Tag Archives: urbanism

Durobrivae

Just to the west of Peterborough lies the Roman town of Durobrivae. This town is one of the so-called ‘small towns’, i.e., not one of the public towns with an administrative function.  It is, however, somewhat larger than some of the public towns such as Caistor-by-Norwich, the civitas-capital of the Iceni.  Duriobrivae had a town wall, parch marks from which can be seen on the Google Earth image along with Ermine Street (the straight line through the town from SE to NW) and the irregular street plan (Fig. 1).

Fig. 1: Google Earth image of Durobrivae.

Fig. 1: Google Earth image of Durobrivae.

Oblique aerial photography over the years has revealed much about the interior of the town, as well as extensive suburbs, prehistoric features, villas and so on.  Fig. 2 shows an oblique image of the town.

Oblique aerial photograph of the town. Reproduced courtesy of Stephen Upex.

Fig 2: Oblique aerial photograph of the town. Reproduced courtesy of Stephen Upex.

I was particularly fascinated to see a large group of circular features to the south of the town, some of which appear to be the ditches around round-barrows, but others are far too large and are tentatively suggested to be some form of henge (Figs. 3 and 4).

Fig. 3: Google Earth image of the field to the south of the town showing circular prehistoric features.

Fig. 3: Google Earth image of the field to the south of the town showing circular prehistoric features.

Fig. 4: Oblique aerial photograph of the field to the south of the town showing the Roman suburbs and earlier prehistoric circular features. Photograph courtesy of Stephen Upex.

Fig. 4: Oblique aerial photograph of the field to the south of the town showing the Roman suburbs and earlier prehistoric circular features. Photograph courtesy of Stephen Upex.

Ruth Halliwell (WAS), who has worked with CAGG regularly, is working on the town for her dissertation and we arranged to go and team-up with local archaeologists to undertake some survey.  It was very much a “proof of concept” trip: which techniques would work best there? Would the surveys add to what could be seen from the air?  We undertook three days survey running all three main machines (magnetometry, resistance and GPR), and Peter Alley also used his UAV to take high-level photographs, partly with a view to creating topographic maps.

Despite early problems with the mag, we managed to survey an 80m wide, 360m long strip NS across the town.  The overall results can be seen in Fig. 5.

Fig. 5: the magnetometry survey.

Fig. 5: the magnetometry survey.

There is a great deal going on in the results.  Ermine Street shows clearly running across the NE corner of the survey transect and matches the parch mark beautifully.  Either side of Ermine Street are a series of buildings with their gable ends onto the road in the approved Romano-British manner.  Other streets can be seen, again matching the parch marks.  Not all the buildings are so clear, but there are clearly other walls that can be seen in the data.  Towards the south, the pattern is more complex.  The results could be cleaned-up a little more.  In places we were suffering from some stagger, partly as a result of the fine reddy-brown dust that settled over all the machines and their operators  (Fig. 6) which, combined with the lubricant we use on the cogs, turned to a sticky slurry.

Fig. 6: the red dust all over the GPR. Image courtesy of Mike Smith.

Fig. 6: the red dust all over the GPR. Image courtesy of Mike Smith.

We undertook a radar survey using the Mala GPR we have on loan from SEAHA.  Pushing the GPR was quite hard work in the long grass, especially as one goes over the agger on which Ermine Street appears to have been constructed (Fig. 7).  The team did, however, manage to complete an excellent six blocks of data.

Fig. 7: Pushing the GPR over Ermine Street.

Fig. 7: Pushing the GPR over Ermine Street.

The images on the screen of the GPR shows that we were getting reasonable depth penetration. I created amplitude maps in 3ns thick slices.  The third slice map (Fig. 8) clearly shows the surface of Ermine street, but for most of the area surveyed the radar signal is still in the ploughsoil.

Fig. 8: time slice 3, 10.5-13.5ns.

Fig. 8: time slice 3, 10.5-13.5ns.

In the fourth time slice (13.5–16.5ns, Fig. 9)  some of the other roads are starting to show, and odd bits of wall.  One very curious feature is the lighter coloured band across the middle of the southern area.  Although it would appear to be related to our grid, our survey was conducted NS across that band.  The aerial photograph (Fig. 2) does show a band across the field so perhaps this is related to some sort of cultivation pattern?

Fig. 9: time slice 4, 13.5-16.5ns.

Fig. 9: time slice 4, 13.5-16.5ns.

The fifth slice (16.5–19.5ns) shows more details in the buildings (Fig. 10).  In the centre of the lower block is a square feature.  This is the Romano-Celtic temple known from aerial photographs.  This type of temple, well-known from many sites across the north-western provinces of the Roman Empire and consists of two concentric squares, usually reconstructed as an inner sanctum and an outer ambulatory.  The two roads the the north and south of the temple appear to mark the edges of the temenos or sacred precinct.  There is a hint of a possibly paved area to the west of the temple, and a solid feature between the internal and external walls to the east.  In the northern block there are hints of the walls on either side of the road as seen in the magnetometry data.

Fig. 10: time slice 5, 16.5-19.5ns.

Fig. 10: time slice 5, 16.5-19.5ns.

In the sixth time slice (19.5–22.5ns) we can start to see some of the buildings along Ermine Street not, as I had expected, as black ‘high amplitude’ features shown in black i.e., stone walls, but as low amplitude features, i.e., areas which have fewer items that would reflect radar waves (Fig. 11).  At Verulamium, I have interpreted these as where the stone foundations have been robbed, but here we know less about the construction techniques used.  Part of the difficulty is that Ermine Street is on a quite marked bank which means the radar has a greater depth of deposits to penetrate.  I undertook a topographic survey of just the northern block area (Fig. 12) and in the future will be able to process the GPR data taking into account the topography.

Fig. 11: time slice 6, 19.5-22.5ns.

Fig. 11: time slice 6, 19.5-22.5ns.

Fig. 12: topographic survey of the northern block.

Fig. 12: topographic survey of the northern block.

In the seventh and eighth time slices (22.5–25.5ns, 25.5–28.5ns), the GPR radar waves are starting to attenuate and we are getting quite faint reflections, but some of the deeper foundations show in these lower time slices (Figs. 13–14),  For example, some of the buildings along Ermine Street start to show very well in Fig. 13, and the outer wall of the Romano-Celtic temple shows very well in Fig. 14.

Fig. 13: time slice 6, 22.5-25.5ns.

Fig. 13: time slice 6, 22.5-25.5ns.

Fig. 14: time slice 6, 25.5-28.5ns.

Fig. 14: time slice 6, 25.5-28.5ns.

There is a great deal more which can be extracted from the GPR data, especially by looking at the radargrams (the vertical slices) and comparing them to the time slices.  The results are less “black and white” than at Verulamium and quite complex, but there is a great deal going on in this data which will take a bit of work to tease out all the details.

As well as the magnetometry and radar surveys, we undertook a resistance survey using UCL’s new RM85 meter (Fig. 15).  We took readings every 50cm.  We managed to survey an area 60x by 80m which had also been surveyed using the GPR and the magnetometer.

Fig. 15: Richard Cushing and Stephen Upex working on the resistance survey.

Fig. 15: Richard Cushing and Stephen Upex working on the resistance survey.

The result of this survey was quite surprising (Fig. 16).

Fig. 16: the earth resistance survey.

Fig. 16: the earth resistance survey.

The square within a square plan of the Romano-Celtic temple could not have been more obvious.  The small room on the eastern side, partially seen in the GPR survey, shows clearly.  The temenos is also quite clear.  A spectacular result, but one that raises a question. Why is the inside of the temple such low resistance?  Normally, low resistance like this is related to water retention.  Is the outer wall of the temple causing water to pool within the wall?  It is useful to compare the three surveys (Fig. 17).

Fig. 17: comparing the three survey techniques and the parch marks in the area of the temple.

Fig. 17: comparing the three survey techniques and the parch marks in the area of the temple.

It will take a bit of work to draw-up a composite interpretation plan.

Peter Alley also undertook some surveys with his UAV (Fig. 18).

Fig. 18: Peter Alley using his UAV to map the site.

Fig. 18: Peter Alley using his UAV to map the site.

As well as taking high-level images of sites, the UAVs pictures can be used to create topographic models using a technique called “Structure from motion”.  Fig. 19 shows a topo plan of part of the site derived from the photographs.  The actual heights vary from my plan because the UAV’s plan needs to be corrected against some control points, but the relative heights are great.  This technique is going to prove very useful in future.

Fig. 19: Topo map in QGIS derived from the UAV's aerial imagery.

Fig. 19: Topo map in QGIS derived from the UAV’s aerial imagery.

The aim of this three days of fieldwork was simply to see which survey techniques would provide useful information at this site.  The answer is: all of them!  We already have a huge amount of data to examine in more detail, and a great deal of thinking to do.  It certainly seems that a more extensive programme of geophysical survey would add to our knowledge of the town greatly, as well as other archaeological features such as the “henges” to the south.

As always, many thanks indeed to everyone who came to help, especially Mike Smith for transporting and running the GPR, and Jim West for helping to run the mag.  This was a great team effort between local group members and CAGG, and exactly what our group exists to do.

Fig. 20: the end of day.

Fig. 20: the end of day.

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Three day catch-up

I haven’t managed a Verulamium post for a few days so here is a quick catch-up.

Firstly, the mag has been slowly working its way eastwards along the Macellum Field.  They are getting pretty close to the end of it.

The mag survey in the Macellum field after day 35.

The mag survey in the Macellum field after day 35.

As can be seen, Watling Street stands out very clearly running from near the theatre to the Chester Gate.  There are lots of buildings along the road as would be expected.  Some are less clear than one would hope because they have been partially excavated.  The spec-ally look to the data, almost certainly because of the gravel subsoil, does make it harder to see what is going on here.  The carrot at the end of the stick — apart from just finishing the field of course — is that there are two Romano-Celtic temples known from aerial photographs near the modern road.

The next image is just to show how much of Verulamium we have now completed.  Poster, anyone?

The complete survey so far.

The complete survey so far.

The resistance survey has had a few problems.  The lack of rain has made the top-surface of the field very dry and hard.  It is very slow going, and the data is not as clean as one would like.  Despite the problems, however, some of the buildings along the road, especially at the north side of the plot, are very clear indeed.

The resistance survey after day 35.

The resistance survey after day 35.

Although it doesn’t jump out at one when just looking at the plot, the sinuous ditch does show in the resistance data when one knows where to look!

The GPR team completed some blocks along the hedgeline which I haven’t processed yet… sorry!  They also did one block up next to the Chester Gate to investigate the building here, and one over the sinuous ditch.  The latter did show the ditch but very little else.  Let’s look at the block near the Chester Gate.

The mag survey near the Chester Gate.

The mag survey near the Chester Gate.

This first image shows the mag data.  The building in the middle shows as white lines roughly parallel to the modern drive.  There are lots of other darker features, probably various pits and the sinuous ditch shows to the west.

Day 35 GPR, slice 2 (9.5 to 12.5ns).

Day 35 GPR, slice 2 (9.5 to 12.5ns).

This time slice shows the impact of splitting the survey over two days!  The left hand side was done yesterday afternoon, the right hand side this morning.  The pattern of the ploughing and the tractor’s turning circle in the corner of the field show clearly.  Luckily, the problem is much less acute at lower depths.

Day 35 GPR, slice 3 (12.5 to 15.5ns).

Day 35 GPR, slice 3 (12.5 to 15.5ns).

This time slice now shows the building beautifully.  What a wonky end wall on the north side! There is a long narrow range of rooms to the SW.

Day 35 GPR, slice 4 (15.5 to 18.5ns).

Day 35 GPR, slice 4 (15.5 to 18.5ns).

This time slice shows the SW line of rooms more clearly, although at the southern end they are been partly destroyed.  We can see, however, fainter traces of the walls on the NE, a corridor, perhaps?

Day 35 GPR, slice 5 (18.5 to 21.5ns).

Day 35 GPR, slice 5 (18.5 to 21.5ns).

This time slice does show the “corridor” to the NE much more clearly.  Perhaps it is more deeply buried in the plough bank?   For most of the plot, though, nothing much else is showing.

There has been a little rain this evening.  I have my fingers crossed for more.  Hopefully, tomorrow will be dry so that the mag can plough on eastwards!

As always, many thanks to everyone, especially those working with the res meter.  It is slow and boring at the best of times, but slow+annoying is a great deal to ask.

The mystery deepens

We managed to run all three machines today, although we were short handed on the resistance meter so many thanks to Peter and Ellen for soldiering-on. The unfortunate side-effect of the lovely weather is the top surface of the field is now like concrete!

First of all, the resistance results.

Day 27 resistance results.

Day 27 resistance results.

Day 27 resistance results, high pass filtered.

Day 27 resistance results, high pass filtered.

Not much new showing today, but we are getting a little more of the temple.  Tomorrow’s grids should show quite a bit more, if we can get the probes in the ground!

The GPR has, for the last few days, been working its way along the northern edge of the field in a series of stepped blocks.  With GPR it is not so easy to have a ragged edge.  Mike Smith, our expert GPR cart wrangler, felt sure they had found some stuff today.  Here are the last three days (the three blocks with jagged top edges) with today’s being the easternmost one.

GPR survey, days 25--27 (slices vary).

GPR survey, days 25–27 (slices vary).

Timeslice 3 (12.5 to 15.5ns) is usually a good starting place at this site.  Not much showing in that bottom block.  How about the next slice down?

GPR survey, days 25 to 27. Day 27 (the easternmost block with a jagged north edge), slice 4 (15.5 to 18.5 ns).

GPR survey, days 25 to 27. Day 27 (the easternmost block with a jagged north edge), slice 4 (15.5 to 18.5 ns).

I think Mike is right!  Excellent stuff.  Let us look a little closer at today’s block:

Detail showing day 27 results, 4th time slice (15.5 to 18.5 ns).

Detail showing day 27 results, 4th time slice (15.5 to 18.5 ns).

What is really interesting is not only do we have evidence of yet another building in the western side of today’s block, but the white lines surrounding darker areas on the eastern side suggest robbed walls of a large building.  Tomorrow’s results are going to be very interesting!

And to the topic of the TLA from the post earlier this evening.  Let us recap the story.  Last year we started picking-up a long ditch which ran right across the site.  It had been found in the two transects of mag undertaken by English Heritage in 2000, but at the time they were not linked. Last year we speculated that the ditch ran up the dry valley that the mag team have been laboring up and down the last few days.  On the 19th century maps there is a well marked on the other side of the Fosse field.  In the next image I have marked the line of the ditch.

The sinuous ditch.

The sinuous ditch.

Much to my surprise, the ditch curves around on itself at the western end, perhaps following the contours of the sides/bottom of the valley.  I will have to check that with the dGPS in the near future.  But where is it going?  Is it still an aqueduct as we thought, or something else entirely?

Looking more closely, we can see there are lots of other, smaller linear features in the area, some quite straight, and possibly even another building.  This top-corner of the field is proving very interesting and we will have to overlay the results on the topography to see what is happening.  This bit of the field is anything but flat!

Detail of the western end of the sinuous ditch.

Detail of the western end of the sinuous ditch.

Tomorrow we see the mag team edging closer to completing the Theatre Field, the GPR working along the northern edge towards the theatre and the the resistance meter covering more of the Insula XVI temple.

Apse mad

We had a very successful day. It was grey and a little drizzly first thing, but true to the forecast it warmed up and turned into a gloriously sunny, if a little windy, day.  The mag team got right into the deep south and have made the mag plot look very tidy.

The magnetometry survey at the end of day 23.

The magnetometry survey at the end of day 23.

The team were determined to finish off the last few partial grid squares.  I think they were fed up walking the 650m from the cars!  The next swathe of grid squares will head north completing the Theatre Field at last.  Let’s look at the area surveyed today.

The area surveyed on day 23.

The area surveyed on day 23.

The band of buildings running from the NE seems to meet the 1955 ditch at a point where the magnetic response from the ditch changes markedly.  Perhaps the ditch was more deliberately filled-in here?  To the SW of the 1955 ditch there are fewer clear indications of stone buildings, but there are many linear features (probably ditches), and “blobs”, probably mainly pits but some of them are very large.  Of particular note is the small square enclosure right up against the town wall towards the south (seen as a black square).  One wonders what this was so close to the third century town wall.

At the other end of the field, the GPR crew managed another 80x40m block of data.  Apparently, nothing much was showing.

The day 23 GPR block (fourth slice, 15.5 to 18.5 ns).

The day 23 GPR block (fourth slice, 15.5 to 18.5 ns).

I beg to disagree!  Another small apsidal building is showing in the data, overlying the line of pits.  There is a large, rectangular, building to the west of it.  Are apsidal buildings the geophysicist’s version of busses?  One interesting aspect of this building is that it does not show in the mag data at all.

The same area as the previous image showing the magnetometry data.

The same area as the previous image showing the magnetometry data.

This is a great example of why undertaking both the GPR and the magnetometry surveys is so useful in giving a fuller picture of the town.  The GPR surveys are getting quite extensive!

All the areas surveyed with the GPR on the Gorhambury side of the town.

All the areas surveyed with the GPR on the Gorhambury side of the town.

The earth resistance team completed another excellent five 20x20m squares.

The earth resistance survey after day 23.

The earth resistance survey after day 23.

We can see some more buildings and bits of road, although there are some very high resistance areas.  One weakness of resistance survey is that there can be underlying variations in the data related to factors such as slope or geology which mask the archaeological patterning.  We can, however, apply a “high pass filter” which attempts to remove the underlying background trend and show the sharper differences more clearly.

The earth resistance survey, high pass filtered.

The earth resistance survey, high pass filtered.

The technique can create some artefacts in the data, but is very useful for bringing out some of the buildings.

Tomorrow and Tuesday are our “weekend”.  Many thanks for everyone who joined in during the first week, and here is to the next three!

Yet more 1955

Today was very warm and sunny with white poofy clouds and a gentle breeze. Ideal for sitting in the garden with a large jug of Pimms and a good novel. We, however, were out on site pushing a mag or GPR, or prodding the ground with the resistance meter. It was, however, worth it!  Many thanks to everyone who turned out on such a hot and sunny day.

The mag survey went well with Jim, Dave and Ellen completing eight whole squares and re-doing part of one where the sensors had frozen on Thursday.  This image shows the whole survey of the town, so far.

The overall magnetometer survey at the end of day 22.

The overall magnetometer survey at the end of day 22.

There are times when the “theatre field” feels so huge that we will never finish.  The mag, however, is getting there.  Another day or two and we should complete that swathe, and then we can start working our way north once more until we have finished the whole of that field.

Looking at the area that was surveyed today, the most striking feature is the infamous 1955 ditch.

Detail showing the area surveyed on day 22.

Detail showing the area surveyed on day 22.

The ditch shows clearly as the broad black line with white edges running across the plot.  At right angles to the ditch is a road with buildings (shown as low-magnetic white lines).  Outside the ditch to the south-west are several smaller ditches (shown as thinner black lines) as well as some probable pits (the black blobs).  As we saw in the park, the 1955 ditch seems to mark a clear divide in the town between the busy built-up area inside it and the quieter area outside it, despite the fact that the ditch is supposed to go out of use 250 years before the “end” of the town.

The earth resistance survey continued eastwards and picked-up the building seen in the GPR data last year,  Remember than in our resistance plots the walls show as darker, high resistance, lines whereas in the mag they show as white lines.

The earth resistance results after day 4.

The earth resistance results after day 4.

It is interesting to compare the resistance results to the GPR data of the same area.

The resistance results overlain on the GPR results.

The resistance results overlain on the GPR results.

The same area as the previous image showing just the GPR results.

The same area as the previous image showing just the GPR results.

Last but not least the GPR results.  The team completed another 80x40m block.  There is surprisingly little that is very obvious in the time slices, but the line of pits does show as an area of low reflections (white) and there are hints of robbed walls.  Most fascinating, however, is a small apsidal building about 15m long and 5.5m wide.  Here are the fourth and fifth time-slices.

The fourth time slice (15.5 to 18.5 ns) showing the apsidal building.

The fourth time slice (15.5 to 18.5 ns) showing the apsidal building.

The fifth time-slice (15.5 to 18.5 ns).

The fifth time-slice (15.5 to 18.5 ns).

Tomorrow will see us out in the field for the last day of the first week.  Monday and Tuesday is our “weekend”.

 

Resistance isn’t futile after all.

Today was another successful day. We had a short rain shower just before lunch but other than that, all went well.  We ran all three machines with good results.  Firstly the mag survey.  The first image shows the western side of our survey.  The block of data we collected today is in the lower left corner of the image.

The western side of the magnetometer survey.

The western side of the magnetometer survey.

The most obvious feature is the big, bright black and white object.  That is a galvanized steel water trough!  Across the SW corner is a dark diagonal line.  That is another stretch of the 1955 ditch — the first century boundary of the town — which Martin Atkin traced with a very early magnetometer in 1959 and 1960.  It was also the first feature we found in Verulamium Park in 2013.  Running diagonally across the survey from NE to SW is a linear feature made up of many light and dark lines which cuts across the western end of the hedge line and goes into the unsurveyed  grid square.  This is the line of one of the street which has buildings and other features along its length.  If we zoom in we can see more details.

The area surveyed with the magnetometer on day 20.

The area surveyed with the magnetometer on day 20.

The area to the SE of the water trough has a number of buildings shown clearly by white lines (representing a low magnetic response) especially either side of the NE–SW running road.  To the west, however, is very quiet with very few features showing.  The unsurveyed square, which is first on the list for tomorrow, should be very busy with the road and more buildings.  The eagle eyed amongst you will have spotted some vertical lines of uniform grey.  Ugh.  Another sensor freeze.  We will need to redo about a third of that square tomorrow.

The resistance survey continued on and completed another four 20x20m squares at 0.5m spacing,  Pretty good going.

The resistance survey after day 20.

The resistance survey after day 20.

In the resistance survey plot, black represents high resistance and white low.  High resistance features are things like roads and walls which do not hold moisture.  Low resistance features are things like ditches and pits which do hold moisture.  Generally, I have found, the high resistance features are easier to spot and interpret on the plots.  In the case of the image above, the two roads show clearly, especially the one in the NW corner.  The buildings along the roads also show well, including some internal walls.  Interestingly, not much shows in the middle of the insula (a “block” in a modern town).  Are they yard areas behind the buildings?

The GPR team managed an 80x40m block of data, plotted to the west in the following image.

GPR data. The two squares to the west were completed on day 20. This is the 15.5 to 18.5ns time slice.

GPR data. The two squares to the west were completed on day 20. This is the 15.5 to 18.5ns time slice.

The GPR has picked up the buildings along the two roads very nice.  It is also pleasing to see the plots from this year join up with the plots from last year so well.  Using a high accuracy differential GPS makes this quite easy to do!

In some areas we now have three data sets over the same area.

The earth resistance data overlain on the GPR data.

The earth resistance data overlain on the GPR data.

The resistance, magnetometry and GPR data all overlain.

The resistance, magnetometry and GPR data all overlain.

Obviously, combining all this data is going to be a challenge.  Looking at a few specific details is fascinating and informative, but how does one do this on a large scale?  There are techniques called “data fusion” that I am going to have to look into.

Again, many thanks to all the team who turned out today, and watch out for tomorrow’s exciting installment!