Tag Archives: archaeology

One

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

One what, I hear you say?  Well, 1km2. What is 1km2? Well, that is the area covered by the mag in all the Verulamium-related surveys. Yup, one whole square kilometer. Impressive, eh? By the way, that is about 20,000,000 individual mag readings.  That doesn’t include, of course, squares that had to be re-done due to sensor freezes or areas blanked out where we were wheel spinning for partial grids.  Congratulations to all who have pushed that machine since the summer of 2013.

Figure 1: Jim West and the mag.

Today the mag team completed the far end of Church Meadow (Figure 2).  It is great to see such a huge proportion of the field done, and much of what is left is not worth doing as it is featureless alluvium.

Figure 2: the Church Meadow mag at the end of the 2019 summer season.

Figure 3 shows the details of the southern end of the survey.

Figure 3: Detail of the southern end of the Church Meadow mag survey.

Most of the new area today was either in the area impacted by the pipes, or featureless alluvium.  The little partial near the road, however, found a small feature which looks like a wall with something in the middle.  Given this is right next to the gate of the town, perhaps this is a mausoleum?  No real way of knowing without digging it, but certainly a possibility.

The Earth Resistance team of Debbie, Tim, Denley and Ellen were on form today and completed a super nine grid, thus satisfying my need for a tidy end to a season! Figure 4 shows the results from today.

Figure 4: the Earth Resistance data after day 3.

Figure 5: the Earth Resistance data high-pass filtered.

As can be seen, there are a number of wall showing clearly as dark (high resistance) lines.  The room which shows most clearly is the one which can be seen on the Google Earth image.  A high-pass filter shows the walls even more clearly (Figure 5).

The GPR crew, allowed down from the heights of the Theatre field, picked a 40×80 strip east-west across the middle of the buildings.  Figure 6 shows the first twelve time slices.

Figure 6: GPR time slices across the nunnery.

As can be seen, the building that shows well on Google Earth is visible right from the first time slice.  The stone work must be literally just under the surface.  Slices 7 and 8 shows the buildings in great detail as well as that pipeline running across the plot. Figure 7 shows slice 7 on the Google Earth image.

Figure 7: GPR data across the nunnery. Slice 7.

To close out the 2019 season posts, I asked Mike Smith to take a group photograph.  Not everyone who was involved this summer was there today, but Figure 8 shows a good number of us.

Figure 8: the gang isn’t quite all here. The CAGG team on the last day of the 2019 Gorhambury summer season. (Photo: © Mike Smith).

Many, many thanks to everyone who turned-out over the last four weeks, be it almost every day or for just an afternoon.  Without the CAGG team members, this project wouldn’t achieve anything!  Also, big thanks to Strutt and Parker and the Gorhambury Estate for facilitating access, and to Lord Verulam and his family for all their support.  Lastly, thanks to the AHRC for funding the original project back in 2013, the Institute of Archaeology, UCL for supporting the project and the loan of the GPS and the Earth Resistance meter, and to SEAHA for the loan of the GPR.

I’m off to Sligo tomorrow morning at about 4.30am and will be presenting some of our results to the International Conference on Archaeological Prospection on Wednesday afternoon.

 

 

Done

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

I think Mike Smith would argue that he is “done” in more ways than one!  I feel that a suitably sonorous 1950s newscaster voice ought to be saying “at 1pm this afternoon, members of…”  The reason?  Because at 1pm this afternoon the GPR survey finally surveyed the last part of the theatre field (Figure 1).

Figure 1: Mike Smith crosses the finish line.

That is some 27ha of GPR survey, mostly at 50cm transect spacing.  That works out to pushing the mag some 540km across the field.  Figure 2 shows the final coverage.  As always, the image is a mess because it has been created by different pieces of software at different times and even with slightly different conversions of OS coordinates to lat and long.  My big job now is to turn that pig’s ear (processing-wise) into a clear image.

Figure 2: Done. The complete GPR survey of the theatre field (but needs better processing).

Many members of CAGG have contributed to the GPR survey over the last five seasons.  This season Nigel Harper-Scott, John  Ridge and John Dent have contributed greatly (Figure 3).  The person who really deserves a rest, however, is Mike Smith who has not only led the GPR team over most of the last five seasons, but has also been the main GPR transporter during the season, and has been looking after it during the week.  Many thanks Mike, and well done on a great achievement.

Figure 3: Nigel (left), Mike (centre) and John (right), today’s GPR crew.

The area covered this year is best seen via the stripes in the grass (Figure 4).

Figure 4: Stripy grass left by the GPR.

Now, you would have thought that completing the survey would have persuaded the boss to let them go home early.  Nope.  None of that slacking.  One grid we did at 1m transect intervals had an interesting building in it.  So after lunch the team set to once again to resurvey it at 0.5m intervals.  In figure 5 I have made the high amplitude reflections white so that the building can be seen more easily and placed it on the mag survey for context.  It is a long, thin, building with a corridor just visible on the eastern side and a larger group of rooms to the south.

Figure 5: the re-done block on the mag data.

Tomorrow, Mike gets his pick of which two squares to survey in Church Meadow.

The mag team re-did all the dodgy squares from yesterday, and quite a few more (Figure 6)!

Figure 6: the mag data in Church Meadow after day 5.

The mag team have just three whole squares for tomorrow, and then a few partials at the ends.  For most of its length, it is not going to be worth surveying up to the edge of the field to the NE as it is clearly into alluvium as shown by the flat, featureless mag data.  Figure 7 shows a close-up of the southern end of the survey.

Figure 7: the Church Meadow mag data, southern end details.

The darker curvy line is possibly an old edge to the river.  There are quite a few small dark blobs (“positive magnetic anomalies”) some of which could be graves, and at least part of a ditch feature.  I am still puzzled by the long negative linear.  I guess I’ll have to talk to some other geophysicists!

The res team was joined by Denley Lane of the Arc and Arc,  Denley remembers some of the pipelines being built.  The team completed an excellent seven grid squares.  Figure 8 shows the results, and Figure 9 shows the same results high-pass filtered.

Figure 8: the res results in Church Meadow after day 2.

Figure 9: the res results in Church Meadow after day 2, high pass filtered.

As can be seen, the building visible in the Google Earth image shows very well indeed, but there does not seem to be many more buildings to the south.  Tomorrow we will fill-in the one last block in the eastern corner, and then do a strip of blocks along the northern edge.

Tomorrow is the last day of the 2019 season.  I’ll be speaking about the Project at the International Conference on Archaeological Prospection on Wednesday.  Not much time to fit in the new results!

 

 

The antepenultimate day (Part 5)

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

It was a very warm and sunny day today, and I think we all felt the heat.  We managed to complete quite a bit of work, though, and so congratulations to all the team.

The GPR team filled-in some of the missing bits on the west, north and east sides of the theatre field (Figure 1).  Just a few bits left now.

Figure 1: The almost-complete GPR survey.

One nice feature was a little detail found on the western side (Figure 2).

Figure 2: A detail of the GPR survey from today. The red arrow indicates a small building.

A nice little building is showing-up.  The block to the east may show some robbed buildings. Last year I noticed some curious white lines in that block which could be robbed walls.

The mag team completed an excellent eight grids taking their total area surveyed in four days to 5.09ha.  Frustratingly, however, I found that the sensors had frozen on the first three grid squares.  Horrible waste of time, but nothing to be done about it.  Figure 3 shows the whole survey.

Figure 3: the mag survey in Church Meadow after day 4.

Despite the frozen sensor, we can see some interesting details in the new area to the south (Figure 4).

Figure 4: the southern area of the mag survey in Church Meadow after day 4.

What I am finding curious is that the long linear features are white in the plots, i.e., below average magnetism.  The features look like ditches in form, but do not give the usually positive response one would expect from them.  My worry is land drains… but the features seem to connect with the edges of Watling Street.  How very curious.

Figure 5 shows the Earth Resistance survey after day 1.  The team completed an excellent eight grids.

Figure 5: the res survey in Church Meadow after day 1.

The broad dark line across the western corner is Watling Street.  Further east the various thinner dark lines are the walls of buildings.  Clearly we have parts of a number of structures showing clearly.  Great stuff.  We don’t have a cruciform-shaped building face east-west yet, but give us time.  Hopefully, tomorrow, we will cover the building which shows so clearly in the Google Earth image. Comparing the mag and the res shows how much the pipes obscure, and how they went right through the middle of this complex (Figure 6).

Figure 6: the res survey in Church Meadow after day 1, with the surrounding mag data.

Well it is now 1am, and I have to be up early in the morning for our penultimate day on site.  Many thanks to everyone who worked so hard in the heat today.

 

Being completist

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

The team are very patient with my need to be neat and tidy and do the silly little bits around the edges.  Today was perhaps an extreme example.  Due to yesterday’s little hiccup, I set-up the res kit and, with Graham’s help, surveyed about a sixth of a grid square, then packed it all up again.  Now I can sleep easy.  On a more ambitious note, I have finally put all the res grid squares into one large composite.  The survey now consists of nine hectares, which is about 360,000 individual readings.  Most Earth Resistance surveys use what is known as a twin-probe configuration.  That means that there are two mobile probes on the frame, and two stationary probes on the end of a long cable, normally about half a meter apart.  One mobile probe, and one remote probe set-up an electric circuit part of which is the soil.  The other mobile probe and the other remote probe measure the resistance.  The problem with this “standard” set-up is that when you have to move the remote probes, the readings for the same spot will change.  This leads to endless struggles to “grid-match” each set of squares.  Since 2016 I have gone over to using a pole-pole configuration.  This is basically the same except the remote probes are a long way away (I aim for about 30m or more) and a long way apart from each other (I aim for more than 20m). This helps enormously with the grid matching.  Grids completed on different days of the survey will match quite nicely, usually.  Where this is not true is when (a) there is a lot of rain in the middle of a survey and (b) when the survey is split over multiple years.  In the case of our 9ha block, this has been completed over four seasons.  Unsurprisingly, one can see the edges.  TerraSurveyor has a function called “periphery match” which will, sometimes, do an excellent job of grid matching.  In this case, it was pretty good.  Figure 1 shows the survey with a periphery match applied.

Figure 1: the res survey as of the end of Thursday.

If you click on the image and see it full size you can see the detail of many buildings.  Unfortunately, the range of values makes seeing some buildings quite hard.  A high-pass filter is a background trend removing tool that makes some buildings show more clearly (Figure 2).

Figure 2: the res survey, high-pass filtered.

There are still many features, many buildings, that one can only see when looking at smaller blocks.  With such a big area getting everything to look clear is going to be impossible, I fear.

From tomorrow the res will be working in Church Meadow where we hope it will map the remains of St Mary du Pré.

The GPR team is getting really close to finishing the theatre field (Figure 3).

Figure 3: the GPR survey so far.

The team have some fiddly bits around the edges to complete, one missed block, and one block we want to re-survey at 50cm intervals as there is a building and road in it.  Fingers crossed, two more days should do it.  Then, for a bit of last day fun, the GPR will also have a look at St Mary du Pré.

The mag team completed another eight blocks today.  Since moving to Church Meadow, they have managed to survey four hectares in three days which is very impressive, especially given that one block had to be repeated due to a sensor freeze.  Having lots of whole grids and no partials makes such a difference.  The team are now just 2.5 ha away from completing a square kilometer of mag at Verulamium.  Figure 4 shows the mag results in Church Meadow.

Figure 4: Church Meadow mag survey after day 3.

Three things are of note.  Firstly, earlier today I wondered which of the two raised areas in the field was Watling Street.  Looking at the survey results, it looks like the road splits in two near the edge of our survey.  Perhaps it is two phases?  I’m not convinced.  Secondly, we have some marked linear features showing that almost look like enclosures.  These are, however, low magnetism suggesting and might be yet more pipes, but not of metal this time?  Again, I’m not convinced.  They might well be archaeological features.  I will have to survey all the pipes I can see in the field.  Lastly, as we get closer to the town to the south, there are many more little black blobs.  Seasoned readers of this blog will know that usually little black blobs in mag data are often pits.  In this case, I wonder if we are starting to pick up the edge of the cemetery which is likely to have lined Watling Street?  In the Roman world, the richer you were, the closer you wanted to be buried to the road and the town.

Tomorrow is the antepenultimate day of the 2019 Gorhambury survey season.  The weather looks good so fingers crossed all goes well.

The Great Escape

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

The mag team have pulled-off the Great Escape.  They waved a tearful goodbye to the field they have been working-in all of this season.  Today saw the completion of the available area of Prae Wood Field.  In fact, we did more than we agreed to because I did not know they had grubbed out a hedgerow since the images on Google Earth were taken last summer. Figure 1 shows the entire mag survey from Abbey Orchard (we must finish that!) to Prae Wood.

Figure 1: the entire Verulamium magnetometry survey as of 17/8/2019.

The complete survey now consists of over 93ha of mag data, which is about 18,700,000 readings.  No wonder my computer is feeling the strain!  The Prae Wood field survey is 10.7ha in extent.  Figure 2 shows the whole field.

Figure 2: the Prae Wood Field magnetometry survey.

Compared to the riches of surveying inside the town, the field provided slim pickings.  We did find an enclosure and a large ditch at the eastern end of the field, and some enclosures towards the west.  Much of the field, however, is a remarkably blank canvas, magnetically speaking.  As always, absence of evidence is not evidence of absence, but in this case there is not much to encourage further work here.  Figure 3 shows the western area completed today.  Well done Ruth, Jim, Dave and Pauline, along with Rhian and Ellen on previous days.  Tomorrow sees the great move to Church Meadow.

Figure 3: the western end of Prae Wood Field.

The one new and obvious feature in the field is the big black blob.  It is quite large, about 10m across, and moderately magnetic (-3.5nT to about 8nT).  My best guess this is a ploughed in pit.  Hertfordshire is full of pits, for chalk, gravel or clay (and sometimes all three in the same field).  This one would be quite modest compared to many.

The Earth Resistance team consisted of Ellen and Anne, assisted in the morning by myself until I went to help with the GPR after lunch.  We completed another six grid squares as we work our way north. Figure 4 shows today’s data.

Figure 4: the Earth Resistance survey. Area completed today outlined in red.

Towards the north of today’s area is a nice little apsidal building orientated NW-SE.  Although it reminds me a little of an early church, apses were not uncommon in the Roman world, and we may not be seeing all the building.  It can be seen a little more clearly if we apply a high pass filter (Figure 5).

Figure 5: the 2019 area after the application of a high-pass filter to remove the background trends.

We have seen this little building before in the GPR data, but not quite so clearly.  Once thing that I am finding very intriguing is why some buildings show in the mag data and the res/GPR data, and some buildings do not show at all in the mag data.  Figure 6 shows the mag data from this area.

Figure 6: the mag data from the same area as Figs 4 and 5.

As you can see, there is no sign at all of the apsidal building in the mag data.  Compare this to the buildings further to the south which line the road and show very clearly in both mag and Earth Resistance data.  There is a mystery to investigate!

The GPR team were on some of the steepest slopes at Gorhambury (Figure 7).

Figure 7: John Ridge (SAHAAS) gallantly pushes the GPR up the steep incline.

Not often do I hear survey teams hoping not to find something, but afraid I might make them resurvey the area at 0.5m transect spacing, the team were keeping their fingers crossed that nothing exciting showed!  The radargrams on screen seemed to be fulfilling their hopes (Figure 8).

Figure 8: example radargram from today.

The data were duly processed in GPR Slice.  I present just one slice, No. 4 (Figure 9).

Figure 9: Time slice 4 from today’s survey.

Although there are areas of high reflectance (in red) and low (in blue) I suspect this is mainly to do with geology on the side of this steep dry valley.  The underlying mag data shows a few features (Figure 10).

Figure 10: the mag data in the same area as the today’s GPR survey.

Although there are a couple of pit-like features, including one quite strong one near the northern edge of the block, there isn’t that much to suggest there is much going on here.  Not enitrely surprising given the geology.

Today’s weather was highly variable.  We had some rain just before lunch, but sunny skies in the afternoon (Figure 11).  Fingers crossed for tomorrow.  The afternoon’s weather is currently forecast to be “unsettled”.

Many thanks to everyone for an excellent day’s progress.

Saving the best ’til last?

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

Unfortunately the weather forecast was accurate yesterday.  Far too much rain for us to go out and play.  Today, however, was very pleasant and at least the pegs went in very easily for a change! I’m afraid that a new toy (Figure 1) resulted in me getting a late start processing data this evening, and so I only have the mag and res to report on.  The GPR team, however, did a sterling job and completed a multitude to sawtooth-edged grids along the western side of the theatre field.  I promise I’ll process all that tomorrow!

Figure 1: Kris’ new multiformat pinhole camera being put through its paces.

The Earth Resistance meter was operated by John and Grahame. We have moved north of the hedge line and are close to the cross-roads around which both mag and GPR have shown multiple buildings.  Figure 2 shows the entire res survey, and Figure 3 a zoomed-in view of today’s area.

Figure 2: the Earth Resistance survey so far.

Figure 3: the area surveyed today and some context.

We have just clipped the edge of Street 11 (in the top-right corner of the new block) and have revealed some wonderfully clear images of the buildings to the south of that street.  These are clearer than the GPR survey of the same area, so it is really pleasing to see.  Excellent stuff!

Over in Prae Wood field, the mag team completed two more strips of grid squares.  Figure 4 shows the survey so far.

Figure 4: the Prae Wood Field survey, so far.

Two more days should, weather and equipment willing, see the team finish the field.  After Prae Wood we are heading north into Church Meadow, so-called because St Mary du Pré lies within that field.  Watling Street also runs through it so we should get some exciting results.

But, back to Prae Wood.  Let us look in more detail at the western end of the survey (Figure 5).

Figure 5: the western side of the mag survey of Prae Wood field.

The dark lines in the new area represent ditches, and it looks like we have some enclosures running across this western end of the field.  Given that the major Iron Age settlement lies in the wood just to the SW, these could be of that date, but they could be Roman or medieval as well.  Once more, we need to check out the historic map data.  Still, after many blank grid we now have features in both the western and eastern extensions of this field.  Saving the best for last?

Many thanks to everyone who turned out today.  Tomorrow looks like we might have to pack-up early due to yet more rain. This season is just flying by.

Verulamium 2019, days 1 to 4

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

After some wet weather just before the season was due to start, the first four days have proved to be warm and sunny. So much so, the ground is already drying out so much as to make the Earth Resistance survey a little annoying.  So far, we have been concentrating on Earth Resistance and Ground Penetrating Radar (GPR) in the main “theatre field”, simply due to person-power and logistics.  We are planning to start the mag survey tomorrow with the aim of completing Prae Wood field and then moving to Church Meadow. Mobbs Hole, the field through which the Fosse passes, will be completed at another time of year when we are less likely to disturb the pheasants.

The res survey (Figure 1), firstly concentrated on filling-in the triangle to the east side of the survey area, and then moved to add another strip of grid squares along the western edge.  Figure 2 shows the entire res survey so far.

Figure 1: Jim West and Rhian Morgan running the Earth Resistance meter on Day 3.

Figure 2: the entire Earth Resistance survey after day 4 of the 2019 season.

At the moment, the res surveys have been processed separately and crudely put-together in Google Earth. As a result, you can see the edges between blocks clearly, especially, for example, the triangle to the east. At some point, I need to combine all the grids into one master survey and process them properly!

Figure 3 Looks in more detail at the eastern triangle.

Figure 3: the completed eastern triangle of res data.

The very strong line near the top of the new area is a road partially excavated by Frere.  The lack of clear buildings either side of it near to the modern road is due to the excavations undertaken by Frere.  We have, however, picked up the last bits of the buildings which run parallel to the side of the Insula XVI temple, as well as some new buildings  alongside the modern road.  The building at the south of the new triangle was partially surveyed in 2017, and is not one previously known.

Today we completed five 20x20m blocks on the western edge of the survey area.  The results are shown in Figure 4.

Figure 4: the western survey area.

The buildings lie along a road, although the road is not at all clear.  The diagonal empty area appears to be a eroded channel cutting across the site visible in the modern topography.  The large corridor house to the east was surveyed in 2017 using the multi-depth Earth Resistance survey (aka “the beast”).

The GPR crew have been working south starting near the area which had such exciting results at the end of 2018.  The GPR leaves entertaining stripes in the grass (Figure 5)!

Figure 5: Stripy GPR grass.

Figure 6 shows the entire GPR survey up to the end of today.  This season’s block of GPR is in colour in the SW corner.

Figure 6: the complete GPR survey after day 4 of the 2019 survey.

Even more than the Earth Resistance survey, the crude use of images in GE is visible.  The data has been collected over five seasons, and processed with different software packages.  I am in the process of putting all the data together into one more consistent analysis.  It might take a while.  Figure 7 shows the four blocks completed thus far.

Figure 7: GPR results after day 4. Slice 7.

Despite the mass of buildings just 20m or so to the east of the block, and on the northern edge, very little seems to lie within the block surveyed.  Comparison to the magnetic data from 2016 (Figure 8) makes it possible to see the so-called ‘1955 ditch’, and one of the GPR blobs is matched by the magnetic blob (it is probably a filled-in gravel / chalk pit).

Figure 8: the magnetic data from the same area as Figure 7.

We are going to complete a couple more 40x80m blocks at 0.5m transect spacing, but then swap to using a cruder 1m transect spacing to ensure we complete this field this season.  Should we pick-up further structures, we can always re-survey targeted areas for more detailed survey.

Many thanks to all our volunteers.  The survey would be impossible without you!  Also thanks to UCL Archaeology and SEAHA CDT for the loan of the equipment. Also, big thanks to Lord Verulam, the Gorhambury Estate and the estate managers for enabling the survey to continue.

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.

 

 

Exciting finds in Ashwell

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

Earlier this Spring the group undertook some survey work at Hunts Close, Ashwell.  The results were surprisingly unexciting despite previous finds and the results of earlier test pits.  The one exception was a small group of features on the western edge of the survey which looked like a corner with some sort of irregular feature in the middle.  Gil Burleigh lead a team to put small trenches over these features.  Often this is called “ground truthing”, a term disliked by many archaeological geophysicists.  What we are testing is not the actual data, which is what it is, but our interpretation of that data.  The results were very positive.

Gil Burleigh writes:

Following on from CAGG’s geophysical surveys in March, volunteers from the North Hertfordshire Archaeological Society, led by Gil Burleigh, excavated three further test pits (three others were dug in March) to investigate a few of the features located by the surveys (Figure 1). These had found a double-ditched rectangular enclosure containing a large irregularly-shaped hollow. Slots were excavated into both the enclosure ditches and the hollow (Figure 2).

Figure 1: the excavations underway. Image © Gil Burleigh.

Figure 2: location of the test pits in relation to the geophysics.

Thanks to Kris for both accurately locating these features in the field beforehand to enable the positioning of our slots, and again to him for accurately locating afterwards the positions of our excavated slots using a dGPS (Figure 3).

Figure 3: locating the test pits. Image © Gil Burleigh.

A section across the outer enclosure ditch was completely excavated (Figure 4) and revealed animal bone, Romano-British pottery and a complete 1st century AD Roman bow brooch (Figure 5).

Figure 4: Section across the outer ditch. Image © Gil Burleigh.

Figure 5: 1st century AD Roman bow brooch in situ. Image © Gil Burleigh.

There was time only to excavate a Roman period re-cut of the inner enclosure ditch.
The slot across the hollow revealed its eastern vertical cut edge and a sample of its fills (Figure 6). These contained much fragmented animal bone, weathered, residual, Romano-British pottery and much other pottery that looks Saxon, but requires specialist identification. Several bone pins found, two complete, may be Saxon (Figures 7 and 8).

Figure 6: The central feature. Image © Gil Burleigh.

Figure 7: a bone pin from the central feature. Image © Gil Burleigh.

Figure 8: bone pins or needles from the central feature. Image © Gil Burleigh.

My working interpretation of the double-ditched enclosure, some of which lies outside our field under a neighbouring property, is that it may have contained a Romano-British temple overlooking the spring-head of the river Rhee which lies downslope less than 100 m to the north. The possible Saxon large artificially cut hollow may have been some kind of ceremonial place where religious rituals were performed. It has similarities to the Romano-British cut ceremonial hollow we excavated at Ashwell End, the site of the discovery of Senuna’s temple treasure hoard (Jackson and Burleigh, Dea Senuna: Treasure, Cult and Ritual at Ashwell, Hertfordshire, British Museum, London 2018). The double-ditched enclosure has similarities also to the double-ditched enclosure containing the Iron Age and Romano-British temples at Hayling Island on the Hampshire-Sussex border. This enclosure also contained a series of ritual pits dated to the mid-Saxon period (King and Soffe, A Sacred Island: Iron Age, Roman and Saxon Temples and Ritual on Hayling Island, Winchester 2013).

Having allocated four days to our excavation, it was frustrating to lose a significant amount of time to persistent rain on three of the days (Figure 9). The only completely dry day was the Sunday when in the afternoon an unexpected mini-whirlwind spiralled vigorously around us, spinning buckets, tools, finds trays and other equipment violently into the air, throwing it about in a highly disruptive way. Although some of us were hit by flying objects, luckily there were no injuries. The wrath of the gods perhaps for us disturbing their sacred place!

Figure 9: back-filling in the rain. Image © Gil Burleigh.

Thanks to all our hard-working volunteers, not least for working through the rain, especially to those who did the muddy back-filling in the rain on the Monday: Georgina Brakenbury, Debbie Curtis, Ivor Davies, Brian Dickinson, Keeley Hale, Nigel Harper-Scott, Kris Lockyear, David Morrison, Irina Natarajan and son, David Pengelly, Ellen Shlasko, Graham Spurway, Sarah Talks, and Derek Turner.

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!