Tag Archives: magnetometry survey

CAGG on tour: Chisbury, Wiltshire

At the request of Andrew Reynolds and Stuart Brookes, medieval archaeologists at the Institute of Archaeology, UCL, members of CAGG headed out on tour to Wiltshire.  The site we were asked to survey was the Iron Age hillfort of Chisbury, near Great Bedwyn, Wiltshire.  One may ask why Chisbury is of interest to early medievalists?  An early 10th century document called The Burghal Hidage records a site called Cissanbyrig (Baker and Brookes 2013, p. 228) which may be Chisbury.  The Burghal Hidage records the defended settlements (burhs) of the Kingdom of Wessex set-up after the defeat of the Vikings in the late 9th century.  The Historic England Scheduled Monument listing notes that:

Although no formal excavations have been carried out within the hillfort, observation of 20th century disturbances has produced evidence of urns, bronze swords and of storage pits containing Late Iron Age and Romano- British pottery.

The site also has a well preserved 13th century chapel under HE Guardianship.

Fig. 1: the chapel at Chisbury.

The interior is quite plain but has some interesting details.

Fig. 2: the interior of the chapel at Chisbury.

One of those details is a surviving consecration cross painted on the back wall.  Apparently these were painted on the wall when the church was dedicated.

Fig. 3: The consecration cross.

Peter Alley’s high-level photograph from the UAV shows how the chapel lies right across the defenses of the hillfort.  Maybe this position was significant in somehow slighting the earthworks?

Fig. 4: high level photo showing the location of the chapel.

The defenses are very well preserved around most of the circuit but they are covered in trees so hard to see and photograph.  From the satellite image the site is an extended oval of trees.

Fig. 5: Chisbury from Google Earth.

Jim West took a good image showing part of the defences to the west.

Fig. 6: Chisbury defences from the west (photo: Jim West).

The weather was wonderful and we all got a bit sun burnt.  The bluebells and primroses were out in force.

Fig. 7: bluebells on the defences.

The theme of the week was, however, definitely “horses”!

Fig. 8: Nosey horsey.

Unfortunately, horses are quite magnetic due to their ferrous footwear.

Fig. 9: Horseshoe alert! (Photo: Mike Smith).

Those horses are also a bit careless with their shoes…

Fig. 10: Missing footwear.

The plan was to complete as much of the inside of the fort as we were able with the magnetometer, and to do some selected areas with the GPR and the Earth Resistance meter. Right from the beginning we were beset with problems. We arrived at lunchtime on the Thursday, and managed to complete quite a few squares in the first afternoon, but the odometer started to over-run, eventually by four or five meters.  I swapped a few emails with Pat Johnson from Foerster, and the next day we managed to cure the problem.  A couple of days later, one of the pins in the “spider” — the cable that joins the sensors to the control box — snapped so we were down to three sensors.  On the last full day, the odometer started slipping again…  We did manage to survey the whole available area of the fort, but only just and without much time to try the other methods.  Fig. 11 shows the overall survey.

Fig. 11: magnetometry survey.

Although some of the major features can be seen at this scale, I have created two images with the north and south parts of the survey and some annotations.  (You might like to look at these downloaded and at full size.)

Fig. 12: the mag survey of the northern part of the site.

In Fig. 12 we can see a series of parallel linear features which have been annotated in cyan. These look like field drains to me.  Very faintly, however, there are some circular features.  These may well be the “drip gulleys” of Iron Age circular round houses.  I have marked some of the possible ones in red.  The problem with these is that the more one stares, the more one invents!  I am sure you can spot a few more possibles if you look long enough.  There is a great deal of ferrous noise, especially around the edges from fences, gates and water tanks, but also in the field from old nuts and bolts, horse shoes and the like.  Looking carefully and the little blobs and measuring the minimum and maximum values in nanoteslas (the unit of magnetism), one can start differentiating between bits of old iron and possible pits.  I have marked a small number of the possible pits with green arrows.

Fig. 13: the mag survey of the southern part of the site.

In Fig. 13 the red line is the pipe which joins the main water tank in the middle of the field.  I have marked just one piece of ferrous rubbish with a red arrow, there are lots more.  The cyan lines mark the possible field drains.  The dark blue line is a negative-magnetism feature which runs from the edge of the water tank to the pond.  This can also be seen in the GPR data (below).  I am guessing this is some sort of drainage / outflow from the water tank to the pond. There are some areas with such high ferrous noise it is impossible to see anything, for example the north end of the eastern field.  There are, however, quite a few pits once more, and I have marked just a small selection.  What is curious, however, is how much of this area seems devoid of any features at all.

Although the mag results are not exciting in the sense of being able to clearly see a building, as we often do at Verulamium, there is quite a bit of detailed information buried in the data.

We managed just a couple of days of GPR survey thanks to some local help.

Fig. 14: the GPR survey underway. (Photo: Mike Smith.)

I used the GPR Process and Surfer programs and created 3ns time slices.

Fig. 15: GPR time slice 3 from 10.5 to 13.5ns.

Fig. 16: GPR time slice 4 from 13.5 to 16.5ns.

Fig. 17: GPR time slice 5 from 16.5 to 19.5ns.

Fig. 18: GPR time slice 6 from 19.5 to 22.5ns.

Fig. 19: GPR time slice 7 from 22.5 to 25.5ns.

Fig. 20: GPR time slice 8 from 25.5 to 28.5ns.

To be frank, not a great deal shows.  The last time slice shows the suggested pipe from the tank to the pond.  The southern area has more high reflections, especially in slice 5 (Fig. 17) which one could try to make into buildings, but I find myself suspicious that these features are close to the water tank and they may be something to do with the tank’s construction.

Giving the billing this site has, the results are not all that stunning.  There are, however, features which would be worth investigating further, and hopefully we will get to “ground truth” some of these features as part of Andrew Reynolds and Stuart Brookes’ wider project.

Many thanks to those CAGG members who came all this way to do the survey: Ellen Shlasko, Ruth Halliwell, Peter Alley, Jim West, Nigel Harper-Scott and Mike Smith.  Many thanks too to the members of the local group who came to help: Shaun Wilson, James Kay and Lynn Amadio. Lastly, but certainly not least, thanks to the stud and the farm for allowing us to play on their land.

The results from Darrowfield

First of all, apologies for the delay to this post. Life has been a bit busy, and the data processing got a bit complicated, but I think I have cracked it now.  Our principal aim was to complete the area using magnetometry.  This we did easily.  We also aimed to try and cover as much as possible using GPR and Earth Resistance.  We managed quite a bit of GPR, but not much resistance survey, partly due to manpower and time, and partly due to the stunningly uninteresting results.  Lastly, we did two resistivity pseudo-sections and two topographic surveys, one using the dGPS and one using a UAV and “structure from motion”.  So be prepared for a long post!

Topography

I am going to start by discussing the topography because this has an impact on the results and the interpretation.  We did two types of topographic surveying.  Firstly, I walked back and forth taking readings with the dGPS at six-pace intervals, or closer if there was a change of slope such as a bank.  Secondly, Peter Alley used his DJI Phantom UAV to take a series of aerial photographs which can be used to create a topographic model using a technique called “structure from motion”.

Fig. 1: Peter Alley landing his UAV after taking aerial images of the site.

The two techniques have their strengths and weaknesses.  The model using SfM can be very detailed, far more so than is easily possible with the dGPS, but the model is of the top of the vegetation.  The camera cannot see through the stinging nettles.  Mostly, this does not matter but there are areas where one has to be aware that the nettles are masking the topography.  I, however, could not easily get dGPS readings in the garden or the orchard because the trees blocked the satellite signals.  The UAV could, however, map these areas. Using the two methods side-by-side is thus a useful thing to do.  The other big advantage of the UAV is that the survey took about 20 minutes compared to almost a day using the dGPS!

Fig 2: Darrowfield paddock topographic model from the dGPS survey.

Fig. 3: Topographic map derived from the UAV survey.

Ignoring the areas outside the paddock / gardens in Fig. 3, we can see that the two surveys show pretty much the same thing.  The highest ground is the bank along the SW edge of the paddock with a steep slope along one edge.  The bank is less pronounced as it curves around through the garden and the orchard in the NW corner of the area.  I had initially thought this was a road line, but it doesn’t line up with the roads seen in the wider magnetometer surveys of the park.  It does seem to be mapped as a long thin enclosure on early OS maps. There is a second, much less pronounced bank along the NE side of the paddock, which fades out towards the east corner.  This bank is assumed to be the line of the Roman wall. Between the two higher areas is a lower lying area in the middle of the field.  In the east corner, the UAV seems to show a channel running west-east into the corner (Fig. 3, the dark blue line running through a lighter blue area).  This is a footpath running through an area of high stinging nettles.  On the dGPS survey (Fig. 2) we can see a low channel running north-south in the same area — shown in red — which is masked by the nettles.

The magnetometer survey

The main area of the paddock was surveyed on the normal grid we have been using at Verulamium.  The two small extra areas, on the lower lawn of Darrowfield House and the orchard, were surveyed using small floating grids set-up to fit the awkward areas better.  In the paddock, nettles, rabbit holes and badger burrows all made surveying a bit awkward.  Very many thanks to Pat Johnson of Foerster for bringing the repaired magnetometer down to site from Tamworth.

Fig. 4: Ruth Halliwell and Jim West surveying the Lower Lawn of Darrowfield House.

Given where we are in the Roman town, i.e., just behind the basilica in the centre of the the city, the results were surprisingly unexciting!

Fig 5: the magnetometry results.

The majority of the area seems to be dominated by a faint striping running SW-NE across the middle of the field.  I am guessing these are cultivation marks, and this interpretation is supported, as we will see below, by the GPR results.

Fig. 6: annotated magnetometry results.

In Fig. 6, at point A, we can see a curving “linear magnetic anomaly”.  This matches the curving bank seen in both topo surveys (Figs. 2 and 3).  The clearest indications of buildings are along the SW edge of the paddock, at B.  The light lines mark walls, and the darker areas magnetically enhanced materials between the walls. It is difficult to know if these are Roman or buildings associated with the farm.  If, however, we project the line of the roads from the wider survey (Fig. 7), it seems likely these are Roman buildings lining the SE–NW running road.

Fig. 7: rough projection of the Roman roads into Darrowfield Paddock.

At Fig. 6, C there appears to be a linear feature running across the lawn.  At the west end is a black dot with a bright white halo.  This is something ferrous, it has a range of -104nT to 2371nT!  The linear feature next to it only has readings from -5nT to +15nT and is much more likely to be something like a ditch.  Just inside this linear feature are three black “blobs” which have similar ranges and may be pits.

The steep bank shown on the topographic maps which runs parallel to the SW hedge has a linear low magnetic feature (Fig. 6, D).  This may just an artefact of the bank, but I am beginning to wonder if there is something holding the bank up, especially in the light of the GPR results below.

Lastly, At Fig. 6, E there are two faint linear low magnetic features.  Is this a hint of the wall line?

The GPR survey

Between the animal burrows and the nettles, the GPR survey could be quite a struggle…

Fig. 8: Jim versus the nettles at Darrowfield (image: Mike Smith).

We did, however, get quite a bit of the paddock done.  As usual, I have used Larry Conyers’ GPR Process program and Surfer v.8 to produce the time slices.  I have created five slices 3ns in thickness.  I’ll discuss them in order from the top down.

Fig. 9: time slice 3, 10.5–13.5ns.

The topmost time slice shows the bank parallel to the SW hedge very clearly indeed.  The clarity of this feature makes me think that this bank must be artificially reinforced to create the flat space between it and the hedge. Perhaps this was part of the entrance to the house, or something to make a view from it?  The other feature is the very clear line to the east which matches where the bank curves in the topo data.  I’m unsure what would make a line like this.  It is very close to the surface being in the top slice.

Fig. 10: time slice 4, 13.5–16.5ns.

In the next time slice (Fig. 10), the bank to the SW has turned into two linear features.  The bank to the NW shows as less clear linear features as well.  There are a series of lines running SE-NE across the lower part of the field.  These become clearer in the lower slices.  Finally, in the middle of the NE edge, cut into by the “dog tooth” pattern of the survey edges, is a broader linear feature running NW-SE.  This may be a hint of the town wall.

Fig. 11: time slice 5, 16.5–19.5ns.

In the next slice (Fig. 11), the parallel lines in the lower area are now very clear.  These seem most likely to be some form system of land drains.  To the south are some other linear features which may be the tops of surviving walls.

Fig. 12: time slice 6, 19.5–22.5ns.

In the next slice (Fig. 12) we are getting below the drains, but the signal is starting to attenuate.  There are, however, some linear features to the south and the west which are parts of buildings lying quite low.  A number of the “wiggly” features, particularly to the north, are badger setts.

Fig. 13: time slice 7, 22.5–25.5ns.

The last time slice (Fig. 13) finally shows the buildings that can be seen along the SW edge in the mag data.  The GPR cannot really push into the undergrowth as well as the mag and so the evidence is a little more bitty.  We can see, however, the road which runs from the SW to the NE, almost parallel to SE hedgeline, pretty much where I suggested it should be in Figure 7.

It would appear, therefore, that the archaeology is more deeply buried in most of this field than elsewhere at Verulamium.  Perhaps soil eroding down the slope built-up at the back of the Roman wall?  The wall itself, however, is conspicuously absent.  A massive structure like that should show quite clearly in GPR data.  The one thing that I ought to do is build the topography into the GPR processing, but I cannot do that as yet.  Watch this space!

Earth Resistance survey

Earth resistance came a poor third in the techniques we used on the site.  We managed one block 40m by 60m using the Institute of Archaeology’s RM85 at 50cm intervals.  We have also recently had the Welwyn Archaeological Society’s machine repaired (thanks Bob!), and did just one 20x20m square at 50cm.

Fig 14: Earth Resistance surveys.

After the excellent results from Gorhambury last summer and Durobrivae last October, these are more the sort of thing I expect in Hertfordshire, i.e., nothing very useful!  In this case, I suspect that, based on the GPR data, the interesting archaeology is deeper than the method with a 50cm probe spacing is looking.

Resistance Pseudosections

The West Essex Archaeological Group has kindly lent us their Resistance Pseudosection kit.  The idea of this is that one takes a series of readings in a line with a 1m spacing between the four probes.  One then repeats the operation with a 2m spacing, then again with 3m, 4m, 5m and 6m.  By increasing the spacing between the probes each time one repeats the line, one is looking deeper into the surface.  The data can then be plotted using a program called res2Dinv which creates a “pseudosection”, an image of a vertical slice through the ground.  We decided to try two of these: one across the road and one across the bank where the wall may be.  The idea was that maybe being able to look deeper into the ground we might be able to see something the other techniques missed.

Professional versions of this system use a switching box and are very expensive.  Bob Randall, however, created a useful budget version using his TRCIA resistance meter.  With just a couple of people this can be hard work moving all the probes between pegs, but if you have five people it can be quite quick. It does look, however, quite amusing as four people bob up-and-down changing probes in unison!

Fig. 15: the team undertaking a resistance psuedosection.

Figure 16 shows the location of the two lines we did overlain on the GPR data.

Fig. 16: the location of section A (red) and B (blue).

For both of the sections, I also ran the GPR along the same line so that we would have a direct comparison.  Unfortunately, the free version of res2Dinv will not undertake corrections for topography and will not save images for sections 40 probes (39m) long, although I can get a screen grab and use that, which is good enough for this blog.  The commercial version is extremely expensive.  I hope I may be able to process the data with the topographic correction in the future.

The first thing I did was to compare each section with its companion radargram.  The radargrams have been corrected for topography.

Fig. 17: Pseudosection A and a radargram along the same line.

There isn’t a huge correlation between the two for section A (Fig. 17).  The broad curving layer shown low on the left of the radargram does not have a similar signature in the resistivity data unless the large grey-green area of slightly higher resistivity is reflecting that.  The very high areas of resistivity shown to the right of the section in red similarly do not clearly match anything much in the radargram (although see below about the ‘circular thing’).

Fig. 18: Section B and its radargram.

The second section was remarkably featureless.  There is a large area of lower resistivity to the right which may represent the robbed out wall, but this doesn’t really show in the radargram. The large curved response in the radargram on the left doesn’t really show in the resistivity section.  All rather frustrating.  I decided to put the images onto the Google Earth images to see if that helps.

Fig. 19: Resistivity pseudosection overlain on the mag data. Note that the red line marks the position of the pseudosection and north is to the left.

In Fig. 19 the red line indicates the position of the pseudosection.  The area of medium resistivity on the left shown in a greeny colour matches where the section cuts across the linear area of low magnetism shown in light grey / white.  These are probably, therefore, caused by the same feature which would appear to be something associated with agricultural activity judging by the wider pattern in the mag data.

Fig. 20: The radargram for the same line, marked in red. North is to the left.

Comparison of the radargram to the mag data doesn’t help much.  The large curving feature to the left of the radargram (N), doesn’t really show in the mag data.  The area of low magnetism mentioned above only really correlates with half of that feature, if at all.

Fig. 21: Radargram overlain on the lower time slice (cf. Fig. 13).

Fig. 21 does show, however, that the radargram goes right over one of three clear sub-circular features which can be seen in the bottom time slice (cf. Fig. 13).  This feature shows very clearly in the radargram.  In Fig. 22 I have annotated it radargrams take some use to ‘reading’.

Fig. 22: Radargram showing the position of the subcircular feature.

I don’t know what these three features are, but there are three of them in a line at right-angles to the suggested line of the road.  Very intriguing!

Fig. 23: Pseudosection B on the mag data. The blue line marks the position of the section.

In Fig. 23 the area of slightly higher resistivity to the west matches very well with the bank of lower magnetic responses that run parallel to the bank from NW to SE.  The low resistivity block on the right (east) doesn’t seem to match anything in the mag data.

Fig. 24: Radargram along section line B. The actual line of the GPR transect is the blue line.

Fig. 24 shows the radargram on the mag data.  The curved feature low in the profile on the western side only partially matches the low magnetism band.  I cannot see anything that clearly matches the low resistivity feature seen in the pseudosection.

Conclusions

As I have often said, using more than one technique can be very powerful as the different methods detect different properties of the below-ground stratigraphy, and therefore different aspects of the archaeology.  The downside is the huge increase in complexity in comparing, contrasting and trying to interpret the various results.  This small survey is an excellent example of this,  There is much more that could be done, especially going through the GPR results in more detail and comparing and contrasting them with the magnetometry data.

For now, we can say the following:

  1. There is no clear evidence for the town wall.  There is a good possibility that it has been robbed in the this area as suggested by the fact it only shows in one time slice, and there is a low resistivity feature in the pseudosection in about the position one would expect the wall.
  2. The clearest evidence for buildings is along the SW hedge line.  These show best in the mag data, and parts show in the GPR but difficulties with nettles and burrows prevented the GPR getting closer into the hedge line.
  3. The lower part of the field in the centre mainly shows evidence for agriculture in both the mag and GPR data, including probable land drains.
  4. The deepest time slice does show some evidence of the road and Roman buildings.  If my estimates of the speed of the radar signal are about right, the tops of these remains lie between 90cm and 125cm below the surface.  The extra depth of soil compared to elsewhere may be due to soil creep down the slope building up behind the bank to the NE.
  5. Part of these building remains include three sub-circular features which have strong radar reflections.

Many thanks to everyone who came out to help with the survey.  Many thanks also to Pete and Flora Letanka for allowing us to survey in their paddock and garden, and to Stuart Gray for putting us in touch.  Also, thanks to WEAG for the use of their pseudosection equipment, and SEAHA for the loan of the GPR.  Lastly, thanks to Pat Johnson for bringing us the mag at the start of the survey.

The mystery of Long Shaw

Over the last few weekends members of CAGG have been working with members of the West Essex Archaeological Group on a site called Long Shaw near Theydon Bois (pronounced “boys”, as I discovered).  The site is a scheduled ancient monument and Ralph Potter, a member of both WEAG and CAGG, obtained the necessary S42 licence and permission from the landowner to work on the site.

The site has an interesting back-story.  In 1940, a Hurricane crashed into the field and the pilot was killed.  There is some doubt as to the identity of the pilot, but it is probably John Benzie who was flying with Sir Douglas Bader’s 242 squadron.  There is an article about this story in After the Battle magazine (issue 147, pp. 50–5).  In 1976 a local enthusiast obtained permission from the landowner to try and excavate the crashed plane.  This attracted the attention of the local archaeologists from WEAG and the Passmore Edwards Museum. The farmer couldn’t remember the name of the person undertaking the excavation apart from the fact that “he came from Loughton and drove a Volkswagen Caravette”.  On the 24th October 1976 T.A Betts and F. Clark of WEAG walked the site and reported their findings to Miss Wilkinson.  The Passmore Edwards report form states that they found:

  • 8kg of tegulae and imbrices [i.e., Roman roof tiles]
  • EPRIA [pre-Roman Iron Age] pottery, including one rim
  • mesolithic prismatic core and five flakes
  • Typical R-B [Romano-British] material of local quartz-gritted fabric, mortaria fragments, fine orange wares, one rosette-stamped, two fragments of Niedermendig quern.

The fieldwalkers created a sketch plan of the site and noted one back-filled machine-cut trench. In February 1977 a letter was sent to the Inspectorate of Ancient Monuments and Historic Buildings recommending that the site, thought to be a Roman villa, should be scheduled.  This advice was taken and the site was scheduled effective from the 13th December 1977.  Despite the scheduling, the site was again excavated on September 12th 1981 in an attempt to recover definitive evidence as to the identity of the pilot.

The scheduling was amended in 1994, but I have no information as to the nature of that amendment.  Since then, the site has not been further investigated, until Ralph organised the latest round of fieldwork.

The surface of the field is still littered with masses of Roman roof tiles.

Roman tiles seen on the surface of the field during the survey.

Roman tiles seen on the surface of the field during the survey.

The site lies at the top of a small hill overlooking a valley with quite surprising views for a site in the middle of suburbia.

Ralph Potter (WEAG) using the magnetometer aided by Jim West (CVAHS).

Ralph Potter (WEAG) using the magnetometer aided by Jim West (CVAHS).

The other surprise is the occasional Central Line train passing by the site to the east!

The Central Line

The Central Line

Luckily we were far enough away for the results not to be affected.

On the last Friday of September we started the mag survey and managed seven grid squares during the day.  The next day I was delivering the key-note address at a conference in St Albans and left the mag survey to Ruth (WAS), Jim, Peter (WAS) and Ralph.  Unfortunately, the weather turned foul, but more worryingly, the mag’s intermittent fault with the odometer reappeared. On the Sunday, therefore, we completed six 20x20m grids of Earth Resistance survey. Yesterday, we returned to site with Mike Smith (Wheathampstead History Society) and completed another six resistance grids.  Meanwhile, the mag has been sent for repair.

The mag survey was not very exciting.  The red line in the image below represents the scheduled monument area.

The mag survey results.

The mag survey results.

There is a line of ferrous responses which show in this image as strong black and white spots running diagonally across the survey.  The explanation for these can be seen by looking at the 1945 historical imagery in Google Earth.

The 1945 historical imagery with the scheduled area marked by a red line, and the now-removed fence line marked in blue.

The 1945 historical imagery with the scheduled area marked by a red line, and the now-removed fence line marked in blue.

There is a hedge with trees running across the field in a similar position.  By marking this with a blue line, and then switching back to the standard view with the mag data we can see that they match pretty closely.

Mag results with the old hedge line superimposed.

Mag results with the old hedge line superimposed.

There are, however, a few other anomalies which may be archaeological features which I have marked on the following image.

The mag survey annotated.

The mag survey annotated.

One long linear anomaly (ditch?) and two smaller ones hardly make a villa, but we need to remember that “absence of evidence is not evidence of absence” and it may be that the features simply have no magnetic contrast.

We initially completed an area of 60x40m of earth resistance survey.  Although not a great deal showed, there were hints of something on the northern edge and so yesterday we surveyed a further 60x40m.

Alexandra Lyons of WEAG uses UCL's Geoscan RM85 Resistance Meter.

Alexandra Lyons of WEAG uses UCL’s Geoscan RM85 Resistance Meter.

We continued to use the pole-pole set-up (i.e., having the remote probes 20m away and with a 10m separation) and this nicely saved us from the pain of matching grids between the two days’ worth of survey.

The Earth Resistance survey results.

The Earth Resistance survey results.

The areas of high resistance, shown in black, are probably largely the result of geology.  The lighter, low resistance linear features across them, however, are very curious, especially the one that follows the line of the scheduled area.  Was this the trench dug in 1976? Or the 1981 trench which may have aimed to follow the edge of the scheduled area but be outside of it?  There is one thinner linear feature running SW-NE which might be something archaeological, and it was the bottom part of this that led to the second day’s worth of survey.  It is, however, a bit irregular and not all that convincing.

Here is a composite image showing how the various bits add together.

Mag and resistance surveys with the hedge line.

Mag and resistance surveys with the hedge line.

Peter Alley has been using his UAV to take high level aerial photographs.  Here is one example.

A high level aerial photograph of the site taken using a DJI Phantom quadcopter. (c) Peter Alley.

A high level aerial photograph of the site taken using a DJI Phantom quadcopter. (c) Peter Alley.

Various soil marks can be seen but nothing convincing as archaeological features.  Peter has also been using the images to create contour maps using “Structure from Motion” techniques.  For more on that, come to the Welwyn Archaeological Society’s members evening on the 11th November where Peter will be showing some initial results.

Looking at the historical imagery on Google Earth, one can almost convince oneself there is a rectangular feature showing in the 1999 image, especially if one turns up the contrast in Photoshop.  The only problem is that the southern edge matches the old hedge line.

The 1999 image from Google Earth with the contrast edited in Photoshop.

The 1999 image from Google Earth with the contrast edited in Photoshop.

This site remains a bit of a mystery.  There are large numbers of Roman tile fragments on the surface, but almost no pottery.  We only spotted one tiny sherd of Highgate Wood ware and one sherd of LPRIA grog-tempered pottery.  The mag and res results are not very exciting. Ralph’s GPR is equally unexciting although on these clay soils that is less surprising.  I am torn between extending the surveys in case the actual site lies just outside the area we have completed, and stopping here and concentrating on the many sites where we have had excellent results.  It is a shame that the mag is out of action as a minimum it would be useful to complete the scheduled area.  Ideally, WEAG should organise a pick-up survey so we could at least map where the finds are coming from.

 

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!

A picture is worth a 1000 words

The Verulamium magnetometry survey.

The Verulamium magnetometry survey.

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.

The area surveyed during day 37) high contrast).

The area surveyed during day 37) high contrast).

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.

Low contrast version of the area surveyed on day 37.

Low contrast version of the area surveyed on day 37.

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:

The 2016 resistance survey.

The 2016 resistance survey.

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…!

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.

Three day catch-up

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.

The mag survey in the Macellum field after day 35.

The mag survey in the Macellum field after day 35.

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 complete survey so far.

The complete survey so far.

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.

The resistance survey after day 35.

The resistance survey after day 35.

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.

The mag survey near the Chester Gate.

The mag survey 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.

Day 35 GPR, slice 2 (9.5 to 12.5ns).

Day 35 GPR, slice 2 (9.5 to 12.5ns).

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.

Day 35 GPR, slice 3 (12.5 to 15.5ns).

Day 35 GPR, slice 3 (12.5 to 15.5ns).

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.

Day 35 GPR, slice 4 (15.5 to 18.5ns).

Day 35 GPR, slice 4 (15.5 to 18.5ns).

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?

Day 35 GPR, slice 5 (18.5 to 21.5ns).

Day 35 GPR, slice 5 (18.5 to 21.5ns).

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.