about a meter, and the nuts and bolts needed constant tightening. On the second day I adjusted the odometer settings which improved matters a bit.
We managed to complete 13 grid squares which was pretty good going, especially as the data logger crashed three lines before the end of the fourteenth square and we lost the rest of the grid. The results, after a bit of work in TerraSurveyor, were very interesting.
The broad line running east-west towards the south of the surveyed area is the road. It can be seen in the Google Earth image in the background. Towards the west, the very dark band must be where the road becomes a sunken way as it goes up the slope. What is very obvious is the difference to the planned public town at Verulamium. This site was clearly a very different type of settlement. What we have clearly shown is that it is worth expanding the magnetometry survey to cover as much of the settlement as possible. Hopefully, the field surface will be a little more benign when we return! One thing won’t change, however, and that is the slope…
Many thanks to Jim West for coming all the way from Chorley Wood to run the mag on the first day while I lay-in the grid, and also many thanks to all the members of BAG who joined in. Looks like we’ll be back!
I suppose I cannot really get away with that simple a post.
First of all, CONGRATULATIONS to everyone, it is a fantastic achievement and I am so proud of all of you. Secondly, a big thanks to everyone who turned out for an extra day on Bank Holiday Monday to complete the Macellum field.
How about some numbers? Well, Verulamium is the third biggest Roman town in Britain, after London and Cirencester. It is, however, the largest Roman town in Britain which doesn’t have a modern settlement built over most of it. We have surveyed 64.5ha of the total area of 81ha. It has taken us 83 working days starting in the summer of 2013, but we didn’t do much at Verulamium in 2014. It took 12,900,400 readings to cover those 64.5 ha. That, of course, doesn’t include the grids we did twice because of frozen sensors or other problems. People pushing the cart walked about 322km, not including having to go back to the start for partials, getting to the squares in the first place, or laying in the tapes and strings.
Let us look in more detail at the last bit surveyed in the Macellum Field.
Several things come to mind. Firstly, there is very little there! Towards the NE and along the western side there may be a ditch feature, although it is quite faint. Other than that, the main (and annoying) thing are the strong magnetic anomalies along the edge of the field. Some of you may remember the 12″ gas main which runs across the Park… well here it is again. What I do not entirely understand why there are differences between the negative and positive readings along our grid lines. Jim and I spent some time making sure I put the composite together correctly, and we are sure it isn’t a survey error.
This end of the field is know to contain two Romano-Celtic temples. These are known from aerial photographs taken in the hot summer of 1976. I wonder if this area of the town was kept clear of encroaching buildings, pits, ditches and the like deliberately? If we turn the contrast down (i.e., clip the image at +/- 40nT instead of +/- 7.5nT, we can see one of the temples close to the hedge as a faint white line.
Yet another target for the GPR next year!
Some of the team (many thanks Ellen, Mike and Jim!) helped re-do a number of areas of the res survey, plus one extra bonus square. The biblical deluge of Sunday night (Lamer Lane was flooded once more) was not ideal. This is the final area completed in 2016:
It is a pretty good result. There is almost no use of the “edge match” feature of the software to get the various grids to join neatly. It could be improved. The very high contrast of the temple rather makes the buildings faint, but either the creation of selective composites (i.e., processing bit of the survey separately), or use of a high pass filter, would improve that. The survey is quite big for a res survey: 2.5739ha according to TS (or 2.6ha to sane people who round numbers), which equals about 103,000 resistance readings. That, of course, doesn’t include the large numbers of squares we re-did due to the dry conditions.
There is a great deal more to do in terms of data processing and interpretation, but I think we all deserve a well-earned rest. Well, at least until Thursday…!
… but may be it should have done! Last year, on the last day of the survey, it poured and we cancelled. Today, we thought “it is only a little drizzle!” On occasions, drizzle was more of a deluge. At one point I was about as far from the cars as it is possible to get when on site, and I got soaked. Thankfully, Ellen went and fetched a dry tee-shirt and my waterproof coat. Thank you Ellen, you’re a star!
Peter, one of our volunteers from SWHAS and WAS, has bought himself a UAV fitted with a camera. He has been having some practice flights over the workers and the site. When I have worked out how to edit the video down to a sane size, I’ll post one of those, but meanwhile here is one of the stills. The UAV will prove a very useful tool.
One of the main reasons we persisted in the rain was the fact that we are so very close to completing the mag survey of the Macellum field.
Here is a closer detail of today’s survey.
As can be seen, Watling Street has rejoined the drive. There appear to be many buildings opposite the theatre which isn’t a surprise in the heart of the town. Street 24, which runs NNE from the theatre, has the macellum on the east side of it. This building has been partially excavated and has a complex building history of five phases going from the mid-first century to the early fourth. It has an almost equally complex excavation history: it has been examined by Grove Lowe (1847), John Harris (1869), Kathleen Kenyon (1934) and finally by Miss K. M. Richardson in 1938.
The resistance survey continued. The wet surface was both a boon and a problem. The first two grid squares went very smoothly. Then we moved all the probes and so forth and the machine started to play-up. After lunch, Peter came to the rescue and worked out that there was water where it oughtn’t to be, cleaned and dried connections and so forth, and all was well again. We managed another three grids including one in 35 minutes. Here is the survey.
In the first of the two images, I have not “edge matched” the grids so that you can see the ones which are a problem. Three of the grids we did today fixed existing problems. Although we had not planned to work tomorrow, I am hoping we might manage five more squares to fix the problems and give us a nice tidy survey. Archaeological geophysicists are obsessed by “nice tidy surveys.”
Comparing the mag and the resistance surveys, the end of the “sinuous ditch”, seen snaking in from the top of the mag survey, can be seen in the resistance survey but it seems to continue further to the east. The clear building on the northern edge of the resistance plot also shows pretty well in the mag survey.
Lastly, we learnt one lesson today. The GPR doesn’t work well in the rain! We surveyed a block near the rectangular enclosure I thought might be a temple. Here is the mag:
Speculation has been rife as to what this may be, so we tried using the GPR.
The terrible striping is caused by the rain. We will have to re-do this block another year. We can see, however, a square in the centre of the block, and almost another square around it. Before we get too excited, however, comparison of the two surveys shows that the squares in the GPR data lie outside the NNE edge of the enclosure, and in fact, partly show as light white lines in the mag data. Yet another question to be investigated more fully next year.
Many thanks to everyone who suffered the rain today. You are all stars.
Tomorrow will be our last day. I won’t post the results until Tuesday, however, as we are going for a celebration meal in a local pub.
I haven’t managed a Verulamium post for a few days so here is a quick catch-up.
Firstly, the mag has been slowly working its way eastwards along the Macellum Field. They are getting pretty close to the end of it.
As can be seen, Watling Street stands out very clearly running from near the theatre to the Chester Gate. There are lots of buildings along the road as would be expected. Some are less clear than one would hope because they have been partially excavated. The spec-ally look to the data, almost certainly because of the gravel subsoil, does make it harder to see what is going on here. The carrot at the end of the stick — apart from just finishing the field of course — is that there are two Romano-Celtic temples known from aerial photographs near the modern road.
The next image is just to show how much of Verulamium we have now completed. Poster, anyone?
The resistance survey has had a few problems. The lack of rain has made the top-surface of the field very dry and hard. It is very slow going, and the data is not as clean as one would like. Despite the problems, however, some of the buildings along the road, especially at the north side of the plot, are very clear indeed.
Although it doesn’t jump out at one when just looking at the plot, the sinuous ditch does show in the resistance data when one knows where to look!
The GPR team completed some blocks along the hedgeline which I haven’t processed yet… sorry! They also did one block up next to the Chester Gate to investigate the building here, and one over the sinuous ditch. The latter did show the ditch but very little else. Let’s look at the block near the Chester Gate.
This first image shows the mag data. The building in the middle shows as white lines roughly parallel to the modern drive. There are lots of other darker features, probably various pits and the sinuous ditch shows to the west.
This time slice shows the impact of splitting the survey over two days! The left hand side was done yesterday afternoon, the right hand side this morning. The pattern of the ploughing and the tractor’s turning circle in the corner of the field show clearly. Luckily, the problem is much less acute at lower depths.
This time slice now shows the building beautifully. What a wonky end wall on the north side! There is a long narrow range of rooms to the SW.
This time slice shows the SW line of rooms more clearly, although at the southern end they are been partly destroyed. We can see, however, fainter traces of the walls on the NE, a corridor, perhaps?
This time slice does show the “corridor” to the NE much more clearly. Perhaps it is more deeply buried in the plough bank? For most of the plot, though, nothing much else is showing.
There has been a little rain this evening. I have my fingers crossed for more. Hopefully, tomorrow will be dry so that the mag can plough on eastwards!
As always, many thanks to everyone, especially those working with the res meter. It is slow and boring at the best of times, but slow+annoying is a great deal to ask.