08 December 2014

Latest Results: Ovingdean

Brighton and Hove Archaeological Society are a very active group fieldwork wise, with an insanely long digging season and a lovely supportive approach to new diggers. It was with BHAS that I started doing geophysics, with Bill Santer teaching me how to do earth resistance. Bill sadly passed away this year, so I would like to dedicate this survey to him, the fantastic bloke who started me on this path. He will be sorely missed.

The site that BHAS have been digging this year is at Ovingdean, where there is a medieval manor complex within an earthwork enclosure next to the church. They have done a few seasons of excavations here on one side of the enclosure, based on some earth resitance they did before I started doing archaeology. They have excavated the main manor house, which has very chunky walls and an undercroft, and this year they have been excavating what looks like a post built barn structure next to it. There is still the other half of the enclosure unexcavated, and BHAS wanted to know what was there.

First, the original earth resistance, flattened with a high pass filter so you can see the main manor building amongst the rubble near the churchyard wall to the south-east. A trackway snakes through the middle of the enclosure from an entrance in the south-east and possibly out the other side to the north-east. To the north-west, parts of the enclosure revetments and bits of masonry buildings up against the earthwork enclosure can be seen.

Earth Resistance. Click for a larger image.

This year's surveys include magnetometry and GPR. Being chalk, the magnetometry results are predictably rubbish, but do show hits of the enclosure to the south-west. Much of the north-west part of this survey is obscured by the magnetic halo of a large water unfortunately.

Magnetometry. Click for a larger image.

The GPR was a lot more productive, showing what looks like an open sided barn and an attached dovecote against the north-west enclosure earthwork. Signs of the outer enclosure revetment are visible in other layers further down.

A single GPR slice. Click for a larger image.


05 December 2014

NSGG Conference 2014

The time came once again to attend the Near Surface Geophysics Group conference, which is held every two years, the last being in 2012. As always, the chats in between lectures were the best bit. My ego was thoroughly strokes by three people coming up to me out of the blue and complimenting me on Snuffler. It's a wonder I got my head out of the door at the end of the day. I also spoke at length to Erica Utsi to enquire about attaching my forthcoming purchase of survey grade GNSS to my GPR.

The talks were many and varied. There were 12 of them! My favourites were :

Kris Lockyear talked about getting community archaeology groups involved in geophysics and some of the results they had. The survey of part of Verulamium. A grant bought a fancy mag cart, which seem to be everywhere these days, which the various groups share, with training days on how to use it. This sort of work is excellent for getting smaller groups who otherwise would not have access to geophysics. They have an excellent blog.

The talk I was most looking forward to was Armin Schmidt (of course) talking about inversion modelling for magnetometry. It seems the subject is a lot more subjective than I imagined, with various models of 3D interpretation potentially fitting the 2D data. I asked where I could find out more about how this was done, as I was thinking of incorporating this into Snuffler. The ripple of laughter that went around the room suggests that this is a black art best left to the damned.

Pope-Carter (I think) spoke about some open source software for geophysics being developed in python by students at Bradford called ArchaeoPY. It was explained that a lot of the work of writing display stuff was already done and available in easy to use python libraries. A lot of the functions already available in the software were shown. The whole thing is impressive and has a lot of potential. Being open source, if there is a feature you think is missing, you can go ahead and add it yourself!

There were two interesting talks from overseas. Someone from Italy talked about the sites found there, including GPR over a proper Roman road with kerbstones and all, which was very nice to see. I do like a bit of Roman. Someone from Canada produced some actually really good plots using EM of buildings, probably because the remains were shallow. Perhaps EM isn't so bad after all, or maybe you just have to do it at the insane resolution that they did. Both speakers lamented the lack of understanding of geophysics there by the relevant cultural authorities.

More Roman from the Canterbury Hinterland Project, both close to me and Roman, my favourite. It is run by institutions outside of Kent, since no-one in Kent seems to do geophysics. Some of the buildings they found using high res radar were quite impressive, and odd looking. There was also an interesting attempt to clean up the rather messy GPR data, using a different method than Armin suggested two years ago. I think this was my favourite talk of the day.

Then, of course, there was the usual posters, many more this time around. The judging was by popular vote rather than the usual panel, so I'm wondering if it will end up a bit like the Eurovision Song Contest with everyone voting for their mates. I certainly did :)

Hardware wise, mag seems to be heading for multi-sensor carts, some of which were evident in the talks, posters and commercial exhibitors. The greater speed and resolution can only be a good thing. I just don't understand how people afford these things.

All in all, another good year.

06 October 2014

Latest Results: Barcombe

This summer, despite the aweful August, I found time to return to my favourite Roman part of Sussex, Bridge Farm, Barcombe. I wanted both to extend the magnetometry survey even further east along the roadside settlement than last time. I also wanted to examine parts of the settlement with the radar, to see what I could find out about the roads and buildings.

We were also lucky enough to have some interesting crop marks making an appearance in the latest (2014) Google Earth imagery, alongside an aerial of this years excavation by the Culver Archaeological Project. Parts of the main road through the settlement are visible heading WNW-ESE. To the south, the edge of the massive pit(s) associated with the big industrial feature that showed so well on the magnetometry. This extended further to the north than the magnetometry suggested, which showed two conjoined pits, with a third phase showing on the aerial data attached to the north end of them both. This pit, taken as a whole, is about 40m across each way, so this is a massive industrial undertaking. It is likely that clay is being extracted, but whether it for making tiles, pottery or bloomery kiln linings is unclear. In the field to the south of the industry was a number of linear crop marks that didn't look very natural, so that is one of the areas we targetted with the magnetometry. Finally, a new trackway appeared as a crop mark in the field to the east, with a small enclosure attached to it, which is also an area we targetted with magnetometry.

Bridge Farm crop marks. Click for larger image.

Let's start with the field to the south of the industrial area. These crop marks didn't look very natural to me, but as you can see from the geophysics below, they are. There is a hint of something aligned to those crop marks, which is probably geological. Archaeology wise, there are a couple of pits to the north-west and the track coming through from the field to the north in the north-east part of the survey. No need to return for further work in this field, but it does confirm the southern extent of the settlement.

Field south of industrial area. Click for larger image.

The new area at the eastern end of the settlement was much more fruitful with a good number of settlement realted pits visible. The main road heading east towards Arlington and Pevensey shows nicely, with the outer ditches 22 metres apart and a slight remnant of the inner ditches showing at 10 metres apart. Also making an appearance are two new side tracks on the north side of the main road. They seem quite different in construction. The long thin track heading east is only 6 metres wide. This is the track that appeared on the aerial photographs. The small enclosure also appears, but unlike the aerials, where it appears continuous, the rather chunky ditches seem broken here. The second track, which meets the first, is 13 metres wide on the outer ditches and 3 metres wide on the inner ditches. These two tracks seem to disappear as they meet. It is unclear whether either of them continue, or what their function is. Slag metalling can be seen where the larger track meets the main road. It looks like slag has been used to repair the road here, where the road has become worn due to the presence of the junction here. The settlement surrounding the junction seems stronger too.

 The eastern end of the settlement. Click for larger image.

The iron slag is scattered liberally on the surface in the south east corner of the field too, along with a couple of other interesting finds, which you can see below. The item on the left is the iron slag. The item in the middle seems to be opus signinum, the distinctive pink concrete the Romans used for fancy flooring. This seems to have something accreted to it. The item on the right seems to be a small amount of opus accreted to a piece of dressed stonework. Fancy stuff. None of this is likely to have come from the Bridge Farm settlement though. The source will be one of the larger iron working sites somewhere up in the weald.

Slag & opus. Click for larger image.

Finally on the magnetometer front, we resurveyed the area where this year's Culver Archaeological Project excavation took place, at a higher resolution. As well as the posthole building showing up nicely, there are a variety of ditches and pits for the diggers to get their teeth into. The dig was a fantastic success, with the highlight being a number of waterlogged leather and carved wood finds. You can read some of the project's blog posts here, here, here, here, here, here, here, here, here and here.

Higher resolution magnetometry of 2014 excavation area. Click for larger image.

Ortho'd drone photo of the dig after machining. Click for larger image.


Now onto the radar. I have been waiting to do this for quite a while. The wet August weather stopped me getting this done earlier, so I was glad to get out and do this in September. As well as examining the roads, I wanted to look for buildings with masonry foundations. Unfortunately, scanning around the defended part of the settlement with the radar, I found none of the latter, and the former were somewhat less in evidence than expected. I have intended to survey more areas, but scanning across them found nothing worth surveying.

Three radar areas in the centre of the settlement. Click for larger image.

You can see the three radar areas in the above image.  The left most survey area was disappointing. Only a small hint of a surface showed of the main road coming in from the west. The other two roads I was expecting (more on these in a bit) did not show at all. The right most survey area was more interesting, with the main road through the settlement showing nicely. Attached to this was a metalled extention of the road from London, which originally stopped at the road coming in from the west. There are a couple of patches of repair to the north of this, but the other roads don't seem to have been metalled other than that. They don't seem to have been ploughed away, and robbing doesn't seem to be the answer with what is left, so it seems these road were not metalled in the area of the settlement, which is most odd. Most of the other tracks within the defended area of the settlement don't seem to be metalled either, even the main north-south track to the west of the London road. Going back to the extension to the Barcombe to London road, apart from being metalled, which the main road north wasn't, the ditches are narrower and the road is on a slightly different alignment, reinforcing the different date for construction. If you look closely, you can see Ivan Margary's 'section 14' through the road surface, unknowingly only metres from the end of the road. The pottery he found in the road ditches lead me to find this settlement in the first place.

Talking of dates, or at least sequence, the order of building seems to be as follows. First, the road came in from the west and stopped just to the east of the defended area of the settlement. Next, the settlement started, followed by the road heading north up to London. Next, the main road through the settlement was built, with metalling this time, and this headed east towards Pevensey. This would most likely have all taken place in the first century. Finally, the defences were dug in the late second century.

Back to the radar, I surveyed the final area of the three, in the centre, because I could see something going on south of the main east-west road by scanning around with the radar. You can see the metalling of the main east-west road, but scanning in this field and the field to the west showed that this did not extend west out of the defended area. People would most likely have continued on and up to the main road west, despite the lack of metalling, but a formal road never seems to have been built. Attached to, what seems to be, the very western end of the road, a further road has been built at a diagonal to the settlement layout, heading south-west. Metalled surfaces on roads were mainly intended for cart traffic, so why was it heading south rather than north to the main road west out of the settlement? The answer may lie in an area close to the water and just to the south of this year's excavation.

Possible port area. Click for larger image.

That diagonal road leads to another east-west track, which was found to be lightly metalled in last year's excavation. Now look at the radar on the left of the above image. Ignore the two linear features, they are modern cow tracks, but in the north-east corner of that radar suvey is the end of the metalled surface of the east-west track. Also visible is light metalling from the end of another track coming in from the north-east, which is one of the three expected tracks that don't appear in the first radar survey I talked about. So what are these tracks heading for? On the magnetometry in this area are a group of strong metal dipoles, out in what would have been the water in the Roman period. My guess is that there would have been a small port here, which those carts would have been heading for. It seems to be roughly where you would expect such a feature, being downstream from the bridge and close to the centre of the settlement.

Finally, jumping over the river to the west of the settlement. I'd had a theory that the settlement extending east-west in this field was actually roadside settlement along the edge of the Greensand Way, so I decided to test this with the radar. The results weren't exactly spectacular, but did explain what had been going on with this field. The magnetometry results in this field were patchy at best, compared to the excellent results in the field to the south. Because of the gravels in this area, the radar was able to show why. Plough lines were visible 90cm down in the survey area, which is a huge depth for ploughing. Was this field steam ploughed in the past? The surface of the north-south road was visible, but only really at the southern end of the survey area. Similarly, the east-west road was only visible at its western end, as it seemed to be diving down in this direction somewhat, protecting it from the plough above.

Radar west of the river. Click for larger image.

If this is the real course of the Greensand Way, then where does this leave the road network in the area? In the map below, you can see how the road network interacts with the settlements east and west of the river, plus the villa complex. Deprecated parts of Margary's road course are shown in blue. The new course of the road avoids the stream valley north of Barcombe plus the steepness of Crink Hill.

The Barcombe settlement area. Click for larger image.

Finally, a big thankyou to all of the people from the Culver Archaeology Project and the Roman Ringmer Study Group who helped with the surveys. It wouldn't have been possible without you.

12 July 2014

Messing around with GPR

I'm still learning about my GPR system and what it is capable of, so recently I have been trying out a few things to find out what the results are like.

Whilst you wont see much of a cut in clay with GPR, since the geology and clay soil have similar EM properties, I wanted to try out what a cut in chalk looked like, since chalk bedrock is going to have a different response to chalk soil. Firstly I visited the BHAS dig at Ovingdean where they are excavating a medieval manor complex. They had a barn like structure with large square post holes, which showed up nicely. While the normal soil-chalk boundary was fuzzy due to the top layers of the underlying geology breaking down into a soil, the bottom of the post holes were a much cleaner cut with a much clearer change on the radar.

Pleased with this success, and happy that I now have something that will quickly assess chalk sites (unlike magnetometry, which is rubbish on chalk without tertiary geology), I decided to try it out on another chalk site. This time, a neolithic causewayed enclosure. There were sights (and sites) to see on the way. Firstly, climbing the hill itself was illuminating. Below you can see layers of flint within the chalk. To the left and in the middle are two layers of nodular flint, which look broken up. To the right, you can see a more solid looking layer of tabular flint. These layers seem to be diving down into the ground, but as I was climbing the hill, they are actually pretty level.

Flint Layers. Click to Enlarge

Reaching the top of the hill was a Bronze Age barrow, unfortunately looted in antiquity. Again, because there is no height data, it looks like a pit. The layer visible is the underlying chalk. It seems there is not much solid chalk in the mound of the barrow.

Bronze Age Barrow. Click to Enlarge

Finally, I reached the neolithic causewayed enclosure, and was rather disappointed. I couldn't make out much difference in the ditch cuts. It didn't help that flint layers kept getting in the way, pretending to be archaeology and becoming the latest in a long line of 'annoying geology'. I later saw an old picture of a ditch section from Whitehawk Hill, which ASE are digging at the moment, and it looked like the ditch fill was mostly chalk rubble rather than soil, which would explain the lack of difference in EM response and why I couldn't see much.


Another thing I wanted to try out was a bit of Roman road hunting (of course). A lot of the Roman road network has unfortunately been built on, making it difficult examine. Unlike with earth resistance and magnetometry, it is possible to use GPR to examine built-up but flat surfaces, such as a modern road or pavement. I took the opportunity to visit the excavation at Ewell, where Stane Street and associated roadside settlement is being excavated in a small field (church meadow) near the church. The course of the road to the south-west is somewhat unclear, as there is a turn in the road somewhere in the area. With the help of the local archaeologists, we found the Roman road crossing the more modern Church Street. You can see the curve of the agger in the centre of the image below.

Stane Street. Click to Enlarge

I've still got a lot more to learn about GPR. I hope to learn a lot more when I do a week of GPR surveying at Bridge Farm shortly. Watch this space.

12 May 2014

Digging up the Geophysics at Oaklands Park

Readers of my blog will remember the magnetometry and radar at Oaklands Park, Sedlescombe. As well as the final report being published, some excavation has been going on, run by the Independent Historical Research Group. Just for a change, I was the site director! Despite thinking that geophysics is way better than excavation, this site provided a genuinely interesting archaeological question that only excavation could answer, which is whether or not the site was run by the Classis Britannica. It is an industrial scale iron working site, connected by iron-slag metalled roads to other Classis Britannica iron working sites and situated on what would have been a navigable river. The answer was surely yes and this could be proved by finding the special CL:BR stamped tiles that can be found on the sites that they ran. Here is the trench layout.

Trench Map. Click for a larger image

Trench A) This was the main trench. I was hoping the building shaped structure on the geophysics would turn out to be some sort of administrative building with a roof of CL:BR tiles. The walls appeared on both the magnetometry and radar, so I was expecting the foundations would be constructed of iron slag, which they were. Apart from that, things were quite different than expected. The feature in the centre of the building, which I had presumed was a central support was actually a trample layer. A smith had worn a shallow groove in the ground whilst standing at his anvil, leaving a thin layer of iron-slag, charcoal and hammer scale. The inner north wall (there are two) was quite substantial, being composed of layers of iron slag, on top of which a wooden building would have stood. The outer north wall was more slight and was most likely a lean-to against the main building. This only went half way across the trench from the east, so the entrance to the building may have been here. Unfortunately, the floor layer of the building had been ploughed away, but the lack of floor material and the trample layer in the centre of the building suggest that the floor was bare earth. No tiles were found in the area, so the building most likely had a wooden shingle roof. The identification of the building as a smithy raises further questions, such as what they were making. It may have been as simple as hammering the raw blooms into a shape suitable for export. On the geophysics, the middle of the southern wall of the building shows a strong feature jutting slightly into the building. This is most likely the forge.

A slot through the inner north wall in Trench A

Trench B) The original purpose of trench B was to target what seemed to be a pile of moderately magnetic material to the west of the smithy, in case a tile roof had been stripped and dumped to one side before the building was robbed, which the radar suggested had happened on the west side. The results of the excavation were slightly different. At the lowest level, at the level of the natural, some stake holes were found. The remains of a burnt plank was found resting on top of the natural. On top of this, a layer of redeposited clay and earth mixed together was dumped, presumably to level the area out in order to construct a wooden building on top. Unlike the main building, there was no foundation, just a beam slot and some postholes cut into the redeposited clay. On top of this, a destruction or occupation layer filled with charcol  contained a lot of pottery and iron slag. The form of the building is unclear given the small area sampled, but it is more flimsy than the smithy.

A complete pot found at the bottom of the occupation layer in Trench B

Trench C) This trench targeted a massive pit feature, about 35m x 10m, to the north of the smithy. The radar suggested that it was lined with a lense of dense material and it was hoped that some tiles had found their way in. The very deep (1.5m) test pit found pretty much what was expected. The bottom of the pit contained thin layers of burnt clay, slag and charcoal, unfortunately tile free. Above this was a very thick layer of silt, presumably because the pit filled with water, followed by a layer of hillwash with a much greater clay component. The natural here was a very clean clay. As this massive pit was only partially filled with rubbish and lacked any iron stone in the natural (the quarries for the iron stone were at the top of the hill), the quarry was most likely for the extraction of clay to build the bloomeries. To further vex us, we found a modern cable and a land drain cut without a land drain in it.

Trench C: Hello down there!

Trench D) After quickly exhausting the first 3 planned trenches much quicker than anticipated, thoughts turned to what to do next. I toyed with the idea of opening up a huge trench over a large (3m x 7m) bloomery, but decided against it, as that would not have furthered the research question. It was decided to further sample other parts of the site in the hope of finding the CL:BR stamped tiles, so trench D never happened.

Trench E) This trench was in a similar massive pit to trench C, but within the rectangular enclosure to the north-east, near the well. This quarry was 20m x 10m and the test pit found a similar thin layer of material at the bottom, but this time composed entirely of burnt clay from a destoyed bloomery. Below this layer was a thick layer of redeposited clay with little in it apart from pottery at the very top. We never reached the natural. Unlike the quarry at trench C, this quarry did not silt up due to being filled with water but was filled with hill wash.

 Some of the burnt clay layer in Trench E

Trench F) This trench targeted the main slag heap against the northern edge of the field. It was recorded that coins and other occupation material had been found in the slag heap as it was being removed for road building, so it was hoped that some tiles would be found. After about a metre of topsoil that had been ploughed down the hill, the slag bank was reached. It was black with charcoal and contained few finds, with only two pieces of pottery being found in 30cm of excavation. It was decided to close this test pit before the slag heap bottomed out.

 Trench F. Nothing to see here, move along.

Trench G) Targetting a different feature type and a different part of the site, what looked like a rubbish pit on the geophysics, about 2 metres wide, was half sectioned at the western end of the site. It did indeed turn out to be a rubbish pit as expected, with a huge amount of pottery in a black charcoal layer at the bottom. There was a dump of burnt clay on top of this on one side of the pit follwed by a silting up of the feature and some sandstone blocks being dumped near the top. There was quite a bit of post-medieval tile in the plough soil above the feature, but no Roman tile in it unfortunately.

A partially excavated Trench G

We didn't manage to prove the site was Classis run by finding their stamped tiles, but we found out a lot about the site while we were looking. Finding a Roman site with so few tiles of ay sort is quite an oddity in itself. A big thankyou must go to Pestalozzi for letting us dig up their land. Thanks to all the diggers, especially Robin, Brian and Cameron for all their sterling work.

13 April 2014

Latest Results: Plumpton

Late last year, I was involved with a magnetometer survey of Plumpton Roman villa with Chris Butler Archaeology Services on behalf of Plumpton Agricultural College. This means that his team did all the hard work and I processed the data. As the villa foundations were constructed of flint, it doesn't show up, but the enclosure around it does. There are also strong magnetic readings at either end of the enclosure, which may relate to a hypocaust system. There were also signs of field systems associated with the villa to the south, and a possible Roman road to the west, which I was very pleased about. This pattern of a road passing rather than heading to a villa is repeated elsewhere, such as Barcombe.

Plumpton Villa magnetometry

At the time time, David Millum of the Culver Archaeology Project led a team surveying the villa itself with an earth resistance meter, which showed up the walls very nicely. It seems to be a standard winged villa, but with the western wing extending further to the north and west than it should. The villa as a whole looks like it has at least two phases. At either end, where we get the strong magnetic responses on the magnetometry, we also get strong high resistance areas, showing more than just the walls. Maybe it is just rubble, maybe it is something to do with a hypocaust.

Plumpton Villa earth resistance

This weekend, I taught a geophysics dayschool here as part of the Sussex School of Archaeology. As well as covering the usual earth resistance and magnetometry, we also did some resistance tomography and a single line of radar, both along the green line in the images below. I have layed them out horizontally at the correct position next to the green line that they were surveyed along, so just imagine that they are vertical slices through the ground along that line.

First the tomography. Several of the walls show as red points towards the top, plus at either end of the villa, we get broader and deeper high resistance confirming the standard earth resistance. There is also a hint of something heading further down under the ground at either end.

Plumpton Villa resistance tomography

Finally, the single line of radar we did. The ground was a bit wet, so the top layer is a bit of a mess, but many of the walls are still visible below that. The two walls visible in the centre of the villa are at the expected depth, but those to the west and east are deeper, further suggesting a hypocaust system.

Plumpton Villa GPR traverse

The Sussex School of Archaeology will be running a training excavation on the villa in the summer of 2014.







09 March 2014

Version 1.13 of Snuffler released

It's time for a new version of Snuffler, and this version is all about importing stuff. There are two new files imports for getting data into Snuffler.

The first is for importing TerraSurveyor (was ArchaeoSurveyor) grids and composites. TerraSurveyor is a (very good) commercial equivalent to Snuffler. Someone sent me some data in that format, so I took the opportunity to write an import for it.

The second is Surfer ASCII grid files, not to be confused with the ASCII Grid files that ESRI produces. Surfer is software for displaying 3D geographical data. It wasn't data from that software specifically that I was trying to import though. ReflexW, the GPR software I use, has an export function for time-slices in Surfer ASCII grid format and I wanted to play around with slice data in Snuffler. Here is a time slice from the site I covered in my last post display in Snuffler.


You can download the new version at the usual place.