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:

Verulamium

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.

Wing

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.

Ashridge

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!

 

Cholesbury, Buckinghamshire

At the request of the Buckinghamshire Archaeological Society‘s Active Archaeology Group, members of CAGG, the Chess Valley Archaeological and Historical Society and the BASAAG got together at Cholesbury to survey a field close to the hillfort.

Cholesbury Camp is one of the hillforts that is being looked at as part of the Chilterns Hillforts Project.  It is a scheduled ancient monument and therefore has legal protection requiring a licence from Historic England to undertake a survey within the protected area.  The field we worked in, however, lies just outside the hillfort and is of potential interest.  The historic map data I have been able to access, just shows this as an empty field.  Over many years, however, marks in the grass, dew and frost had led the owners, Bob and Mary, to wonder if there was anything under the surface. They kindly allowed us to come and survey for the day, and between us we undertook magnetometry, resistance and ground penetrating radar surveys.

As the field was not nicely aligned on the Ordnance Survey grid, I had to set-up a “floating grid” using the dGPS.  Unfortunately, the guide for how to do this provided by Leica is terrible and it took a while to work out what to do.

Putting in the grid. Image (c) Mike Smith.

Putting in the grid. Image (c) Mike Smith.

The mag survey went quickly once we had got started.  The field was only four grids in size, and only one of those was a whole 40x40m.  By early afternoon the job was done.

Pauline Hey (CAGG, BASAAG) operating the magnetometer.

Pauline Hey (CAGG, BASAAG) operating the magnetometer.

The resistance survey was undertaken by John Gover and members of CVAHS.  They completed a number of 20x20m grid squares at 1m intervals, although at times we were tripping over each others lines and cables!

Members of CVAHS operate a TRCIA resistance meter.

Members of CVAHS operate a TRCIA resistance meter.

The Ground Penetrating Radar on loan from SEAHA was operated by members of CAGG and completed an area 76m by 40m.  We had the option of a second day if the results warranted it.

Jean Bluck (CAGG/BASAAG) operating the Mala GPR.

Jean Bluck (CAGG/BASAAG) operating the Mala GPR.

The grass was short enough to not be too much of a problem.  Unfortunately, the moles were less forgiving…

Jim West (CAGG/CVAHS) pushes the GPR over some unforgiving mole hills.

Jim West (CAGG/CVAHS) pushes the GPR over some unforgiving mole hills.

Muddy wheels from the mole hills.

Muddy wheels from the mole hills.

The location of the survey can be seen in the next image.  The oval of trees follows the line of the ramparts of the hillfort.

The location of the field next to Cholesbury camp.

The location of the field next to Cholesbury camp.

Undertaking magnetometry surveys in relatively small fields is often less successful than one would hope.  The metal associated with fences, along with other ferrous material can lead to quite “noisy” surveys.  The image below shows the magnetometry plot overlain on the Google Earth image.  As can be seen, all around the edges of the area surveyed are strongly magnetic features shown in black or white, associated with the fences, gates and so forth.

Results of the magnetometry survey.

Results of the magnetometry survey. The mag plot has been clipped to +/- 4nT, with black as the positive readings, white negative.

There are, however, a few potential archaeological features.  There are a number of irregular areas of high magnetism, especially towards the north (see labelled plot below), which might just be the remains of bonfires. Excavations by Day Kimball within the fort in 1932 revealed some industrial features including some hearths and some bloomery slag from iron working. These features have readings in the range of -4 to +12nT, which does not seem strong enough for iron working, but may be from other processes involving fire.  One clear linear feature can be seen, with ranges from -3 to +5nT, which is also visible on the GPR time slices, along with a couple of fainter and less convincing linear features.  These are all labelled on the next image.

The principal features in the mag survey.

The principal features in the mag survey.

The resistance survey was initially downloaded into Snuffler and then the data exported as text files, which were in turn imported to TerraSurveyor. The grids were range matched to make the edges merge nicely, despiked to get rid of odd data, interpolated, smoothed and clipped.  The resultant image shows a few possible features but may, as is often the case in our region, reflect the underlying geology as much as anything.

The resistance survey. The image has been clipped to 16-32 ohms, black is high.

The resistance survey. The image has been clipped to 16-32 ohms, black is high.

There are a few possible features, including two curved / circular ones and a possible platform.  In the raw data, these look a little less convincing.  The “circular feature” (see below) is very close to the end of a linear feature seen in the GPR data.  It would be helpful to extend the survey to pick up more of these features.  The most convincing feature is the large low resistance area (i.e., wet like ditch fill) with a high resistance area running along the edge (i.e., dry like a bank).  This lines up nicely with an enigmatic spur which extends from the ditch and bank circuit of the hillfort on its western side.  We may have found a bit more of the fort’s earthworks.

The resistance survey with labels.

The resistance survey with labels.

The GPR data was processed using Jeff Lucius and Larry Conyers’ free software.  Once converted from Mala format to GSSI format, time slices were in 4ns bands from 6–10ns, 10–14ns and so on.  The top five slices showed features.  Below that the GPR signal was attenuated and nothing can be seen.

Top GPR time slice, 6-10ns.

Top GPR time slice, 6–10ns.

The top time slice, which is essentially the topsoil, shows hints of what is coming below but is largely dominated by two areas of strong responses along the NE edge of the grid.  Yup, its them moles again…

Second GPR time slice, 10--14ns.

Second GPR time slice, 10–14ns.

The second time slice starts to show a regular grid work of lines.  Some are especially strong, e.g., the one that runs roughly parallel to the SE edge of the plot.  That one also shows in the mag data, unlike the others.  The lines, however, do not seem to easily resolve into buildings.

Third GPR time slice (14--18ns).

Third GPR time slice (14–18ns).

The third time slice shows how shallow many of these linear features are. The one to the north of the second time slice is not really visible, although the one to the south that shows in the mag data persists quite strongly.

The forth GPR time slice (18--22ns).

The fourth GPR time slice (18–22ns).

By the fourth time slice, the plot is largely noise with the occasional  feature showing, the most obvious one being the linear feature mentioned above.

The fifth time slice (22--26ns).

The fifth time slice (22–26ns).

The last slice I bothered plotting shows the GPR signal almost completely attenuated and only a few strong features remaining.

What are we to make of this?  The linear feature that shows on the mag and the GPR surveys must be something both magnetic and reflective.  It could be a brick wall, or perhaps a land drain filled with brick rubble?  The remaining regular grid-work of lines doesn’t really resolve into clear building outlines.  My best guess (and it is a guess!), is that these represent field drains leading down towards the ditch of the hillfort.

Sadly, nothing in the surveys really leaps out at one as “wow, we found…” There are tantalizing hints and some possibilities, but in general it is a little disappointing.  It is great to see the strength of using multiple techniques, however, and how the combined data sets help with the interpretation.

Many thanks to everyone who came out and helped, to Peter Marsden for organizing it, and especially to Bob and Mary for not only letting us play in their field, but also for supplying tea, cake, and somewhere dry to sit!

Insane?

The sun was shining, we have a GPR and a Park to play in, so why not? Mike, Peter and I headed out to Verulamium Park to do a little more GPR, although with a difference. We used a 25cm spacing between lines instead of 50cm. We managed a 40m x 60m block.  Without taking into account turning around, or getting to the spot and back again, that is 9.6km of walking, or just under six miles in old money. The results were great, however.  Later, I will do a posting comparing the 0.5m spacing with 0.25m to see if the extra 4.8km was worth it.

First things first.  Where in the Park were we?  I am especially interested in Insula XXIV simply because the Wheeler’s did not dig anything there and we have very good mag results.

Location of the mag grids in the northern half of Insula XXIV.

Location of the mag grids in the northern half of Insula XXIV.

Here is a more detailed view of the mag results.

Detail of the magnetometry results from the northern half of Insula XXIV.

Detail of the magnetometry results from the northern half of Insula XXIV.

The stark black-and-white feature in the NE corner is a practice cricket wicket made of concrete and green carpeting. It overlies the Roman road.  The diagonal feature in the NW corner is the other Roman road, and at least two buildings show clearly as white lines against a dark background.  The western 40m square was surveyed by Ralph Potter at 0.5m intervals.  The middle square and the western 20m of the eastern square were surveyed yesterday.

I used Larry Conyer’s program to slice the data we collected.  The slices start at 8ns and go down in 4ns thick pieces.  Ground surface in the Mala radargrams is usually at about 7ns and so the first slice is very much the top surface of the field.

Slice 1: 8 to 12ns.

Slice 1: 8 to 12ns.

The cricket wicket can be very clearly seen in the top-right corner.  The change across the plot at about 27m is where we went for lunch.  But what on Earth is the big circle?  Here is a clue…

GPR in the Park.

GPR in the Park.

Yup, it is the centre circle of the soccer pitch.  The second slice reveals the striping we have often seen in the Park.

Slice 2: 12 to 16ns.

Slice 2: 12 to 16ns.

I am guessing these are cultivation marks before the land was turned into the Park in the 1930s.  How old they are I am unsure, but could they be the residue of cultivation ‘strips’ as seen at Ashwell?

In the third slice, we begin to see the Roman archaeology.

Slice 3: 16 to 20ns.

Slice 3: 16 to 20ns.

Notice how there is a white line running through the lower building cutting the black lines representing internal walls.  This is a wall which has been partly robbed.  We also appear to have some surviving floors as shown by the larger black areas in the building.

Slice 4: 20 to 24ns.

Slice 4: 20 to 24ns.

In the fourth slice down we can now see the road running across the top-right hand corner in a similar, but not identical position to the cricket wicket.  I wonder if we might be seeing evidence of a hypocaust in the middle of the lower building?

Slice 5: 24 to 28ns.

Slice 5: 24 to 28ns.

In slice five we can see the outer walls of the building are much fainter suggesting shallower foundations, but the main wall which was only a light line previous is now showing more clearly.  This is presumably what is left at the bottom of the robbed wall.  There is also a building along the road which was not visible in the higher time slices.

Slice 6: 28 to 32ns.

Slice 6: 28 to 32ns.

Slice six is very black-and-white.  This is because the radar signal is now very weak and the slices are either showing something or nothing.

All in all, a very nice set of results.  As always, these are my rather rough-and-ready time slices.  With some experimentation and practice they could be better.

Here are Ralph’s grid square (processed by Mike Langton of Mala) and our new one roughly plotted together in Google Earth.

Ralph's and our blocks of GPR data plotted together using Google Earth.

Ralph’s and our blocks of GPR data plotted together using Google Earth.

I think it would be worth surveying the whole of Insula XXIV if we can!

 

Processing mag data (Part 1)

I thought it would be useful to outline how I have processed the magnetometry data from our surveys.

The first stage is to download the data from the Foerster datalogger.  I use their simple and free program IFR Dataload to do this.  Foerster sell a more complex program for processing data from the Ferex, but I find TerraSurveyor easier to use, as well as having the convenience of being able to process data from the resistance meters and other magnetometers.  When I download the files, you can give them a suffix, and they are numbered in sequence.  Usually I use some like “gorday4_” standing for Gorhambury survey, day four. I then end up with a sequence of files: gorday4_1.fdl, gorday4_2.fdl and so on.  FDL is Foerster’s own file format. You can look at each square in Dataload.

Screen grab from Dataload (best seen full size).

Screen grab from Dataload (best seen full size).

Although one can play around with the image at this point, there is no reason to.  If the square is a partial, it is worth making a note of its dimensions.  Each grid square is then exported as a “Text table” which is a fairly simple ASCII file.  I just number each text file sequentially, e.g., gor001.txt, gor002.txt and so on.  If I know that two or more text files are eventually going to be combined into one grid in TerraSurveyor, e.g., when we have surveyed a partial grid square in bits, I use gor003a.txt, gor003b.txt and so on.  You can see the importance of keeping good field notes or processing the data very soon after the day’s survey.

The next stage is to import the text files into TerraSurveyor.  If it is a new site, we need to create it.

Creating a new site in TerraSurveyor.

Creating a new site in TerraSurveyor.

Having created the new site, we have to import the Foerster text files.  First click on the import button.

The download button is TS.

The download button is TS.

This will make the import window pop-up. A Foerster template is not currently part of the default installation, you’ll need to get it from David Wilbourn or myself. Once you have it, just select it from the list.

Selecting the Foester text table import template.

Selecting the Foerster text table import template.

In the next screen, you need to navigate to where you saved the text files exported from dataload and then choose which ones you want to import.  On the first day it will be all of them, but subsequently just the grids since the last time you processed the data.  After that, just keep clicking next and accept all the defaults until the data is imported. The TS grid files will take the name of the text files, so gor001.txt becomes gor001.xgd.

Choosing text files to import.

Choosing text files to import.

The next stage is to assemble the grid into a composite.  Either click on “Assemble grids” to create a new composite, or select the existing one and click on “Open Grid Assembly” (all in the navigation window).

Grid assembly (part 1).

Grid assembly (part 1).

In the Grid Assembly window you can see thumbnails of the individual grids.  These can be drag-and-dropped into the grid in the correct pattern.  The direction of first traverse is always from left to right, so if we are surveying south-to-north on the first line of the grid, north will be to the right.  To make the grid bigger, use the blue arrows on the right.

Grid assembly (part 2).

Grid assembly (part 2).

Having completed adding the grids, click on Save or Save As.

Grid assembly (part 3).

Grid assembly (part 3).

At first, the composite just looks uniform gray.

First look at a new composite.

First look at a new composite.

This is because the extreme values from pieces of iron in the ground are being plotted as black and white.  The majority of the values which are much closer to zero are squeezed into a small number of mid-grays.  To see the pattern in the majority of the data, we need to clip the display.

Clipping the display values.

Clipping the display values.

The clip button on the processes toolbar on the left is indicated above.  You can see from the graph in the pop-up window how the values are compressed.  I have entered values of -9 and 9 for the clipping.  Depending on the site, you may need to clip down to as far as +/- 1.5nT.

In the image above I have cheated a bit as the composite has already been clipped.  You can see the archaeology, but the image is very stripy.  This is caused by the sensors being imperfectly compensated, by walking in zig-zags, and by sensor drift.  It is perfectly normal in mag data and can be removed using the destripe command in TS.  Most processing packages call this zero mean traverse as the process alters each traverse (a line of data within a grid square) so that its mean is zero.  TS offers ZMT, but also zero median traverse which is often more robust.  TS, therefore, bundles all the options in the destripe window.

Zero median traverse.

Zero median traverse.

I have indicated the position of the destripe button in the above image.  Often, just accepting the default values is fine but sometimes large numbers of large values can mess this up, and so I set absolute values for the process.  In this case I have used +/- 10nT.

Destriped and clipped.

Destriped and clipped.

The resulting image is rather flat.  This is because we have performed the processes in the wrong order.  We should only clip the final image, not the data on which other processes are working.  I did this in the wrong order so that you can see what was happening at each stage, and also to demonstrate the Modify command.  TS, very sensibly, does not alter the base values.  This allows us to edit the processes via the Modify command indicated above.

Modifying and re-ordering processes.

Modifying and re-ordering processes.

In this case we just want to move the destriping command down to below the clipping command.  Clipping should always be the last command.

Interpolating data values.

Interpolating data values.

The resulting image is pretty good and for quite a while that is all I would do.  Jarrod, however, persuaded that better images could be obtained by interpolation and smoothing.

Interpolating data values.

Interpolating data values.

The first step is to interpolate some new values.  The Foerster collects readings 10cm apart along the traverse, and the traverses are 50cm apart.  This is a rather unbalanced grid.  The traverses are the y-axis in TS and so by selecting the Interpolate button (shown above) and choosing to double the values on the y-axis, we create a grid which is now 25cm by 10cm.

Applying a low pass filter.

Applying a low pass filter.

The next stage is to smooth the data a little. Obviously, we don’t want to smooth the data so much we get rid of the archaeology! We use a low pass filter.  Click on the button indicated above and select low pass in the window.  I use values of x=7 and y=3.  This is because x=7 is 70cm and y=3 is now 75cm, i.e., close to the ideal of a circular “window” for the filter.

The resulting image will look rubbish.  This is because the processes are again in the wrong order.  Using the modify command, the processes should be in the following order (from bottom to top!)

  1. Destripe
  2. Interpolate
  3. Low pass filter
  4. Clip

I have done things a bit backwards so that you can see the effect of each stage.  Normally, I would just do everything in the right order from the start.  If you are adding new squares to an existing composite, the processes will be automatically applied when you save it at the grid assembly stage.  Normally one would only do all this once a survey.

In the next posting I will explain how I deal with partial grid squares.

Apologies to anyone who looked at this soon after I posted it as something went wrong and all the work I did this morning vanished.  I had to re-write half of it again!

Bank Holiday Monday

The August Bank Holiday lived up to its reputation.  It rained, pretty much all day.  According to our rain gauge we had another centimeter and the roads were all flooded once more.  So, folks, that it is for Gorhambury for the next 11 months.  Plenty to do, however, with data processing, analysis and interpretation.

Today Ellen and I went back to site to collect the last remaining pegs.  Ellen organised us into walking back and forth across the field systematically to see if we could find my missing survey book.  On our second pass…

A very damp notebook!

A very damp notebook!

It was very soggy, but thanks to being a “rite-in-the-rain” product it looks OK and is now drying in the airing cupboard.  Yay!  Inevitably, as we were walking around picking up pegs and finding the book, it started to rain and we were soaked by the time we left.

On Sunday, I had some fun making timelapse videos.  Here is a condensed version of surveying three squares.

I did a poor job of setting it up for the GPR, but here it is anyway:

I have quickly processed the GPR data from using the large 160mhz antenna. We surveyed a block 20x40m which went over the “sinuous ditch” in the hopes we might be able to see it more clearly. We have picked up the walls that we saw with the 450mhz antenna.  In the radargrams (the vertical slices which is how the data is collected) I think I can see the ditch… but I need to check with someone more experienced at reading them.  Here are the time slices.  Note that we used a 1m spacing between lines which leads to a cruder picture.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

I thought it would be useful to show one radargram from the day 15/16 block.  This is line 1893 which was surveyed from north to south, i.e., from the top of a block (as I usually present them) to the bottom.  Here is a screen grab from “radexplorer” showing unprocessed data.

An unprocessed radargram of line 1893.

An unprocessed radargram of line 1893.

It is very difficult to see much.  This is because most of the signal is the loud “noise” near the surface.  The first thing to do is tell the software where ground level is, i.e., the start of the first ‘loud’ reflection shown as black band at the top of the radargram.  Then we need to apply ‘gain’.  This is simply amplifying the lower reflections which are much weaker than the ones near the surface.

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

We can see much more now, but there is still a great deal of banding.  The bands are, essentially, echos and can be removed with the “background removal” function.

The processed radargram of line 1893.

The processed radargram of line 1893.

There is a great deal going on in this line of data.  Note, however, how the vertical column of echos under where I have put the label “wall” starts down into the radargram where as the column of echos where I have put “ring” starts at the top.  The “ring” is a small incipient mushroom ring which I noticed as we pushed the GPR over it.

A small incipient mushroom ring.

A small incipient mushroom ring.

If we look at the upper time slices we can see the ring:

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

For the wall, however, we need to look at a deeper slice.

Slice 3, 14-18ns.

Slice 3, 14-18ns.

In this slice we can see a small rectangular building showing faintly in the data. This building explains some of the things we can see in the radargram.  Remember that north is to the left of this radargram.

The arc we can see at the bottom of the top slice starting at about 27m west and going to 34m west is the northern edge of a very big mushroom ring.

A large mushroom ring.

A large mushroom ring.

The interaction between the fungus and the grass is quite complex.  The rings are showing in the radargrams probably because they are retaining water.  This can make the grass grow lush, or in extreme circumstances kill the grass.

A mushroom ring killing the grass.

A mushroom ring killing the grass.

Some of the rings merge and become quite complex.

A complex mushroom ring.

A complex mushroom ring.

And some of them cause mad photographers to get to their knees.

yummy circle

Yet another mushroom ring.

Yes, I became a bit obsessed.

Although we have finished for this season, there are lots of other things on the horizon, as well as working through all this great data.

I know I have said it before, but it is worth saying again.  Many thanks to everyone who contributed.  We collected great data, and we have found some really intriguing things.

In the frozen north?

A short while ago we were in the bottom pointy bit of the theatre field. We are now in the top pointy bit! We have surveyed with the mag a substantial proportion of the field. A quick check with QGIS shows that we have surveyed 17.3 hectares of a 27.3 hectare field.  Good job everyone.

To start, let us see the overall progress.

The area surveyed after day 18.

The area surveyed after day 18.

The northern area has some very interesting features.

Detail of the northern area.

Detail of the northern area.

The sinuous ditch continues westwards up the dry valley towards the wall.  A basic aquaduct seems a good possibility.  Less Pont du Gard and more muddy ditch, as aquaducts in Britannia tended to be.  There are also quite a few small and quite faint ditches which look like small field boundaries.  Perhaps evidence of agriculture outside the town when the ‘1955 ditch’ was the boundary and before the wall was built?

There is a nice find in the topmost area.

A building parallel to the road just inside the Chester gate.

A building parallel to the road just inside the Chester gate.

A rectangular building can be seen a little back from the line of Watling Street just inside the Chester Gate.  Snack bar or tax office?  This will be a nice building to target with the GPR… next year!

On the subject of GPR, the crew managed a 20×40 block with the large 160mhz antenna, and two 40x40m  blocks with the usual 450mhz antenna.  Here are the time slices from the two blocks with the 450.

The more southerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

The more southerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

The more northerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

The more northerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

An excellent result showing a whole series of buildings.  Joining up the 16-20ns slices in Google Earth we can see how they relate to the magnetic data.

The day 18 GPR blocks overlaid on the magnetic data.

The day 18 GPR blocks overlaid on the magnetic data.

The same area as the previous image for comparison.

The same area as the previous image for comparison.

And finally for this evening, the context of those GPR blocks.

The GPR results around the day 18 data.

The GPR results around the day 18 data.

Quite an impressive series of buildings in the heart of the town.  Clearly, expanding the GPR survey next year will be well worth the effort.

The weather for tomorrow, our final day, is somewhat uncertain.  We will see…

 

The antepenultimate day

“Antepenultimate” is my second favourite word. At least I get more use of it than I do of my favourite word, “defenestrate”. I used to defenestrate my Christmas tree until I moved and now I can only deport it. Anyway, enough of this nonsense.

Today the mag re-did a few problematic blocks of data before moving on the survey a batch of new grids.  Here is the overall plan.

The total area surveyed after day 17.

The total area surveyed after day 17.

I think we should all be very pleased with the progress we have made.  Looking at the new area in more detail.

Detail of the area surveyed on day 17.

Detail of the area surveyed on day 17.

The large ditch which runs from the SW to the NE of the westernmost part of the survey is the return leg of our old friend, the 1955 ditch.  It looks like the northern part of this feature has been substantially backfilled with non-magnetic material as it is only visible faintly.  The sinuous ditch clearly crosses the 1955 ditch  and heads west up the shallow valley.  At least we can be sure now that the sinuous ditch post-dates the 1955 ditch.  Part of the western part of the sinuous ditch was found in the 2000 English Heritage survey.  The line of pits which appears to join to the thinner NW-SE ditch could be all one long linear feature.  This area may not have the big stone buildings found in the central part of the town, but it still asks a great many questions.

The GPR has now covered quite a large area.

All areas surveyed using the GPR overlain on the mag data.

All areas surveyed using the GPR overlain on the mag data.

Today they covered another 80x40m block to the south of the “uber magnetic” building.

The GPR block surveyed on day 17.

The GPR block surveyed on day 17.

The same area for comparison to the previous image.

The same area for comparison to the previous image.

We picked up quite a bit more of the building which was in the SE corner of yesterday’s block, as well as this missing corner of the corridor building to the SE.  The road does show, but not very clearly in the time slices.  There is also quite a solid area in the western part of today’s block; it maybe another building.

Tomorrow may be the last day if the weather forecast is accurate.  Fingers crossed that it isn’t!