Forthcoming talk

On Friday 29th November 2019, Kris Lockyear will be giving a talk on the results of the survey work entitled “Verulamium: busy places and empty spaces.”  The meeting will start at 7.45pm, United Reformed Church hall, Church Road, Welwyn Garden City.  WAS members free, visitors £3.

One

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

One what, I hear you say?  Well, 1km2. What is 1km2? Well, that is the area covered by the mag in all the Verulamium-related surveys. Yup, one whole square kilometer. Impressive, eh? By the way, that is about 20,000,000 individual mag readings.  That doesn’t include, of course, squares that had to be re-done due to sensor freezes or areas blanked out where we were wheel spinning for partial grids.  Congratulations to all who have pushed that machine since the summer of 2013.

Figure 1: Jim West and the mag.

Today the mag team completed the far end of Church Meadow (Figure 2).  It is great to see such a huge proportion of the field done, and much of what is left is not worth doing as it is featureless alluvium.

Figure 2: the Church Meadow mag at the end of the 2019 summer season.

Figure 3 shows the details of the southern end of the survey.

Figure 3: Detail of the southern end of the Church Meadow mag survey.

Most of the new area today was either in the area impacted by the pipes, or featureless alluvium.  The little partial near the road, however, found a small feature which looks like a wall with something in the middle.  Given this is right next to the gate of the town, perhaps this is a mausoleum?  No real way of knowing without digging it, but certainly a possibility.

The Earth Resistance team of Debbie, Tim, Denley and Ellen were on form today and completed a super nine grid, thus satisfying my need for a tidy end to a season! Figure 4 shows the results from today.

Figure 4: the Earth Resistance data after day 3.

Figure 5: the Earth Resistance data high-pass filtered.

As can be seen, there are a number of wall showing clearly as dark (high resistance) lines.  The room which shows most clearly is the one which can be seen on the Google Earth image.  A high-pass filter shows the walls even more clearly (Figure 5).

The GPR crew, allowed down from the heights of the Theatre field, picked a 40×80 strip east-west across the middle of the buildings.  Figure 6 shows the first twelve time slices.

Figure 6: GPR time slices across the nunnery.

As can be seen, the building that shows well on Google Earth is visible right from the first time slice.  The stone work must be literally just under the surface.  Slices 7 and 8 shows the buildings in great detail as well as that pipeline running across the plot. Figure 7 shows slice 7 on the Google Earth image.

Figure 7: GPR data across the nunnery. Slice 7.

To close out the 2019 season posts, I asked Mike Smith to take a group photograph.  Not everyone who was involved this summer was there today, but Figure 8 shows a good number of us.

Figure 8: the gang isn’t quite all here. The CAGG team on the last day of the 2019 Gorhambury summer season. (Photo: © Mike Smith).

Many, many thanks to everyone who turned-out over the last four weeks, be it almost every day or for just an afternoon.  Without the CAGG team members, this project wouldn’t achieve anything!  Also, big thanks to Strutt and Parker and the Gorhambury Estate for facilitating access, and to Lord Verulam and his family for all their support.  Lastly, thanks to the AHRC for funding the original project back in 2013, the Institute of Archaeology, UCL for supporting the project and the loan of the GPS and the Earth Resistance meter, and to SEAHA for the loan of the GPR.

I’m off to Sligo tomorrow morning at about 4.30am and will be presenting some of our results to the International Conference on Archaeological Prospection on Wednesday afternoon.

 

 

Done

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

I think Mike Smith would argue that he is “done” in more ways than one!  I feel that a suitably sonorous 1950s newscaster voice ought to be saying “at 1pm this afternoon, members of…”  The reason?  Because at 1pm this afternoon the GPR survey finally surveyed the last part of the theatre field (Figure 1).

Figure 1: Mike Smith crosses the finish line.

That is some 27ha of GPR survey, mostly at 50cm transect spacing.  That works out to pushing the mag some 540km across the field.  Figure 2 shows the final coverage.  As always, the image is a mess because it has been created by different pieces of software at different times and even with slightly different conversions of OS coordinates to lat and long.  My big job now is to turn that pig’s ear (processing-wise) into a clear image.

Figure 2: Done. The complete GPR survey of the theatre field (but needs better processing).

Many members of CAGG have contributed to the GPR survey over the last five seasons.  This season Nigel Harper-Scott, John  Ridge and John Dent have contributed greatly (Figure 3).  The person who really deserves a rest, however, is Mike Smith who has not only led the GPR team over most of the last five seasons, but has also been the main GPR transporter during the season, and has been looking after it during the week.  Many thanks Mike, and well done on a great achievement.

Figure 3: Nigel (left), Mike (centre) and John (right), today’s GPR crew.

The area covered this year is best seen via the stripes in the grass (Figure 4).

Figure 4: Stripy grass left by the GPR.

Now, you would have thought that completing the survey would have persuaded the boss to let them go home early.  Nope.  None of that slacking.  One grid we did at 1m transect intervals had an interesting building in it.  So after lunch the team set to once again to resurvey it at 0.5m intervals.  In figure 5 I have made the high amplitude reflections white so that the building can be seen more easily and placed it on the mag survey for context.  It is a long, thin, building with a corridor just visible on the eastern side and a larger group of rooms to the south.

Figure 5: the re-done block on the mag data.

Tomorrow, Mike gets his pick of which two squares to survey in Church Meadow.

The mag team re-did all the dodgy squares from yesterday, and quite a few more (Figure 6)!

Figure 6: the mag data in Church Meadow after day 5.

The mag team have just three whole squares for tomorrow, and then a few partials at the ends.  For most of its length, it is not going to be worth surveying up to the edge of the field to the NE as it is clearly into alluvium as shown by the flat, featureless mag data.  Figure 7 shows a close-up of the southern end of the survey.

Figure 7: the Church Meadow mag data, southern end details.

The darker curvy line is possibly an old edge to the river.  There are quite a few small dark blobs (“positive magnetic anomalies”) some of which could be graves, and at least part of a ditch feature.  I am still puzzled by the long negative linear.  I guess I’ll have to talk to some other geophysicists!

The res team was joined by Denley Lane of the Arc and Arc,  Denley remembers some of the pipelines being built.  The team completed an excellent seven grid squares.  Figure 8 shows the results, and Figure 9 shows the same results high-pass filtered.

Figure 8: the res results in Church Meadow after day 2.

Figure 9: the res results in Church Meadow after day 2, high pass filtered.

As can be seen, the building visible in the Google Earth image shows very well indeed, but there does not seem to be many more buildings to the south.  Tomorrow we will fill-in the one last block in the eastern corner, and then do a strip of blocks along the northern edge.

Tomorrow is the last day of the 2019 season.  I’ll be speaking about the Project at the International Conference on Archaeological Prospection on Wednesday.  Not much time to fit in the new results!

 

 

The antepenultimate day (Part 5)

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

It was a very warm and sunny day today, and I think we all felt the heat.  We managed to complete quite a bit of work, though, and so congratulations to all the team.

The GPR team filled-in some of the missing bits on the west, north and east sides of the theatre field (Figure 1).  Just a few bits left now.

Figure 1: The almost-complete GPR survey.

One nice feature was a little detail found on the western side (Figure 2).

Figure 2: A detail of the GPR survey from today. The red arrow indicates a small building.

A nice little building is showing-up.  The block to the east may show some robbed buildings. Last year I noticed some curious white lines in that block which could be robbed walls.

The mag team completed an excellent eight grids taking their total area surveyed in four days to 5.09ha.  Frustratingly, however, I found that the sensors had frozen on the first three grid squares.  Horrible waste of time, but nothing to be done about it.  Figure 3 shows the whole survey.

Figure 3: the mag survey in Church Meadow after day 4.

Despite the frozen sensor, we can see some interesting details in the new area to the south (Figure 4).

Figure 4: the southern area of the mag survey in Church Meadow after day 4.

What I am finding curious is that the long linear features are white in the plots, i.e., below average magnetism.  The features look like ditches in form, but do not give the usually positive response one would expect from them.  My worry is land drains… but the features seem to connect with the edges of Watling Street.  How very curious.

Figure 5 shows the Earth Resistance survey after day 1.  The team completed an excellent eight grids.

Figure 5: the res survey in Church Meadow after day 1.

The broad dark line across the western corner is Watling Street.  Further east the various thinner dark lines are the walls of buildings.  Clearly we have parts of a number of structures showing clearly.  Great stuff.  We don’t have a cruciform-shaped building face east-west yet, but give us time.  Hopefully, tomorrow, we will cover the building which shows so clearly in the Google Earth image. Comparing the mag and the res shows how much the pipes obscure, and how they went right through the middle of this complex (Figure 6).

Figure 6: the res survey in Church Meadow after day 1, with the surrounding mag data.

Well it is now 1am, and I have to be up early in the morning for our penultimate day on site.  Many thanks to everyone who worked so hard in the heat today.

 

Being completist

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

The team are very patient with my need to be neat and tidy and do the silly little bits around the edges.  Today was perhaps an extreme example.  Due to yesterday’s little hiccup, I set-up the res kit and, with Graham’s help, surveyed about a sixth of a grid square, then packed it all up again.  Now I can sleep easy.  On a more ambitious note, I have finally put all the res grid squares into one large composite.  The survey now consists of nine hectares, which is about 360,000 individual readings.  Most Earth Resistance surveys use what is known as a twin-probe configuration.  That means that there are two mobile probes on the frame, and two stationary probes on the end of a long cable, normally about half a meter apart.  One mobile probe, and one remote probe set-up an electric circuit part of which is the soil.  The other mobile probe and the other remote probe measure the resistance.  The problem with this “standard” set-up is that when you have to move the remote probes, the readings for the same spot will change.  This leads to endless struggles to “grid-match” each set of squares.  Since 2016 I have gone over to using a pole-pole configuration.  This is basically the same except the remote probes are a long way away (I aim for about 30m or more) and a long way apart from each other (I aim for more than 20m). This helps enormously with the grid matching.  Grids completed on different days of the survey will match quite nicely, usually.  Where this is not true is when (a) there is a lot of rain in the middle of a survey and (b) when the survey is split over multiple years.  In the case of our 9ha block, this has been completed over four seasons.  Unsurprisingly, one can see the edges.  TerraSurveyor has a function called “periphery match” which will, sometimes, do an excellent job of grid matching.  In this case, it was pretty good.  Figure 1 shows the survey with a periphery match applied.

Figure 1: the res survey as of the end of Thursday.

If you click on the image and see it full size you can see the detail of many buildings.  Unfortunately, the range of values makes seeing some buildings quite hard.  A high-pass filter is a background trend removing tool that makes some buildings show more clearly (Figure 2).

Figure 2: the res survey, high-pass filtered.

There are still many features, many buildings, that one can only see when looking at smaller blocks.  With such a big area getting everything to look clear is going to be impossible, I fear.

From tomorrow the res will be working in Church Meadow where we hope it will map the remains of St Mary du Pré.

The GPR team is getting really close to finishing the theatre field (Figure 3).

Figure 3: the GPR survey so far.

The team have some fiddly bits around the edges to complete, one missed block, and one block we want to re-survey at 50cm intervals as there is a building and road in it.  Fingers crossed, two more days should do it.  Then, for a bit of last day fun, the GPR will also have a look at St Mary du Pré.

The mag team completed another eight blocks today.  Since moving to Church Meadow, they have managed to survey four hectares in three days which is very impressive, especially given that one block had to be repeated due to a sensor freeze.  Having lots of whole grids and no partials makes such a difference.  The team are now just 2.5 ha away from completing a square kilometer of mag at Verulamium.  Figure 4 shows the mag results in Church Meadow.

Figure 4: Church Meadow mag survey after day 3.

Three things are of note.  Firstly, earlier today I wondered which of the two raised areas in the field was Watling Street.  Looking at the survey results, it looks like the road splits in two near the edge of our survey.  Perhaps it is two phases?  I’m not convinced.  Secondly, we have some marked linear features showing that almost look like enclosures.  These are, however, low magnetism suggesting and might be yet more pipes, but not of metal this time?  Again, I’m not convinced.  They might well be archaeological features.  I will have to survey all the pipes I can see in the field.  Lastly, as we get closer to the town to the south, there are many more little black blobs.  Seasoned readers of this blog will know that usually little black blobs in mag data are often pits.  In this case, I wonder if we are starting to pick up the edge of the cemetery which is likely to have lined Watling Street?  In the Roman world, the richer you were, the closer you wanted to be buried to the road and the town.

Tomorrow is the antepenultimate day of the 2019 Gorhambury survey season.  The weather looks good so fingers crossed all goes well.

Mea culpa

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

Firstly, an apology to those to whom I said “no, we’ve done that” this afternoon when we hadn’t.  Oops.

Just a quick posting tonight as it is late.  The mag team completed an excellent nine grids in Church Meadow.  They are half-way along the field already (Figure 1).

Figure 1: the mag results in Church Meadow.

Sadly, the most obvious things are the two pipelines, and the dark stripes across one grid square.  The latter is our old friend the frozen sensor.  Way back during the survey in the Park we once managed twelve grids in a day only to have to re-do a bunch of them thanks to the sensor.  Of the archaeology, we cam most clearly see the edge of Watling Street heading SSE towards the Chester Gate.  The ditch we picked-up on Sunday appears to be the SW edge of an enclosure, possibly part of the nunnery.

The GPR crew managed an excellent five grids today despite only having one battery.  Figure 2 shows the entire GPR survey in the theatre field to date.

Figure 2: the GPR survey to date.

I’d like to give a big thank you to Jimmy Adcock of Guideline Geo for going above and beyond in finding a second battery for us as one of ours has died.  As can be seen from the figure, we are within spitting distance of finishing this field, and it would be a shame if we missed that target due to a dodgy battery.

The res team made it to the fence line. We would have finished the new transect if it wasn’t for my incompetence (see the opening paragraph!).  Figure 3 shows the res survey in the theatre field.

Figure 3: the resistance survey to date.

The funny little notch in the top by the fence is my little oppsie. Figure 4 shows the northern area with a high-pass filter applied to the new data.

Figure 4: the new area (high pass filtered).

The most obvious feature in the new data is the aqueduct showing as a very low resistance feature in white cutting across the transect.  There are, however, other buildings showing very subtly in the data that are very hard to pull out in an overall plot.  They show best in the thumbnails as one is putting together the data in TerraSurveyor.  Apart from the little partial which I’ll sort out tomorrow (actually, later today!), the Earth Resistance meter will move into Church Meadow tomorrow to try and retrieve a plan of St Mary du Pre denied the mag by the pipelines.

Just four days left!

Pipe dreams

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

Saturday night I said the weather was predicted to be “unsettled”.  Well on Sunday at 10am it was raining cats and dogs.  (I wonder why cats and dogs?  Why not ducks and pigeons?, or frogs and mice?)  I was determined to set-out grids in Church Meadow and so I soldiered-on.  Up until now, I have stuck to the 40m grid based on the OS for all the fields we have surveyed in Verulamium Park and in Gorhambury.  Church Meadow, however, is a long thin field at approximately 45º to the OS grid.  Additionally, the fence along Gorhambury drive is very straight for much of its length.  I decided, therefore, to use a floating grid to minimise partial grid squares and wheel spinning.  The lack of an “end line” function is the Foerster’s Achilles’ heel.  We must have wasted hundreds of hours spinning the wheel due to that one simple omission.  Although the data processing involves some extra steps and jiggery-pokery to get the plot in the right place, it seems worth it in this case.  I think my decision was vindicated when the team completed nine complete 40x40m grids despite the wet start to the day, and the lunchtime deluge. Figure 1 shows the team in action.

Figure 1: the mag team in action in Church Meadow. Image © Mike Smith.

Figure 2 shows the location of Church Meadow.

Figure 2: the location of Church Meadow, outlined in red.

Looking closely at the Google Earth image reveals some features in the field (Figure 3).

Figure 3: Features visible on Google Earth.

With these features we had high hopes.  Figure 4 shows the results of the mag survey from the first day.

Figure 4: Mag survey results.

Sadly, the plot is dominated by the two pipelines running through the field (Figure 5).

Figure 5: Heavy metal. Image © Mike Smith.

One of the pipes clearly runs straight through the building seen in the GE image. There are, however, some archaeological features to be seen (Figure 6).

Figure 6: mag survey in Church Meadow with some labels.

It is very frustrating that we can see the walls in the mag data to the SW and between the pipelines, but the image is so dominated by them that it is hard to make sense of anything.  Hopefully the res or the GPR will show the details better.  The ditch is interesting, however.  Could this be the vallum monasterii? It could, perhaps, be related to Watling Street, or it could simply be the remains of the earlier route of Gorhambury drive.  It will be fascinating to see where it goes.

The Earth Resistance team completed an excellent six blocks of data.  Figure 7 shows the whole res survey.

Figure 7: the whole Earth Resistance survey after Sunday 18th.

With good luck and a fair wind we should reach the hedge line on the next survey day.  Figure 8 shows the grids completed on Sunday.

Figure 8: The grids completed on Sunday.

Not a great deal is showing in those grids apart from the faint line across the top corner.  Let’s look at the mag data from that area (Figure 9).

Figure 9: the mag data from the same area as Figure 8.

The light line in the res data is matched by the dark line of “the sinuous ditch”, which is exactly what we would expect.  The sinuous ditch is, we think, the town’s aqueduct.  We should pick-up much more of this on Wednesday.

The GPR team have been working down the western edge of the town with the end in sight.  Soon, soon, they hope, they can escape the theatre field and its rugged terrain (Figure 10).

Figure 10: GPR and the rugged terrain.

The GPR team have been doing a lots of sawtooth edges as well as extreme hill-climb GPR.  Figure 11 shows recent results.

Figure 11: the western edge completed between Thursday and Sunday (colour section).

GPR, perhaps even more than Earth Resistance, is affected by the weather and ground conditions.  It seems very difficult to get different days to match-up.  I tried three methods with this data collected over four days, and none were perfect.  This image was created by just treating everything as one big data set. It doesn’t help that each tweak to see what works takes half an hour to process!

Looking back over the first three weeks, we have managed to achieve quite a bit despite dry weather, wet weather and endless partials.  Many thanks to everyone who has been involved, especially those stalwarts who come most days (you know who you are!).

Nigel wonders what next week will bring…

Figure 12: The Thinker. Image © Mike Smith.