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Peter Alley using the mag cart at Willington with the dovecot in the background.

A weekend at Willington

Firstly, apologies to everyone for the delays in posting. The day job got in the way, but I am hoping to catch-up with several posts over the next week or so.

In late April, we spent a pleasant weekend surveying at Willington, just south of Bedford, for the local group that have been investigating the medieval complex, of which the dovecote and stable block survive.  Both listed monuments and are looked-after by the National Trust. We had to obtain a Section 42 licence from Historic England to undertake the work.  The site has been surveyed once before: an earth resistance (“resistivity”) survey was undertaken in 1997.

Peter Alley using the mag cart at Willington with the dovecot in the background.

Fig. 1: Peter Alley using the mag cart at Willington with the dovecot in the background.

The area available for survey was not very large, a mere seven of our 40x40m grids covered the area we needed to investigate and we completed the mag survey in a day.  Historic sites are, however, often poor candidates for magnetometry due to the large quantities of ferrous material, such as nails, that get left behind.  We decided, therefore, to undertake a GPR survey as well.  Many thanks, once more, to SEAHA for lending us their Mala system.  The GPR survey took us two days to complete a slightly smaller area.  We also undertook some survey inside the buildings.  That will be the subject of another post.

Results of the magnetometry survey at Willington.

Fig.2: Results of the magnetometry survey at Willington.

The first impression of the survey, shown above, is how “noisy” it is.  The large quantities of ferrous metal fragments, and possibly even relatively magnetic bricks, along with two roads and some services, make for a very messy result.

Interpretation of some of the main features showing in the survey.

Fig. 3: Interpretation of some of the main features showing in the survey.

The northern road marked on the image above is the modern drive to the buildings to the SW. The older road which curves around the stables dates back at least as far as the Ordnance Survey maps in the 1880s, and was still in use in the 1970s as it is marked on the maps of that date.  The one feature our survey shows, which does not appear on the early OS maps from the 1880s and 1900s, is that the older road forks to the SW.  This split, however, shows clearly on the Google Earth image, which dates from 2009, shown below.

Google Earth image showing the fork in the old road as a parch mark.

Fig. 4: Google Earth image showing the fork in the old road as a parch mark.

This extra road does, however, show on the maps of the 1920s and must have been built sometime in the early 20th century.

As the survey progressed I only surveyed in one of the manhole covers, which is marked in Fig. 3.  One can see how clearly these show in magnetic data.  The large and very noisy area represents a pond which is shown on many historic maps including the OS maps from the 1920s, but has been filled-in by the 1970s map.  A more thorough investigation of the map evidence that I was able to do online might tie down these dates more closely.

In general, it is hard to see any details of the buildings we know existed from the 1779 estate map.  I have re-drawn it for comparison here (Fig. 5).

Fig 5: The 1779 Estate map redrawn.

Fig 5: The 1779 Estate map redrawn.

The GPR survey took two days and we covered two 74m long strips.

Fig. 6: the GPR starting the survey near the stable block.

Fig. 6: the GPR starting the survey near the stable block.

The GPR results are affected by changes in ground and surface water.  It can be quite difficult to get surveys split over several days to match, and this is the case here.  I have created time-slices 4 ns thick.  It would be possible to spend quite some time adjusting the slices and the contrast to make the edges match more cleanly than is shown here.

GPR time slice 1: 8 to 12 ns.

Fig 7: GPR time slice 1: 8 to 12 ns.

The topmost time slice (Fig. 7), merely shows the two road surfaces, the modern one to the north and the older one to the south.

GPR time slice 2: 12 to 16 ns.

Fig. 8: GPR time slice 2: 12 to 16 ns.

The second slice (Fig. 8) shows the southernmost road more clearly.  The lighter band to the SE is partly where we stopped for lunch:  as the sun dries the upper layers we can see this in the GPR results.  But this doesn’t explain why there is a change back.  I doubt this is archaeological as it follows the line of the GPR transects.  We are, however, probably still quite close to the surface at this point.

GPR time slice 3: 16 to 20 ns.

Fig. 9: GPR time slice 3: 16 to 20 ns.

The third time slice (Fig. 9) is starting to get down a bit as we are beginning to see through the road. The area to the east of the stable block is still remarkably noisy.  If we compare this image with the 1779 map (Fig. 5) we can see that “noisy” parts of the time slices are roughly where there are buildings, and the quiet parts of the slices are where we have empty spaces on the map. Sadly, however, we cannot really see any buildings clearly in the GPR data.  There is a broad stronger response (shown in dark tones) heading towards the entrance of the stables (may be a track?) and a linear feature running away from that at roughly a right angle which is much narrower.  Maybe this is an earlier wall?

GPR time slice 4: 20 to 24 ns.

Fig. 10: GPR time slice 4: 20 to 24 ns.

The fourth timeslice (Fig. 10) shows a little bit more towards the northern corner of the survey.  The thin line is almost certainly a modern service.  There is, however, a broader linear feature running SW towards the stables.  Perhaps this is another track?

GPR time slice 5: 24 to 28 ns.

Fig. 11: GPR time slice 5: 24 to 28 ns.

The last time slice (Fig. 11) shows a scatter of strong reflections but generally very little.  The signal has “attenuated”, and we are getting little sensible information.  Some parts of the road is still showing as an “echo” of the upper features.  One useful thing, however, is the strong linear feature heading towards the southern corner of the stables.  This is probably a modern service but it does show clearly on the radargram (Fig. 12).

Screen grab of one radargram in RadExplorer.  the strong reflection in Fig. 11 is indicated by the red arrow.

Fig. 12: Screen grab of one radargram in RadExplorer. the strong reflection in Fig. 11 is indicated by the red arrow.

Why do we care?  This strong reflection can be used for hyperbola matching, i.e., drawing a curve on-screen which matches that in the data indicated by the red arrow in Fig. 12.  This process provides a speed for the radar waves through the soil at this site.  I tried several spots and radargrams, and a speed of about 8.5 to 9cm per ns seems about right.  Using RadExplorer I can then see the strong reflector (?pipe) is about 60cm deep.  Assuming the top of our top slice is about ground level, each of our time slices is about 12cm deep.

A maximum depth of only about 60cm is not all that much for GPR, and we must presume that the soils are not that conducive to this sort of survey.  Having said that, I would be surprised if the top levels of the archaeology were any deeper than 60cm.

Unfortunately, the surveys have not provided the impressive and exciting results that we have obtained at some sites which is a shame.  Both the magnetometry and the GPR surveys show noisy areas which match well with the areas of buildings shown in the 1779 Estate Map, but we do not have any clear indications of surviving building remains, although there are a couple of possible track ways.


Looking forward to 2016

Firstly, I’d like to wish all our readers, and especially all our volunteers, a very happy New Year and all the best for 2016.

This past year has been very productive with our big survey at Verulamium, but also other surveys around the region.  We have had an article published in issue 310 of Current Archaeologyand another article awaiting publication in the journal Archaeological Prospection,  as well as a short piece in the International Society for Archaeological Prospections newsletter.  I have given lectures about our work to the St Albans and Hertfordshire Architectural and Archaeological Society, the Römisch-Germanische Kommission in Frankfurt, the Norton Community Archaeology Group and to the Chess Valley Archaeological and Historical Society.  Several more bookings are in the pipeline.

One excellent addition to our work since August has been access to SEAHA’s Ground Penetrating Radar (GPR).  This has been an excellent addition to the range of techniques we can regularly employ on our surveys providing some superb results at Verulamium and Bushey Hall.  Hopefully, we can continue to access this machine in 2016, and perhaps even raise funds to purchase our own.

What does 2016 hold in store for us?  There are a number of sites which we are hoping to survey:


We are hoping to continue the survey of the Gorhambury estate side of the town in August.  We should be able to complete the mag survey of the Theatre Field and begin on the field to the north of the drive.  GPR surveys, perhaps resistance cart surveys, and even a magnetic susceptibility survey are all desirable as well providing we can access the equipment.

Willington (Bedfordshire)

We have been asked if we would be willing to undertake some surveys near the dovecot and stables at Willington, Bedfordshire.  The stables and the the dovecot are looked after by the National Trust and are listed buildings.  The area around them has been part of a community-based project for a few years looking for the Tudor manorial complex which goes with the standing buildings.

The dovecot at Willington.

The dovecot at Willington.

The stables at Willington.

The stables at Willington.

Hogshaw (Buckinghamshire)

Following our two successful days last February, it would be good to go back and finish the bits of the survey area we were unable to do last time, and perhaps try the GPR out over some of the areas where walls had been detected previously.

Surveying at Hogshaw.

Surveying at Hogshaw.


We have been asked if we would like to work with the Wing Heritage Group to do some surveys around this very important village with some great Anglo-Saxon archaeology.  It seems an exciting chance to try our equipment out on a different period of archaeology, and to work with a very active local group.


The Berkhamsted and District Archaeological Society have been working on the site at the Ashridge Management College for a few years.  They have asked if we would be willing to undertake surveys in the gardens.

Little Hadham

We undertook a survey at Little Hadham on a late Iron Age and Roman site a couple of years back.  The site merits more work to extend the survey.

Ashwell End

The site at Ashwell is like the road that goes for ever on.  It would be good to complete Great Buttway and move into nearby fields.

These are just some possibilities!  Hopefully I have remembered the main sites…  From a personal note I’d like to do some more work at Broom Hall Farm, and in the eastern half of the county more generally.  We seem to have been slowly drifting west!

Another aspect of our work also needs addressing, and that is writing-up the reports.  We need to develop ways to work on mapping the results of our surveys collaboratively, and to investigate archive materials, especially at Verulamium.  Help with drafting reports, again especially at Verulamium, is going to be needed.  All this makes me think that we probably need to put in for some more grant money to allow us to expand our equipment and software, and to develop ways of sharing the post-fieldwork analysis and reporting.

Last, but not least, up to now CAGG has only existed as an informal network based around our mailing list and the blog.  Should we be working towards creating a more formal group? Should planning and prioritizing surveys be placed in the hands of a steering committee? How to we maintain our identity of a cross-arch soc group, rather than being just another arch soc? Ideas and thoughts on a postcard to Kris!

I would like to thank everyone involved with our work.  Without you, there is no group and there are no surveys.  It has been a privilege to work with you all.

Here is to another successful year. May our anomalies be clear, our batteries charged and enthusiasm unwaning!


GPR data!

Just a quick posting to say I have managed to get something from the GPR data! The attached image is from today’s block and consists of three time slices.  The top image just shows the plough marks and some fairy rings, but the middle and bottom slices clearly show the buildings we picked up in the mag survey.  These images could be greatly improved with more processing, but at least we can see the effort is worth it.

GPR slices from day 5 at Verulamium/Gorhambury.

GPR slices from day 5 at Verulamium/Gorhambury.


Today we ran the mag and the GPR on a warm, if not very sunny, day.  All went well, and we completed an area 80m by 160m with the mag, and 80m by 40m with the GPR.  All in all a good day’s work.  Many thanks to everyone who came out and helped.  The GPR results will take a little longer to process, but the mag data is looking great.

The mag survey after day 3.

The mag survey after day 3.

The temple in Insula XVI shows in the mag data quite nicely, if not very strongly.  We hope that the GPR and resistance surveys may show more details.  The line of buildings which runs SW to NE that we started to see on day 1 is showing very nicely.  Surely they must lie along a road?  Other buildings are also showing, and I have marked just one of them.  This is Monument 744 in the “Urban Archaeological Database”.  Just to the north is a long sinuous ditch.  We have about 65m of it so far.  The UAD only has 31m of it, recorded as Monument 740.  Unfortunately, both these monuments post-date the publication of Niblett and Thompson’s Alban’s Buried Towns so I am trying to find out some more information.

The weather forecast for tomorrow (Thursday) is looking grim, so we may not go out.  If so, I’ll work on the GPR data!  So far, however, the results are looking great.

News Update

End of term madness as well as conferences in Basel and Reading have stopped me from working on the project for a few weeks. Yesterday I spoke to the Roman Archaeology Conference in Reading and there is great interest in our results.

A couple of upcoming events.  On Thursday evening (3rd April 2014) I will be delivering a lecture about the project as part of Verulamium Museum‘s Thursday evening lecture series.  It starts at 7.30pm at the Museum, all welcome.  Then on Saturday 5th April I will be giving the Gordon Moodey Lecture after the AGM of the East Herts Archaeological Society which is to be held in Great Amwell Parish Hall, 2.30pm.

We plan to start surveying again very soon and we will be emailing the trimmed down mailing list about that soon.  Meanwhile, a few photographs to remind you all of the joys of working in Verulamium Park in the wettest winter on record.

Ralph Potter (WEAG) teaching UCL students how to use the GPR.

Ralph Potter (WEAG) teaching UCL students how to use the GPR. Note the mud on the wheels!

Jim West (CVAS) negotiating the trees and undergrowth near the southern edge of the site.

Jim West (CVAS) negotiating the trees and undergrowth near the southern edge of the site.

Rainbow over the Park.

Rainbow over the Park.

Half way mark

As July draws to a close we have reached the half-way mark of the project.  We have bought the machine and run the course.  Even if we never surveyed another block in the Park, we have added significantly to what we know about Verulamium.  I now know more about the site at Six Acres and am looking forward to finishing the survey when the crop comes off.  The first site of several near Braughing has been completed.  Plans for surveying at Ashwell are advancing well, and we have agreement in principal for Wheathampstead.  Although the next couple of months are going to be extremely busy, I think we can be pleased with what what have achieved so far.

All this makes me think of the future.  What happens after the project has ended?  We are developing into a good group, and it would be a shame to lose that, even if people can still borrow the machine.  How can we collaborate?  Can we sustain the momentum we have built up?  Obviously there are many projects we could pursue: the rest of Verulamium, the landscape around Wheathampstead, the plethora of sites near Braughing, to name but three.  Having thought about this a fair bit lately, I wonder if a new group would be the best way to go? How about the Community Archaeology Geophysics Group or CAGG for short?  We could keep using this website, and we already having a mailing list.  Should we be just a virtual group, or should be have a formal membership? We can collaborate on surveys, or design and implement larger survey projects, we could share expertise and equipment, and perhaps meet now and again for mini-conferences looking at results etc.

Is this a good idea?  Is anyone prepared to run it at first? Would you all join? Let me know what you think.