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.

Batford Mill

I have a number of small surveys which remain unreported that I need to catch-up on, and so here is the first of them.

Earlier in the year, Mike Smith and I assisted Alex Thomas (University of Bristol) in undertaking a Ground Penetrating Radar survey of land lying to the north of the B653 at Batford, Harpenden, Hertfordshire (TL 148150, Fig. 1). The survey was undertaken over the weekend of 2nd/3rd April 2016. Earth resistance and magnetometry surveys had been undertaken in the area previously.

Fig. 1: Location of the site at Batford.

Fig. 1: Location of the site at Batford.

The underlying geology of the site is Lewes Nodular Chalk formation overlain in places by the Kesgrave Catchment Subgroup sand and gravel.

A Mala GPR with a 450mhz antenna was used, identical to the one CAGG borrow from SEAHA. The survey transects were at a 0.5m spacing collected in a zig-zag fashion. The survey started in the NE corner and proceeded east-west. Radar pulses were set for 0.05m intervals with a time-window of 73ns. The newer Mala systems do not allow manual selection of sample numbers which are determined by the machine, in this case 376 samples per trace.

For the amplitude slices presented here, as usual, the software system developed by Jeff Lucius and Larry Conyers was used (http://www.gpr-archaeology.com/software/). This necessitates the conversion of the Mala rd3 files into GSSI dzt files using the companion conversion program.

For this posting, the slices were 3ns in thickness starting at 3.5ns  From these, it appears that the second slice, 6.5–9.5ns represents the immediate ground surface. This agrees with the estimate of the first reflection from the individual radargrams at about 8ns as examined using RadExplorer. Beyond slice 7 (>24.5ns) the signal has completely attenuated. This means that all the usable returns lie in the band between c.6.5 and 24.5ns. This is not unusual for Hertfordshire where the clay soils do not allow for the GPR surveys to penetrate particularly deeply.

As with most of the GPR surveys reported in this blog, the numerical output from that software was turned into images using Surfer v.8. Kriging was used to interpolate the values into a 0.1 x 0.1m grid. The resulting images where then imported into Google Earth.

Six amplitude maps or `time slice’ maps were produced and are shown in Figures 2–7. The topmost map (Fig. 2) shows two strong reflections to the north-east and the south. The second map (Fig. 3), which represents the 9.5–12.5ns range, has the clearest set of features. A number of long linear features are visible, two of which I have labelled A and B. There is a odd-looking curved linear feature with two parallel lines, labelled C, into which a pair of parallel lines cuts, labelled D. Further fainter linear features can be seen, such as those at E.

slice-2

Fig. 2: Time slice 2 (6.5–9.5ns).

Time slice 3 (9.5–12.5ns).

Fig. 3: Time slice 3 (9.5–12.5ns).

The third map (Fig. 4) has fewer clear features, most of which are probably `echoes’ of the features seen in the previous map. The next three maps (Figs. 5–7) have successively fewer features in them, none of which are especially clear. By the last map, the GPR signal has largely attenuated and little can be seen. At best, we are getting a depth penetration of about a meter, probably somewhat less.

Fig. 4: Time slice 4 (12.5–15.5ns).

Fig. 4: Time slice 4 (12.5–15.5ns).

Fig. 5: Time slice 5 (15.5–18.5ns).

Fig. 5: Time slice 5 (15.5–18.5ns).

Fig. 6: Time slice 6 (18.5–21.5ns).

Fig. 6: Time slice 6 (18.5–21.5ns).

Fig. 7: Time slice 7 (21.5–24.5ns).

Fig. 7: Time slice 7 (21.5–24.5ns).

The question arises, therefore, as to what the long linear features may be. If Fig. 3A is a wall, it would be at least 35m long, and Fig. 3B would be at least 55m long. One possibility is that they represent old field boundaries. Looking at the 1898 OS map (Fig. 8), there is nothing to suggest an origin for those features. The 1799, map now in the Westminster Abbey Muniments Room does, however, show a field boundary behind some buildings to the north of the road. A crude overlay of an extract of this map (Fig. 9) on the Google Earth image with the GPR data, shows a remarkably good correlation between the field boundary and the one of the linear features (Fig. 3A).

Fig 8: The survey overlain on the 1898 OS map.

Fig 8: The survey overlain on the 1898 OS map.

Fig. 9: The 1799 Westminster Abbey map overlain on the Google Earth image.  Westminster Abbey map used with permission.

Fig. 9: The 1799 Westminster Abbey map overlain on the Google Earth image.

The origins of the curved and parallel linear features can be seen if one takes into account the location of the machine-dug test trench marked in Figure 10.  These parallel lines, only some 1.8m apart, represent areas of soil compression from the machine used in the excavation of the test trench.  Examination of one of the radargrams (Fig. 11) would seem to confirm this.  The origin of the reflections, marked with blue arrows, occurs at the very surface and is highly suggestive of compression rather than construction.

Fig. 10. Slice 2 showing the location of the machine dug test hole.

Fig. 10. Slice 2 showing the location of the machine dug test hole.

Fig. 11: Radargram showing areas of surface compression.

Fig. 11: Radargram showing areas of surface compression.

The survey results appear to be largely connected to (a) earlier agricultural use of the land in the form of hedgerows and so forth or (b) the recent impact of the excavation of the test trench.  It appears highly unlikely the GPR results indicate any sort of structure although the golden rule of ‘absence of evidence is not evidence of absence’ must be applied.  The lack of pottery or ceramic building materials on the surface makes it unlikely that a building is indicated.

Perhaps I should have saved a more exciting post for #100!

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.