Tag Archives: Geoscan

Neat and tidy

Due to being rained off on our last day, a small team of us decided to go out and finish off some things on Bank Holiday Monday. Many thanks to Pauline, Judith, Ruth, Dave and Jim for turning out to do “just one more grid.”  I think it must be a geophysicists ailment that we always would like to be able to just a little bit more…

The mag team completed an impressive ten grids including two awkward partials.  Figure 1 shows the entire survey at the end of the 2018 season.

Figure 1: the mag survey after day 19.

The team have managed to add 19 ha to the survey in the last month.  Figure 2 shows the southern area that we have been surveying this week.  (This field is, confusingly, called “Prae Wood”.)

Figure 2: the southern area (Prae Wood) after day 19.

The team have picked-up an area of intense ferrous noise.  This looks like a small historic period site.  We will have to check out some old maps to see if we can work out what that might be.  The one hiccup in a brilliant last day of work is a single line of data where the sensor froze.  It is very annoying and I’ll have to find some way of fudging that until next summer!

The Earth Resistance survey had one last little block left to make the plot look all neat and tidy.  Many thanks to Pauline and Judith for helping me fill that in (Figures 3 and 4)!

Figure 3: Kris, Judith and Pauline (out of shot) extended the resistance survey. Image © Mike Smith.

Figure 4: the main block of Earth Resistance data collected 2016–2018.

The data collected shows some faint indications of buildings in that corner (Figure 5).

Figure 5: the northern area of the res survey. The NW corner was completed on day 19.

Although my trick of spreading the remote probes wide apart has worked on the whole, this year there is a bit of an edge.  This is because we started with a block in the SW corner, worked eastwards, and then when we had got to the corner, worked back along the hedge line westwards.  Between when we started this block and yesterday we have had in excess of 100mm of rain (or about 4 inches in old money) so it isn’t surprising this shows in the results.

We have now cleared away all the pegs and flags, packed-up the machines and left Gorhambury for another year.  It is a beautiful place to work and we are very grateful to Lord and Lady Verulam and their family for allowing us to extend the survey, to those who work the estate and put up with us getting in the way, and to the estate managers, especially Stuart Gray. Thanks to the Institute of Archaeology, UCL, for lending us the dGPS and the res meter, and SEAHA for the loan of the GPR.  I hope everyone involved thinks the results are worth all the effort. Most of all I would also like to thank all the volunteers who came this year, whether you only managed a day or two, or you came for the whole season.  You are what makes this project so much fun!

 

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“The way I see it, if you want the rainbow, you gotta put up with the rain.”

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

As I start this entry of the blog, the rain is splashing against my windows as was predicted by the Met Office. Although we might question Dolly Parton’s grammar, the sentiment seems true enough.  Yesterday, however, was a superb day with all three techniques collecting data across the site.

After yesterday’s excellent results, the GPR crew had great expectations.  The only problem was a tree in the way under which the shepherdess had put hay when the grass in the field was dead from lack of rain.  Unfortunately, sheep mean sheep droppings (Figures 1 and 2).

Figure 1: Mike on sheep poo removal duty.

Figure 2: Check out those wheels!

Luckily for everyone concerned, I think the effort was worth it (see Figure 3)!

Figure 3: GPR time slices from Day 18.

I could misquote Dolly along the lines of putting-up with sheep poo if you want excellent GPR results but I might be pushing my luck…  The many buildings are quite obvious in this data set.

Figure 4 shows this grid in context of the other GPR grids in this area.

Figure 4: GPR results including the day 18 data (SW corner).

We have added a very large number of new buildings to the map of Verulamium.  As I was only just starting with GPR data when we started collecting it in 2015, the processing keeps changing a bit from block to block. One of my jobs is to start from scratch and reprocess the whole thing so that the maps are consistent.  Should keep me busy for a while.  Figure 5 is a crude mosaic of images just showing the entire area surveyed so far.

Figure 5: crude mosaic of GPR time slices at the end of the 2018 season.

This represents 19ha of GPR data collected at 0.5m transect intervals.  Just pushing the machine along the lines, not including getting to the block, setting-up, moving strings etc. is 380km.  It also means 380km of radargrams!  No wonder the data takes-up 33gb of my hard disk and consists of over 70,000 files.

The mag team completed nine 40x40m grid squares which is 1.44 hectares.  Excellent progress!

Figure 6: the mag team in the southern field.

Figure 7 shows the whole of the 2018 survey (along with a big chunk of Verulamium).

Figure 7: the mag survey after day 18.

Even though we have been using the machine for some years now, and it does have its frustrations, when all is going well we can really cover some ground.  The season was planned for 20 days: we lost 3 days to rain, and most of a day to testing the mag at the start.  Despite this, the team have managed to collect 17.7 hectares of mag data.  Without actually getting to the grids and back (which is quite a bit of walking in itself), the team have pushed the cart 88.5km over the past four weeks.

Figure 8 shows the southern area in more detail.

Figure 8: the southern area of mag data after day 18.

The blue arrows in Figure 8 indicate the lines of old field boundaries.  These can be seen on old maps such as the 1699 parish map.  The yellow arrows mark ferrous objects.  Some are very big, but there are a scatter of smaller ones too.  Last, but definitely not least, there are a few magnetic features which may be archaeological, such as pits.  I have picked a few out with red arrows.  Although they look small at this scale, they are probably 1m to 2m across, a quite respectable size for a pit.

Although large mainly  blank areas are disappointing to collect, they are important nonetheless. The immediate environs of Verulamium are extremely rich, archaeologically. The field lies:

  • 360m W of the busy area of buildings recorded by the GPR discussed above;
  • 600m NE of the major Iron Age settlement at Prae Wood;
  • 600m N of the fields at Windridge Farm where metal detecting rallies have taken place;
  • 500m NW of the major cemetery at King Harry Lane;
  • 1,100m SE of Gorhambury Roman villa;
  • 1,000m NE of the new villa found at Windridge Farm.

Also, the Fosse, which is preserved in the woodland along the NE edge of the field, is a really very impressive earthwork.   We just seem to have hit an empty bit of landscape between all these sites!

The res survey now covers some 6.58ha, that is about 263,200 earth resistance readings.  Not into the millions like the mag and GPR, but this is res after all!  Figure 9 shows the entire survey.

Figure 10: the entire Earth Resistance survey after day 18.

At this scale the roads show very nicely as do some of the more substantial buildings.  Figure 11 is the area surveyed in 2018.

Figure 11: Res survey after day 18.

Given that the fields were baked hard and the grass was dead at the start of the season, I am pleased we managed any Earth Resistance survey at all this season.  The team yesterday put-up with my geophysics OCD and completed right into the corner by the theatre. We then doubled-back and started filling-in between the top of the survey block and the drive.  We have picked-up some parts of buildings seen in grids to the south, but in general along the edge the deep colluvium, as shown by the sunken nature of the drive, is to some extent masking the archaeology.

Many thanks to everyone on the team who made the 2018 season such a success.  A especially big thanks to those who helped move the equipment about including Ellen, Mike, Jim and Ruth.

For those who haven’t been involved but would like to join future surveys, do get in touch.  We are a friendly group, and provide on-the-job training.

And finally… (as they used to say on the news)

 

A busy day

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

We had a large team today and as a result we managed ten mag grids, two and a bit GPR grids and seven earth resistance grids. Good job everybody!

First to the mag.  The team extended their survey in the field to the south of Mobbs Hole.  Figure 1 shows the overall survey and Figure 2 zooms in on this field.

Figure 1: the 2018 magnetometry survey.

Figure 2: the survey in the field to the south after day 2.

I have annotated Figure 2.  The red arrows indicate the line of the ditch of the Fosse.  It is salutary to note that a feature as big as the Fosse barely shows in the mag data.  Clearly the upper fills of the ditch are largely the same soil as the surrounding topsoil.  We can normally see pits and ditches on archaeological sites because they are filled with more organic, and thus more magnetic, soils, the result of nearby human occupation.  The green arrow shows a “blob” of higher magnetic readings. The rather diffuse edges to this feature make me suspicious that this might be a “tree throw”, i.e., where a tree has blown down.  The yellow arrow marks two strongly magnetic parallel lines.  At first I thought these might be something metallic but checking their actual values shows they vary from -10 to +29 nT.  Certainly strong, but unlikely to be metal.  The blue indicates something which is definitely metal; it has values of -1543 to +680nT!  The dark pink arrows indicate a faint line, possibly an old fence line.

The res team consisting of Deborah, Tim, Julia and Anne completed seven squares.  Figure 3 shows the whole survey from 2016–2018.

Figure 3: the earth resistance survey 2016–2018.

We have now covered 6.3ha.  For a resistance survey at 0.5m spacing between readings, that is pretty impressive.  Res has always been a poor third to mag and GPR in this survey.  We didn’t get started until a year after the other techniques when UCL purchased a new RM85, and we have had problems with weather.  Hopefully we can fill in the top corner on Saturday.

Figure 4 shows a detail of the area completed this year.

Figure 4: the northern area completed so far this season.

The street shows very clearly in Figure 4 running SW-NE, and slightly more faintly we can see the buildings either side. One problem to tackle in processing data is that very high areas, like the road, can make the more subtle stuff harder to see.  If we “clip” the image to bring-up the details of the buildings, the road area becomes one big black blob!  One way to get around this is to use a high-pass filter.  Figure 5 shows the same area with the high-pass filter applied.

Figure 5: the 2018 survey area after the application of a high-pass filter.

As you can see, the buildings show much more clearly but the road much less so.  Especially with resistance data, it is worth looking at several versions of the data processing to get the most detail from the survey.

The GPR crew finished off the grid from yesterday and did another 40x80m block.  Figures 6 and 7 are the time slices from the two days.

Figure 6: time slices from day 16 of the GPR survey.

Figure 7: time slices from day 17 of the GPR survey.

As you can easy see, we have some sweet buildings showing.  Figure 8 is a rough composite of the sites in this area.

Figure 8: composite of slices in the area of the day 16 and 17 survey blocks.

I need to do some cleaning-up of the various blocks as they were processed at different times and with different software packages, but in general you can see the mass of buildings crowding along this section of road.  Very nice!

Signing off now so we can go and start day 18.  This may be our last day as the weather forecast for Sunday is dire…

 

End of week two, part 2

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

Just a quick update as week 3 will be starting in about eleven hours and I’d like some sleep!

The GPR crew on day 10 completed three areas of “sawtooth”.  Well done all for putting up with such an annoying, fiddly job, but it does look good along the edge of the survey.  It took a bit of setting-up, processing-wise, but all was well.  Sadly, not much showing (Figure 1).

Figure 1: the GPR survey in the northern area after day 10.

Starting from tomorrow, the crew will be working their way slowly southwards, back up the hill.  The downside is the hill, the upside is that they will be covering areas which clearly have buildings in them!

The earth resistance meter, operated by myself and Ellen, managed a modest two grids once we had set-up the other two machines.  The results were good, however, and clearly show many of the details of this building in the top-corner of the Theatre field.  The next three images show the mag, GPR and earth resistance results for this area.

Figure 2: mag data in the top corner. the building shows as white lines of low magnetism.

Figure 3: the GPR data showing this building very clearly as black lines of strong radar reflections.

Figure 4: the earth resistance data for the same building.

Although the GPR data appears very clear, the Earth Resistance and mag data appear to show more walls between the main range and the road.  There is a suggestion, also, that the “corridor” to the SW of the main range is in fact another phase.  It would be odd for a corridor to have subdivisions.  Plenty of room for debate over the details of this building.

Many thanks to all for your excellent work in the first two weeks.

One hundred and fifty

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

We managed a full day today, and I’m just about keeping up! The mag team completed two grids yesterday, half of one in the aforementioned deluge. Today they completed 11 grids: three partials and eight complete ones. Way to go! Well done everyone. Figure 1 shows the survey so far.

Figure 1: the mag survey after day 9.

One really does wonder if that break in the mag data is an entrance.  It doesn’t seem like it on the ground.  I have downloaded the LiDAR data but haven’t had a chance to process it yet.

The GPR crew finished their 80x40m block, and then did some of the next “sawtooth” section, another 14m worth.  Figure 2 shows the time slices.

Figure 2: day 9, time slices 3 to 6.

Nothing jumps out at one, although there are some curious “light” lines in the fourth slice (top-right) which are parallel to the aqueduct.  Figure 3 shows that slice in context.

Figure 3: GPR survey after day 9, slice 4.

After all the rain I thought it would be worth trying the Earth Resistance survey (Fig. 4).  I spent the morning laying in grids for the mag, but managed some survey in the afternoon.

Figure 4: Earth Resistance survey in action.

Although the rain has softened the surface, it won’t have penetrated 50cm yet, and I was concerned that there would be no contrast at that depth.  I decided to survey a grid where we knew there was a building.  Fig. 5 shows the comparison between the GPR survey and the two squares of res I managed to complete (thanks Anne!).

Figure 5: Earth resistance survey compared to GPR results.

Given the drought, the results are pretty good.  It would be interesting to compare these to results from a normal English summer!

Tomorrow isn’t looking great.  We might get some work done in the morning.  Fingers crossed.

Many thanks to everyone who helped out today.  Especially big thanks to Mike, Ellen, Jim and Ruth who take on the responsibility of shipping the equipment back and forth.

By the way, this is the 150th blog post…

And now for somewhere completely different

Although this isn’t CAGG related, or Hertfordshire, I thought members of the group might be interested in my latest geophysical adventures.

Some 15 years ago I undertook a survey in Alba Iulia, Romania, for a colleague.   The site was part of one of the Roman cities at Apulum which grew-up alongside the legionary fortress.  The results were pretty good, but I was only taking one reading per square meter.  Since getting the RM85 I have been wanting to return and re-do the survey at higher resolution.  Well, be careful what you wish for!  Last Saturday, I found myself on the way…

Fig. 1: On my way…

Yes, you did read the time correctly.  I flew to Cluj-Napoca via Munich.  Sadly, when I got to Cluj, my luggage was still in Munich.  Thankfully, they delivered it all safe-and-sound the next day but it did mean I lost a half day of survey.

Alba Iulia has changed quite a bit in the fifteen years.  The citadel, especially, has been restored beautifully and now has a series of bronze statues decorating the area.

Fig. 2: scrumping.

Having lost half a day, we got started in the afternoon.  Three whole grids and partial that day, seven whole grids a four partials (including one very silly small one) the next day, and eleven yesterday.

Fig 3: Wyatt helping with the Earth Resistance survey.

Yesterday, going along the first line seemed fine with the wind at my back.  Then I turned into the howling gale and snow…  The effect was like star-trails in a science fiction movie as the snow blew past me horizontally.  Thankfully, the weather got better during the day.

There were some software issues to begin with, but thanks to David Wilborn’s excellent customer support, those were quickly resolved.  The results look pretty good.  In the next image I have applied a high-pass filter to even out the big changes in range that occur in this data set.

Fig. 4: the Earth Resistance survey results at the end of Day 3.

I’ll update you all as I go along.  I have six more days to try and complete the whole survey.  I suspect I’ll be a little tired by the end.

Your foreign correspondent.

Back to Durobrivae

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

Firstly, many apologies for the time it has taken to write this report.  The data were quite complex, and the day job comes first.  This is a monster blog post, so make a cup of tea and settle back somewhere comfy!

Last year CAGG teamed-up with local groups to undertake some survey at the Roman ‘small town’ of Durobrivae, near Peterborough. If you have ever driven on the A1(M) past Peterborough you would have passed by the site. Our aim was simply to determine which geophysical survey techniques would give good results at this site.  The answer was: all of them!  A short note on the results has just been published in the International Society for Archaeological Prospection‘s newsletter.

We decided to add to our original survey by undertaking another three days work from 4th to 6th November.  Unfortunately, it decided to rain in the morning of the first day and so we lost some time.  We had enough helpers to run the mag, two Earth Resistance meters (the Welwyn Archaeological Society‘s and UCL‘s) and the Malå GPR we had on loan from SEAHA.  A small group of us returned on 26th November to expand the magnetometry survey and undertake a topographic survey using the dGPS.   The first three figures just show how much we have done so far.

Fig. 1: the complete mag survey as of the end of November 2017.

Fig. 2: the complete GPR survey as of the end of November 2017.

Fig. 3: the complete Earth Resistance survey as of the end of November 2017.

In the following post I am going to firstly discuss the western block of data, and then the eastern block over “the tumulus”.

Last year we completed a 80m x 360m transect of mag data across the town.  We also completed two blocks of GPR data, one 80×80, and one 80x40m.  This year we wanted to fill in the gap between those two blocks so surveyed another 80x40m block giving us one contiguous 80x160m survey.  Unfortunately, matching GPR grids is quite difficult, especially when there is a year between when they were collected, and therefore quite different ground conditions.  I did, however, manage to produce some “OK” time slices by applying a zero-mean traverse to each line of GPR data.  Hopefully, I will be able to create better slices in future, but these will do for now.  I also noticed that Larry Conyers had produced a much clearer plot of the temple by using a much thicker time slice.  I usually aim for 3ns thick slices.  Larry, however, used 8ns slices.  Here I have compromised by using 5 1/4 ns slices with a 50% overlap.  Fig. 4 is a composite of 12 slices starting at the surface.

Fig. 4: twelve time slices of the western area. Each slice is 5.27ns in thickness.

Apart from Ermine Street cutting across the top right hand corner, the first three slices are not really showing anything much of interest. Let us now look at the individual slices in more detail.

Fig. 5: GPR time slices 4 and 5.

In Figure 5, left, we can see the temple (A) starting to show as an area of lower reflections.  Larry Conyers was able to demonstrate that the interior of the temple building was clear of rubble, and thus there is little to reflect the radar waves.  In slice 5 on the right, we can see a linear feature (B) to the west of the temple.  This lines up perfectly with a strongly magnetic feature and is therefore a narrow cut feature like a ditch or possibly a wall foundation.  There are faint hints of buildings with robbed-out walls at C and D, showing as light areas of low reflections.  Similarly, at E, we can see some of the buildings alongside Ermine Street.

Fig. 6: GPR time slices 6 and 7.

In Figure 6, left, at A, we can see one of the minor side roads off Ermine street starting to show.  The building at B is still visible (just), and we are can start to see a wall parallel to the linear feature we saw previously (Fig. 5, B).  This suggests to me that we are dealing with a cut feature rather than a robbed wall.  Slightly deeper, in Fig. 6, right, at D we can see more buildings alongside Ermine Street.  The square building at E is now showing more clearly.  The large building at G is beginning to show quite clearly at this depth.  I am puzzled, however, that the road coming from the east seems to end in a sharp angle at F, as though something has cut through it.

Fig. 7: GPR time slices 8 and 9.

In Fig. 7, left, at A we can now see the building to the north of the temple very well.  It would appear the walls have been robbed but some of the floors left intact.  At B, the large building to the east of the temple is showing very clearly now. The wall along the west side of the temenos of the temple (C) is showing clearly at this depth.  Starting to show, but more clearly in the next depth slice at D, is a long wall running across the site.  There seems to be an almost entrance-like feature in it at the western end.

Fig. 8: GPR time slices 11 and 12.

Skipping a slice and moving to No. 11 (Fig. 8, left), we can see the wall to the west of the emple at A very clearly.  The possible floor of the building to the north at B still shows.  In the deepest slice I have generated, we have a curious series of curved features at C.  I have no idea what these are.  Answers on a postcard, please, to…

Fig. 9: the Earth Resistance survey (lower half) overlain on the GPR data (faded out a little).

Figure 9 shows the Earth Resistance survey.  We added a single line of grids on the eastern edge of the block we did last year.  There is a strange speckly effect in the new strip.  This block of grids we did with WAS’s TRCIA meter.  The resistance values were very high, and the meter had to keep swapping range which, apart from slowing us down in the field, may be the cause of the rather odd looking results.  The main result in the new strip is the high-resistance line running WSW to ENE which is probably a road.

A comparison with Stephen Upex’s transcription from aerial photographs is quite informative (Fig. 10).

Fig. 10: Transcription of the aerial photographs for the temple complex by Stephen Upex.  The image has been rotated to match the geophysical surveys. © Stephen Upex, reproduced with permission.

Some of the details between the aerials and the geophysics agree quite well.  The temple itself, and the temenos are pretty good.  What about the circular shrine?  I reprocessed the GPR data from just that section using 6.5ns thick slices this time.  I have produced an image of slice 6 (16.04 to 22.49ns) in the variety of palettes offered by GPR Slice (Fig. 11).

Fig. 11: GPR time slice of the area to the immediate north of the temple in a variety of palettes.

The building to the north of the temenos shows quite well.  There are hints of a circular structure just to the north of the main temple building lying underneath a robbed rectangular building.  The circular feature shows quite well in the last palette, and the antepenultimate one.  The rectangular building is clearer in the second slice where black is showing areas of low reflections.

There is clearly a great deal which can be teased out of this data, but let us move on!

The second area we surveyed was over the so-called tumulus towards the western side of the town.  The mound showed quite nicely last year when the evening mist rolled in on the last day (Fig. 12).

Fig. 12: The mist shows the location of the “tumulus” beautifully.

Our aim was to survey the mound using all three techniques.  Unfortunately, the half day we lost to rain resulted in not covering quite as much ground as we hoped.  The mag results were especially interesting, hence our return to expand the survey area a few weeks later.

Fig. 13: the magnetometry survey of the eastern area over the “tumulus”.

Figure 13 shows the magnetometry results.  Ermine Street and the minor road running off it show well.  The town wall also shows clearly.  The zig-zag look to the wall is not “stagger” in the usual sense of the odometer being incorrect, but a result of the cart going up and down a steepish slope resulting in the sensors not being vertical.  There are indications of more long, thin buildings coming off Ermine Street at right angles, and plenty of other pits, ditches and other features.  The really curious aspect though, is the empty space in the middle, under the “tumulus”.  This seems to have a polygonal linear feature around it, showing as a magnetic positive and therefore either a cut feature, or a brick-built wall. To the east / south-east of the tumulus is a largely open area, somewhat fan shaped in appearance.  How very curious.

I wanted to check the relationship between the results and the topography so I undertook a topographic survey with the dGPS taking readings every six paces (just under 5m).  This differs from using the UAV.  The GPS survey will give us a digital terrain model (DTM) which is the actual surface, but at a cruder resolution.  The UAV will give us a digital surface model (DSM) which gives the surface and thus maps the tops of stinging nettle patches and so on, but at a much higher resolution.  We saw this at Darrowfield. Neither method is better than the other, it depends on ones aims, but using the UAV is certainly very much quicker in the field!

Fig. 14: dGPS topographic survey of the area around the tumulus.

As can be seen from Figure 14, the tumulus shows as an elongated feature running SW-NE.  My guess is that this shape is a result of plough damage.  How does this relate to the mag results?

Fig. 15: the topography with the mag data overlain on it and made partially transparent.

In Figure 15 I have overlain the mag data on the topography, and then made it partially transparent.  As can be seen, the mound is smack in the middle of the polygonal magnetic feature.

The Earth Resistance survey adds a little to the picture (Fig. 16).

Fig. 16: the Earth Resistance survey overlain on the mag data.

The resistance data shows little in the way of positive features apart from an area of high readings towards the south.  There does seem to be a oval of low resistance readings, normally indicative of a ditch-like feature.  There are faint hints of this in the mag data, but they are obscured by other magnetic features running up to the polygonal feature.  If the ultimate origin of the “tumulus” is a prehistoric burial mound, perhaps the res survey is showing us the outer ditch surviving, in part, below the Roman levels?  Fig. 17 shows the resistance survey with contours from the topo survey.

Fig. 17: contours overlain on the Earth Resistance data.

We managed three 40x40m blocks of GPR data.  Unfortunately, the GPR had a glitch in the second grid resulting in a single line of very high values.  This glitch caused some problems in the processing.  I have tried to get rid of them, but the line still shows, especially in the lower slices.  Fig. 18 shows the composite of 16 slices.

Fig. 18: all GPR slices from the eastern survey.

Surprisingly little shows in this survey.  The two main features are the rectangular building which shows from slice 5 onwards, and the general lack of anything much under the tumulus part from some general reflections suggesting some hard material (stones, rubble?) under the mound.  Let us look at slice 6 in more detail.

Fig. 19: GPR time slice 6.

The building towards the south of the image is fairly clear.  I wonder if it might be a bath house?  The area of higher reflections under the mound have faint hints of straight lines and rectangles, but this only shows in this one slice and my guess is that these are fortuitous rather than archaeology.  How does the GPR data relate to the topography?

Fig. 20: GPR time slice 6 with the contours superimposed.

As can be seen from Fig. 20, the higher reflections do not lie below the main part of the mound but slightly to one side.  The building lies outside the polygonal feature seen in the mag data.

A slightly deeper time slice (Fig. 21) shows the strip buildings along Ermine Street starting to show.  They appear to be missing their back walls which might be one impact of ploughing.

Fig. 21: GPR time slice 8 with topographic contours.

I made a crude interpretation map in Google Earth by marking the polygonal feature from the mag data, the building from the GPR data, and, with some guess work, the outer feature from the Earth Resistance data (Fig. 22).

Fig. 22: rough interpretation of the three data sets.

It is impossible from the data to tell if the outer feature from the res survey goes under or around the building, so I may have been a bit generous there.

So what is it?  One possible interpretation could be that we have a prehistoric feature with a mound and a ditch, presumably a round barrow.  The ditch silts up before the Roman occupation.  The mound is then fenced off and kept completely clear of structures or negative features like ditches and pits.  A building is constructed to the south of this mound, and a viewing area to the east.  Stephen Upex, solely on the basis of the aerial imagery, suggested that the feature was prehistoric, and re-used as either a amphitheatre in the Roman period or a small castle in the medieval period.  With the new data, I think we can rule out the castle (unless it was much more substantial at that date).  Although the phrase “ritual” is greatly over-used in archaeology, maybe in this case we are looking at an earlier mound which continued to be venerated into the Roman period?  Baths are often associated with religious sites.  At this stage, this is purely guesswork at the end of a long blog post.  This feature is, really fascinating and quite enigmatic.  Extending the earth Resistance and GPR surveys would, obviously, be very helpful.

The landscape around Durobrivae is fascinating from an archaeological point of view.  The nearby Roman fort is only known from aerial photographs, and just across the river and the Nene Valley Railway lies the site of Castor (Fig. 23) with its huge Roman building complex.

Fig. 23: Castor as seen from Durobrivae.

Last, but not least, many thanks to all those who helped push the mag and the GPR, and who aerated the grass with the resistance frame, or flew UAVs to map the topography.  Although the site is a long way for all of CAGGs volunteers, the site is both stunning and intriguing and, I think, worth the effort.  We hope to return to collect some more data soon.

Fig. 24: collecting Earth Resistance data with WAS’s machine.