CAGG on tour

Mike and I undertook a survey for a colleague of mine on a site in Sussex. Last year, a Roman stone sarcophagus had been found (Fig. 1) and the question was “are there any more?” As stone sarcophagi are not magnetic, it seemed that GPR would be the ideal method to use.

Fig. 1: the sarcophagus (photo: Mike Smith).

I had hoped to be able to run the GPR over the site of the burial before they cleared it for excavation, but the timings did not work. It would have been ideal to be able to see what this looked like in radar data.

When we arrived, the GPS decided it was going to play-up. It took over an hour on the phone to get it to work once again. As a result, on the first afternoon we did a small grid 42m x 8.5m at 0.5m intervals. The top eight slices are shown in Figure 2.

Fig. 2: the initial grid, top eight slices.

There wasn’t anything too exciting in the data but I did wonder if the “blob” (“high reflectance feature”) indicated with the red arrow might be something. With the transects running E–W, only one or two would actually hit a sarcophagus also lying E-W. In order to test this, we did a second small detailed grid the next day, only 5m x 8.5m but this time at 0.25m transect intervals and N-S. The top eight slices are shown in Figure 3.

Fig. 3: the detailed grid.

The feature starts showing in 3, is clear in 4 and 5 but has gone by slice 6. Sadly, I don’t think this is a sarcophagus. Firstly, it’s shape doesn’t seem all that convincing, and secondly, it is quite thin, which shows even more clearly in the radargrams (Fig. 4).

Fig. 4: radargram across feature (NB N is to the left.)

It also does not show in the one Earth Resistance grid we had completed earlier in the training dig (Fig. 5).

Fig. 5: Earth Resistance grid.

Oh well, we tried! We also completed a larger block on the second day, some 50m x 40m at 0.5m transect intervals (Fig. 6).

Fig 6: the GPR in action (photo: Mike Smith).

The top eight radargrams are shown in Fig. 7. Despite there being features in the magnetic data (not collected by us), nothing much shows in the data beyond plough marks and cultivation patterns.

Fig. 7: the main grid.

It was quite a shame that we were unable to detect something more exciting. As always, being able to survey a bigger continuous area would be ideal. Odd grids here and there are hard to put in context and a bit hit or miss. It is always preferable if one has time and resources to survey large areas.

The magnetic susceptibility survey

“Mag sus” is not used as commonly in archaeology as some of the other survey techniques such as magnetometry. It can, however, be very useful in specific circumstances such as the project I did in Italy a couple of years ago. Magnetometry (or to give its full name, magnetic gradiometry) is a passive technique. It measures the strength of the local magnetic field whatever the source of the magnetism, be that pits full of soil which is slightly more magnetic than the surrounding subsoil, or the keys in your pocket. At the start of each day, we find a “quiet spot” and adjust the magnetometer so that it reads zero whatever direction we are facing, and so all the sensors are reading zero over the same point. In that way, our readings are all variations against that local zero point. Even at the data processing stage, we make the general background level of magnetism be zero. As a result, we can see the “anomalies” (ditches, pits, kilns, old horseshoes etc.) as local variations against the background.

On occasions, however, we might want to see the broad pattern of magnetism that we cannot see in the gradiometry data. In other words, is the soil on this part of the site potentially more magnetic than the soil on a different part of the site, perhaps as a result of burning, or simply where people dumped their rubbish? To do this, a magnetic susceptibility meter creates a magnetic field, and then measures how magnetic the sample within that field becomes. In this way we can plot variations in absolute magnetism rather than patterns in the local field. The size of the sample measured depends on the size of the loop the meter is using. The MS2D sensor we were using last week has a diameter of 20cm and measures the top 10cm of the soil. In Italy I only had the MS2K sensor which has a diameter of only 25mm and is measuring only the top few millimetres which is why we had to use a mattock to allow us to take a reading from bare earth.

So, why were we trying this at Gorhambury? Fig. 1 is a slide I use in a lecture which shows the Earth Resistance survey results for part of the Theatre Field. I have circled two buildings.

Fig. 1: The Earth Resistance survey with two buildings indicated.

In the Earth Resistance data the foundations are showing as slightly darker lines, i.e., higher resistance features. We know from excavations that buildings at Verulamium have foundations of flint resulting in the higher resistance values. Now let us look at the magnetometry data (Fig. 2).

Fig. 2: the magnetometry survey with the same two red circles as Fig. 1.

In Fig. 2 I have circled the same two locations as Fig. 1. In the eastern circle we can see the building foundations as white, i.e., negative magnetism, but in the western circle we cannot see the building here at all. What is going on? We have to go back to first principles. We are measuring magnetism, and flint foundations are not magnetic so we should not be able to see them. My idea is that the soil around the eastern circle is quite magnetic in absolute terms, so when one digs a foundation trench one is removing magnetic soil and replacing it with non-magnetic material, hence the walls showing as white lines. In the western circle, however, the soil may be much less magnetic so that digging some away and replacing it with flint might not be visible. The magnetic susceptibility survey is an attempt to test this idea. If I am right, the soils near the western circle should have a lower magnetic susceptibility than those near the eastern one.

But what about the area of strong magnetic responses circled in yellow, and a similar area in the SW corner? These look like buildings which have burnt down, and were never replaced. If we look at the Earth Resistance data again (Fig. 3), it doesn’t appear that the area was built-on again. This is just an interpretation, and one that could only be securely tested by excavation.

Fig 3: the Earth Resistance data showing the location of the possible burnt building (circled in yellow).

To test my idea about variation in the background magnetic susceptibility, I picked out an area which contained the above buildings and we took 1,301 mag sus readings over a few days in the last week (Fig. 4). Once one got going, it was quite quick although a team of three does seem a bit overkill.

Fig. 4: the location of the mag sus readings.

By drawing a rough line around the points we can see the area surveyed on the mag data (Fig. 5). It includes both burnt buildings as well as the building we can see and the one we cannot.

Fig. 5: the mag sus survey area shown by the red line.

Fig 6 shows the results of the survey. White/red are areas of high magnetic susceptibility and blue is low.

Fig 6: the magnetic susceptibility survey results.

The results are a bit blobby — well a lot blobby! — but there is a clear trend for the higher readings to be in the NE half of the area and the lower ones to the west and south with one exception. The strong blob in the SW corner matches very closely to the location of the smaller of the two hypothesized burnt buildings. That gives us some faith in the method. Figure 7 shows the same results with the locations of the two buildings mentioned earlier indicated with red boxes (Google Earth doesn’t do circles!).

Fig. 7: the two buildings shown on the mag sus map.

The results are a little inclusive. Although the larger building to the east lies within the generally higher area of mag sus, its immediate location is not especially strong. The smaller building to the west, however, does lie in an area of low readings apart from one hot spot. If one looks at the survey points (Fig. 8), you can see that “spike” is the result of one reading. The magnetometry survey (Fig. 5) shows some spikes in the same area, probably modern iron.

Fig. 8: the mag sus sample points overlain on the results.

I do have some reservations. Firstly, the MS2D is only measuring the top 10cm. The grass and turf is quite thick in the field as it has been pasture since 2000. In Verulamium Park, the wear on the grass from walkers and footballers probably means the turf is less dense and thick, and the grass is certainly cut shorter. Perhaps we might get clearer results if we could measure a larger or deeper sample? Secondly, the topography of the field means that there has been soil wash / creep downslope (Fig. 9). Might the trend be a result of that?

Fig 9: topography in the survey area from the dGPS data.

The last image is just to show how the aqueduct follows the contours (bar the odd dog-leg around something).

Fig. 10: topography with the line of the aqueduct indicated in blue.

Many thanks to all those I cajoled into helping me complete this little experiment. I shall keep pondering…

I have results from other surveys and projects we have undertaken, and so will post those as and when I can. My desire to write blog posts rather died during covid, but now I am going again I’ll try and keep posting the occasional piece beyond the results of the Gorhambury survey.

A happy ending

Most years two things have happened. Firstly, we have lost two days to bad weather and secondly, we’ve had some little job to finish off on the Bank Holiday. This year, although the dead Foerster mag resulted in us spending the first three days on another site using the Bartington magnetometer instead, and the late hay harvest delayed our start for the Earth Resistance survey in the Theatre Field, we have not lost any days for bad weather and we have met all my targets for the year so will not be back tomorrow. Go team!

Before I get stuck into the results and the usual end-of-season stats, I’d like to say a big thank you to all the team. Twenty people were involved in the work this season, some just for the odd day, some for almost every day. I’d especially like to thank those who helped ferry stuff and look after it over night, who helped fix stuff, and for ferrying me. You all know who you are. All your efforts are really appreciated and the project would not be the same, or even possible, without you. I’d also like to thank Lord Verulam for facilitating access to his land. There are some days when I am high on the hill overlooking the Roman town, with the cathedral on the horizon, that I cannot believe I can be so lucky as to be able to work here each summer.

The Earth Resistance team managed an excellent seven grids today to complete the southern transect to the hedge line (Fig. 1).

Fig. 1: the res team on the last day.

Over the last 17 days at Gorhambury the resistance team have collected data from 75 grids. Since moving into the Theatre Field they have averaged six grids a day. Those 75 grids cover 30,000m2 or 3 ha. The team have stabbed the ground with the machine some 60,000 times, collecting 180,000 readings, 120,000 0.5m spacing readings and 60,000 1m spacing readings. The area covered is a bit odd looking because we have been adding to the edges of the area previously surveyed (Fig. 2).

Fig. 2: the 2021 Earth Resistance survey.

The GPR team had a few problems with the new system at the start of the season, but they soon got into the swing of things and have completed 4.12ha of Blacklands Field (Fig. 3). The field is 7.97ha, so they have completed 51% of the field, some 82,400m of radargrams which occupy some 3.14gb of my hard-disk!

Fig. 3: Blacklands Field GPR at the end of the 2021 season.

Despite Fergus’ indifference (Fig. 4), the res team did a great job today. The results are quite subtle (Fig. 5).

Fig. 4: Unimpressed.
Fig 5: the results from the Earth Resistance survey.

Although the results from the survey aren’t hugely obvious, we may have found a couple of buildings. I had indicated them with the res arrows in the image. I’ll have to cross-reference them with the mag and GPR images to be sure.

Figure 6 shows nine time slices from the GPR survey.

Fig. 6: nine time slices for the GPR survey at the end of the 2021 survey.

Today’s survey has revealed a series of buildings on the eastern edge of the survey area. Figs. 7-9 show three time slices in more detail.

Fig. 7: slice 5.
Fig. 8: Slice 9.
Fig. 9: slice 11.

We clearly have a series of buildings around a open area with Watling Street on the SW side. To the east, there appears to be a walled enclosure. Some of the buildings appear to have columns in front, especially the small one in the SE corner. The buildings do not look like the big town houses we have found in much of the rest of the town. I’m going to have to do some research into the plans of other types of building.

We also finished the mag sus survey today, We collected some 1,300 readings which I’ll have to type in. Unlike our other survey methods, the mag sus meter uses a Mark 1 pencil data logger…

I’ll post updates on the data processing, and on some other surveys I haven’t had a chance to write-up over the following weeks. Meanwhile, we’ve pulled-up our last flag at Gorhambury for another year (Fig. 10).

Fig. 10-: the last flag.

Just one day more

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I always have mixed feelings at this time of year. I so enjoy spending the days out in the fields at Gorhambury with the gang, and the end of the season marks, for me, the end of summer and the start of getting ready for the new term. On the other hand, I’ve had, essentially, one day off in a month so I am looking forward to putting my feet up, at least for a day!

We ran all three machines today: the Earth Resistance Meter, the GPR and the magnetic susceptibility meter. All three managed a good area and hopefully we’ll all meet our targets by the end of tomorrow so we can avoid the traditional extra part-day on the Bank Holiday. The mag sus has now taken well over 1100 readings, but I’ll make a separate posting about that in due course.

Fig. 1: Gill, Pauline and the resistance meter, watched by Fergus.

The Earth Resistance team are a very long way from the rest of us, at the top of the hill overlooking the town. They are working eastwards towards Bluehouse Hill.

Fig. 2: the Earth Resistance survey after day 16.

The survey (Fig. 2) shows the 1955 ditch and the buildings I mentioned yesterday. There is quite a contrast between some areas on the edge of the dry valley and the buildings making them harder to see. With a bit of careful data processing I can, hopefully, bring them out more clearly.

The GPR survey’s two 40x40m blocks had one or two buildings in them. Fig. 3 shows time slice 9.

Fig. 3: GPR slice 9.

We seem to be in an area of public buildings and open spaces, rather than private houses like the “motorway services” (which I think is actually a very nice house and not services at all!). We are going to be left with very tantalising half-surveyed buildings until next summer (we hope).

Short posting tonight as it is getting late and tomorrow is our last day.


For those unfamiliar with geophysical survey techniques in archaeology, please visit the “geophysical survey in archaeology” page for an introduction.

I know it is getting towards the end of August and the nights are drawing in, but who turned the heat off? It was really rather chilly out on site today, and the wind didn’t help.

The magnetic susceptibility survey continued today thanks to the help of Gill and Julia. At my request, Julia came in today specifically to help with the survey, so many thanks. The survey technique is a peculiar mixture of high and low tech. The mag sus meter itself looks like a thirty-year-old design, and probably is! It has no data logger or GPS. The survey method, therefore, is as follows:

  1. Hold the loop of the meter high in the air and hit zero.
  2. Hold the loop of the meter as flat on the ground as possible and hit measure.
  3. Shout out the reading and the second person writes it down in the notebook.
  4. Take a GPS measurement where the reading was taken.
  5. Pace out roughly five meters to the next point and repeat.

The easiest way to do this is with three people (Fig. 1).

Fig 1: the mag sus survey in action (photo: Mike Smith).

I haven’t had a chance to type in the 892 readings we have taken so far to process them, but I have downloaded the coordinates to see how well our system of pacing out distances towards a ranging rod is working. There are a few gaps, but not too bad (Fig. 2)!

Fig. 2: the location of the magnetic susceptibility readings.

The GPR team spent the day in doing the saw tooth southern edge of the Blacklands field. The lines started quite short, but ended-up at 52m. Not that much longer than 40m, really… (Fig. 3).

Fig. 3: collecting long GPR transects (photo: Mike Smith).

The buildings on the edge of the GPR survey yesterday continued to show-up very nicely. Figures 4, 5 and 6 show slices 5, 9 and 11. Remember that the higher slice numbers are the deeper ones.

Fig 4: GPR slice 5 after Day 15.
Fig. 5: GPR Slice 9.
Fig. 6: GPR Slice 11.

The strong (dark) response across Watling Street seen most clearly in Slice 5, and as more of a blob in slices 9 and 11 is the northern triumphal arch excavated in 1961 by Frere. It had been seen in an aerial photograph by Dr J. K. S. St Joseph.

Many of these buildings do not seem to appear in Alban’s Buried Towns but we have seen some of them before. Fig. 7 shows the magnetic gradiometer data from 2016 in the same area.

Fig. 7: the mag data in the same area.

The mag and GPR together do seem to suggest that there is a large open space here, with buildings along the sides, especially to the south of Watling Street. Lots of things to speculate about!

The res team completed another six grids to hit the target I set them a while ago two days early (Fig. 8).

Fig. 8: the complete res survey with the mag survey in the background.

Figure 9 zooms in on the southern area to show the grids collected recently.

Fig. 9: the southern edge of the res survey.

The latest blocks have picked-up the 1955 ditch (as seen a bit more clearly in the mag data) and interestingly, a road which runs behind the ditch. It has also completed some buildings that were surveyed in earlier seasons. The team now have a bonus prize (!) of completing a line of 12 grid squares along the southern edge of the survey, ending-up at the side of the road up Bluehouse Hill. To encourage the team, some nice buildings can be seen in the magnetic data!

Those of you who have followed this blog over the years may be surprised that I haven’t used one of my favourite words, as is traditional on the last Friday of the season. Well, I decided using antepenultimate was getting a bit old… oops. I just used it again. Oh well…

A bit busy

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It seemed as though today was quite busy. There are times when I wish I could clone myself, or at least we had two dGPS units! It wasn’t helped by leaving the all important “brown bag” at home with my notebooks, so big thanks to Ellen for bringing it over. Both teams completed excellent areas, two 40×40 grids for the GPS and six 20x20m grids for the Earth Resistance. Pauline, Georgina and I also completed some more magnetic susceptibility survey despite a large haystack in the way. I think Thistle (Fergus, CAGG’s watchdog’s best friend) thought we were utterly mad dragging her back and forth across the same bit of field.

The res team is still heading south, although they only have six grids left to complete the transect I hoped they would complete (Fig. 1).

Fig. 1: The Earth Resistance survey to date.

With luck the team will complete the last six grids tomorrow and then what will they do on the weekend? (At this point imagine an evil Mutley-like laugh…) Yes, I have an idea…

Fig. 2 shows a detail of the southern end of the transect the team have been working on.

Fig. 2: detail of the survey. The darker strip to the west is the 2021 survey.

As you can see, the survey picked-up the southern end of the buildings, but has gone back into an area of nothingness. What is not visible in the image is that there is an undulation in the field surface, and the diagonal SW-NE area of blankness south of the hedge is a slight dry valley. The question arises, therefore, is the “blank” area actually empty, or can we not see the archaeology due to the build-up of colluvium? We may get the answer from Oxford Archaeology’s test pits excavated in 2000 (Fig. 3).

Fig. 3: Oxford Archaeology’s test pits from 2000.

At 10pm after a busy day I’m not going to start looking at that data now, but it is obviously quite important. Verulamium Museum also has a large box of 35mm slides from the test pitting exercise, if someone fancies a job scanning them…

The GPR team had a slight problem at the start of the day when one of their antenna batteries had not charged properly overnight. Thankfully, the second battery lasted the rest of the day… just! Here are the usual nine time slices (Fig. 4).

Fig. 4: Nine time slices from the survey up to the end of day 14.

Today’s survey revealed a mass of buildings to the south of Watling Street. It looks like the buildings to the north of Watling Street might be arranged around a large open area. Cutting across Watling Street is a curiously strong response, some sort of quite substantial foundation. Is this just a spot where the robbers missed a section of road, or something else? Figs. 5-7 show three slices of increasing depth so that you can see how this changes as we go deeper.

Fig. 5: Day 14, time slice 4.
Fig. 6: Time slice 9.
Fig. 7: Time slice 11.

I spent the last hour of the day laying out the “saw tooth” edge to the field ready for the team to survey today. Given the buildings going into that area, it should be an exciting set of data tomorrow.

A glorious day

The weather today was lovely and reminded me of how much I love working in the Theatre Field. Although the hill leaves one a little breathless (especially when carting gear up it) the view across to the abbey is just lovely (Fig. 1).

Fig. 1: St Albans abbey in the sunshine.

Many thanks to Mia, Gill and Georgina for helping me with the magnetic susceptibility survey. No results to show as yet (I have to type them in!) but we did cover an area 140m x 60m at roughly 5m intervals, about a third of the area I’d like to get done this summer. Hopefully, this survey will help us understand why some buildings show in the magnetic survey data, and some do not.

The GPR crew had a screw loose this morning. No, I mean an actual screw. Well, probably is it technically a bolt. I looked down from the hill to see them scouring the grass. Thankfully, we had another bolt which sort of fitted and allowed them to carry on and complete another 80x40m block. Fig. 2 shows the time slices.

Fig. 2: nine time slices from the survey up to the end of day 13.

They found lots of bits of buildings, but it looks like whatever was here has been a bit more robbed out than some of the other buildings we have seen. For a change, Fig, 13 shows time slice 13.

Fig. 3: time slice 11.

The building parallel to the wall at the northern edge of the survey area, towards the eastern side, appears to have a large apsidal room. This looks like the sort of dining room we get in the fourth century such as the one at Lullingstone villa. Traditional Roman dining rooms had three couches arranged around three sides of a square, hence triclinium. In the fourth century the fashion changed to have a U- or omega-shaped couch, and hence the dining rooms became apsidal.

The Earth Resistance team completed an excellent six grids, although one had to be in two parts as it straddled the hedge line. Sadly, there wasn’t much in it (Fig. 4).

Fig. 4: the Earth Resistance survey. Today’s grids are the slightly darker block to the west.

OK. I lied. The grids are stuffed full of buildings with Street 25 running right through the middle. Bits of these buildings could be seen in aerial photographs from 1976, but with nothing like the clarity we see here. These are super results.

Tomorrow sees us back on site as the end of the season rapidly approaches. Hopefully, I’ll get a bit more mag sus done, and the other teams will continue their excellent progress.

A little bit of history

A week or two ago on Twitter, Tom Higham posted a photograph of Martin Aitkin and his father, Charles Higham, using Aitkin’s proton magnetometer at Verulamium. As a result, I have been swapping emails with Charles, who now lives in New Zealand and writes a column for the magazine Current World Archaeology. The photograph shows Charles, Martin and Felicity Wild using the machine (Fig. 1).

Fig.1: Charles Higham, Martin Aitkin and Felicity Wild using the magnetometer.

Charles worked at Verulamium with Sheppard Frere in 1958 and 1959. His diary entry for 1st September 1959 reads:

I go on at St Albans until 23rd of this month at least. Today a change. I helped Dr Martin Aitkin… manage his proton magnetometer. This is a machine to measure subsoil disturbance. We tried with partial success to trace a ditch for a 1/3 of a mile.

The ditch is the confusingly named “1955 ditch”, so-called because Frere excavated a section of it under Insula XX, Building 1 in 1955 (see Plate 1b in the first interim report, Antiquaries Journal 1956). This proved to be the early boundary of the Roman town, and on the basis of two sections excavated across it later, was dated by Frere to c.AD 80, and going out of use in c.AD 125. I have mentioned this boundary in many previous posts as it is such a prominent feature of the magnetic survey data.

Aitkin’s survey was also an important one for the development of the method. In 1957 J. C. Belshe had suggested that kilns could be detected magnetically. As a result, Aitkin undertook the first magnetometry survey in March 1958 at Durobrivae (another site we have worked on) and found it was very successful in finding pits and other features. During the next year Aitkin tried the method on 16 other sites. In 1959 he returned to Durobrivae when the A1 was to be widened and would destroy an area of the suburbs. With two machines, each with three operators, they covered four acres in eight hours taking some 900 readings. Having narrowed down the area of interest, they then surveyed a section in greater detail pinpointing kilns, a ditch and other features. The early development of the method, from initial suggestion to a practical tool for archaeology was surprisingly quick.

Aitkin’s discussion of his surveys in the 1959 and 1960 interim reports from Verulamium (Antiquaries Journal for 1960 and 1961) make for interesting reading. In Fig. 2 is the results from our survey for the southern corner of the 1955 ditch in Verulamium Park.

Fig. 2: the southern corner of the 1955 ditch as shown in the magnetic data.

I have marked the line of the ditch with yellow arrows. I was always puzzled as to why published plans of Verulamium showed this corner as a sharp bend rather than the gradual curve seen in our plots. Aitkin gives us the answer:

The disturbance continued in a straight line for 1,100 feet (110 traverses) at which point the municipal cricket pitch had been reached; here, fearful of the groundsman’s eye, traverses were interrupted and continued on the far side of the pitch. However, the disturbance did not reappear there. Measurements made with care on the pitch itself showed that the ditch turned eastwards through an angle of approximately 107°, whence, despite some interference from iron piping, it was followed for 950 feet…

Aitkin, 1960, pp. 22–24.

The area marked marked “disturbance” in Fig. 2 underlies the eastern edge of the cricket pitch and probably represents some hardcore used to level the area. We also found the metal pipe to which Aitkin refers.

Mostly, Aitkin’s surveys were not illustrated, but for the northern corner of the 1955 ditch he produced the following image, here compared with our survey (Fig. 3).

Fig. 3: The northern corner of the 1955 ditch as surveyed in 1960 and 2016.

As can be seen, Aitkin’s survey compares very well with ours undertaken some 56 years later.

Aitkin was very pleased that in 1959 they measured 302 traverses which constitute some 3,000 measurements. Each traverse was usually 50 feet long, with a reading taken at 5-foot intervals. Each traverse took less than two minutes. That is about 5 readings per minute. At that rate I think Martin’s helpers were kept pretty busy! In comparison, when our Foerster is working (which it isn’t at the moment — again!), we collect 3,000 readings every four minutes, with 10cm along the traverses and 50cm between them. This isn’t to boast, but just to show that the developments in the last six decades have often revolved around the speed of collection allowing us to survey greater areas at greater data densities. Aitkin’s work, however, led the way.

Charles is including the story of helping Aitkin at Verulamium in a future column for Current World Archaeology. I look forward to seeing it.

Two hundred

Two hundred what?, I hear you ask. Well, this is the 200th posting to the hertsgeosurvey blog. Who knew I could be so verbose? Well, generations of Institute of Archaeology students probably have [impolite] opinions on that!

In the morning, Mike, Nigel and I worked with the GPR. Normally, I’m off with my “staff of position” as Peter would call it, but GPR with only two people is not fun, especially in the long grass. Many thanks to Sonia for turning out this afternoon to help move strings having only got back from holiday yesterday. It was most appreciated. The GPR completed two 40x40m blocks, the second with “saw teeth” along the northern edge. Fig. 1 shows nine time slices of the whole survey area.

Fig. 1: nine time slices of the whole survey area.

Figure 2 shows my favourite slice, No. 9, in context.

Fig. 2: time slice 9 at the end of week 3.

Unusually, the GPR has found traces of a building near the northern edge of the town. So far, the area behind where we think the wall would be has been pretty empty. Fig. 3 shows this in a little more detail.

Fig. 3: detail of the area surveyed this weekend.

There are some tantalising faint marks as well as the more obvious walls, which might represent robbed walls. I’m not entirely sure, but I think this area is just marked as “area of occupation” in Niblett and Thompson (Alban’s Buried Towns). Even more tantalising are the parch marks which can be seen in the unsurveyed area to the south. Comparison to the mag data shows (Fig. 4) that the walls sort of show if you know where they are and squint, but you might not put much faith in them without the GPR. Multi-instrument survey rocks.

Fig. 4: The mag survey for the same area as Figure 3. The red line indicates the edge of the GPR survey.

The weather today was, at times, a bit wet (especially immediately after lunch) but at times was very pleasant (Fig. 5). The very definition of changeable!

Fig. 5: ominous clouds (image © Mike Smith).

The Earth Resistance team completed an excellent eight grids, making-up for my errors of yesterday. Fig. 6 shows the entire res survey on the Gorhambury side of the town.

Fig. 6: the Gorhambury Earth Resistance survey.

The survey is now an impressive 11.5ha which is pretty good going for a twin-probe Earth Resistance survey. At the end of the season I will add the new blocks into the master composite which should get rid of the difference between the old and new parts of the survey in the Theatre Field. Fig. 7 shows a detail of the new area.

Fig. 7: the area surveyed recently (to the west) and adjacent blocks from the master survey.

The diagonal line running NW-SE across the plot is Street 11. It has been robbed to the east of the cross-roads, but appears to be pretty much intact to the west. Although subtle, there are clear buildings in the new area, one just south of Street 11 on the western edge of the survey, and one disappearing into the hedge on the southern edge of the new area. Street 25 can be seen running NE-SW and appearing as a parch-mark south of the hedgerow. There are also clear parch-marks of buildings just off that street which are right in our transect. Exciting prospects for next week’s survey. At the end of week three we finally headed down the hill to the cars (Fig. 8).

Fig. 8: Home from the Hill. (Image © Mike Smith).

Now for a well earned two days off. I’ll be spending them going to Sussex to teach… umm… geophysics. Oh well, no rest for the wicked. Look out for a special bonus post tomorrow.

A bit damp

Today started badly. Somehow I had managed to leave the Earth Resistance meter on and the batteries were flatter than the proverbial pancake, and the weather in the morning was miserable. It was the sort of rain that was light enough that one thought soldiering on was OK, but heavy enough that everything was a bit damp by lunchtime. During lunch the weather dried-up, and apart from one short shower in the afternoon it was not too bad in the afternoon, although the team might not agree with me!

Despite the problems, the res team completed four grids before the dry-cell batteries gave out. They don’t last long. The results of the Earth Resistance survey are shown in Fig. 1.

Fig. 1: The Earth Resistance survey. The strip to the west is the 2021 results so far.

Sadly, the grids are mostly the same odd “noise” as yesterday’s data, although the SW corner clips Street 11 shown as the black (high resistance) triangle in the corner. There is also a large roughly circular feature which shows in the res data and the mag data as shown in Fig. 2.

Fig. 2: the blob (outlined in red) in the res (left) and magnetic data (right).

I have no idea what it is, but it is big (some 9m across, roughly). Some form of burnt surface, maybe?

From the top of the hill where the res team are working, the GPR team in Blacklands Field seem a long way away. Fig. 3 shows the GPR team at work, and this image was taken with a telephoto lens!

Fig. 3: the GPR team in Blacklands Field.

The GPR completed the “saw teeth” on the southern edge of the transect, and started the new transect by surveying yet more saw teeth on the northern edge! Fig. 4 shows nine time slices.

Fig. 4: GPR time slices for days 1 to 11.

Processing the GPR data in one go has some advantages, and some disadvantages. Despite having a fairly powerful computer, it takes a while now to process the whole survey. I also deleted some 30gb of duplicate files this evening. The software makes many copies of the data as it works. Fig. 5 shows my favourite slice, No. 9.

Fig. 5: GPR slice 9.

The completed saw-tooth section on the southern side of the survey area has revealed a nice small building, albeit with its western wall missing. There are also traces of Street 26 running SW-NE. The new transect has started like most of the data close to the line of the wall, with nothing showing. Although we presume the steep bank down to the flood plain of the river represents the line of the wall, we have yet to find anyone mentioning any actual evidence of it.

For people who “know” GPR, you may be wondering why I just say “slice 9” rather than give actual depths. “Slice 9” is pretty meaningless as it depends on the settings one uses in the software (for the 2021 season I have kept these the same). To calculate the actual depths, one needs to know the speed that the radar waves are travelling through the soil. To calculate this, one uses the software to find hyperbolic curves in the data, and then match them on-screen which will give you the speed. The only problem is that the software I use assumes your mouse has a central scroll wheel. Mine does not — I use a trackball. The alternative method involves clicking the mouse, but the curve changes so fast that it is almost impossible to get a match. I’ve given-up for now.

After the hay was baled, the local red kites come to look for dead things in the grass. I get a bit worried when they circle over the team… I’m not really a wildlife photographer, and I don’t really have a long enough lens, but I had a go this lunchtime (Fig. 6).

Fig, 6: a red kite looking for lunch.

Hopefully tomorrow will be a more straightforward day.