Tag Archives: WAS


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.




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.

Saving the best ’til last?

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

Unfortunately the weather forecast was accurate yesterday.  Far too much rain for us to go out and play.  Today, however, was very pleasant and at least the pegs went in very easily for a change! I’m afraid that a new toy (Figure 1) resulted in me getting a late start processing data this evening, and so I only have the mag and res to report on.  The GPR team, however, did a sterling job and completed a multitude to sawtooth-edged grids along the western side of the theatre field.  I promise I’ll process all that tomorrow!

Figure 1: Kris’ new multiformat pinhole camera being put through its paces.

The Earth Resistance meter was operated by John and Grahame. We have moved north of the hedge line and are close to the cross-roads around which both mag and GPR have shown multiple buildings.  Figure 2 shows the entire res survey, and Figure 3 a zoomed-in view of today’s area.

Figure 2: the Earth Resistance survey so far.

Figure 3: the area surveyed today and some context.

We have just clipped the edge of Street 11 (in the top-right corner of the new block) and have revealed some wonderfully clear images of the buildings to the south of that street.  These are clearer than the GPR survey of the same area, so it is really pleasing to see.  Excellent stuff!

Over in Prae Wood field, the mag team completed two more strips of grid squares.  Figure 4 shows the survey so far.

Figure 4: the Prae Wood Field survey, so far.

Two more days should, weather and equipment willing, see the team finish the field.  After Prae Wood we are heading north into Church Meadow, so-called because St Mary du Pré lies within that field.  Watling Street also runs through it so we should get some exciting results.

But, back to Prae Wood.  Let us look in more detail at the western end of the survey (Figure 5).

Figure 5: the western side of the mag survey of Prae Wood field.

The dark lines in the new area represent ditches, and it looks like we have some enclosures running across this western end of the field.  Given that the major Iron Age settlement lies in the wood just to the SW, these could be of that date, but they could be Roman or medieval as well.  Once more, we need to check out the historic map data.  Still, after many blank grid we now have features in both the western and eastern extensions of this field.  Saving the best for last?

Many thanks to everyone who turned out today.  Tomorrow looks like we might have to pack-up early due to yet more rain. This season is just flying by.

Half-way point

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

It is amazing (and slightly scary) that we have already reached the half-way point in the 2019 Gorhambury survey season.  To give the surveyor a chance to get a little ahead of the game, we pulled the mag team off Prae Wood Field this morning and got them to help the GPR crew and to do some res.  After lunch, the mag finished Prae Wood and the GPR completed their 80 x 80m block.

The mag completed the last two triangles in the far eastern part of Prae Wood, and re-did one square for which the mag cart had, for some inexplicable reason, developed a horrible stagger error (Jim puts it down to cosmic rays).  I wouldn’t have been too bothered, apart from the fact that one edge of our enclosure passes through that square.  I have started to lay-out the grids for the western edge of Prae Wood field.  Hopefully, by next weekend we can be out of that field and into Church Meadow.  Figure 1 shows the whole mag survey of the field, and Figure 2 the detail of the eastern extremity.

Figure 1: the mag survey of Prae Wood field.

Figure 2: detail of the mag survey of Prae Wood field.

Not much new has shown-up in the eastern extremity of that field, but the enclosure we found a couple of days ago is now a little clearer than before.  The mass of modern noise is unfortunate.

The res survey was extended up to the hedge line in the theatre field, with hints of what is to come.  Just to the north of the hedge line their are many buildings which the res should pick-up nicely.  Figure 3 shows the entire res survey, and Figure 4 a closer view of the second block of the 2019 survey so far (the strip to the west).  As always, the edges are because the blocks have been processed separately and joined-up in Google Earth.  I’ll start to put everything together soon.

Figure 3: the Earth Resistance survey after day 10.

Figure 4: detail of the western edge of the survey showing the new data.

The res survey is now 8ha in extent.  Not counting grids we have had to do twice for various reasons, this is 320,000 measurements for the 0.5m mobile probe separation survey.  (We only started using the 1+2 survey method last summer.)

GPR this year, and last, can be a little challenging (Figures 5 and 6).

Figure 5: Dave Minty (WAS) pushing the GPR in week 1. Image © Mike Smith.

Figure 6: Jim West (CVAHS) pushing the GPR today. Image © Mike Smith.

For most of this week, we have been using a 1m transect spacing with the GPR in order to finish the field this season.  We can do this because (a) the features we know about are big such as the 1955 ditch; (b) we aren’t expecting any stone buildings in this area and (c) if we do find something interesting, it will show in the 1m data, but just won’t be very clear.  Today saw the GPR reach the bottom of the dry valley across which the aqueduct dog-legs, and also joins up with last years survey.  Figure 6 shows the entire GPR survey (very crudely!).

Figure 7: the entire mag survey after day 10.

Figure 8 shows today’s block along with the neighbouring ones.

Figure 8: the day 10 GPR block, slice 4.

The eagle-eyed amongst you would have spotted a long building running SW–NE with its short end on the big black blob.  Yup, we have a building.  Oops.  Well, at least that proves that we can see buildings even with 1m transect spacing, although it does look a bit dot-to-dot.  What are the other things?  Figure 9 shows the mag data.

Figure 9: Mag data in the area of the Day 10 GPR survey. Red square marks today’s block, blue rectangle the building.

The mag data shows us that part of the big black blob is the aqueduct.  We have been speculating whether the aqueduct had some sort of structure to carry it across the dry valley.  This may help us address this question.  The long building, however, does not show in the mag data at all.  We have come across this previously.  The further buildings are from the core of the town, the less likely it is we will see them in the mag data.  This is because, I think, the surrounding soils are less magnetic than in the core of the town.  The mag survey processes out these broad scale background changes in magnetism.  We need to undertake a magnetic susceptibility survey!

Across the SE corner of the mag plot is a long linear feature which is the 1955 ditch shown with green arrows in Figure 9.  As can be seen, some parts are strong and easy to see, other parts are much fainter.  Are we dealing with parts of the ditch that have been filled-in, or parts that were never really dug in the first place?  I suspect the former, but it is only a suspicion. Towards the south part of the 1955 ditch in Figure 9 are two strongly magnetic features in line with the ditch, either side of a low area of magnetism.  This is where Street 11 appears to cross the ditch as shown by the red arrow in Figure 9.  The street barely shows in the mag data but Figure 10 shows how clear it is in the GPR data.

Figure 10: today’s GPR block with some labels.

We have had a very successful two weeks.  Rain has only lost us a few hours (so far… touch wood!) although the wind yesterday was trying.  Many thanks to everyone who has made this all possible.





Partial madness

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

Apologies for not posting the results from yesterday’s survey.  Prae Wood field is an odd shape, leading to a plethora of partial grids for the mag, and the consequent head-scratching as to how it all fits together back at base last night.

Starting with the mag data.  The team have had two days of multiple partial grids, many of which have to be split into several smaller blocks in order to be surveyed.  At times, the sea of flags looked a bit like bunting at a village fete.  The team, however, have made excellent progress and we should finish the field in a few days (weather willing).  Figure 1 shows the results for the whole field.  Mainly, we have found pipelines (the black-and-white linear features) and clearly something big happened in the field near the gate at the south edge (an old building, perhaps?).  Traces of more interesting archaeology are hard to come by, and I have pointed out one with the red arrow.  Slim pickings in this big field, but important to see the empty spaces.

Figure 1: the mag survey in Prae Wood field after day 7 (day 3 of mag survey).

The Earth Resistance survey took a back seat on Wednesday with Graeme and I completing a token single square after lunch.  Today, we did not even try to do any more, the ground is so dry and hard.  Tomorrow the frame is off being repaired.  The single square we did manage showed nothing at all (Figure 2, top left square).  It lies in the valley between the buildings that line streets 23 (with all the buildings that can be seen Figure 2) and street 25 which we have not yet surveyed with the res.

Figure 2: Earth Resistance survey after day 6.

The GPR team, completed a block 40 x 80 at half meter transects yesterday.  As they did not find any buildings, and generally the evidence seems to suggest that the NW side of the town is free of them, we swapped to 1m transect spacings today enabling the team to cover an area 80 x 80m.  We would very much like to complete the GPR of this field this season.  This afternoon the team was joined by Sandy Walkington, President of the Arc and Arc (Figure 3).

Figure 3: Sandy Walkington (SAHAAS) operates the Mala GPR.

Figure 4 shows the 40 x 160m block surveyed over the last three survey days (on the left) next to some blocks surveyed last year.

Figure 4: the GPR data from the last three days of survey.

Although nothing stunning jumps out, unlike the buildings to the east found last year, there are some things to note.  There are clearly some linear features showing in the NW block of data.  Figure 5 shows the mag data from the same area.

Figure 5: the mag data from the same area as Figure 4.

The large dark linear feature that turns a right angle in Figure 5 is our old friend, the 1955 ditch, the late 1st century boundary of the town (Fig. 6, red arrows). There is a second, much fainter, linear feature running parallel to the ditch, that clearly must be related to it (Fig 6, blue arrows).  Note the three faint circular features (Fig. 6, green arrows).

Figure 6: Figure 5 with colourful arrows (see text).

Looking back at the GPR data (Figure 7) we can see all these features reflected in the GPR data, albeit subtly.

Figure 7: GPR data with arrows (cf. Fig 6 and text).

The three circular things would appear to be depressions or pits into which slight more magnetic topsoil has collected.  These is going to be much correlating of features between surveys on the horizon!

Finally, the St Albans and Hertfordshire Architectural and Archaeological Society (aka the “Arc and Arc”), the oldest archaeological society in Hertfordshire will celebrate its 175th anniversary next year.  They have recently launched an updated website.  John Dent, Arc and Arc member and CAGG volunteer right from the beginning, has his 15 minutes of fame on the front page (Figure 8), pushing the GPR at Gorhambury in the first season of our survey in 2015.  Go John!

Figure 8: The Arc and Arc’s new front page featuring John Dent and the GPR.

Tomorrow may, or may not be a bust.  The weather forecast has heavy rain over night but dry during our working day.  What the reality will be, who knows!

Congratulations everybody for some excellent surveying.

The smallest grid ever?

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

Well, possibly not. The mag team’s first grid this season was a 1.5m x 40m grid. Why? Well, in the penultimate grid of the very last day last year we had one single frozen sensor for one line of data. For the last 11 months this has annoyed me every time I saw it. Finally, I have been able to fix that grid! Yay.  Having completed that line, the team went on to complete another seven grids of data.  Well done team (Figure 1)!

Figure 1: Jim West and the mag.

Figure 2 shows the whole of Prae Wood Field and the survey completed so far (but without the dodgy line!), and Figure 3 zooms into the area completed on Sunday.

Figure 2: the mag survey of Prae Wood Field.

Figure 3: detail of the mag survey of Prae Wood field.

The overall impression one gets from both Figures 2 and 3 is a whole lot of nothing.  There are the occasional strong magnetic responses from iron objects, and on Sunday we picked-up two pipelines (shown in Figure 3 with yellow arrows).  The larger area of noisy magnetic data near the southern edge of the survey might be an historic structure.  In the new area, there is a very faint line (as indicated by the blue arrows) which might be an old fence line, or might be my imagination.  There are some “monopolar positive” features (i.e., ones which are mainly positive but with a slight negative response on the north side) which could well be pits.

What makes this all fascinating is that the Urban Archaeological Database (the UAD), suggests that the field is within a “rectilinear  enclosure”, Monument Number M27.  At the moment, I’m not sure where this idea comes from, but at the moment it seems as though it is an enclosure around not very much! This field, and part of Prae Wood itself, are within the area mapped by the Environment Agency using LiDAR (Figure 4).

Figure 4: LiDAR image of Verulamium. Data from the Environment Agency, image courtesy Mike Smith.

The Fosse, running through the woodland along the NE edge of the field shows nicely (Figure 4, right-hand red arrow).  The little fragment of Prae Wood itself shows a mass of features in the woodland, some of which are parts of the Iron Age settlement (blue arrow). Our field shows the faint hint of the ploughed-in Fosse (left-hand and central red arrow), and a whole lot of not-much-else.  How very curious!

Despite the very dry ground surface and the small team, we did manage a further three res squares on Sunday.  Many thanks to Pauline for putting-up with my cursing as we did the work. Figure 5 shows the results.  The edge between the earlier survey to the east and the current block of eight squares is due to my processing differing between the two seasons.

Figure 5: the Earth Resistance survey after day 4.

As can be seen, we have a line of buildings along the SW–NE road.  This road, Street 23 in Niblett and Thompson’s Alban’s Buried Towns, shows very poorly on all three survey techniques.  In the res survey, it almost looks like an eroded channel, and I have often been a bit confused as to the where the dry undulation (seems a bit grand to call it a valley) lies.  It lies, however, behind these buildings and those that face onto Street 25 to the west.  I have indicated the valley in question with a yellow arrow in Figure 4.

A further source of data is the work undertaken by the Oxford Archaeological Unit in January of 2000.  They excavated 379 1.6m x 1m test pits using a mechanical excavator to strip the topsoil.  As they were investigating plough damage, they did not excavate the features revealed.  The distribution of test pits is shown in Figure 6.

Figure 6: test pits excavated in 2000 by the OAU.

If we zoom into the area we have surveyed using res over the weekend we can see which test pits are relevant (Figure 7).

Figure 7: test pits and the res survey (click on the image to see a larger version).

Test-pit 268 is described as showing a possible floor and a wall foundation of chalk. Judging by its position in the middle of a small building that seems appropriate. Test pit 258 is described as having “?floors” and “?Fill of wall trench”, whereas pit 257 which lies either on, or more probably just outside the wall of the long building, just has a “layer”. In all three test pits the topsoil was between 27 and 32cm deep.  I have yet to see the plans of these trenches, but clearly combining the evidence of the trenches with the geophysics data is going to be very informative.

The GPR team jumped a few grids to complete a block next to the one that they  completed on the last day last season.  We intend to swap to 1m transects soon, and I wanted to catch the details of the buildings that clearly intruded into this block.  Figure 8 shows the location (the bit sticking out to the west).

Figure 8: the GPR survey showing the location of the block surveyed on Sunday.

As can been seen in Figure 8, we have managed to complete the building which lies over the grid edges, but there doesn’t appear to be much more.  Lets look at the first 12 slices (Figure 9).

Figure 9: the first 12 time slices from Sunday’s survey.

Slices 1 and 2 are basically showing the top surface and the topsoil.  In slice 3 we can start to see the building and a long, wide, linear feature.  These show in slices 4 and 5 too.  By slice 6, we are already in the natural and/or where the signal has started to attenuate.  Slices 7  to 12 are basically a few deeper things and echoes / attenuated signal.  The only thing of technical interest is the semicircle which shows on the eastern edge of slices 7 to 12, and also shows in slices 1 and 2.  This is an “airwave” caused by the radar signal bouncing off the underside of the tree canopy.

GPR slice comes with a plethora of palettes for display time slices.  Figures 10 and 11 show slices 4 and 5 in the first 12 palettes.

Figure 10: slice 4 shown in 12 different colour palettes.

Figure 11: slice 5 shown in 12 different colour palettes.

The building along the eastern edge shows well in palette four, and the big linear thing running across the plot shows well in slice 4, palette 11. So what is the big linear thing?  Figure 12 has a clue…

Figure 12: the GPR data with the line of the 1955 ditch indicated.

Yes, you’ve guessed it (or at least read the caption), the “long linear thing” is our old friend, the 1955 ditch.

Well I think that is enough for now.  We will be back on site again tomorrow, hopefully running all three machines if the long promised rain actually happens this evening.  Many thanks to everyone who has contributed this week.  It was a great beginning to the 2019 season!

Verulamium 2019, days 1 to 4

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

After some wet weather just before the season was due to start, the first four days have proved to be warm and sunny. So much so, the ground is already drying out so much as to make the Earth Resistance survey a little annoying.  So far, we have been concentrating on Earth Resistance and Ground Penetrating Radar (GPR) in the main “theatre field”, simply due to person-power and logistics.  We are planning to start the mag survey tomorrow with the aim of completing Prae Wood field and then moving to Church Meadow. Mobbs Hole, the field through which the Fosse passes, will be completed at another time of year when we are less likely to disturb the pheasants.

The res survey (Figure 1), firstly concentrated on filling-in the triangle to the east side of the survey area, and then moved to add another strip of grid squares along the western edge.  Figure 2 shows the entire res survey so far.

Figure 1: Jim West and Rhian Morgan running the Earth Resistance meter on Day 3.

Figure 2: the entire Earth Resistance survey after day 4 of the 2019 season.

At the moment, the res surveys have been processed separately and crudely put-together in Google Earth. As a result, you can see the edges between blocks clearly, especially, for example, the triangle to the east. At some point, I need to combine all the grids into one master survey and process them properly!

Figure 3 Looks in more detail at the eastern triangle.

Figure 3: the completed eastern triangle of res data.

The very strong line near the top of the new area is a road partially excavated by Frere.  The lack of clear buildings either side of it near to the modern road is due to the excavations undertaken by Frere.  We have, however, picked up the last bits of the buildings which run parallel to the side of the Insula XVI temple, as well as some new buildings  alongside the modern road.  The building at the south of the new triangle was partially surveyed in 2017, and is not one previously known.

Today we completed five 20x20m blocks on the western edge of the survey area.  The results are shown in Figure 4.

Figure 4: the western survey area.

The buildings lie along a road, although the road is not at all clear.  The diagonal empty area appears to be a eroded channel cutting across the site visible in the modern topography.  The large corridor house to the east was surveyed in 2017 using the multi-depth Earth Resistance survey (aka “the beast”).

The GPR crew have been working south starting near the area which had such exciting results at the end of 2018.  The GPR leaves entertaining stripes in the grass (Figure 5)!

Figure 5: Stripy GPR grass.

Figure 6 shows the entire GPR survey up to the end of today.  This season’s block of GPR is in colour in the SW corner.

Figure 6: the complete GPR survey after day 4 of the 2019 survey.

Even more than the Earth Resistance survey, the crude use of images in GE is visible.  The data has been collected over five seasons, and processed with different software packages.  I am in the process of putting all the data together into one more consistent analysis.  It might take a while.  Figure 7 shows the four blocks completed thus far.

Figure 7: GPR results after day 4. Slice 7.

Despite the mass of buildings just 20m or so to the east of the block, and on the northern edge, very little seems to lie within the block surveyed.  Comparison to the magnetic data from 2016 (Figure 8) makes it possible to see the so-called ‘1955 ditch’, and one of the GPR blobs is matched by the magnetic blob (it is probably a filled-in gravel / chalk pit).

Figure 8: the magnetic data from the same area as Figure 7.

We are going to complete a couple more 40x80m blocks at 0.5m transect spacing, but then swap to using a cruder 1m transect spacing to ensure we complete this field this season.  Should we pick-up further structures, we can always re-survey targeted areas for more detailed survey.

Many thanks to all our volunteers.  The survey would be impossible without you!  Also thanks to UCL Archaeology and SEAHA CDT for the loan of the equipment. Also, big thanks to Lord Verulam, the Gorhambury Estate and the estate managers for enabling the survey to continue.

Exciting finds in Ashwell

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

Earlier this Spring the group undertook some survey work at Hunts Close, Ashwell.  The results were surprisingly unexciting despite previous finds and the results of earlier test pits.  The one exception was a small group of features on the western edge of the survey which looked like a corner with some sort of irregular feature in the middle.  Gil Burleigh lead a team to put small trenches over these features.  Often this is called “ground truthing”, a term disliked by many archaeological geophysicists.  What we are testing is not the actual data, which is what it is, but our interpretation of that data.  The results were very positive.

Gil Burleigh writes:

Following on from CAGG’s geophysical surveys in March, volunteers from the North Hertfordshire Archaeological Society, led by Gil Burleigh, excavated three further test pits (three others were dug in March) to investigate a few of the features located by the surveys (Figure 1). These had found a double-ditched rectangular enclosure containing a large irregularly-shaped hollow. Slots were excavated into both the enclosure ditches and the hollow (Figure 2).

Figure 1: the excavations underway. Image © Gil Burleigh.

Figure 2: location of the test pits in relation to the geophysics.

Thanks to Kris for both accurately locating these features in the field beforehand to enable the positioning of our slots, and again to him for accurately locating afterwards the positions of our excavated slots using a dGPS (Figure 3).

Figure 3: locating the test pits. Image © Gil Burleigh.

A section across the outer enclosure ditch was completely excavated (Figure 4) and revealed animal bone, Romano-British pottery and a complete 1st century AD Roman bow brooch (Figure 5).

Figure 4: Section across the outer ditch. Image © Gil Burleigh.

Figure 5: 1st century AD Roman bow brooch in situ. Image © Gil Burleigh.

There was time only to excavate a Roman period re-cut of the inner enclosure ditch.
The slot across the hollow revealed its eastern vertical cut edge and a sample of its fills (Figure 6). These contained much fragmented animal bone, weathered, residual, Romano-British pottery and much other pottery that looks Saxon, but requires specialist identification. Several bone pins found, two complete, may be Saxon (Figures 7 and 8).

Figure 6: The central feature. Image © Gil Burleigh.

Figure 7: a bone pin from the central feature. Image © Gil Burleigh.

Figure 8: bone pins or needles from the central feature. Image © Gil Burleigh.

My working interpretation of the double-ditched enclosure, some of which lies outside our field under a neighbouring property, is that it may have contained a Romano-British temple overlooking the spring-head of the river Rhee which lies downslope less than 100 m to the north. The possible Saxon large artificially cut hollow may have been some kind of ceremonial place where religious rituals were performed. It has similarities to the Romano-British cut ceremonial hollow we excavated at Ashwell End, the site of the discovery of Senuna’s temple treasure hoard (Jackson and Burleigh, Dea Senuna: Treasure, Cult and Ritual at Ashwell, Hertfordshire, British Museum, London 2018). The double-ditched enclosure has similarities also to the double-ditched enclosure containing the Iron Age and Romano-British temples at Hayling Island on the Hampshire-Sussex border. This enclosure also contained a series of ritual pits dated to the mid-Saxon period (King and Soffe, A Sacred Island: Iron Age, Roman and Saxon Temples and Ritual on Hayling Island, Winchester 2013).

Having allocated four days to our excavation, it was frustrating to lose a significant amount of time to persistent rain on three of the days (Figure 9). The only completely dry day was the Sunday when in the afternoon an unexpected mini-whirlwind spiralled vigorously around us, spinning buckets, tools, finds trays and other equipment violently into the air, throwing it about in a highly disruptive way. Although some of us were hit by flying objects, luckily there were no injuries. The wrath of the gods perhaps for us disturbing their sacred place!

Figure 9: back-filling in the rain. Image © Gil Burleigh.

Thanks to all our hard-working volunteers, not least for working through the rain, especially to those who did the muddy back-filling in the rain on the Monday: Georgina Brakenbury, Debbie Curtis, Ivor Davies, Brian Dickinson, Keeley Hale, Nigel Harper-Scott, Kris Lockyear, David Morrison, Irina Natarajan and son, David Pengelly, Ellen Shlasko, Graham Spurway, Sarah Talks, and Derek Turner.