15 September 2019

Latest Results: Hébécrevon, France

It's not often I leave my comfort zone of doing geophysics surveys on Roman sites in the south-east of England, but my latest survey was definitely outside, being on a WWII site in France. At Hébécrevon was part of the German main line of resistance against the allied advance into France after D-Day. It was designed to slow down the advance rather than completely stop it, and in that regard was successful. The section being looked at near Hébécrevon came under attack by US forces, and is being investigated by the Battlefield Archaeology Group.

The first site is a small hill overlooking a stream valley. The side of the hill had German positions among the trees, which kept the US forces at bay for a while. The field at the top of the hill was originally 3 fields, divided by hedgelines running north-south, which have now been removed. It is known that WWII positions were dug into the side of the hedgerows, to help hide them from aerial reconnaissance. Therefore, it was important to survey the field walking east-west, to highlight the ditches either side of the hedgerows into which the foxholes were dug. The northern part of the field was already under archaeological investigation, so the geophysics was limited to the southern part of the field. So what does the magnetometry show?

Field A: +/-5nT

The first thing to notice is the huge number of metal dipoles across the field, due to shrapnel from bombing and the battle at the site. This makes it difficult to spot weaker features. The concentration of metal drives up the spread of readings, so if you use standard deviations to set your display bounds, you would end up around +/-10nT. You will only end up seeing the metal, and any softer features will be invisible. Conversely, if you set the display bounds too low, you would see some softer features, but you would also be swamped by thousands of tiny pieces of shrapnel, as shown below.

 Field A: +/-2nT

The trick is to look at images using multiple display bounds in order to get a better picture of what is going on. Take the below image for example. The display bounds are much higher, and you are left with the larger pieces of iron. This is important information, because all of the shrapnell will be small, but larger dipoles could represent pieces of iron equipment from WWII. Couple this with the position of the hedgerows, where you would expect WWII features, and you have a set of targets to excavate. A list of likely targets, A-K are shown on the image below.

 Field A: +/-30nT

Staking out the targets can provide further information. The rough position is found and marked, then the precise position is found by setting the range on the magnetometer higher and scanning for the highest reading. We then used a metal detector to get information about depth, size and composition of the target, to decide whether or not to dig it. Below is what we learned about each target. Many thanks to Brian for the pictures.

Feature A is the first of five features arranged in a crescent shape on an old hedgeline. It was a horizontal dipole and the metal detector suggested it was a metre deep. Excavation found the remains of an air to ground rocket fired from a typhoon, buried in the natural.

 Feature A

Feature B was a vertical dipole, with the metal detector suggesting the feature was half a metre down. Excavation revealed a foxhole dug into the hedgeline which contained American 105mm artillary shell cases and other artifacts.

Feature B

Feature C was a vertical dipole, with the metal detector suggesting it was half a metre long and a metre down.  Excavation revealed the long artifact the metal detector was picking up was a case for a German M42 machine gun barrel within another foxhole.

Feature C

Feature D was a vertical dipole, and the metal detector suggested the feature was sizable and half a metre down. Excavation revealed 3 gas masks and infantry equipment for a german soldier.

Feature D

Feature E was a horizontal dipole. The metal detector suggested the feature was half a metre down. Excavation found a foxhole containing more American 105mm artillary shell cases.

Feature E

Feature F was by far the strongest feature on the magnetometry results. The metal detector suggested it was 1m down and roughly 2m square. Unfortunately, you can't win them all, as it turned out to be a section of modern metal cable.

Feature F

Feature G is a feature of two parts. There is a horizontal metal dipole on the west side of what appears to be a sizable cut. Cut features are difficult to see in these results with the amount of metal everywhere. I suggested that the cut might have been where a tank had been dug in. The metal detector suggested that the dipole was only 20cm down. At that depth, it was likely to be modern, and it did indeed turn out modern, as it was a metal pole used to hold up an electric fence. A slot was dug across the cut, but the results were inconclusive.

Feature H was a horizontal dipole in the eastern of the two hedgelines that once divided this field. The metal detector suggested it was only 20cm down, so it was decided to quickly dig it out. The metal detector turned out to be picking up something closer to the surface, and the feature generating the large dipole was actually a metre down. It was an open ammunition case lying on top of a large amount of belted ammunition.

Feature H

Feature I initially looked like a cut feature in the middle of the field. We later found the spot was once occupied by a large tree, so is probably a silted up tree throw. We decided not to dig it.

Feature J was a horizontal metal dipole which the metal detector found to be 0.5m down and quite dispursed. It was a good target, being on the hedgeline, but it was decided that enough trenches had been opened to cover the week.

Feature K was a horizintal metal dipole which the metal detector suggested was small and 1m down. It wasn't on the hedgeline, so wasn't seen as a priority to dig.

The geophysics was a great success, identifying several features by finding their contents. Finding the ditches of the old field boundary was important to identify which features should be a priority, so walking across rather than along them was important. The combination of the magnetometer and metal detector was very useful in assessing what features should be dug and what should be ignored. It's also important on a site like this to not just accept the default display bounds for the magnetometry results, but to try different settings in order to highlight different types of feature.

Further to the west of the excavation side is a field known to have been used as part of a German road block, with tanks and machine gun positions stopping the advance of the American troops. The field was much larger than the first, and judging by the amount of metal lying about on the surface, it was going to be just as noisy as the first field.

Field B: +/-2nT

Field B: +/-5nT

Field B: +/-30nT

Again, looking at images with different display bounds is useful, with cut features showing best at lower values and the larger chunks of metal showing best at higher values. Of particular interest is a set of features highlighted with the red arrows on the 5nT image, on which it shows best. There are two sets of broken linears here, which fit within one of the historical smaller fields that make up this now much larger field. This could be a German 'square wave' trench. As I was walking north-south, and east-west sections of trench show best, with the short connecting pieces of trench between the two lines falling between my survey lines, and are thus invisible in some cases. That's an excavation for another year.

22 November 2018

Latest Results: Brough

Sometimes I get asked to survey a bit further afield than my home county of Sussex. I will pop over the border to Kent, Surrey or Hampshire. Recently I was asked to do a survey in Yorkshire! Despite being a very long way, it seems odd for someone from the south coast to be asked to do a geophys survey so close to Bradford, but apparently my results from Chichester made me famous. I'm glad I did though, as the site is very interesting. My contribution is only a small part, as part of a larger project involving Petuaria Revisited, Elloughton cum Brough PFA, the Roman Roads Research Association, Hull University, Geophiz.biz and East Riding Archaeological Society.

At the site of a Roman period ferry crossing of the River Humber, grew a town by the name of Petuaria, progressing from a military fort to the civitas capital of the Parisi tribe. Most of the town is now covered by the modern town of Brough, leaving just the eastern side located under a playing field. Magnetometry and earth resistance have been done at the site, but these are somewhat limited by the depth of made ground used to level the site to make the current playing field. Enter myself, with a GPR, to try to get the depth required to find the most interesting features. Despite the rain, the results were good, as the grass was short, and the geology was sandy. Some features had already been excavated back in the 1930's, before the area was scheduled, but there were many gaps in the map that needed to be filled in.

Click for larger image

The eastern edge of the site is marked by a wall (pink) and associated ditch systems (purple), within which can be seen a number of building revealed by walls (light blue) and demolition rubble (dark green) aligned to a road system (light green). One building, at the western edge of the survey area is particularly large and has surviving floors (dark blue). The layout is certainly striking, but its function is not quite clear. The curving nature of part of this building possibly marks this as a theatre, which matches with a piece of reused inscription found nearby, but a forum, macellum or town house with courtyard are also possibilities.

The BBC have picked up the story in the local news, and I managed to make a television appearance. It would be nice to see the larger building excavated to work out its function, but that seems unlikely as the site is scheduled.

22 July 2018

Digging Up The Geophysics: 2018 Edition

Recently, I realised that there were currently five sites undergoing excavation, all in Sussex, that had targetted geophysics I had done. With my ego approaching dangerously unsafe proportions, I resolved to go on a quest, to visit all five sites in one day, and make a blog post about it. All of these sites have volunteer positions, so if you fancy a dig next year, you can join up and join in.

Site 1: Priory Park, Chichester

After the GPR survey in 2015, and a successful first season of excavation in 2017, CDAS returned for a second season of excavation looking at the east site of the bath house. The excavation trench was much bigger this time, and they are looking for the connection between the bath house and the Roman town house to the south. There are what seem to be robbed out walls connecting the two, which need to be confirmed on the ground. Judging by the GPS survey, the new trench covers part of last year's excavation, and the entire bath house.

Priory Park excavations, 2017 and 2018 trenches

When I visited, they were only half way through the excavation, so the layout of the building wasn't completely clear yet. The picture below is of the re-cleared area from last year, showing pilae, with some floor still surviving, sticking out of the baulk. I gather from later reports that the building was not completely excavated by the end of the second week, but they had certainly made progress. Some of the pilae seemed to extend out of the south wall, which was very odd, as no additional rooms appeared on the GPR. Perhaps the building was bigger than the tiny footprint I had found.

Priory Park bath house

Site 2: Rocky Clump, Stanmer

This is a Romano-British farmstead site on the Downs north of Brighton that has been under excavation for many years by BHAS. Historically, the excavation started in the clump itself, then in more recent time, moved north of the clump, and then after some geophysics, excavations have moved to an enclosed settlement to the south-east of the clump. The currently open trench, which is tacked onto the east edge of the last trench, covers several ditches within the settlement, in an attempt to find evidence of occupation.

Rocky Clump excavations and ditches found so far

As you can see, a number of ditches have already been found. The feature in the corner of the inner enclosure turned out to be a solution hollow. Some of the ditches have a high flint content, seemingly placed in such a way as to suggest that their placement is deliberate. Could it be to support something above? If so, why the need for a ditch in the first place. As ever Rocky Clump excavations providing as many questions as answers.

 North ditch under excavation

 Flint layer on top of inner south ditch

I also resolved to investigate further the area between the current excavation and the clump. There are a number of large features on the earth resistance and radar where it is not quite clear whether they are archaeological or geological. I tried radar as a further complement to the earlier surveys, which managed to answer the question. The geological features are all clay-with-flints, and there is enough moisture left in the ground, despite the hot weather, to render the clay inpenetrable to radar. These are easy to spot in the results. There is a strong signal up top, then a blank area below, as the signal is attenuated by the clay. This contrasts nicely with the surrounding messiness of the chalk, which is visible to a much greater depth.

Clay with flints in a solution hollow surrounded by chalk

One of the features did turn out to be much more interesting. Situated right next to Rocky Clump itself, was a circular feature. It is barely visible as an amplitude change, but is very clear as a phase change. It has a shape of a shallow, round bowl, and my best interpretation is a dew pond. There are plenty of animal bones at Rocky Clump, so it makes sense that there is a water source for the animals, if it is Roman in date.

Vertical section across the centre of the possible dew pond

Possible dew pond, top left

Site 3: Plumpton Roman Villa

Ok, for this site, I only set out the grids and processed the data. The survey itself was done by someone else. That still counts, right? The site has been under excavation by the Sussex School of Archaeology for a few years now, starting at the east end of the villa building and working west. The western end, which is now under excavation, seems to be interpreted as a bath house. Below is the current excavation trench, overlayed on earth resistance of the site. Some of the southerly walls do not show up well on the geophys, as there is a higher content of chalk there compared to the use of flint in the north. I have not recorded the more substantial western end in the plan below, as that is still to be fully revealed by excavation.

Plumpton excavation trench and exposed walls, 2018

 View from the south, looking as the chalk wall foundations

More substantial foundations on the north side

Site 4: Barcombe Roman roadside settlement

I started surveying this massive site back in 2011, and it is currently under excavation by the Culver Archaeological Project. This year, they moved to the centre of the defended settlement, in the hopes of finding the footprints of buildings. What postholes they have found so far don't quite resolve themselves into buildings yet. Instead, there seems to be a lot of kiln features, though whether these are industrial features, or something domestic, like corn drying, is not quite clear yet. You can see the current excavation area highlighted in the images below. If we change the display bounds to a higher value, it makes it easier to see which features are more likely to be industrial in nature. The black demolition layer visible in other trenches is also present here, so it must have been quite an event. What's left of the road surface is very close to the surface, so no surprise it is being plouged away. The trench is a rather large 20m x 45m, so they certainly have a large area to investigate. They will be back here next year.

Magnetometry, 3nT display boundary

Magnetometry, 20nT display boundary

One of the kilns. Site supervisor for scale

Site 5: Jevington Church

Jevington are extending their churchyard, and I was asked by ENHAS to survey the area ahead of an investigation. The magnetometer was pretty useless in such a small fenced field, but the GPR came up with some results, despite problems with the encoder wheel, thankfully now fixed. At the northern end of the field seemed to be an old track, leading to the church, surrounded by some solid features. I originally thought these might be grave furniture, but one of them, under excavation, turned out to be the rather boring concrete footing for an old fence post. The track itself was composed of gravel. Further to the south, the tenuously identified plague pit turned out to be a scatter of neolithic struck flint, which was much more interesting than a fence post.

 Some of the features found at Jevington and the trenches opened to investigate them

Concrete post footing

Neolithic flint scatter. Photo courtesy of Martin Jeffery

14 August 2017

Latest Results: Bodiam

I was recently asked by the owners of Quarry Farm, Bodiam, to look at the Roman roadside settlement on their land, which is right up my (Roman) street. They were kind enough to put me up in their fantastic glamping site, with views across the settlement to the castle :

View to the castle from the glamping hut

The site was excavated back in the 1960's, when the Battle and District Historical Society dug up a Roman building with Classis Britannica stamped tiles associated with it. The Roman road broadly follows the modern road across the floodplain, and the presence of the CL:BR stamped tiles in the floodplain has led to the interpretation of the site as a port. Further to the south, as the land rises out of the floodplain, some iron slag has been found, suggesting an iron working site.

Lidar shows up an interesting feature in the floodplain, an embanked paleochannel that cuts much further south than the current course of the river. It has been suggested that the river was redirected further north in medieval times to help fill the moat. Also look at the two fields just south of the floodplain, and notice that the field on the left is significantly higher than the field on the right.

lidar of the site, click for larger image

So what about the geophysics? Starting with magnetometry, the reason for the height difference between the two fields is obvious, as there is a medium sized iron working site, whose slag heap has raised the level of that field compared to the one next door. You can also see a slag metalled track leading from the Roman road (to the east) to the iron working site. The floodplain is not so easy to interpret. There is a lot of metal junk, geological features, and material from the railway and modern road. It's difficult to say with certainty from the magnetometry that anything in there is Roman, despite the amount of Roman material found in excavation.

Magnetometry, click for larger image

It's down to the radar to help sort the mess out. The orange lines mark out the areas surveyed with GPR. The black rectangle is the Battle and District Historical Society excavation area, in which was found the building they excavated. That building doesn't show up on the radar, which is not surprising, as the excavations were described as waterlogged, and GPR doesn't do well with wet alluvium. There is what looks like new building just next to it though, closer to the road. Most interesting though is an oval bowl shaped feature surround on three sides by hard standing. My gut says this is an upstream port, but the paleochannel seems to go through it rather than by the side of it, so I'm not sure what to make of it.

Interpretation of floodplain GPR, click for larger image

Back at the iron-working site, the radar showed the extent of the slag heap as an amplitude change on the enveloped data, and also showed the location of three bloomeries as circular phase change features on the unenveloped data.

Interpretation of iron working GPR, click for larger image

So there's a lot going on, but not quite as large a settlement as expected. If you want to know more, you can read the full report here.

16 June 2017

Digging Up The Geophysics: Chichester

A couple of years ago, I did some radar surveys in Priory Park, Chichester. After a small test pit dug by CDAS last year, there was an official press release by Chichester District Council, which then led to an awful lot of press coverage (some of them actually managed to spell my name right), and me appearing on South Today briefly. Then this year, there was a bigger excavation run by the council and CDAS that produced even more press coverage. Here are a few more details about it all than you may not have heard about in the press.

Click for larger image

If you look at the above image, the test pit from last year is on the western end of the building marked 'B', targetting the surviving floors, while the larger trench from this year targets the smaller building marked 'C'.

Click for larger image

What is left of the floor fills most of the test pit in the above image, unfortunately with no surviving mosaics. There are walls to the west and north, with a cut representing a robber trench for another wall to the east, over which can be seen part of the remaining floor in the next room

Click for larger image

The above image shows the southern end of the larger trench from this year, looking west. The funny shape of the end of the wall visible in the radar is also visible in plan. The stacks of tile are pilae forming part of a hypocaust heating system, over which would have been an opus floor, part of which is visible sticking out of the baulk at the bottom of the image. That floor, and the pilae, don't appear on the radar data though, so clay products don't seem to have a difference in contrast to the local soil. The building is most likely a small bath house, which despite being built next to the building to the north, is actually attached to the building to the south. This distance is so that if the building goes up in flames because of the under floor heating, the rest of the building is not threatened. Dating seems to suggest a date for the building in the last Roman period.

Congratulations to Chichester on a fantastic dig, and watch this space for more results if they dig more next year.

06 April 2017

NSGG Conference 2016

It's been a few months since the NSGG conference in December 2016, so it's about time I did my usual post about my favourite bits.

Erica Utsi, whose name is on my GPR and is shortly to publish a book recently became a TV star after appearing on a program about William Shakespeare's grave, and given we were a specialist audience rather than the TV viewing public, we got a slightly more in depth explanation of what it was all about, which was that Shakespeare's head may have been nicked due to a fashion for collecting the skulls of famous people.

Adam Booth treated us to some technology not often linked to the sort of geophysics we do, portable x-ray fluorescence, which can be used to identify the elements present in a sample, without having to a lab. His test site was the site of a WWII plane crash. Parts of the plane were visible on magnetometry, so what did the new tech turn up to go with that? In a transect across the site, a spike in copper and zinc from the remains of the plane leaching into the surrounding soil was visible. I've seen a similar talk where the technology was used in industrial sites, where it was suggested that it was useful for identifying prehistoric metal working, which may have been little more than a campfire affair. Always nice to see new tech explained.

My favourite subject, Roman roads, got a mention by Joep Orbons, who had thrown quite a few geophysics techniques (EM, Mag, ER, ERT and GPR) at a section of Roman road in Belgium. The sort of results he got were very familiar to me, with differences in preservation and different soil conditions giving different results of quite a small area, with some techniques (GPR, ER) performing better than others. Sometimes even massive features like this can be hard to find. Not content with one talk on Roman roads, that last talk was immediately followed by Michal Pisz taking about the Roman fort of Tibiscum in Romania with the Roman roads and surrounding vicus being surveyed and excavated.

Already mentioned in this blog, Chris Lockyear has been producing some amazing results on the Roman town of Verulanium here in the UK, with multiple buildings, roads and an aqueduct visible in surveys carried out using ER, Mag and GPR. The preservation is fantastic, and I really hope they find a lot more like it. If you haven't seen it already, check out their blog. Talking of big, pretty surveys, Tomasz Herbich spoke after Chris and has been researching ancient towns in Egypt, with predictably decent results from magnetometry due to the use of fired brick.

H Webber suggested a new avenue of research for archaeologists, using the vast geophysical surveys, such as EM, carried out for the benefit of farmers in modern day precision agriculture. Phosphates present in occupation material may highlight areas of occupation that the archaeological community were not previously aware of. Of course, the farmers would have to be approached in order to get this data, and someone in the audience pointed out that if all of this was explained to the farmers, some of them might deep plough the sites away in order to bring the free fertiliser to the surface.

Petra Schneidhofer gave us a talk about the state of geophysics in Norway and Denmark. Apparently, igneous geologies make our usual favourite, magnetometry, rather pointless, so GPR is commonly used instead. despite that, the natural variation over an area in GPR is quite extreme, and it can be quite difficult to pick out features. Thanks goodness for the boring sedimentary geologies in my part of the world.

All the little geophysical surveys

It's that time of year when the weather is getting a bit warmer and it is time for me to wander once more into the green fields of England, with a machine that goes beep, to find the lost wossnames of times past. It isn't just my own Roman period projects that I work on though, I also do work for various local societies, as many don't have their own geophysics equipment. Here's a selection of projects that I've been involved with recently.

The Pepperpot, Brighton

At the end of Tower Road, Brighton, there is a tower (no surprise there) which apparently used to house pumping equipment for a well that supplied the Attree Villa and estate. There was apparently a water tank and an underground tunnel under what is now the road, and Brighton and Hove Archaeological Society along with the Friends of the Pepperpot asked me to take a look with my radar. There were signs of rubble in the area where the water tank would have been, and very vague signs of the tunnel, but the results weren't all that clear. You can see the full report here.

The interpretation of the GPR survey over an old map of the area around the Pepperpot

Butts Brow Neolithic Enclosure, Eastbourne

Though mostly filled with a combination of car park and a clump of trees, there is a second neolithic enclosure above Willingdon, Eastbourne (the first being the more well known Combe Hill causewayed enclosure). After a season of excavation targetting the surviving sections of bank and ditch by the Eastbourne Natural History and Archaeology Society, I was asked to see if I could find them some internal features to dig up. It's rather difficult to see cuts in chalk with radar, especially with modern tracks and bands of natural flint around, but the ditch was slightly visible as a negative feature cutting through the flint layers. It's the dark band in the image below. The contrast between the ditch and surrounding chalk was very slight though, so smaller internal features were not visible. You can see the full report here and you can see a video of the results here. Details of a dig this summer will be published here at some point.

The neolithic ditch cutting through a band of natural flint

Southborough Post Mill

Just over the border into Kent this time, the Southborough and High Brooms Amateur Archaeological Society asked me to look at a platform in the woods of Southborough Common, the site of a post mill. Geophysics surveys in woodlands are never easy, and while the woods had been cleared, some trees remained. Both earth resistance and magnetometry were used, the results of which are in the channel merge image below. The magnetometry didn't show much apart from a big chunk of metal and some surrounding (no longer visible) fencing, the earth resistance showed a high resistance area on the east side of the platform, which may have been the site of the mill. You can see the full report here.

Earth resistance in green and magnetometry in red.