Tag Archives: magnetometry

Filling in the gaps

Thanks to Stuart Gray of Strutt and Parker, I was able to contact Pete and Fiona Letanka and obtain permission for us to survey the paddock of Darrowfield House which lies immediately behind Verulamium Museum. It is the last large-ish area of Verulamium left that has not been totally surveyed using magnetometry. Members of CAGG are, therefore, spending this week filling in the gap.  Our mag was away being repaired, but Pat Johnson of Foerster kindly agreed to bring it down to us today.  We started, therefore, with Earth Resistance and GPR, but after lunch the poor old res meter was left abandoned on the grass while the team got started with the mag survey.

The paddock ought to be exciting.  It lies just behind the line of the Roman wall, and there is a road shown in publications running SW-NE across it.  The first mag results are shown in Fig. 1.

Fig. 1: the mag survey.

They are surprisingly unexciting.  The diagonal lines running SW-NE look like cultivation marks of some sort.  There is a quiet area behind the supposed line of the wall, running NW-SE, and then one high-magnetic curved feature which may match a bank we can see in the field.  No real sign of the wall, or of the road,

The GPR survey has been processed in 3ns thick slices.  As usual, I’ll work my way down from the top.

Fig. 2: GPR survey, time slice 3, from 10.5ns to 13.5ns.

Fig. 3: GPR survey, time slice 4, from 13.5ns to 16.5ns.

The third slice shown in Fig. 3 really shows surprisingly little. I’m guessing we are only looking into the topsoil here.  The fourth slice (Fig. 3) starts to show some diagonal striping similar to what we can see in the mag data.  Again, this is probably agricultural.

Fig. 4: GPR survey, time slice 5, from 16.5ns to 19.5ns.

Fig. 5: GPR survey, time slice 6, from 19.5ns to 22.5ns.

The fifth slice (Fig. 4) shows the striping very strongly, with two alignments at right angles.  I think these must be land drainage.  They look too straight for “ridge and furrow” and do not have a headland. What is also interesting — or is that worrying? — is that the wheel-ruts which show so clearly on the Google Earth image in the background, also show in the GPR data at this depth.

The sixth slice (Fig 5) faintly shows echoes of the drainage.  To the north are some black squiggly features.  Yes, folks, we have mapped the badger setts!  Nothing very archaeological, so far.

Fig. 6: GPR survey, time slice 7, from 22.5ns to 25.5ns.

In the final slice (Fig. 6), we still have the setts in the north, but we have a very strong linear feature to the south running SW-NE.  Comparing this to the proposed line of the road in Fig. 7, we can see that this would appear to be one edge of it.

Fig. 7: GPR survey, time slice 7, from 22.5ns to 25.5ns, with overlain map of Verulamium.

Right at the edge of the GPR’s signal depth where the returns are very attenuated, we are starting to see the archaeology. Unfortunately, I cannot yet easily convert the time (22.5ns–25.5ns) into physical depth.  It would seem, however, that the archaeology in the lower area may well be at the limit of what we can pick up.  If we look at the radargrams, however, we can see a big feature running across our vertical slices and I would guess this is the road surface.

Fig. 8: an example radargram showing the possible road.

Many thanks to everyone who turned out today and enjoyed the beautiful sunshine.  Also, big thanks to Pat for bringing the mag back for us, and Pete and Fiona for arranging for us to be able to survey the Paddock.

There probably won’t be a posting tomorrow, but I hope on Weds to catch-up.

February fun

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.

The Abbey Orchard survey.

The Abbey Orchard survey.

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.

The Earth Resistance survey underway.

The Earth Resistance survey underway.

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 mag results in the area of the Earth Resistance survey.

The mag results in the area of the Earth Resistance survey.

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.

The Earth Resistance survey results.

The Earth Resistance survey results.

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.

GPR slice 3, 10.5 to 13.5ns.

GPR slice 3, 10.5 to 13.5ns.

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.

GPR slice 4, 13.5 to 16.5ns.

GPR slice 4, 13.5 to 16.5ns.

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.

GPR slice 5, 16.5 to 19.5ns.

GPR slice 5, 16.5 to 19.5ns.

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).

GPR slice 6, 19.5 to 22.5ns.

GPR slice 6, 19.5 to 22.5ns.

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.

GPR slice 7, 22.5 to 25.5ns.

GPR slice 7, 22.5 to 25.5ns.

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,

 

Infamy, infamy (they’ve all got it in for me)?

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.

The Institute of Archaeology, UCL.

The Institute of Archaeology, UCL.

Just to add to my joy, the staircase between levels is just by my window, so they have covered it in plastic…

The view from my office.

The view from my office.

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.

Jarrod surveying, December 2016.

Jarrod surveying, December 2016.

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…

The Verulamium survey on the hoardings.

The Verulamium survey on the hoardings.

Yay, fame at last!  Sadly, the only caption near the image of our survey doesn’t mention CAGG, Verulamium or geophysics.

The caption.

The caption.

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.

Four days in the Park (part 1)

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.

Fig. 1: The survey team in Abbey Orchard.

Fig. 1: The survey team in Abbey Orchard.

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.

Fig. 2: the mag survey results.

Fig. 2: the mag survey results.

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).

Fig. 3: GPR survey in the Park, November 2016.

Fig. 3: GPR survey in the Park, November 2016.

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.

GPR time slice 3 (10.5--13.5ns).

Fig 4: GPR time slice 3 (10.5–13.5ns).

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.

GPR time slice 4 (13.5--16.5ns).

Fig 5: GPR time slice 4 (13.5–16.5ns).

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.

GPR time slice 5 (16.5--19.5ns).

Fig 6: GPR time slice 5 (16.5–19.5ns).

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.

GPR time slice 6 (19.5--22.5ns).

Fig. 7: GPR time slice 6 (19.5–22.5ns).

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.

GPR time slice 7 (22.5--25.5ns).

Fig. 8: GPR time slice 7 (22.5–25.5ns).

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?

cagg-logo-3

T-shirts anyone?

Durobrivae

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).

Fig. 1: Google Earth image of Durobrivae.

Fig. 1: Google Earth image of Durobrivae.

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.

Oblique aerial photograph of the town. Reproduced courtesy of Stephen Upex.

Fig 2: Oblique aerial photograph of the town. Reproduced courtesy of Stephen Upex.

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).

Fig. 3: Google Earth image of the field to the south of the town showing circular prehistoric features.

Fig. 3: Google Earth image of the field to the south of the town showing circular prehistoric features.

Fig. 4: Oblique aerial photograph of the field to the south of the town showing the Roman suburbs and earlier prehistoric circular features. Photograph courtesy of Stephen Upex.

Fig. 4: Oblique aerial photograph of the field to the south of the town showing the Roman suburbs and earlier prehistoric circular features. Photograph courtesy of Stephen Upex.

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.

Fig. 5: the magnetometry survey.

Fig. 5: the magnetometry survey.

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.

Fig. 6: the red dust all over the GPR. Image courtesy of Mike Smith.

Fig. 6: the red dust all over the GPR. Image courtesy of Mike Smith.

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.

Fig. 7: Pushing the GPR over Ermine Street.

Fig. 7: Pushing the GPR over Ermine Street.

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.

Fig. 8: time slice 3, 10.5-13.5ns.

Fig. 8: time slice 3, 10.5-13.5ns.

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?

Fig. 9: time slice 4, 13.5-16.5ns.

Fig. 9: time slice 4, 13.5-16.5ns.

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.

Fig. 10: time slice 5, 16.5-19.5ns.

Fig. 10: time slice 5, 16.5-19.5ns.

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.

Fig. 11: time slice 6, 19.5-22.5ns.

Fig. 11: time slice 6, 19.5-22.5ns.

Fig. 12: topographic survey of the northern block.

Fig. 12: topographic survey of the northern block.

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.

Fig. 13: time slice 6, 22.5-25.5ns.

Fig. 13: time slice 6, 22.5-25.5ns.

Fig. 14: time slice 6, 25.5-28.5ns.

Fig. 14: time slice 6, 25.5-28.5ns.

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.

Fig. 15: Richard Cushing and Stephen Upex working on the resistance survey.

Fig. 15: Richard Cushing and Stephen Upex working on the resistance survey.

The result of this survey was quite surprising (Fig. 16).

Fig. 16: the earth resistance survey.

Fig. 16: the earth resistance survey.

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).

Fig. 17: comparing the three survey techniques and the parch marks in the area of the temple.

Fig. 17: comparing the three survey techniques and the parch marks in the area of the temple.

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).

Fig. 18: Peter Alley using his UAV to map the site.

Fig. 18: Peter Alley using his UAV to map the site.

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.

Fig. 19: Topo map in QGIS derived from the UAV's aerial imagery.

Fig. 19: Topo map in QGIS derived from the UAV’s aerial imagery.

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.

Fig. 20: the end of day.

Fig. 20: the end of day.

Another day, another town

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.

The survey underway on Wickham Hi

The survey underway on Wickham Hill.

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 survey results.

The survey results.

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…

On top of the hill.

On top of the hill.

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!

Rain didn’t stop play

… 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.

Verulamium Theatre.

Verulamium Theatre.

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.

The magnetometer survey of the Macellum field.

The magnetometer survey of the Macellum field.

Here is a closer detail of today’s survey.

The mag survey after day 36.

The mag survey after day 36.

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.

The res survey after day 36.

The res survey after day 36.

The same area as the previous image showing the mag data.

The same area as the previous image showing the mag data.

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:

Rectangular enclosure, near the southern side of the town, seen in the mag data.

Rectangular enclosure, near the southern side of the town, seen in the mag data.

Speculation has been rife as to what this may be, so we tried using the GPR.

Day 36 GPR data.

Day 36 GPR data.

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.