Tag Archives: Hertfordshire

More on mills

Jonathan Neville, who runs the website on Norfolk mills, made the following interesting comments having seen the blog post.

The Tottenhill trestle base was enclosed within the roundhouse while the mill was in use.  The Little Snoring open trestle mill did not have a roundhouse so is a slightly better example

The trackway around the mills was narrower when the mills used a fantail to automatically turn the mill to the wind. This system only came into use after 1807. Sometimes the trackway for the necessary wheels would be paved, e.g.,  www.norfolknmills.co.uk/Windmills/garboldisham-postmill. If the mill was turned by hand the pathway would almost certainly have been wider.

Many thanks to Jonathan for the extra information.

Also, I love the place name “Little Snoring”.  I wonder if there is a Much Snoring?


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.

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.


Pipe dreams

Yes, I know, blog posts are like busses…

Some of you may have seen a recent BBC News item about the holes for the gas pipe in Verulamium Park.

Fig. 1: BBC News item about Verulamium Park.

I must admit to being a little peeved that a few small roughly dug holes to look at a gas pipe got more media attention than CAGG’s extensive surveys in Verulamium!  The gas pipe has always been, however, the bane of our lives as it’s magnetic field is so strong it masks a 30m wide strip of the archaeology in our magnetometer survey.  That field, however, has no impact on the GPR and I have been intending to extend the GPR survey to cover the areas affected by the pipe. This, and the forthcoming meeting of the Roman Finds Group in Verulamium this Friday, prompted me to see if anyone could come out at short notice and survey around one of the pipe holes.

Fig. 2: Surveying the pipeline (photo: Mike Smith).

So, Mike Smith and I spent a day surveying around the site of the hole in which the traces of a Roman building were discovered.  It got a little fiddly at times, especially as the pipe had to be dragged along 11m every 30 minutes as they joined the lengths together.  The location of our new survey is given in Fig. 3.

Fig. 3: the location of the latest GPR survey shown in red.

As usual, we surveyed in 40m lengths (where possible) at 0.5m transect spacing.  The data were sliced using GPR Process and the images produced using Surfer v. 8, and imported into Google Earth via Adobe Photoshop.  The last step is needed to make the missing bits transparent. Unusually, the deeper slices (nos. 8 and 9) still showed useful information.

One annoying thing is that Google Earth have recently updated the satellite image for Verulamium.  It is a bit dark to my eyes, but more seriously it is in the wrong place.  In the time slices I could see the relationship between the path and the image was clearly wrong.  I spent some time checking my notes, but when I checked the historical imagery, I found the 2009 images matched the plots much better.  I have, therefore, used the older imagery for the figures below.

Fig 4: the magnetometer survey in the area of the new GPR survey.

In Fig. 4 we can see the magnetometer survey for the area in question.  The pipe line, running under the path and then diagonally across the football pitch, dominates the plot.  We can however, see the road running clearly SW–NE across the map (Niblett and Thompson’s Streets 16 and 17 crossing Street 12), and bit of buildings showing as the white lines against the grey and black.  As well as the gas main, there are three pipelines running from the cafe in the NE corner.

Fig. 5: third time slice, 10.5 to 13.5ns.

In Fig. 5, the topmost time slice in the sequence, we can already see the road surface running SW–NE, and we can also see signs of the pipe running across the plot although this might be a marker in the upper levels as the pipe itself shows more clearly lower down.

Fig. 6: fourth time slice, 13.5 to 16.5ns.

The next slice (Fig. 6) shows the road more clearly, and some hints of buildings.  I am unsure what the area of high reflections is to the south-east and parallel to the road is.  It is almost as if the road line was changed a little at some point.

Fig. 7: fifth time slice, 16.5 to 19.5ns.

Fig. 7 is starting to show the buildings up near the path much more clearly.  These are buildings that front onto Street 12 to the north.  Street 12 eventually leads into the forum and is probably part of Watling Street.  At the southern edge of the survey there are some white areas which may be robbed walls, with perhaps a surviving floor surface.

Fig. 8: sixth time slice, 19.5 to 22.5ns.

In Fig 8. we can now see two very clear lines either side of the road.  These maybe stone conduits or roadside drains?  The buildings near the path are starting to show more clearly, and now we can see the gas pipe quite clearer.  The fact that is disappears to the west is probably due to topography.  Ideally, time slices would be topographically corrected.

Fig. 9: seventh time slice, 22.5 to 25.5ns.

Fig. 9 shows the gas main beautifully, and more of the buildings near the path.  Those to the east of Street 16 and south of Street 12 are labelled “baths” by Niblett and Thompson.  The GPR data would make that interpretation a little more tentative, I think.

Fig. 10: eighth time slice, 25.5 to 28.5ns.

Fig. 11: ninth time slice, 28.5 to 31.5ns.

Figs 10 and 11 show some of the deeper features.  Some hints of more buildings are showing, especially in the NW corner of the plot.

Personally, I think this is much more interesting and informative than the gas pipe trench!

Fig. 12: the gas pipe.

The one thing that will be useful, is that by knowing how deep the gas pipe is below the surface, we can calibrate the speed of the GPR signal and convert nanoseconds into depth.

I am hoping to continue to expand the GPR survey when I can over the coming months.

Many thanks to Mike for coming out for the day in the cold sunshine.

The results from Darrowfield

First of all, apologies for the delay to this post. Life has been a bit busy, and the data processing got a bit complicated, but I think I have cracked it now.  Our principal aim was to complete the area using magnetometry.  This we did easily.  We also aimed to try and cover as much as possible using GPR and Earth Resistance.  We managed quite a bit of GPR, but not much resistance survey, partly due to manpower and time, and partly due to the stunningly uninteresting results.  Lastly, we did two resistivity pseudo-sections and two topographic surveys, one using the dGPS and one using a UAV and “structure from motion”.  So be prepared for a long post!


I am going to start by discussing the topography because this has an impact on the results and the interpretation.  We did two types of topographic surveying.  Firstly, I walked back and forth taking readings with the dGPS at six-pace intervals, or closer if there was a change of slope such as a bank.  Secondly, Peter Alley used his DJI Phantom UAV to take a series of aerial photographs which can be used to create a topographic model using a technique called “structure from motion”.

Fig. 1: Peter Alley landing his UAV after taking aerial images of the site.

The two techniques have their strengths and weaknesses.  The model using SfM can be very detailed, far more so than is easily possible with the dGPS, but the model is of the top of the vegetation.  The camera cannot see through the stinging nettles.  Mostly, this does not matter but there are areas where one has to be aware that the nettles are masking the topography.  I, however, could not easily get dGPS readings in the garden or the orchard because the trees blocked the satellite signals.  The UAV could, however, map these areas. Using the two methods side-by-side is thus a useful thing to do.  The other big advantage of the UAV is that the survey took about 20 minutes compared to almost a day using the dGPS!

Fig 2: Darrowfield paddock topographic model from the dGPS survey.

Fig. 3: Topographic map derived from the UAV survey.

Ignoring the areas outside the paddock / gardens in Fig. 3, we can see that the two surveys show pretty much the same thing.  The highest ground is the bank along the SW edge of the paddock with a steep slope along one edge.  The bank is less pronounced as it curves around through the garden and the orchard in the NW corner of the area.  I had initially thought this was a road line, but it doesn’t line up with the roads seen in the wider magnetometer surveys of the park.  It does seem to be mapped as a long thin enclosure on early OS maps. There is a second, much less pronounced bank along the NE side of the paddock, which fades out towards the east corner.  This bank is assumed to be the line of the Roman wall. Between the two higher areas is a lower lying area in the middle of the field.  In the east corner, the UAV seems to show a channel running west-east into the corner (Fig. 3, the dark blue line running through a lighter blue area).  This is a footpath running through an area of high stinging nettles.  On the dGPS survey (Fig. 2) we can see a low channel running north-south in the same area — shown in red — which is masked by the nettles.

The magnetometer survey

The main area of the paddock was surveyed on the normal grid we have been using at Verulamium.  The two small extra areas, on the lower lawn of Darrowfield House and the orchard, were surveyed using small floating grids set-up to fit the awkward areas better.  In the paddock, nettles, rabbit holes and badger burrows all made surveying a bit awkward.  Very many thanks to Pat Johnson of Foerster for bringing the repaired magnetometer down to site from Tamworth.

Fig. 4: Ruth Halliwell and Jim West surveying the Lower Lawn of Darrowfield House.

Given where we are in the Roman town, i.e., just behind the basilica in the centre of the the city, the results were surprisingly unexciting!

Fig 5: the magnetometry results.

The majority of the area seems to be dominated by a faint striping running SW-NE across the middle of the field.  I am guessing these are cultivation marks, and this interpretation is supported, as we will see below, by the GPR results.

Fig. 6: annotated magnetometry results.

In Fig. 6, at point A, we can see a curving “linear magnetic anomaly”.  This matches the curving bank seen in both topo surveys (Figs. 2 and 3).  The clearest indications of buildings are along the SW edge of the paddock, at B.  The light lines mark walls, and the darker areas magnetically enhanced materials between the walls. It is difficult to know if these are Roman or buildings associated with the farm.  If, however, we project the line of the roads from the wider survey (Fig. 7), it seems likely these are Roman buildings lining the SE–NW running road.

Fig. 7: rough projection of the Roman roads into Darrowfield Paddock.

At Fig. 6, C there appears to be a linear feature running across the lawn.  At the west end is a black dot with a bright white halo.  This is something ferrous, it has a range of -104nT to 2371nT!  The linear feature next to it only has readings from -5nT to +15nT and is much more likely to be something like a ditch.  Just inside this linear feature are three black “blobs” which have similar ranges and may be pits.

The steep bank shown on the topographic maps which runs parallel to the SW hedge has a linear low magnetic feature (Fig. 6, D).  This may just an artefact of the bank, but I am beginning to wonder if there is something holding the bank up, especially in the light of the GPR results below.

Lastly, At Fig. 6, E there are two faint linear low magnetic features.  Is this a hint of the wall line?

The GPR survey

Between the animal burrows and the nettles, the GPR survey could be quite a struggle…

Fig. 8: Jim versus the nettles at Darrowfield (image: Mike Smith).

We did, however, get quite a bit of the paddock done.  As usual, I have used Larry Conyers’ GPR Process program and Surfer v.8 to produce the time slices.  I have created five slices 3ns in thickness.  I’ll discuss them in order from the top down.

Fig. 9: time slice 3, 10.5–13.5ns.

The topmost time slice shows the bank parallel to the SW hedge very clearly indeed.  The clarity of this feature makes me think that this bank must be artificially reinforced to create the flat space between it and the hedge. Perhaps this was part of the entrance to the house, or something to make a view from it?  The other feature is the very clear line to the east which matches where the bank curves in the topo data.  I’m unsure what would make a line like this.  It is very close to the surface being in the top slice.

Fig. 10: time slice 4, 13.5–16.5ns.

In the next time slice (Fig. 10), the bank to the SW has turned into two linear features.  The bank to the NW shows as less clear linear features as well.  There are a series of lines running SE-NE across the lower part of the field.  These become clearer in the lower slices.  Finally, in the middle of the NE edge, cut into by the “dog tooth” pattern of the survey edges, is a broader linear feature running NW-SE.  This may be a hint of the town wall.

Fig. 11: time slice 5, 16.5–19.5ns.

In the next slice (Fig. 11), the parallel lines in the lower area are now very clear.  These seem most likely to be some form system of land drains.  To the south are some other linear features which may be the tops of surviving walls.

Fig. 12: time slice 6, 19.5–22.5ns.

In the next slice (Fig. 12) we are getting below the drains, but the signal is starting to attenuate.  There are, however, some linear features to the south and the west which are parts of buildings lying quite low.  A number of the “wiggly” features, particularly to the north, are badger setts.

Fig. 13: time slice 7, 22.5–25.5ns.

The last time slice (Fig. 13) finally shows the buildings that can be seen along the SW edge in the mag data.  The GPR cannot really push into the undergrowth as well as the mag and so the evidence is a little more bitty.  We can see, however, the road which runs from the SW to the NE, almost parallel to SE hedgeline, pretty much where I suggested it should be in Figure 7.

It would appear, therefore, that the archaeology is more deeply buried in most of this field than elsewhere at Verulamium.  Perhaps soil eroding down the slope built-up at the back of the Roman wall?  The wall itself, however, is conspicuously absent.  A massive structure like that should show quite clearly in GPR data.  The one thing that I ought to do is build the topography into the GPR processing, but I cannot do that as yet.  Watch this space!

Earth Resistance survey

Earth resistance came a poor third in the techniques we used on the site.  We managed one block 40m by 60m using the Institute of Archaeology’s RM85 at 50cm intervals.  We have also recently had the Welwyn Archaeological Society’s machine repaired (thanks Bob!), and did just one 20x20m square at 50cm.

Fig 14: Earth Resistance surveys.

After the excellent results from Gorhambury last summer and Durobrivae last October, these are more the sort of thing I expect in Hertfordshire, i.e., nothing very useful!  In this case, I suspect that, based on the GPR data, the interesting archaeology is deeper than the method with a 50cm probe spacing is looking.

Resistance Pseudosections

The West Essex Archaeological Group has kindly lent us their Resistance Pseudosection kit.  The idea of this is that one takes a series of readings in a line with a 1m spacing between the four probes.  One then repeats the operation with a 2m spacing, then again with 3m, 4m, 5m and 6m.  By increasing the spacing between the probes each time one repeats the line, one is looking deeper into the surface.  The data can then be plotted using a program called res2Dinv which creates a “pseudosection”, an image of a vertical slice through the ground.  We decided to try two of these: one across the road and one across the bank where the wall may be.  The idea was that maybe being able to look deeper into the ground we might be able to see something the other techniques missed.

Professional versions of this system use a switching box and are very expensive.  Bob Randall, however, created a useful budget version using his TRCIA resistance meter.  With just a couple of people this can be hard work moving all the probes between pegs, but if you have five people it can be quite quick. It does look, however, quite amusing as four people bob up-and-down changing probes in unison!

Fig. 15: the team undertaking a resistance psuedosection.

Figure 16 shows the location of the two lines we did overlain on the GPR data.

Fig. 16: the location of section A (red) and B (blue).

For both of the sections, I also ran the GPR along the same line so that we would have a direct comparison.  Unfortunately, the free version of res2Dinv will not undertake corrections for topography and will not save images for sections 40 probes (39m) long, although I can get a screen grab and use that, which is good enough for this blog.  The commercial version is extremely expensive.  I hope I may be able to process the data with the topographic correction in the future.

The first thing I did was to compare each section with its companion radargram.  The radargrams have been corrected for topography.

Fig. 17: Pseudosection A and a radargram along the same line.

There isn’t a huge correlation between the two for section A (Fig. 17).  The broad curving layer shown low on the left of the radargram does not have a similar signature in the resistivity data unless the large grey-green area of slightly higher resistivity is reflecting that.  The very high areas of resistivity shown to the right of the section in red similarly do not clearly match anything much in the radargram (although see below about the ‘circular thing’).

Fig. 18: Section B and its radargram.

The second section was remarkably featureless.  There is a large area of lower resistivity to the right which may represent the robbed out wall, but this doesn’t really show in the radargram. The large curved response in the radargram on the left doesn’t really show in the resistivity section.  All rather frustrating.  I decided to put the images onto the Google Earth images to see if that helps.

Fig. 19: Resistivity pseudosection overlain on the mag data. Note that the red line marks the position of the pseudosection and north is to the left.

In Fig. 19 the red line indicates the position of the pseudosection.  The area of medium resistivity on the left shown in a greeny colour matches where the section cuts across the linear area of low magnetism shown in light grey / white.  These are probably, therefore, caused by the same feature which would appear to be something associated with agricultural activity judging by the wider pattern in the mag data.

Fig. 20: The radargram for the same line, marked in red. North is to the left.

Comparison of the radargram to the mag data doesn’t help much.  The large curving feature to the left of the radargram (N), doesn’t really show in the mag data.  The area of low magnetism mentioned above only really correlates with half of that feature, if at all.

Fig. 21: Radargram overlain on the lower time slice (cf. Fig. 13).

Fig. 21 does show, however, that the radargram goes right over one of three clear sub-circular features which can be seen in the bottom time slice (cf. Fig. 13).  This feature shows very clearly in the radargram.  In Fig. 22 I have annotated it radargrams take some use to ‘reading’.

Fig. 22: Radargram showing the position of the subcircular feature.

I don’t know what these three features are, but there are three of them in a line at right-angles to the suggested line of the road.  Very intriguing!

Fig. 23: Pseudosection B on the mag data. The blue line marks the position of the section.

In Fig. 23 the area of slightly higher resistivity to the west matches very well with the bank of lower magnetic responses that run parallel to the bank from NW to SE.  The low resistivity block on the right (east) doesn’t seem to match anything in the mag data.

Fig. 24: Radargram along section line B. The actual line of the GPR transect is the blue line.

Fig. 24 shows the radargram on the mag data.  The curved feature low in the profile on the western side only partially matches the low magnetism band.  I cannot see anything that clearly matches the low resistivity feature seen in the pseudosection.


As I have often said, using more than one technique can be very powerful as the different methods detect different properties of the below-ground stratigraphy, and therefore different aspects of the archaeology.  The downside is the huge increase in complexity in comparing, contrasting and trying to interpret the various results.  This small survey is an excellent example of this,  There is much more that could be done, especially going through the GPR results in more detail and comparing and contrasting them with the magnetometry data.

For now, we can say the following:

  1. There is no clear evidence for the town wall.  There is a good possibility that it has been robbed in the this area as suggested by the fact it only shows in one time slice, and there is a low resistivity feature in the pseudosection in about the position one would expect the wall.
  2. The clearest evidence for buildings is along the SW hedge line.  These show best in the mag data, and parts show in the GPR but difficulties with nettles and burrows prevented the GPR getting closer into the hedge line.
  3. The lower part of the field in the centre mainly shows evidence for agriculture in both the mag and GPR data, including probable land drains.
  4. The deepest time slice does show some evidence of the road and Roman buildings.  If my estimates of the speed of the radar signal are about right, the tops of these remains lie between 90cm and 125cm below the surface.  The extra depth of soil compared to elsewhere may be due to soil creep down the slope building up behind the bank to the NE.
  5. Part of these building remains include three sub-circular features which have strong radar reflections.

Many thanks to everyone who came out to help with the survey.  Many thanks also to Pete and Flora Letanka for allowing us to survey in their paddock and garden, and to Stuart Gray for putting us in touch.  Also, thanks to WEAG for the use of their pseudosection equipment, and SEAHA for the loan of the GPR.  Lastly, thanks to Pat Johnson for bringing us the mag at the start of the survey.

Filling in the gaps

Thanks to Stuart Gray of Strutt and Parker, I was able to contact Pete and Fiona Letanka and obtain permission for us to survey the paddock of Darrowfield House which lies immediately behind Verulamium Museum. It is the last large-ish area of Verulamium left that has not been totally surveyed using magnetometry. Members of CAGG are, therefore, spending this week filling in the gap.  Our mag was away being repaired, but Pat Johnson of Foerster kindly agreed to bring it down to us today.  We started, therefore, with Earth Resistance and GPR, but after lunch the poor old res meter was left abandoned on the grass while the team got started with the mag survey.

The paddock ought to be exciting.  It lies just behind the line of the Roman wall, and there is a road shown in publications running SW-NE across it.  The first mag results are shown in Fig. 1.

Fig. 1: the mag survey.

They are surprisingly unexciting.  The diagonal lines running SW-NE look like cultivation marks of some sort.  There is a quiet area behind the supposed line of the wall, running NW-SE, and then one high-magnetic curved feature which may match a bank we can see in the field.  No real sign of the wall, or of the road,

The GPR survey has been processed in 3ns thick slices.  As usual, I’ll work my way down from the top.

Fig. 2: GPR survey, time slice 3, from 10.5ns to 13.5ns.

Fig. 3: GPR survey, time slice 4, from 13.5ns to 16.5ns.

The third slice shown in Fig. 3 really shows surprisingly little. I’m guessing we are only looking into the topsoil here.  The fourth slice (Fig. 3) starts to show some diagonal striping similar to what we can see in the mag data.  Again, this is probably agricultural.

Fig. 4: GPR survey, time slice 5, from 16.5ns to 19.5ns.

Fig. 5: GPR survey, time slice 6, from 19.5ns to 22.5ns.

The fifth slice (Fig. 4) shows the striping very strongly, with two alignments at right angles.  I think these must be land drainage.  They look too straight for “ridge and furrow” and do not have a headland. What is also interesting — or is that worrying? — is that the wheel-ruts which show so clearly on the Google Earth image in the background, also show in the GPR data at this depth.

The sixth slice (Fig 5) faintly shows echoes of the drainage.  To the north are some black squiggly features.  Yes, folks, we have mapped the badger setts!  Nothing very archaeological, so far.

Fig. 6: GPR survey, time slice 7, from 22.5ns to 25.5ns.

In the final slice (Fig. 6), we still have the setts in the north, but we have a very strong linear feature to the south running SW-NE.  Comparing this to the proposed line of the road in Fig. 7, we can see that this would appear to be one edge of it.

Fig. 7: GPR survey, time slice 7, from 22.5ns to 25.5ns, with overlain map of Verulamium.

Right at the edge of the GPR’s signal depth where the returns are very attenuated, we are starting to see the archaeology. Unfortunately, I cannot yet easily convert the time (22.5ns–25.5ns) into physical depth.  It would seem, however, that the archaeology in the lower area may well be at the limit of what we can pick up.  If we look at the radargrams, however, we can see a big feature running across our vertical slices and I would guess this is the road surface.

Fig. 8: an example radargram showing the possible road.

Many thanks to everyone who turned out today and enjoyed the beautiful sunshine.  Also, big thanks to Pat for bringing the mag back for us, and Pete and Fiona for arranging for us to be able to survey the Paddock.

There probably won’t be a posting tomorrow, but I hope on Weds to catch-up.

Batford Mill

I have a number of small surveys which remain unreported that I need to catch-up on, and so here is the first of them.

Earlier in the year, Mike Smith and I assisted Alex Thomas (University of Bristol) in undertaking a Ground Penetrating Radar survey of land lying to the north of the B653 at Batford, Harpenden, Hertfordshire (TL 148150, Fig. 1). The survey was undertaken over the weekend of 2nd/3rd April 2016. Earth resistance and magnetometry surveys had been undertaken in the area previously.

Fig. 1: Location of the site at Batford.

Fig. 1: Location of the site at Batford.

The underlying geology of the site is Lewes Nodular Chalk formation overlain in places by the Kesgrave Catchment Subgroup sand and gravel.

A Mala GPR with a 450mhz antenna was used, identical to the one CAGG borrow from SEAHA. The survey transects were at a 0.5m spacing collected in a zig-zag fashion. The survey started in the NE corner and proceeded east-west. Radar pulses were set for 0.05m intervals with a time-window of 73ns. The newer Mala systems do not allow manual selection of sample numbers which are determined by the machine, in this case 376 samples per trace.

For the amplitude slices presented here, as usual, the software system developed by Jeff Lucius and Larry Conyers was used (http://www.gpr-archaeology.com/software/). This necessitates the conversion of the Mala rd3 files into GSSI dzt files using the companion conversion program.

For this posting, the slices were 3ns in thickness starting at 3.5ns  From these, it appears that the second slice, 6.5–9.5ns represents the immediate ground surface. This agrees with the estimate of the first reflection from the individual radargrams at about 8ns as examined using RadExplorer. Beyond slice 7 (>24.5ns) the signal has completely attenuated. This means that all the usable returns lie in the band between c.6.5 and 24.5ns. This is not unusual for Hertfordshire where the clay soils do not allow for the GPR surveys to penetrate particularly deeply.

As with most of the GPR surveys reported in this blog, the numerical output from that software was turned into images using Surfer v.8. Kriging was used to interpolate the values into a 0.1 x 0.1m grid. The resulting images where then imported into Google Earth.

Six amplitude maps or `time slice’ maps were produced and are shown in Figures 2–7. The topmost map (Fig. 2) shows two strong reflections to the north-east and the south. The second map (Fig. 3), which represents the 9.5–12.5ns range, has the clearest set of features. A number of long linear features are visible, two of which I have labelled A and B. There is a odd-looking curved linear feature with two parallel lines, labelled C, into which a pair of parallel lines cuts, labelled D. Further fainter linear features can be seen, such as those at E.


Fig. 2: Time slice 2 (6.5–9.5ns).

Time slice 3 (9.5–12.5ns).

Fig. 3: Time slice 3 (9.5–12.5ns).

The third map (Fig. 4) has fewer clear features, most of which are probably `echoes’ of the features seen in the previous map. The next three maps (Figs. 5–7) have successively fewer features in them, none of which are especially clear. By the last map, the GPR signal has largely attenuated and little can be seen. At best, we are getting a depth penetration of about a meter, probably somewhat less.

Fig. 4: Time slice 4 (12.5–15.5ns).

Fig. 4: Time slice 4 (12.5–15.5ns).

Fig. 5: Time slice 5 (15.5–18.5ns).

Fig. 5: Time slice 5 (15.5–18.5ns).

Fig. 6: Time slice 6 (18.5–21.5ns).

Fig. 6: Time slice 6 (18.5–21.5ns).

Fig. 7: Time slice 7 (21.5–24.5ns).

Fig. 7: Time slice 7 (21.5–24.5ns).

The question arises, therefore, as to what the long linear features may be. If Fig. 3A is a wall, it would be at least 35m long, and Fig. 3B would be at least 55m long. One possibility is that they represent old field boundaries. Looking at the 1898 OS map (Fig. 8), there is nothing to suggest an origin for those features. The 1799, map now in the Westminster Abbey Muniments Room does, however, show a field boundary behind some buildings to the north of the road. A crude overlay of an extract of this map (Fig. 9) on the Google Earth image with the GPR data, shows a remarkably good correlation between the field boundary and the one of the linear features (Fig. 3A).

Fig 8: The survey overlain on the 1898 OS map.

Fig 8: The survey overlain on the 1898 OS map.

Fig. 9: The 1799 Westminster Abbey map overlain on the Google Earth image.  Westminster Abbey map used with permission.

Fig. 9: The 1799 Westminster Abbey map overlain on the Google Earth image.

The origins of the curved and parallel linear features can be seen if one takes into account the location of the machine-dug test trench marked in Figure 10.  These parallel lines, only some 1.8m apart, represent areas of soil compression from the machine used in the excavation of the test trench.  Examination of one of the radargrams (Fig. 11) would seem to confirm this.  The origin of the reflections, marked with blue arrows, occurs at the very surface and is highly suggestive of compression rather than construction.

Fig. 10. Slice 2 showing the location of the machine dug test hole.

Fig. 10. Slice 2 showing the location of the machine dug test hole.

Fig. 11: Radargram showing areas of surface compression.

Fig. 11: Radargram showing areas of surface compression.

The survey results appear to be largely connected to (a) earlier agricultural use of the land in the form of hedgerows and so forth or (b) the recent impact of the excavation of the test trench.  It appears highly unlikely the GPR results indicate any sort of structure although the golden rule of ‘absence of evidence is not evidence of absence’ must be applied.  The lack of pottery or ceramic building materials on the surface makes it unlikely that a building is indicated.

Perhaps I should have saved a more exciting post for #100!