Tag Archives: geophysics

Of snow and sheep

Yesterday started off well. The weather was a little cold, but other than that it wasn’t too bad.  Then it started to snow.  The snow was OK, not too much.  Then it snowed some more.  And a bit more…

Fig. 1: surveying in the snow.

Just to add to the delights, we had a visitation from some sheep.  They charged past me where I was laying in the grid for the next square, and surrounded the team using the res.

Fig. 2: The sheep visit the survey.

Fig. 3: The sheep visit the survey (heavily cropped photo).

By lunchtime, it was getting silly, and we packed-up.

Fig. 4: Getting silly.

We did, however, get some very nice results from both the 0.5m probe-spacing data and the 1m -probe spacing data (the latter “looks” a bit deeper into the soil).

Fig. 5: the 0.5m probe-spacing data.

Fig. 6: the 1m probe-spacing data.

The dark line running diagonally across the plot is a Roman road with a subsidiary road running off it at an angle.  The excavations were at the far left-hand side of the image.  The buildings along the road are pretty clear.  Both these images have been high-pass filtered to bring out the structures.

Although the landscape is covered in snow, it isn’t actively snowing at the moment, so I suspect we will be going out again today.

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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.

 

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.

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!

Sawtooth Sunday

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

Today saw the GPR team doing the sawtooth edges of the field. A bit fiddly to do, but very fiddly to survey-in and process.  I have only processed one block of the three, I’ll work on the others tomorrow and also start joining the various blocks together into one big survey.  The  Resistance team also managed a very respectable six grid squares, including some that covered the east wing of the “House on the Hill”, otherwise known as Insula XXVI Building 2 or even Niblett and Thompson Monument 445…  This building was first published by Corder in 1941 and has only been seen through aerial photographs until we came on the scene.

First the underlying magnetometry data from 2015.

Fig. 1: the magnetometry data underlying the 2017 Earth Resistance survey.

As can be seen from Fig. 1 we can see the building quite clearly in the mag data as a series of white (low magnetism) lines.  The east wing does not show very clearly.

Fig. 2: the Earth Resistance survey.

The results from the Earth Resistance survey show the building beautifully.  The image has been high-pass filtered.  The large room on the east wing shows very clearly, although the “apse” on the west wall of it is still a little unclear.  It looks like a large dining room.

Let’s look at the 2015 and the accidental 2017 GPR surveys.

Fig. 3: the 2015 survey of the “House on the Hill”.

The slice from the 2015 survey (Fig. 3) shows some of the detail more clearly than the Earth Resistance survey, and some less so.  The other details may show more clearly in other slices and I’ll reprocess the data in due course. It will be interesting to see what the Resistance survey makes of the building to the far east of the GPR data.

Fig. 4: The 2017 GPR survey overlain on the other data.

Just for completeness sake, here is one of the time slices from the 2017 survey (Fig. 4).

Further south (and further up the hill!), the team completed a block to the south of the nice building we saw yesterday.  The new software can create an image with all the slices on.

Fig. 5: slices 3 to 10 of the day 4, block 3, GPR data.

This is useful to be able to see how the details change as one goes deeper.  (Take the depths given with a pinch of salt as 0.09m/ns is just a guess at the moment.)

The next series of images shows slices 4-8 from the above plotted next to yesterday’s building (just yesterday’s slice 5). The joins between blocks will improve when I process it all as one big survey.

Fig. 6: slice 4 (see Fig. 5 for depths).

Fig. 7: slice 5 (see Fig. 5 for depths).

Fig. 8: slice 6 (see Fig. 5 for depths).

Fig. 9: slice 7 (see Fig. 5 for depths).

Fig. 10: slice 8 (see Fig. 5 for depths).

As can be seen there are various buildings in this area.  It looks like there might be another small town house just the SW of the one we saw yesterday.  There is also something quite large parallel to the SW-NE road which shows in the deeper slices.  The large ditch which runs parallel to that building, just to the west which can be seen in the mag data, shows as a whiter line of “no reflections” in the GPR data.  There is quite a lot going on in this little block of data.

Tomorrow I will work on the other blocks and try to integrate the first four days of survey.  We are not out on site again until Wednesday.

Many thanks to Ellen, Jim, Mike, Graham, Nigel, David and Pauline for all their hard work, and especially to Mike, Jim and Ellen for transporting all the equipment as well as myself!