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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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Just too claggy

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

A group of us headed out to Little Hadham today with the aim of extending both the Earth Resistance and magnetometry surveys.   By the time Nigel had pushed the mag across the field, the wheels had diameters several inches larger than they should.  Jim tried a line or two of data collection, but the odometer was over-running by two meters and the wheel needed trowelling clean every transect.  We decided, therefore, to abandon the mag survey, and concentrate on enlarging the area of the Earth Resistance survey.  We managed another six 20x20m squares giving us a total of 100m by 80m, some 32,000 readings in total.

Figure 1, below, shows the initial results.  As before, the data is dominated by striping caused by the cultivation pattern.  A 2D fast fourier transform (as implemented in TerraSurveyor), quickly removed these stripes.

Fig. 1: the Earth Resistance data overlain on the mag data.

In Figure 2 I have applied the filter to remove the striping.  To the right I have put the mag data for comparison.

Fig. 2: The resistance data after processing with the 2DFFT. The mag data of the same area is shown to the right.

Unusually, most of the features show in the res and mag data.  The res data has nicely picked-up many of the linear features more normally only clearly seen in mag data.  In Figure 3 I have labelled a few points.

Fig. 3: the res and mag data with labels.

Ditch features A and B show nicely in both the res and mag data.  What is clear from the res data, however, is that the ditch continues between the two and they are one distinct linear feature.  If one draws a straight line along A and B, it lines up perfectly with the linear feature C we found across the road in 2014.   Linear feature D shows equally well in both data sets.  At E, something complex is happening.  In the mag data it looks almost as if A is turning and runs alongside E, whereas in the res data is looks more like AB cuts across the linear to the west of E.  The parallel lines to the west of E show quite well in both, and are probably some form of trackway.

Many thanks to Jim (CVAHS; both for surveying and transporting the equipment and myself), Nigel (NHAS), Caroline, Peter, Amanda and Mark (BAG).  Hopefully we can get to do some more when the field is less claggy.

Tilting at windmills?

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

Firstly, apologies to anyone waiting on the Durobrivae report.  I am trying to resolve a small problem with the GPR data and will post something soon.  We did get some good results, if not quite so spectacular as the temple we found last year.  The “tumulus” is proving very intriguing.

Back in April 2014 we surveyed part of a site in Little Hadham.  We had always intended to go back, but never quite managed to get our act together.  Last weekend we finally managed to plan another three days at the site, working in the field to the west of the road.  We were mainly intending to undertake magnetometry (Fig. 1), but as we had enough people we also did some Earth Resistance survey (Fig. 2).

Fig. 1: Jim West (CVAHS) moves the string for Ruth Halliwell (WAS).

Fig. 2: Peter Alley (WAS) ably uses the Earth Resistance meter aided by Caroline and Peter Baigent (Braughing Archaeological Group).

The three days were about as different as you could get.  The first day was quite nice, the second day wet, drizzly and foul (we had to keep trowelling the wheels of the mag clear of mud) and the last day was absolutely glorious.

The results from the magnetometry survey were excellent (Fig. 3).  The features are relatively subtle, however.  The image below is clipped to +/- 1.5nT.  In others words, all readings above 1.5nT are plotted solid black, and all readings below -1.5nT are plotted white.  At Gorhambury, I clip the images at +/- 5nT.  The pottery kilns are Verulamium have very strong values of -15 to +150nT.

Fig. 3: the magnetometry results from 2014 (east of the road) and 2017 (west of the road).

As can be seen from Fig. 3 there are lots of mainly linear features, some very straight, and some quite sinuous.  We are clearly dealing with a multi-period site.  The faint striping running west-nor-west to east-sou-east are a result of the harrowing of the field, made more visible by the extreme clipping of the image.  To make the discussion easier, I have labelled up the figure.

Fig. 4: the mag results.

One of the first things to note is how different all the linear features are.  The one indicated by blue arrows is quite straight and for some of its length, at least, very magnetic (-6nT to +11nT).  The one labelled with red arrows is, however, very sinuous and only faintly more magnetic than the background (about +/- 1nT).  That ditch seems to continue as indicated by the green arrows, which in places seems to break up into a series of linked “blobs”, either patches of more magnetic material dumped in the ditch, or perhaps pits within the line of the ditch.

The strength of the magnetic values is dependent on two things: firstly, the source of the magnetism.  Soils may be strongly magnetically enhanced by burning or intense occupation, for example, or may only be weakly magnetically enhanced if they contain just slightly more rotted organics than the background.  Secondly, size can also be a factor.  A large feature can contain more magnetically enhanced soil than a very small, shallow feature.

Features C and D are very straight, and are unlikely to be pre-Roman but they could be Roman or later.  I wonder if E could be a drove-way leading up from the valley to the west?  We need to do some work in the archives and see how much the field systems in this area have changed.

We have two circular features: A and B.  My initial quick thought is that these are both round barrows.  The majority of barrows are Bronze Age, but we do get barrows in the Roman and Saxon periods too.  Their location on a ridge with excellent views would support their interpretation as barrows.  The fact that the two features looked so different worried me, and then I remembered a lecture I used to give on aerial photography.  Could this be a windmill?  A quick comparison with an image published by Wilson (2000, Fig. 58) strongly suggested this interpretation.  Of course windmills also want to be up high!  We’ll come back to the putative windmill below.

Fig. 5: Members of BAG running the Earth Resistance meter.

We initially decided to use the Earth Resistance meter over a patch of the field where the farmer had noted it was difficult to plough, and where there were a large number of flints on the surface.  Over the three days we completed 14 20x20m grids.

Fig. 6: The Earth Resistance results overlain on the mag data.

As can be seen from Figure 6, we have detected the ring ditches quite clearly, and some of the linear features.  The stripes are plough scars.  This makes it quite difficult to see what is happening in many places.  TerraSurveyor can apply a Two-dimensional Fast Fourier Transform (2D FFT) to the data to try and removing striping such as this.  Using a 2D FFT doesn’t always help, but in this case the results were excellent (Fig. 7).

Fig. 7: The Earth Resistance results after applying a 2D FFT.

To aid discussion I have labelled-up the plot as before (Fig. 8).

Fig. 8: Resistance results, labelled.

The two circular features (A and B) show clearly in the res data.  Feature B appears to have taken a “bite” out of the high resistance area F.  This area is where the flints were on the surface, and rather than being a building, it seems more likely we are dealing with a pocket of flints in the periglacial drift geology. The ditch which runs ENE–WSW just above the letter F shows very clearly where it has cut through these flints.  The ditch shown with green arrows in both Figs, 4 and 8 is quite clear.  What is intriguing is that ditch C, which appears only faintly to the west of the road in Fig. 4, and then peters out, clearly continues as shown by the pink arrows in Fig. 8.  We must never forget that ditches show well in mag data because they act as “traps” for more organic, magnetically enhanced soils.  If the fill is not magnetically enhanced, as may happen when one moves away from occupation sites, we may not be able to detect them.

The res survey is particularly pleasing from a teaching point of view.  Not only has the 2D FFT worked very well, but this is the exception to my usual statement that “res is less good at picking-up ditches and pits…”.

Fig. 9: Kai makes sure I am putting the grid in correctly.

The Windmill

When I first emailed the people helping on the survey and said I thought feature B was a windmill, Peter Alley immediately pointed out that the lane which runs through the site is “Millfield Lane.”  Well done Peter!  This is what Wilson (2000, p. 108) says:

Medieval post mills stood on crosstrees whose foundation-trenches formed a cross measuring about 10m wide overall.  The crosstrees were usually embedded in, or set in the top of, a low mound surrounded by a ditch.  The higher the mound, the broader its ditch, but the less likely that the timbers have penetrated the subsoil.  Crop-marks of windmill-mounds thus fall into two groups: those with proportionally broad ditches that usually display no central cross… and those with modest ditches (2 – 3m wide) and a cross within.  The ditch is ordinarily 25m in diameter; it may have two even three entrances.

Our feature is almost exactly 25m in diameter but the longest part of the surviving crosstree foundation is slightly less at about 8.5. The ditch is between 3 and 4m wide.  The “ditch”, however, does not look like a classic ditch feature in the mag data, and it may have other origins.

I checked the book by Howes (2016) but he only discusses the smock mill known from elsewhere in the parish which was burnt down in 1981 (pp. 132–4).

Jim West wrote:

I have been looking at windmill design to try to identify what would create the large circle (dia about 20m)  in the mag results.  Thoughts so far:

The windmill was probably a post mill, i.e., the whole of the upper structure rotated on a single post.  The post was often supported on a cruciform base rather than set in a hole. This design was in use for several hundred years until c.19 when the more powerful smock mills were introduced.

An example of a trestle base (this one is on brickwork which seems to be a modern improvement)
Post and trestle 18th May 1979from: http://www.norfolkmills.co.uk/Windmills/tottenhill-postmill.html

with the upper structure it looks like this (different mill)

from: https://en.wikipedia.org/wiki/Post_mill

These mills had to rotated to face the wind which was done by pushing on the long beam (threaded through the steps) on right in the image above.  This beam (or tailpole in miller speak) had to be long enough to get the leverage to move 15-35 tons of mill.  Most of the larger mills had a wheel on the end of the tailpole, usually with an iron rim.

Some later mills used wind power to rotate the mill.  The example below gives some idea of the length of the tailpole.

Fantail trolley June 1936
from: http://www.norfolkmills.co.uk/Windmills/tottenhill-postmill.html

My initial conclusion is that the large circle in the results is a record of the arc of the tailpole;  there may have been a surface laid to reduce rolling resistance because the width of the “path” is too great for a wheel rut and with a prevailing SW wind in the UK more wear would be expected in the NE sector.

The results also show what could be one of the beams of the trestle; the beam at right angles to it is less clear but I think supports the ides of a post mill on a trestle.

Having overlaid the res and mag data, the feature suggested by the res does seem a little larger than that suggested by the mag, although they do overlie to some extent.  The outer ring, therefore, may be a complex mix of the outer ditch (likely to be more irregular) and the sweep of the tail pole (which would be a perfect circle).

Ruth examined some of the historical evidence:

I have been looking at the old maps I have access to, to see if I could work out when the lane was named ‘Millfield Lane’, the house ‘Millfield Cottage’ and see if I could find any mills in the area. Working back from the modern OS maps, the house only became identified on the map as ‘Millfield Cottage’ between 1960–80. It was previously ‘Millfield Houses’. [NB: you can browse through old maps on the National Library of Scotland website]. I looked back through 20th century maps and back to 1870’s and the name of the lane was consistently Millfield Lane, but there was never a mention of a windmill at that site. On the 1880 Shire view map, there is a windmill (corn) just to the NW of Little Hadham.  It is beside Mill Common. The Bryant Map of 1822 has a windmill drawn at, what is probably, the same place as the 1880 map – but none in our field.

Dury and Andrews, 1766 [NB: available to view online here]  does record windmills as there is one at Hadham Lordship, but none in the area of Hadham Ford, Berry Green, Hadham on Ash and Green Street. Rowe and Williamson (2013, p. 261) mention that there was a mill in Little Hadham built 1786–7, which is likely to be the one in the Bryant Map and the 1880 OS map. The mill must post-date the map by Dury and Andrews (1766) [NB: available to view online here]. The construction of 1786–7 must have been a ‘new build’.

It may also be the one I found mentioned in Wikipedia entry – Hertfordshire Windmills, Little Hadham, which gets listing from Moore (1999). Only two are mentioned: the first a smock mill that was built in 1786 and burned down in 1981 that matches the location and description of the one at Mill Common mentioned by Rowe and Williamson.

The second entry is that listed by Moore (1999) and dates to before 1700.   Moore (p. 77) notes that ‘… just north of Bury Green there is a house today, which O.S. maps name Millfield Houses. Fields on both sides of the road in this position are names ‘Mill feelde’ on a map dated 1588 but it is possible that the field could have been names from a horse mill situated nearby in medieval times. There is no doubt that there was a windmill in medieval times and possibly two sites. The only miller’s name found was in the 1587 Muster Roll Richard Howell – myller.’  There are a couple of references to a 13th century mill, but Moore was unable to show that these refer to this site.

The Domesday Book does not list any sort of mill within Little Hadham.

There is clearly some more historical work to follow-up on.  It would be good to see the 1588 map mentioned by Moore which is at HALS, and I’d like to see the references in Holt’s 1988 book cited by Moore.

Hopefully, it won’t take us three-and-a-half years to return to this fascinating site.

Fig. 10: At the end of day 3.

References

Historic England (2011). Mills. Available online.

Holt, Richard (1988). The Mills of Medieval England.  Blackwell.

Howes, Hugh (2016). Wind, Water and Steam.  The story of Hertfordshire’s mills. Hatfield: University of Hertfordshire Press.

Moore, Cyril (1999). Hertfordshire Windmills and Windmillers. Bishops Stortford: Windsup Publishing.

Rowe, Anne and Tom Williamson (2013). Hertfordshire: a landscape history. Hatfield: University of Hertfordshire Press.

Wilson, D. R. (2000). Air photo Interpretation for Archaeologists, second edition.  Stroud: Tempus.

 

Ivinghoe Aston

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

Last Sunday we undertook a small survey in the parish of Ivinghoe Aston, Buckinghamshire. Earlier in the year, following a find by a metal detectorist, the Buckinghamshire Archaeological Society’s Active Archaeology Group undertook a small excavation and recovered a Roman cremation burial.  As is always the way, tucked into one corner was the edge of a second burial.  This was left in situ and we were asked if any of the survey techniques might detect cremations.  Of the techniques we use, magnetometry is the most likely to be able to find cremations but given how small they are, the standard 0.5m spacing between sensors might just be too big.  We can configure our cart to have a 0.25cm spacing between sensors (Fig. 1) at the cost of having to walk twice as far.  In this case, we were not aiming to cover a large area and so the trade-off seemed reasonable.  Jim West (CVAHS)  Pauline Hey (LBDAHS), Peter Alley (WAS) and myself (WAS), along with Jean Bluck (CVAHS and BASAAG), Rhian Morgan (CVAHS and BASAAG) and Piotr Sobisz (the person who found the site) surveyed a 80 x 80m area at 40 readings per meter.  Peter Alley also undertook a survey with his UAV to map the lumps and bumps in the field.

Fig. 1: Jim West operating the mag with a 25cm sensor spacing. Fergus is supervising closely.

The basic results are shown in Figure 2.  This image has had the readings clipped to +/- 2.5nT.

Fig. 2: the magnetometry survey results.

The first and most obvious things are the two strong linear features across the bottom and top left hand corners.  There are also some fainter linear features.  I have marked these in Fig. 3.

Fig. 3: the results with the linear features marked. Yellow: fence lines etc.; red: more subtle linears, possibly archaeology.

The lower feature marked with yellow arrows clearly lines up with a hedge in the next field and is almost certainly a grubbed-out hedge line.  The upper line marked with yellow arrows is less  certain.  It isn’t that strong, and it isn’t that continuous.  I still have my doubts, however.  It may be an old field drain?  There are many subtle linear features marked with red arrows.  These do not make a coherent pattern and may well be earlier drainage or other agricultural work, but some could be something more interesting archaeologically.

But what about the grave?  Fig. 4. marks the position of the excavation trench.

Fig. 4: the results with the excavation trench marked.

You can just see the little red square in the middle of the plot. (It might be worth clicking on the image to see it in more detail, and then clicking on that version of the image to zoom it to maximum size.)  Fig. 5 shows the area in more detail (again you might like to click on it).

Fig. 5: zoomed in view of the area of the trench.

It is interesting to see that the excavation does not show in the data at all.  The second, unexcavated cremation was in the SW corner.  As can be seen from these plots, there are lots of “little blobs”, some of them are stronger than others.  Little blobs which are strongly magnetic, shown with the positive pole in black and the negative in white, are likely to be metal, especially when the negative (white) part is not north of the black part.  In order to see the subtle stuff, we have clipped the image to +/- 2.5 nT (which isn’t much!) but this makes it hard to see the difference between strong and moderately magnetic “anomalies”.  To make this clearer, I have created an image where the strong values (greater or less than 5nT) are in red and green (Fig. 6).

Fig. 6: strong positive and negative magnetic values shown in red and green.

This allows us to discount a number of features, but there are still lots of possible blobs.  If we look at Fig. 5 closely there is a small blob on the SW corner, just about where the second cremation is likely to be.  Using TerraSurveyor, we can measure the magnetic profile of the feature and draw a graph of it (Fig. 7).

Fig. 7: Readings across the feature to the SW of the trench.

From this graph we can see that the burial, if that is what it is, has a “signature” about 75cm across and ranging from -1nT to a maximum of about 5nT.  From this we can suggest some possible extra burials.  I have marked just a few in Fig. 8.

Fig. 8: Red arrow: likely cremation next to the trench; yellow arrows: some possible cremations; blue arrow: larger feature.

There are clearly quite a few more in the survey, I have just marked five.  Unfortunately, the law of equifinality comes into play, i.e., very different things can have very similar magnetic signatures.  All sorts of things might make small, subtle magnetic features, like old animal burrows for example.  There are also a small number of larger features which might be interesting, for example the one marked in blue.  This is bigger than a single cremation, being 2m north-south and 1.5m east-west but has a similar magnetic range (i.e., -1 to -5nT).  The reason why I am curious about it is that cremation cemeteries sometimes also preserve evidence for the pyre site.  Rarely, we also get what are called bustum burials where the pyre is built over a small pit and the pyre remains, along with possibly extra grave goods, and placed in the pit.  Hopefully, the AAG might be able to test a few of these features to “ground truth” the results.

All in all we had a grand day, especially Kai (Fig. 9)!  We have several possible surveys in the pipeline.  If you want to be put on the mailing list, please email us.

Fig. 9: Kai being spoilt.

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.