Tag Archives: Roman town

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.

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Day 54

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

Yesterday was the last day of the 2017 season at Gorhambury. Apologies for the slight delay in posting… we went down the pub for supper!  We have completed 54 days of survey at Gorhambury over the three seasons.  As well as the 35.7ha of magnetometry we completed last season, we have now completed 14ha of GPR survey.  Just pushing the machine along the strings is about 280km.  We didn’t manage any usable Earth Resistance survey in 2015, but we have done quite a bit in 2016 and 2017.  Last year we had terrible problems with the very hard and dry soil.  As a result, many grid squares have been done twice.  We have, however, completed a 5.6ha survey at 0.5m intervals.  That is 224,000 resistance readings, or sticking the machine into the soil 112,000 times.  That doesn’t include three days with the beast which is a further 9,600 survey points resulting in 67,200 readings.  Here is the story in pictures.

Fig. 1: crude mosaic of GPR time slices showing the extent of the survey so far.

Fig. 2: the 2016 and 2017 Earth Resistance surveys at Gorhambury.

Figure 1 shows a (very crude) mosaic of time slices for Gorhambury just to show the entire extent of the survey.  There is going be a great deal of work reprocessing these to get the best out of them and to get the various blocks to match.  I also need a faster computer.  I tried out the kriging option last night and it took several hours to process the data, but the images were no doubt sharper.

Figure 2 shows the 2016 and 2017 Earth Resistance surveys.  At the moment the two seasons are just images put together in Google Earth.  I need to see if I can join the two into one big survey and get the edges to match properly.  I also need to see if I can get rid of the line caused by the deluge this season.

Firstly, let us look at the Earth Resistance survey results.  On the last day we redid four squares from last year, and then completed five awkward partials around the corner of the enclosure for the theatre.  Why did we redo those four?  Figure 3 shows last year’s survey with the block marked.  The hot dry conditions gave very noisy and unsatisfactory results.  I thought it was worth a morning’s effort to get those re-done.

Fig. 3: the 2016 resistance survey showing the duff grids.

Now the improved grids.  Note that the slight difference between the two surveys is due to minor differences in how I processed the data.  I will produce a more standardised plot.

Fig. 4: detail of the area surveyed at the end of the 2017 season.

Fig. 5: the Earth Resistance survey with the blocks from 2017 high-pass filtered.

There are some interesting things happening at the north-eastern corner of the plots, and into the area to the north we haven’t surveyed.  My guess is that the stratigraphy is probably deep and complex in this area.  Let us compare this area to the magnetic data (Fig. 6) and the GPR data (Fig. 7).

 

Fig 6: the magnetic survey with the area of the Earth Resistance survey from days 17 and 18 indicated by the cyan outline.

Fig. 7: the GPR survey with the 2017 Earth Resistance survey area indicated in red.

Fig. 8: the GPR survey with the res data overlain on it.

From all three data sets we can see that there is a lot going on in that bit of the field near the drive and the theatre, but it also appears there has been a good deal of robbing to add confusion to the picture.

How about the GPR team?  A month of nice weather with a bit of rain has made the grass green and lush.  Lovely for sheep, but a pain to push the GPR through.  They completed two 40x40m grids on a hot humid day, excellent progress in the conditions.

Firstly, here are a set of time slices (Fig. 9).

Fig. 9: GPR time slices from the area surveyed on Day 18.

The vast majority of the area surveyed appears to be empty.  There are hints of earlier agricultural practice but not much else apart from the top edge where part of a building can be seen.  This connects to the area surveyed earlier in the season.  I reprocessed the earlier version of that block today using kriging to give a sharper image.  Here are the slices together (Fig. 10).

Fig. 10: the GPR survey from day 18, along with a re-processed block from day 12.

We clearly have a nice L-shaped building.  I suspect this is Insula XXX, Building 4, Niblett and Thompson Monument No. 461 which is known from aerial photographs from 1976.  They only have part of the plan, however, and ours looks quite different (hence my doubts).  The mag data shown in Fig. 11 shows some slighter, more ephemeral buildings to the SW along the line of the SW-NE street 25 which can be seen very easily.

Fig. 11: the mag data in the area of the GPR survey.

The SSW–NNE running street 25 which runs along the NW side of the building we have been discussing, shows very clearly indeed but street 10, which is supposed to have run WNW–ESE just to the south of our new building and the “House on the Hill”, does not show at all in either the mag or the GPR data.  It was observed in excavations by Frere near the modern road, but not this far west.  In terms of the town plan, we seem to have two lines of buildings running SW-NE, one along Street 25, and another along street 23, but a large open area with nothing very much it in apart from a few quite large pits.  One can almost see some alignments in those pits.  Are we seeing backyard areas divided into blocks?

Although our season at Gorhambury has come to an end, we will be undertaking surveys elsewhere, and probably in Verulamium Park once more.  I started this posting with some numbers, so I thought I would end with some as well.  This is the 131st posting on the blog.  Those postings take-up 583mgb of our 3gb free allowance and include 693 images.  There have been 68 comments, but we have been protected from 9,402 pieces of spam!  The blog has been viewed 32,150 times by 11,016 separate visitors (in practice, this means that number of IP addresses).  Our best month was the first season at Gorhambury in 2015.  This August has been down on the previous two (2015: 1.8k, 2016: 1.7k and 2017: 1.4k) but the average number of visitors per day has gone-up over the year and we are likely to reach 9,000+ views by year’s end.

I would like to thank everyone who helped this season once more, both with pushing machines, moving strings, laying-tapes and moving equipment.  You are all stars in my eyes, and I think we have created a stunning survey.  We all got a bit tired towards the end, especially in the rather hot and humid conditions over the couple of days, including CAGGs loyal follower, Fergus (Fig. 12).

Fig. 12: Fergus sleeps off a busy day on site.

If anyone is interested in joining in with some of CAGG’s activities, drop us an email.  We are a friendly bunch, and on-the-job training is given.

Buildings galore

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

Before looking at today’s results, I thought everyone might be interested in seeing the reaction to our survey at Verulamium at the Near Surface Geophysics Group conference last December. Not being someone who twits, I hadn’t realised that a webpage of postings from the conference had been put together at https://storify.com/girlwithtrowel/tweets-from-the-nsgg-recent-work-in-archaeological You have to scroll down a long way, but look out for the comment by Magnitude Surveys.

Today saw the GPR crew tackle an awkward block that has the hedgerow half way across it.  Here are nine time slices from it.

Fig. 1: Nine time slices from the survey on day 17.

There are some very clear buildings.  Look, for example, at slice 7 (right hand column, middle slice).  In proof of the only universal law, look at the first “sawtooth” on the south side of slice 8 (bottom left hand corner).  There is a lovely little apse just peeping out into the plot.  Typical… the building is under the hedge.

Let us see where this block fits in the overall GPR survey (Fig. 2).

Fig. 2: all areas surveyed using GPR at Gorhambury. Today’s block is in colour.

The absolute area we have now surveyed is quite impressive thanks to the efforts of members of CAGG.  Well done all.  Let us now have a look at a couple of time slices from today.

Fig. 3: GPR time slice 4 from the day 17 survey,

The fourth time slice (Fig. 3; roughly 0.4 to 0.5cm below surface), mainly shows the road running diagonally SW to NE.  The cross-roads just to the north of today’s block is a very busy locality with lots of buildings clustered around it. The block just to the south of today’s survey also has some pretty substantial buildings.

Fig. 4: time slice 8 from the GPR survey on day 17.

Time slice 8 (Fig. 4, roughly 0.8 to 1.0m below the surface) shows some of the buildings alongside the road to the south of the hedge line.  The road is squeezed in between the building found earlier in the block to the south and the new rooms found today.  We also have a little more of the building to the west.  The two lonely walls in the southern block would seem to be related to that building too.  All-in-all, some very nice results.

The Earth Resistance team of Ellen, Pauline and Graham headed north to tidy-up the top-edge of last year’s survey.  They managed a surperb eight 20x20m grid squares.  Here are the results.

 

Fig. 5: Earth Resistance data from day 17.  The pink line marks the 2016 survey.  The lone grid square on the northern edge was re-surveyed.

Fig. 6: the Earth Resistance data from day 17, high pass filtered.

As can be seen, especially from Fig. 5, we have picked-up some more details of the nice large building in the middle of the plot, as well as other buildings such as the small one at the western end of the strip of grid squares.  At the eastern end we have a large square high resistance feature.  A surviving floor, perhaps?

Fig. 7 shows the mag data from this area.

Fig. 7: the mag data from the same area as Figs. 5 and 6.

Not much sign of the small building to the west, or the “floor” to the east in the mag data, although the “floor” seems to be associated with quite a few walls.

The “sinuous ditch” cuts across the line of today’s plot (seen as the broad dark linear feature entering Fig. 7 top centre, and heading to the SE).  This is almost certainly the town aqueduct as it lies along the 300ft contour. Comparison to Fig, 8, however, shows we we do both mag and res…  no sign of the aqueduct in the res data at all.

Fig. 8: today’s Earth Resistance data overlain on the mag plot.

One last push tomorrow and we are “done” at Gorhambury for 2017.  Many thanks to everyone who has worked so hard, and also thanks to the Earl of Verulam for allowing us access.

 

Just two more days

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

I did consider using antepenultimate again, but I thought you might think me pretentious…

Everyone worked extremely hard today.  Mike and Jim on the GPR finished yesterday’s block and managed another 40x40m grid square.  No easy task over the long grass and thistles.  Ruth, Dave and Julia completed five earth resistance squares, including two that had to be done in two parts and joined together in the software later due to an inconvenient hedge!  Good job everybody, and many thanks.

Here is the Earth Resistance survey, both normal, high-pass filtered, and the magnetometry data from the same area.

Fig. 1: the Earth Resistance data at the end of day 16.

Fig. 2: the Earth Resistance data at the end of day 16, high pass filtered.

Fig. 3: the magnetometry data for the area shown in Figs. 1 and 2.

I hate to say it, but our five squares, including the two annoying partials, appear to lie between the buildings along the road to the north of the hedge line, and to the north of the buildings we found yesterday. We do, however, have a nice tidy area surveyed now.  We couldn’t have left quite such a silly hole in our survey data.  Tomorrow we head north to survey along the northern edge of the block we did last year.

The first block of GPR data from today was a continuation of yesterday’s

Fig. 4: the GPR time slices from the block completed on day 16.

Nothing very exciting jumps out from the plots, although there are some things to check out.  Slice 6 (second from top on left) has a strange upside-down M shaped feature (in black) and slice 8 (bottom-left) has something semi- or sub-circular near the northern edge.

How about the second block?

Fig. 5: the GPR time slices from the second block completed on Day 16.

Not a great deal there either.  Sorry guys!

Tomorrow sees the GPR crew filling-in an odd gap between last year’s survey and this years.  The plus side is that the mag shows lots of buildings, so tomorrow’s results ought to be much more interesting!

Many thanks to everyone who worked so hard in the sun today.  Just two more days.

Templeless

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

The rather odd title will become clearer! Today saw Graham, Dave and Ruth on the Earth Resistance meter, Jim, Mike and Robert on the GPR, and myself rushing around with the dGPS. I managed to find time to do a little topographic surveying, the purpose of which will be revealed in due course.

First, the resistance survey.  The team managed an excellent seven 20x20m grid squares.  Thankfully, all whole grids today, although we are back in partial-land tomorrow.  Here are the “normal” and the high-pass filtered versions of the data.

Fig. 1: Earth resistance survey at the end of day 15.

Fig. 2: Earth resistance survey at the end of day 15, high-pass filtered.

Two things can be seen from today’s survey.  Firstly, there are some nice buildings showing on the western edge of the survey area, especially in the ‘sticky-out’ bit (technical term that) which was the last grid square we did.  They show great.  The second thing that arises, is: what on earth is going on the the south-west corner?  The irregular lines of high resistance running down slope from SW to NE do not look like archaeology, so are either geology or erosion.  Very curious!  I need to drape the results onto a topographic map (probably the LiDAR data) to see what the relationship is.

The mag data (Fig. 3) also shows the buildings on the west, but not so clearly.

Fig. 3: Mag data in the same area as Fig 1 and 2. The 2017 survey area marked in red.

The strong linear things in the SW corner do not show at all.  In fact, it is rather bland!

Lastly, the GPR data (Fig. 4).  Please note that the image is made of a variety of time slices created by two separate systems. Eventually, I will process it all to make a nice clean image, but this will do for now.

Fig. 4: GPR data in the same area as Fig 1 and 2. The 2017 survey area marked in red.

Interestingly, the plans of some of the buildings look clearer in the GPR than the res, and for others it is the other way around!  The “muddier” looking walls in the western side of the GPR plot are because I am using inverse weighting to interpolate the results as the kriging in the new software is impossibly slow. The old software’s kriging routine was much faster and gives crisper looking walls.  I probably need a faster computer!

The GPR team finished one block they started yesterday, and started a second block today.  Here are the slices.

Fig. 5: GPR survey time slices. Yesterday’s second block and today’s first block.

Fig. 6: GPR survey time slices, today’s second block.

You can be forgiven for being underwhelmed by the results.  I am a bit puzzled.  The radargrams look quite busy in places, but the slices look very dull.

Now comes the explanation of the strange title of this post.  Let us first look at the slices in position (Fig. 6), and then at the mag data for the same area (Fig. 7).

Fig. 6: a crude mosaic of the time slices in Google Earth.

Fig. 7: the mag data in the same area as Fig. 6.

In Fig. 7 you can see a small square of in black on the lefthandside, probably representing a small square enclosure.  I had hoped this was another Romano-Celtic temple like the one we surveyed at Durobrivae.  Not much sign of anything is showing in the GPR data, however.  Shame!  I had hoped for a small temple overlooking the Insula XVI temple and looking across the valley to the one at Folly Lane.  Sadly, not to be.  The little square enclosure remains a mystery.

Many thanks to everyone who helped out today. Tomorrow is our antepenultimate day.  Fingers crossed for good weather.

One more week

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

As we start our final week of the 2017 season at Gorhambury, our minds turn to what we can finish this year. I have a master plan for the Earth Resistance meter which aims to complete a sensible looking block of data to join to last year’s survey.  Thanks to the amazing efforts of Ellen, Pauline and Julia we managed not only to complete two awkward blocks with trees, nettles and thistles in them, but another four blocks too.  This means we are two grids ahead of schedule compared to the master plan.  Worry not…  I have plenty I would like to get done to fill the time available!

The next three images show the Earth Resistance data, firstly as normal (Fig. 1), then high-pass filtered (Fig. 2) and finally the mag data from the same area.

Fig. 1: the Earth Resistance data at the end of day 14.

Fig. 2: the Earth Resistance data at the end of day 14, high-pass filtered.

Fig. 3: the mag data for the area covered by the Earth Resistance data shown in the previous two figures.

One interesting feature is the building which is to the south of the obvious east-west road (Street 11 in Niblett and Thompson, Alban’s Buried Towns) in between the two easternmost clumps of trees.  We did not know about this building because neither the mag or the GPR really like fighting with nettles and thistles.  A res meter wielded by someone with good boots and trousers can venture where other machines fear to tread!

The other interesting feature is the lack of anything much on the western edge.  The pattern looks like erosion and water-borne sediments.  This is one of the dry valleys which come down the slope from the deep south.  We are still unable to say whether there were no buildings in that area, or if they have been eroded away or buried.  My guess is the former, but it is really only a guess.

The GPR team in the deep south did an amazing job completing the saw-tooth section into the far corner, and starting the next line.  Well done Mike, Jim and Robert.  Excellent job.  They are getting tantalizingly close to my mystery enclosure which shows in the mag.  Here are two sets of time slices.

Fig. 4: All 12 time slices for the survey in the deep south.

Fig. 5: All 12 time slices for the second block of GPR survey.

In Fig. 4 one can see the impact of splitting a survey over several days (let alone years!).  The second slice shows that today’s data has very strong reflections early on.  That is the impact of the rain over our days off, and the generally damp conditions today.  It is interesting — well at least to me it is! — that the literature on the various survey techniques talk a great deal about the impact of rain and weather on Earth Resistance survey, but don’t really mention the problems of GPR surveys run over multiple days.  The lower slices (e.g., slice 9) do show the boundary discussed in the previous post continuing south to the Roman wall.

The second block (Fig. 5) seemed as though it was going to be exciting.  The radargrams are full of strong reflectors and we are starting to go over the enclosure seen in the mag data.  The slices are less than exciting, however. There are a few strong reflections, e.g., near the top of slice 7, but nothing too much to write home about.

I had hoped to get this posting done quickly tonight as I have a report to finish, but the new software refused to play ball for ages and now it is too late and I am too tired!  Only four days left for the 2017 survey season at Gorhambury.

 

 

Just in time

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

Not long after we arrived home after a busy week on site, the heavens opened. We’ve had 7.2mm of rain in the last 24 hours. Hopefully, not enough to create problems with the Earth Resistance survey, unlike the deluge we had the other week.

Mike Smith sent me this entertaining picture.

Fig. 1: Red Flag © Mike Smith.

The Earth Resistance meter was manned by Ellen, Pauline, Graham and Fergus (CAGG’s mascot).  They completed an excellent seven 20x20m grid squares including two partials with a big oak tree in the middle.  Figs. 2 and 3 show the data “normally” and high-pass filtered.

Fig. 2: the Earth Resistance survey.

Fig. 3: the Earth Resistance survey, high-pass filtered.

The main feature of interest in the new area is the nice building in the top left-hand corner of the plot.  The apsidal end of a room facing NE shows especially clearly.  This building was known previously, partly from an aerial photograph taken in 1977 and partly from the English Heritage survey undertaken in 2000.  We have much more of it, however.  The buildings also show nicely in the mag data (Fig. 4).

Fig. 4: the magnetometry data for the area of the Earth Resistance survey.

We will fill in the odd missing squares on Wednesday.

The GPR was in the deep south of the Gorhambury side of the town, up on the hill near the King Harry Lane roundabout with marvellous views down across the theatre and the River Ver with the fields of ripe wheat on the opposite hill slope.  That is after you have caught your breath…

The GPR team completed the block I was hoping to get done, and started on the next one slated for Wednesday. Good job!  Here is the composite of six time slices (Fig. 5).

Fig. 5: The GPR time slices 3 to 8. See labels for times and approximate depths.

Slice 4 (Fig. 5, top right) appears to show a wall running NNW–SSE with a rounded corner.  Here it is on Google Earth.

Fig. 6: time slice 4.

Slice 6 (Fig. 5, middle right, and Fig. 7) starts to show a clear building in the NE corner. This is supposed to be along a road which runs approximately parallel to the modern road, but which I cannot see in the geophysical data. The wall seen in the previous image is still visible on the northern side but does not show on the west.

Fig. 7: time slice 6.

Slice 8 (Fig. 5, bottom right and Fig. 8) shows the wall again on the western side as well as the northern.  Curiously, the corner appears different. Comparison to the mag data (Fig. 9) is informative.

Fig. 8: time slice 8.

Fig. 9: the mag data in the area of the GPR survey.

The line of the wall is clearly following the magnetic anomaly in this area, with two large circular anomalies on the corner.  I had thought the anomaly was a ditch, but now I am less sure.  It is clearer to see the relationship if one traces over the wall line from slice 8 (Fig. 10).

Fig. 10: the mag data showing the wall line from GPR time slice 10 marked in yellow.

The differences in the corner can be seen if one then looks at slice 4 with the wall line from slice 8 marked (Fig. 11).

Fig. 11: the wall line from slice 8 marked on slice 4.

As can be seen from Fig. 11 the wall lines match perfectly apart from the corner.  Was it rebuilt at some point, or do the reflections in slice 4 represent some wall collapse?  I’ll need to examine the radargrams carefully to decide what is happening.

The ditches are marked on the “Urban Archaeological Database”.  Isobel Thompson informs me that they have been interpreted as field boundaries for “Little Wynyards” in the 17th century.  Perhaps a vinyard? As Isobel points out, there are the Vintry Gardens near the abbey which are in a walled enclosure.  Just one more avenue of research to pursue to be able to interpret our complex data set.

Many thanks to Pauline, Ellen, Graham, Mike, Jim and Nigel for all their help. We’ll be back on Wednesday for the final week.  Doesn’t time fly when you are enjoying yourself!