Tag Archives: GPR survey

Half-way point

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

It is amazing (and slightly scary) that we have already reached the half-way point in the 2019 Gorhambury survey season.  To give the surveyor a chance to get a little ahead of the game, we pulled the mag team off Prae Wood Field this morning and got them to help the GPR crew and to do some res.  After lunch, the mag finished Prae Wood and the GPR completed their 80 x 80m block.

The mag completed the last two triangles in the far eastern part of Prae Wood, and re-did one square for which the mag cart had, for some inexplicable reason, developed a horrible stagger error (Jim puts it down to cosmic rays).  I wouldn’t have been too bothered, apart from the fact that one edge of our enclosure passes through that square.  I have started to lay-out the grids for the western edge of Prae Wood field.  Hopefully, by next weekend we can be out of that field and into Church Meadow.  Figure 1 shows the whole mag survey of the field, and Figure 2 the detail of the eastern extremity.

Figure 1: the mag survey of Prae Wood field.

Figure 2: detail of the mag survey of Prae Wood field.

Not much new has shown-up in the eastern extremity of that field, but the enclosure we found a couple of days ago is now a little clearer than before.  The mass of modern noise is unfortunate.

The res survey was extended up to the hedge line in the theatre field, with hints of what is to come.  Just to the north of the hedge line their are many buildings which the res should pick-up nicely.  Figure 3 shows the entire res survey, and Figure 4 a closer view of the second block of the 2019 survey so far (the strip to the west).  As always, the edges are because the blocks have been processed separately and joined-up in Google Earth.  I’ll start to put everything together soon.

Figure 3: the Earth Resistance survey after day 10.

Figure 4: detail of the western edge of the survey showing the new data.

The res survey is now 8ha in extent.  Not counting grids we have had to do twice for various reasons, this is 320,000 measurements for the 0.5m mobile probe separation survey.  (We only started using the 1+2 survey method last summer.)

GPR this year, and last, can be a little challenging (Figures 5 and 6).

Figure 5: Dave Minty (WAS) pushing the GPR in week 1. Image © Mike Smith.

Figure 6: Jim West (CVAHS) pushing the GPR today. Image © Mike Smith.

For most of this week, we have been using a 1m transect spacing with the GPR in order to finish the field this season.  We can do this because (a) the features we know about are big such as the 1955 ditch; (b) we aren’t expecting any stone buildings in this area and (c) if we do find something interesting, it will show in the 1m data, but just won’t be very clear.  Today saw the GPR reach the bottom of the dry valley across which the aqueduct dog-legs, and also joins up with last years survey.  Figure 6 shows the entire GPR survey (very crudely!).

Figure 7: the entire mag survey after day 10.

Figure 8 shows today’s block along with the neighbouring ones.

Figure 8: the day 10 GPR block, slice 4.

The eagle-eyed amongst you would have spotted a long building running SW–NE with its short end on the big black blob.  Yup, we have a building.  Oops.  Well, at least that proves that we can see buildings even with 1m transect spacing, although it does look a bit dot-to-dot.  What are the other things?  Figure 9 shows the mag data.

Figure 9: Mag data in the area of the Day 10 GPR survey. Red square marks today’s block, blue rectangle the building.

The mag data shows us that part of the big black blob is the aqueduct.  We have been speculating whether the aqueduct had some sort of structure to carry it across the dry valley.  This may help us address this question.  The long building, however, does not show in the mag data at all.  We have come across this previously.  The further buildings are from the core of the town, the less likely it is we will see them in the mag data.  This is because, I think, the surrounding soils are less magnetic than in the core of the town.  The mag survey processes out these broad scale background changes in magnetism.  We need to undertake a magnetic susceptibility survey!

Across the SE corner of the mag plot is a long linear feature which is the 1955 ditch shown with green arrows in Figure 9.  As can be seen, some parts are strong and easy to see, other parts are much fainter.  Are we dealing with parts of the ditch that have been filled-in, or parts that were never really dug in the first place?  I suspect the former, but it is only a suspicion. Towards the south part of the 1955 ditch in Figure 9 are two strongly magnetic features in line with the ditch, either side of a low area of magnetism.  This is where Street 11 appears to cross the ditch as shown by the red arrow in Figure 9.  The street barely shows in the mag data but Figure 10 shows how clear it is in the GPR data.

Figure 10: today’s GPR block with some labels.

We have had a very successful two weeks.  Rain has only lost us a few hours (so far… touch wood!) although the wind yesterday was trying.  Many thanks to everyone who has made this all possible.

 

 

 

 

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Like a welcome summer rain

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

Langston Hughes probably didn’t have geophysics in mind when he wrote that. I thought we might lose today altogether, but actually it was fine until the afternoon. Then the showers got a bit much and we headed home mid-afternoon. Hopefully, however, it might mean the Earth Resistance survey can make some progress soon.

The mag team are still working their way around the edges of Prae Wood field.  They are making great progress despite the rain and the machine crashing today and loosing a whole grid of data.  Figure 1 shows the whole field.

Figure 1: The Prae Wood field mag survey.

The SE of the field has quite a few services (the dark black and bright white lines) but lets zoom in a little (Figure 2).

Figure 2: the SW corner of the mag survey in Prae Wood field.

As might be expected near to the buildings there are blobs and bits of metal and all that.  There is, however, what might be a sun-rectangular enclosure.  I don’t think I am making it up…  Figure 3 shows the possible outer boundary.

Figure 3: possible enclosure in the mag data.

It looks pretty good to me!

The GPR team were aiming to do another 80 x 80m block at 1m intervals, but the rain stopped them short.  Using the GPR on the steeper bits of hillside is quite a challenge (Figure 4)!

Figure 4: Pushing the mag up the hill.

The collected three 40x40m blocks and the first 10m of the fourth before the rain drove us all away.  Figure 5 shows the GPR blocks from the last two days (left) and last year (right).  Figure 6 shows the mag data from the same area.

Figure 5: the GPR data.

Figure 6: the mag data in the same area as the GPR data in Figure 5.

One of the obvious things in both data sets is the “1955 ditch” running across the survey area north-south.  There are some subtle things happening too.  For example, in Figure 5 there is a strong blob in the ditch which I have marked with a red arrow.  In Figure six we can see a circular feature in the mag data in the same place.  I’m not sure what it is (yet), but it is one of a thousand details we can see when comparing the two data sets.

Tomorrow is forecast to be very windy.  Let’s hope we don’t end-up gently flying over North Herts!

 

The smallest grid ever?

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

Well, possibly not. The mag team’s first grid this season was a 1.5m x 40m grid. Why? Well, in the penultimate grid of the very last day last year we had one single frozen sensor for one line of data. For the last 11 months this has annoyed me every time I saw it. Finally, I have been able to fix that grid! Yay.  Having completed that line, the team went on to complete another seven grids of data.  Well done team (Figure 1)!

Figure 1: Jim West and the mag.

Figure 2 shows the whole of Prae Wood Field and the survey completed so far (but without the dodgy line!), and Figure 3 zooms into the area completed on Sunday.

Figure 2: the mag survey of Prae Wood Field.

Figure 3: detail of the mag survey of Prae Wood field.

The overall impression one gets from both Figures 2 and 3 is a whole lot of nothing.  There are the occasional strong magnetic responses from iron objects, and on Sunday we picked-up two pipelines (shown in Figure 3 with yellow arrows).  The larger area of noisy magnetic data near the southern edge of the survey might be an historic structure.  In the new area, there is a very faint line (as indicated by the blue arrows) which might be an old fence line, or might be my imagination.  There are some “monopolar positive” features (i.e., ones which are mainly positive but with a slight negative response on the north side) which could well be pits.

What makes this all fascinating is that the Urban Archaeological Database (the UAD), suggests that the field is within a “rectilinear  enclosure”, Monument Number M27.  At the moment, I’m not sure where this idea comes from, but at the moment it seems as though it is an enclosure around not very much! This field, and part of Prae Wood itself, are within the area mapped by the Environment Agency using LiDAR (Figure 4).

Figure 4: LiDAR image of Verulamium. Data from the Environment Agency, image courtesy Mike Smith.

The Fosse, running through the woodland along the NE edge of the field shows nicely (Figure 4, right-hand red arrow).  The little fragment of Prae Wood itself shows a mass of features in the woodland, some of which are parts of the Iron Age settlement (blue arrow). Our field shows the faint hint of the ploughed-in Fosse (left-hand and central red arrow), and a whole lot of not-much-else.  How very curious!

Despite the very dry ground surface and the small team, we did manage a further three res squares on Sunday.  Many thanks to Pauline for putting-up with my cursing as we did the work. Figure 5 shows the results.  The edge between the earlier survey to the east and the current block of eight squares is due to my processing differing between the two seasons.

Figure 5: the Earth Resistance survey after day 4.

As can be seen, we have a line of buildings along the SW–NE road.  This road, Street 23 in Niblett and Thompson’s Alban’s Buried Towns, shows very poorly on all three survey techniques.  In the res survey, it almost looks like an eroded channel, and I have often been a bit confused as to the where the dry undulation (seems a bit grand to call it a valley) lies.  It lies, however, behind these buildings and those that face onto Street 25 to the west.  I have indicated the valley in question with a yellow arrow in Figure 4.

A further source of data is the work undertaken by the Oxford Archaeological Unit in January of 2000.  They excavated 379 1.6m x 1m test pits using a mechanical excavator to strip the topsoil.  As they were investigating plough damage, they did not excavate the features revealed.  The distribution of test pits is shown in Figure 6.

Figure 6: test pits excavated in 2000 by the OAU.

If we zoom into the area we have surveyed using res over the weekend we can see which test pits are relevant (Figure 7).

Figure 7: test pits and the res survey (click on the image to see a larger version).

Test-pit 268 is described as showing a possible floor and a wall foundation of chalk. Judging by its position in the middle of a small building that seems appropriate. Test pit 258 is described as having “?floors” and “?Fill of wall trench”, whereas pit 257 which lies either on, or more probably just outside the wall of the long building, just has a “layer”. In all three test pits the topsoil was between 27 and 32cm deep.  I have yet to see the plans of these trenches, but clearly combining the evidence of the trenches with the geophysics data is going to be very informative.

The GPR team jumped a few grids to complete a block next to the one that they  completed on the last day last season.  We intend to swap to 1m transects soon, and I wanted to catch the details of the buildings that clearly intruded into this block.  Figure 8 shows the location (the bit sticking out to the west).

Figure 8: the GPR survey showing the location of the block surveyed on Sunday.

As can been seen in Figure 8, we have managed to complete the building which lies over the grid edges, but there doesn’t appear to be much more.  Lets look at the first 12 slices (Figure 9).

Figure 9: the first 12 time slices from Sunday’s survey.

Slices 1 and 2 are basically showing the top surface and the topsoil.  In slice 3 we can start to see the building and a long, wide, linear feature.  These show in slices 4 and 5 too.  By slice 6, we are already in the natural and/or where the signal has started to attenuate.  Slices 7  to 12 are basically a few deeper things and echoes / attenuated signal.  The only thing of technical interest is the semicircle which shows on the eastern edge of slices 7 to 12, and also shows in slices 1 and 2.  This is an “airwave” caused by the radar signal bouncing off the underside of the tree canopy.

GPR slice comes with a plethora of palettes for display time slices.  Figures 10 and 11 show slices 4 and 5 in the first 12 palettes.

Figure 10: slice 4 shown in 12 different colour palettes.

Figure 11: slice 5 shown in 12 different colour palettes.

The building along the eastern edge shows well in palette four, and the big linear thing running across the plot shows well in slice 4, palette 11. So what is the big linear thing?  Figure 12 has a clue…

Figure 12: the GPR data with the line of the 1955 ditch indicated.

Yes, you’ve guessed it (or at least read the caption), the “long linear thing” is our old friend, the 1955 ditch.

Well I think that is enough for now.  We will be back on site again tomorrow, hopefully running all three machines if the long promised rain actually happens this evening.  Many thanks to everyone who has contributed this week.  It was a great beginning to the 2019 season!

Kennel Farm, Little Missenden

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

As a result of the hot, dry weather (sound familiar?) last year, the Chess Valley Archaeological and Historical Society noticed some parch marks in the fields along the River Chess near Little Missenden.  They undertook some Earth Resistance survey in those fields.  Earlier this year they asked if CAGG would be willing to come and undertake further surveys at the site.  At the end of May we undertook three days of survey completing an area of mag survey, two blocks of GPR survey, and extending the Earth Resistance survey undertaken by CVAHS.  Peter Alley also undertook a topographical survey using his UAV.

To take the topography first, we have two sources of information: the LiDAR data which can be downloaded from the data.gov.uk website and the UAV aerial photogrammetry data collected by Peter.  Each method has its strengths and weaknesses.  I have posted the LiDAR image created by John Glover and Peter’s imagery below.

Figure 1: LiDAR image. The field we were working in is to the west of the buildings in the centre of the plot. Image courtesy of John Glover.

Figure 2: Geo-rectified composite image of the area. Image by Peter Alley.

Figure 3: Elevation map created using Structure from Motion (photogrammetry). Image courtesy of Peter Alley.

Figure 4: hillshade representation of the photogrammetry data. Image courtesy of Peter Alley.

From these various images we can see that the site consists of a flat area either side of the river (essentially the flood plain) and then a steeper slope up to the houses and the main road. The flat area is alluvium, the slopes are “New Pit Chalk”.  Although there are some undulations, most clearly shown in Figure 4, nothing much suggests archaeology.  Some of the features, especially in the field to the east, are probably alluvial or periglacial.

The mag, working as quickly as ever, covered two blocks either side of the river (Figures 5 and 6).

Figure 5: Yvonne Edwards and Rhian Morgan running the magnetometer. Image © Mike Smith.

Figure 6: the magnetometry results.

There are a few features showing in the mag plot, especially the modern ferrous features to the south of the river.  I have put arrows on the plot to make the discussion easier (Figure 7).

Figure 7: the mag results with arrows.

South of the river is a long line of strong magnetic features (red arrows).  I’m not entirely sure what this was, perhaps an old fence line?  Maybe an obsolete pipe which has been partially removed? Whatever they are, they have largely obliterated any other magnetic features in that area, as well as causing problems with the destriping routine!

North of the river there are two very strong features marked with dark blue arrows.  These are easy to interpret (see Figure 8)!

Figure 8: the explanation for the strong magnetic features indicated with the dark blue arrows.

The only interesting thing about these is that the services connected to them do not show on the mag data at all.  The most enigmatic of the features is the linear one indicated with the light blue arrows.  This feature could be seen in the parch marks from 2018. To the west, the straight line looks like it might be a service of some sort, but then it curves up towards the pond, and then continues towards the east.  I suspect that the linear feature indicated by the green arrows might be part of the same thing.  It is very curious, and I am very unsure what it might be.  The low and high magnetic readings keep swapping sides, which suggests it isn’t just the result of soil with a high magnetic susceptibility filling a linear feature.  Sadly, I think the odds are on it being a service again, but it needs to be tested.  The pond, by the way, is a relatively new feature in the landscape (Figure 9).

Figure 9: the pond.

The linear feature indicated by the white arrows in Figure 7 is the build-up of topsoil at the change of slope (cf. Fig. 3). The last remaining feature is a square of magnetic noise shown by the yellow arrow.  I suspect that this might be the location of something like a wooden building, or parhaps a flat platform for something.

As well as the survey which had already been undertaken by CVAHS, we did a small block of Earth Resistance survey on the lower slopes. Thanks to Ruth Halliwell and Peter Alley for helping me with the survey (Figure 10).

Figure 10: the Earth Resistance survey underway. Image © Mike Smith.

As usual, we used the pole-pole configuration for the cables (i.e., the remote probes were at least 30m away and at least 20m apart), and took one 1m mobile probe separation reading and two 0.5m mobile probe separation readings at each survey point.  On this occasion, the 1m readings didn’t add much to the results so I will just discuss the 0.5m survey.  Figure 11 shows the results.  Very unusually, the readings went from very low (6.1 Ω) to very high (250 Ω).  The high readings were so high that I had to change the settings on the res meter, and made my ever patient helpers re-do a couple of grid squares.

Figure 11: the earth Resistance survey results.

A further problem can be seen.  The long tail of high readings which can be seen in the inset graph in Figure 11 means that most of the survey is shown as very light low resistance readings and it is hard to see much of a pattern.  If we simply clip the values (i.e., make everything above a certain value black), the high resistance areas become one nasty blob.  The statistician in me is used to seeing skewed data sets like this in all sorts of cases.  The answer is some form of data transformation, and TerraSurveyor provides this via its “Compress” function.  This can apply a log-transform to pull in that tail and make the data look more normal (in the statistical sense of more normal). Figure 12 is the result.

Figure 12: the Earth Resistance results subjected to a log-transform.

In Figure 12 the transformed data values are shown as the inset graph once more.  Hopefully, you’ll agree it allows us to see patterns in both the high and low values.  There does seem to be a linear feature running across the plot.  I suspect, however, that this is, again, part of the geology.  I doubt that anything we can see in this plot is archaeology.  The high readings might be a deposit of flints.  The blobby mag results in the same area suggest depressions in the surface of the subsoil which have filled with slightly more magnetic topsoil.

The final technique we used was GPR.  We completed two blocks, one to the east of the area, north of the river and the second to the west, again north of the river (Fig. 13).

Figure 13: the GPR being operated by John Glover of CVAHS. Image © Mike Smith.

Figure 14 shows the time-slices from the first block.  Just to confuse you all, north is to the bottom of these plots. They have been created with a 40% overlap between the slices.  Ignore the depths, they are incorrect.

Figure 14: time-slices from the first block of the GPR data. North to the bottom.

Slice 1 is simply the impact of the surface layers on the GPR, probably variations in vegetation and water retention.  Often we can see things like mushroom rings in this slice.  Slices 3 to, probably, about slice 8 are the actual deposits under the surface.  Slices beyond that are probably “echoes” of the upper slices boosted by the software into visible images.  We seem to have three things happening.  Some linear features which are undulating and strong reflections shown in red.  A broad curving low reflection area shown in dark blue, and a linear feature shown in cyan.  Figure 15 shows these on the Google Earth image.

Figure 15: time slice 6 from the GPR survey of Block 1.

As can be seen from the figure, the dark blue swathe comes out from the current course of the river and returns to it.  I suspect we have picked-up an earlier meander of the river.  Similarly, the lines of red might be banks of river gravels from previous meanders.  In Figure 16 I have indicated the outline of the noisy area of mag data shown by the yellow arrow in Figure 7.

Figure 16: the GPR slice with the magnetic feature from Figure 7 indicated by the white line.

As can be seen from Figure 16, the southern edge of the magnetic feature aligns nicely with the northern edge of the dark blue “channel”.  Perhaps we have a building platform on the banks of the river?

The linear feature in light blue could well be another service trench.

The second GPR block was to the east, again on the northern bank.  It was sited to cover one of the parch marks seen in the previous year.  Figure 17 shows the first 12 time-slices.  Again, north is to the bottom and ignore the depths.

Figure 17: First twelve time-slices from the second GPR block. North is to the bottom, depths are incorrect.

The most interesting thing about this block are the linear features which show clearly from about slice 7 onwards.  We have two clear parallel lines running alongside the river, and one which runs at a right angle to the southern (upper) line into the river bank.  Figure 18 shows the eighth time-slice on Google Earth.

Figure 18: time-slice 8 from the second GPR block.

As can be seen, the northern linear feature follows the one seen in the mag data.  This feature is, therefore, something that (a) is more magnetic than the background; (b) a good radar reflector and (c) in places remains green when other areas are parched (see Figure 19).

Figure 19: photo taken using a UAV during the drought in 2018. Courtesy of CVAHS.

Comparing Figures 18 and 19, we can see the southernmost linear feature shows well as a green mark in the field and as a strong reflector.  This feature, however, does not show in the magnetic data.  We can look at the radargram and the time slice at the same time using GPR Slice (Figure 20).

Figure 20: a radargram and time slice seen in pseudo-3D.

Looking at the radargram we can see that the broad reflection layer at the top is continuous over the northern linear feature, but is interrupted over the southern one.  This suggests to me that the southern feature might be relatively recent.

So what have we discovered?  Largely, I feel, most of the features are to do with geology and the river.  The two main mystery features for me at the long linear feature and the shorter one with right-angles.  Both show clearly on the aerial and the GPR.  The longer linear one also shows on the mag.  I’m not sure what either of these are, and only “ground truthing” might give us an answer.  My instinct, however, is that neither are all that old, but this is only an instinct.

As always, many thanks to everyone who helped both during the survey and with moving the gear. Also, many thanks to the Institute of Archaeology, UCL and SEAHA for the loan of the equipment.

For those awaiting the first results from Verulamium 2019, they will be posted in the next day or so.

 

 

The fully at the Folly

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

Mike Smith writes:

The sighting of what looked like an overgrown platform on the north side of the River Lea near Wheathampstead, suggested a previously unknown water-mill site (Fig. 1).  The ‘mill’ site is on Melissa Field on the Lower Luton Road, opposite a Victorian housing development known as ‘The Folly’.

Figure 1: The raised mound at the west end of the Melissa field, believed to be the site of the mill building.

Years later I was doing an unrelated map regression exercise when I found a clue to what the platform might be.  The John Seller county map of 1676 showed a building that was labelled as a ‘fulling mill’.  Later maps, including an estate map and the Dury and Andrews 1766 map, depicted a building but without the ‘mill’ label.  By 1799 the ‘mill’ had vanished.

The mapping evidence suggested a date that fitted in with the fulling mill theory.  Water-driven fulling mills, used for finishing wool, were a pre-industrial technology.  They had largely disappeared by the mid-eighteenth century.

When I revisited the site in the late afternoon, the low sunlight showed up extensive earthworks in the field to the west.  They looked like the remains of a leat, (or millstream) which fed water to the mill.  This was confirmed by Environment Agency LIDAR data that showed the leat snaking across the field.  The LIDAR also revealed the tail-race that returned water to the river from the water-wheel. Figure 2 shows the possible line of the leat.

Figure 2: The site of the mill and the route of the mill stream based on Environment Agency LIDAR data (superimposed on the 19th century Ordnance Survey).

At this stage I decided to ask Kris if we could help me do a GPR survey of the ‘mill’ platform.  As it was a small site (probably only 12 meters square) we were able to do the survey between the two of us on a Sunday morning.  We surveyed across the platform at 50cm intervals.  One problem was the edge of the platform was over-grown with bushes and small trees.

What did I expect to find?  My guess was that the mill building may have dated to the 16th century (or before) and was probably a wooden structure, something that was unlikely to show up on GPR.   However I anticipated finding stone foundations around the working end of the mill where the wheel and machinery were located.  The fulling mill used powerful tilt hammers to pummel the wool and this also suggested substantial foundations.

Unfortunately when Kris analysed the data there was little convincing evidence of a building (Fig. 3).  Although there were strong responses at the south end of the platform, where I was hoping to find evidence of a stonework foundation, it is far more likely that what we were seeing was an ‘airwave’ phenomena.  Leaking radar signals were bouncing back off the nearby trees and bushes.   This ‘airwave’ effect is something we have come across before, most recently during the churchyard surveys at Bovenay and Ashwell.

Figure 3: The GPR data was inconclusive and the high responses might be due to the nearby trees.

Errant radar waves apart, why didn’t we find the remains of the mill?  The answer is that it is likely that the mill owners, the Westminster estate, robbed out the largely wooden building in the late 18th century.  This included the stone foundations that would have been valuable building material.

I asked Peter Alley to fly his UAV over the site to see if he could get more detail about the leat in the field to the west.  Unfortunately I got my timing wrong.  Having not visited the site for several weeks I had not realised how high the vegetation had grown.  This compromised the results.  However Peter’s Digital Surface Model (represented as a hillshade plot, Fig. 4) did hint that the leat could extend further west than I thought, perhaps connecting with the river upstream at Leasey Bridge. If this is true it would mean the leat was over 450 meters long, making it a considerable engineering task.  My map (Fig. 2) shows the earlier interpretation of the leat running west before making a sharp turn south to the river.

Figure 4: Peter’s Digital Surface Model of the topography of the field to the west of the mill (based on a resolution of 1cm in elevation).

This part of the LIDAR data is faint and far from conclusive and Peter may well be right that the leat continues west.  Unfortunately housing has been built on this alternative route, making it difficult to confirm either way.

The GPR on the building may have been inconclusive but there is scope for further work on this site.  The fulling mill was unlikely to be a stand-alone structure and there may be more remains of the medieval and early-modern wool industry on Melissa field.  The middle of the field is rich in ‘lumps and bumps’.

The discovery of the fulling mill does provide an answer to a local mystery.  Where did the place name ‘The Folly’ come from?  Up to now there has been no credible explanation for the curious name of this Victorian housing estate across the road from Melissa field, also known as ‘Folly Fields’.  The obvious answer is that it is a corruption of ‘The Fully’, itself a corruption of ‘the fulling mill’.

Mike has published an article on the Fulling Mill in Herts Part and Present, Spring 2019.

Hogshaw Redux

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

Archaeologists often have skeletons in their cupboards. Sometimes they are real skeletons. Sometimes, as here, they are unfinished jobs that they haven’t quite got around to completing.  There are a few surveys we have undertaken that never quite got finished, and for which there are no blog posts (shock! horror!). Way back when we got together with the Buckinghamshire Archaeological Society’s Active Archaeology Group and CVAHS to undertake some surveys at Hogshaw in Buckinghamshire.  The AAG had undertaken an interesting research project on this site including topographic survey.  We managed some mag (even though the mag was down to three probes) and some Earth Resistance survey (using our old system).  The results were posted at the time.

The following year, in 2016, we returned and expanded the mag survey and undertook some Ground Penetrating Radar survey.  We had only just started using GPR and I was still learning how to process the data.  The following year, Mike and I returned with the GPR to survey another two areas.  Due to problems with that data (we were distracted by lunch), that I couldn’t solve at the time, the results were put on the back burner.  Fast forward two years and I am now a little more confident and have a better handle on the software.  Having finished processing the awkward survey at Bovenay, I thought I would have a go at re-processing the Hogshaw data.  As you might guess from the fact you are reading this post, I had some luck and so, two years late, here are the results! (See the older post for the previous results and the background to the site.)

The magnetometry survey was mainly aimed at finishing the awkward bits around the edges, and an area to the south where the landowner kindly took down his fence so we could survey across it.  The results are shown in Figure 1.

Figure 1: the magnetometry data after the 2016 survey.

At first sight the magnetic survey is rather busy and hard to interpret.  This is not unusual in historic period sites where iron artefacts and fired bricks are relatively common.  I have labelled the plot with some basic interpretative points (Fig. 2).

Figure 2: magnetic data interpretation.

The fence line is where the farmer kindly removed the fence so we could survey.  It is fascinating to see that even when the fence has gone, we still detect the line of it.  Iron rust etc. washes down and permeates the soil, I guess.  The platform is a large flat area in the NW corner of the site.  We do not know what it is for, and the mag does not help a great deal (neither did the Earth Resistance last time).

Perhaps the most interesting feature that we detected last time is the four squares inside a square.  This was quite a surprise.  It looks very much like a formal garden.  If it is a garden, there appears to be a line heading out westwards to an area of magnetic noise.  I rather ignored that last time, but now I wonder if that is where the remains of the manor house were?  It was abandoned in the 18th century.

There are two lines of very noisy magnetic readings, one along the current road and one along the northern edge.  I’d like to see how these relate to the topographic features.  I think they line-up with the banks, and could be lines of brick rubble.  Unfortunately, the LiDAR data for this area does not cover the site, ending just under half a mile to the north (Fig. 3).  Typical!

Figure 3: the LiDAR data overlain on a Google Earth satellite image.

Three blocks of radar data were collected.  We used SEAHA’s Mala GPR, and we thank them for the loan.  The location of the three blocks are shown in Figure 4.

Figure 4: Location of GPR blocks.

The southern block was surveyed in 2016 because an excavation had found a couple of stone walls in this area, and it was suggested this might be the location of the lost chapel. Figure 5 shows the top nine time slices (note that north is downwards in these images).

Figure 5: time slices from the southern block.

The first time slice shows the road nicely.  Also helps build confidence when the method detects the absolutely obvious! By about the fourth slice (second row, leftmost image) the road is largely gone but there are two parallel lines running north south.  Could these be our missing walls?  Perhaps, but I suspect they are compacted earth either side of the fence which the farmer took down for us.  The area of high amplitude reflections in the bottom-right corner (north-west) is the area of wet mud around the various temporary structures that were moved.  All in all, a rather disappointing result.

The platform block was an attempt to see if we could work out the function of the platform in the NW corner of the site.  Figure 6 shows nine time slices.

Figure 6: time slices from the platform block.

Again, note north!  There is a vague hint of something in slice 7 (third row, first image) that might be rectangular, but it is quite low down in the sequence, and a bit amorphous.  Looking at the radargrams (the original vertical slices), I cannot see anything particularly wall-like.  I suspect that what little radar energy has been reflected has been greatly emphasised in these plots creating the illusion of something.  Figure 7 shows slice 7 in context.

Figure 7: platform block, slice 7, in context.

Last, but not least, is the “garden” block (Fig. 8).

Figure 8: six GPR time slices over the “garden” block.

In slice 1 (top left), the results just reflect the uneven surface. In slice 2 we can start to see something, but it is in slices 3 and 4 that we can see the “garden” feature quite clearly.  The whole feature is about 36m across with the internal square about 12m by 12m.  To the south there appears another strong linear reflection.  Maybe a road to the house?

Figure 9 shows slice 3 in context.  I’m glad to say that the mag and GPR data match very closely.  The edging around the features must be something both magnetic and that reflects radar data.  Brick is one possibility, and some form of igneous rock is another.

Figure 9: the “garden” block, slice 6 in context.

One might ask why I am so keen on it being a garden feature.  Looking at another much grander garden, we can see many similar features (Fig. 10).  The part I have outlined in red is approximately the same size as ours.  The inner squares of that garden at Hatfield are 11m across, the enclosing hedge 28m by 42m, the outer edges 37m by 57m.  As always, the only real way to tell is to dig a hole…

Figure 10: the gardens at Hatfield House.

Many thanks to everyone who helped on the four days of survey, especially to the very helpful landowner.  Also many thanks to Anne Rowe for commenting on the “garden” feature and sending me some very useful information. Hogshaw still has some secrets to give up!

A busy day

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

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

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

Figure 1: the 2018 magnetometry survey.

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

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

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

Figure 3: the earth resistance survey 2016–2018.

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

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

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

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

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

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

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

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

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

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

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

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

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