Tag Archives: Mala

Partial madness

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

Apologies for not posting the results from yesterday’s survey.  Prae Wood field is an odd shape, leading to a plethora of partial grids for the mag, and the consequent head-scratching as to how it all fits together back at base last night.

Starting with the mag data.  The team have had two days of multiple partial grids, many of which have to be split into several smaller blocks in order to be surveyed.  At times, the sea of flags looked a bit like bunting at a village fete.  The team, however, have made excellent progress and we should finish the field in a few days (weather willing).  Figure 1 shows the results for the whole field.  Mainly, we have found pipelines (the black-and-white linear features) and clearly something big happened in the field near the gate at the south edge (an old building, perhaps?).  Traces of more interesting archaeology are hard to come by, and I have pointed out one with the red arrow.  Slim pickings in this big field, but important to see the empty spaces.

Figure 1: the mag survey in Prae Wood field after day 7 (day 3 of mag survey).

The Earth Resistance survey took a back seat on Wednesday with Graeme and I completing a token single square after lunch.  Today, we did not even try to do any more, the ground is so dry and hard.  Tomorrow the frame is off being repaired.  The single square we did manage showed nothing at all (Figure 2, top left square).  It lies in the valley between the buildings that line streets 23 (with all the buildings that can be seen Figure 2) and street 25 which we have not yet surveyed with the res.

Figure 2: Earth Resistance survey after day 6.

The GPR team, completed a block 40 x 80 at half meter transects yesterday.  As they did not find any buildings, and generally the evidence seems to suggest that the NW side of the town is free of them, we swapped to 1m transect spacings today enabling the team to cover an area 80 x 80m.  We would very much like to complete the GPR of this field this season.  This afternoon the team was joined by Sandy Walkington, President of the Arc and Arc (Figure 3).

Figure 3: Sandy Walkington (SAHAAS) operates the Mala GPR.

Figure 4 shows the 40 x 160m block surveyed over the last three survey days (on the left) next to some blocks surveyed last year.

Figure 4: the GPR data from the last three days of survey.

Although nothing stunning jumps out, unlike the buildings to the east found last year, there are some things to note.  There are clearly some linear features showing in the NW block of data.  Figure 5 shows the mag data from the same area.

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

The large dark linear feature that turns a right angle in Figure 5 is our old friend, the 1955 ditch, the late 1st century boundary of the town (Fig. 6, red arrows). There is a second, much fainter, linear feature running parallel to the ditch, that clearly must be related to it (Fig 6, blue arrows).  Note the three faint circular features (Fig. 6, green arrows).

Figure 6: Figure 5 with colourful arrows (see text).

Looking back at the GPR data (Figure 7) we can see all these features reflected in the GPR data, albeit subtly.

Figure 7: GPR data with arrows (cf. Fig 6 and text).

The three circular things would appear to be depressions or pits into which slight more magnetic topsoil has collected.  These is going to be much correlating of features between surveys on the horizon!

Finally, the St Albans and Hertfordshire Architectural and Archaeological Society (aka the “Arc and Arc”), the oldest archaeological society in Hertfordshire will celebrate its 175th anniversary next year.  They have recently launched an updated website.  John Dent, Arc and Arc member and CAGG volunteer right from the beginning, has his 15 minutes of fame on the front page (Figure 8), pushing the GPR at Gorhambury in the first season of our survey in 2015.  Go John!

Figure 8: The Arc and Arc’s new front page featuring John Dent and the GPR.

Tomorrow may, or may not be a bust.  The weather forecast has heavy rain over night but dry during our working day.  What the reality will be, who knows!

Congratulations everybody for some excellent surveying.

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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!

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.

 

 

“The way I see it, if you want the rainbow, you gotta put up with the rain.”

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

As I start this entry of the blog, the rain is splashing against my windows as was predicted by the Met Office. Although we might question Dolly Parton’s grammar, the sentiment seems true enough.  Yesterday, however, was a superb day with all three techniques collecting data across the site.

After yesterday’s excellent results, the GPR crew had great expectations.  The only problem was a tree in the way under which the shepherdess had put hay when the grass in the field was dead from lack of rain.  Unfortunately, sheep mean sheep droppings (Figures 1 and 2).

Figure 1: Mike on sheep poo removal duty.

Figure 2: Check out those wheels!

Luckily for everyone concerned, I think the effort was worth it (see Figure 3)!

Figure 3: GPR time slices from Day 18.

I could misquote Dolly along the lines of putting-up with sheep poo if you want excellent GPR results but I might be pushing my luck…  The many buildings are quite obvious in this data set.

Figure 4 shows this grid in context of the other GPR grids in this area.

Figure 4: GPR results including the day 18 data (SW corner).

We have added a very large number of new buildings to the map of Verulamium.  As I was only just starting with GPR data when we started collecting it in 2015, the processing keeps changing a bit from block to block. One of my jobs is to start from scratch and reprocess the whole thing so that the maps are consistent.  Should keep me busy for a while.  Figure 5 is a crude mosaic of images just showing the entire area surveyed so far.

Figure 5: crude mosaic of GPR time slices at the end of the 2018 season.

This represents 19ha of GPR data collected at 0.5m transect intervals.  Just pushing the machine along the lines, not including getting to the block, setting-up, moving strings etc. is 380km.  It also means 380km of radargrams!  No wonder the data takes-up 33gb of my hard disk and consists of over 70,000 files.

The mag team completed nine 40x40m grid squares which is 1.44 hectares.  Excellent progress!

Figure 6: the mag team in the southern field.

Figure 7 shows the whole of the 2018 survey (along with a big chunk of Verulamium).

Figure 7: the mag survey after day 18.

Even though we have been using the machine for some years now, and it does have its frustrations, when all is going well we can really cover some ground.  The season was planned for 20 days: we lost 3 days to rain, and most of a day to testing the mag at the start.  Despite this, the team have managed to collect 17.7 hectares of mag data.  Without actually getting to the grids and back (which is quite a bit of walking in itself), the team have pushed the cart 88.5km over the past four weeks.

Figure 8 shows the southern area in more detail.

Figure 8: the southern area of mag data after day 18.

The blue arrows in Figure 8 indicate the lines of old field boundaries.  These can be seen on old maps such as the 1699 parish map.  The yellow arrows mark ferrous objects.  Some are very big, but there are a scatter of smaller ones too.  Last, but definitely not least, there are a few magnetic features which may be archaeological, such as pits.  I have picked a few out with red arrows.  Although they look small at this scale, they are probably 1m to 2m across, a quite respectable size for a pit.

Although large mainly  blank areas are disappointing to collect, they are important nonetheless. The immediate environs of Verulamium are extremely rich, archaeologically. The field lies:

  • 360m W of the busy area of buildings recorded by the GPR discussed above;
  • 600m NE of the major Iron Age settlement at Prae Wood;
  • 600m N of the fields at Windridge Farm where metal detecting rallies have taken place;
  • 500m NW of the major cemetery at King Harry Lane;
  • 1,100m SE of Gorhambury Roman villa;
  • 1,000m NE of the new villa found at Windridge Farm.

Also, the Fosse, which is preserved in the woodland along the NE edge of the field, is a really very impressive earthwork.   We just seem to have hit an empty bit of landscape between all these sites!

The res survey now covers some 6.58ha, that is about 263,200 earth resistance readings.  Not into the millions like the mag and GPR, but this is res after all!  Figure 9 shows the entire survey.

Figure 10: the entire Earth Resistance survey after day 18.

At this scale the roads show very nicely as do some of the more substantial buildings.  Figure 11 is the area surveyed in 2018.

Figure 11: Res survey after day 18.

Given that the fields were baked hard and the grass was dead at the start of the season, I am pleased we managed any Earth Resistance survey at all this season.  The team yesterday put-up with my geophysics OCD and completed right into the corner by the theatre. We then doubled-back and started filling-in between the top of the survey block and the drive.  We have picked-up some parts of buildings seen in grids to the south, but in general along the edge the deep colluvium, as shown by the sunken nature of the drive, is to some extent masking the archaeology.

Many thanks to everyone on the team who made the 2018 season such a success.  A especially big thanks to those who helped move the equipment about including Ellen, Mike, Jim and Ruth.

For those who haven’t been involved but would like to join future surveys, do get in touch.  We are a friendly group, and provide on-the-job training.

And finally… (as they used to say on the news)

 

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…

 

Seven red kites, two fire engines and a microlight

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

It was an eventful day. At lunch seven red kites descended on some tasty tit-bit not far from where we were sitting, and in the afternoon two fire engines drove up the drive and we were “buzzed” by someone in a microlight. None of this has anything to do with the geophysics, however!

The mag team completed the last two grids in Mobbs Hole (for now), and have started on the field to the south.  The first six grids were all wheel-spinning partials too.  They have, however, only one partial left and then there are eleven whole grids laid-out and waiting.  Partials are not the Foerster’s strong point.  The lack of an “end line” function means hours are wasted spinning the wheel to fool the odometer into thinking we have completed the line.  Open fields, however, are its strength and the team will be glad to be out in the wilds again.  Figures 1 and 2 show the results from Mobbs Hole.

Figure 1: the Mobbs Hole survey in its entirety.

Figure 2: the southern area completed today and the start of the next field.

The GPR team had a partial around the water trough this afternoon and so they didn’t quite complete their usual 80x40m block (I knew I should have kept quiet yesterday).  The next two figures are nine time slices of the western and eastern halves of the block.

Figure 3: time slices from the day 16 GPR data, western block.

Figure 4: Day 16 GPR data, eastern block.

The western block seems to be yet-more blobby stuff, although with some very strong reflections.  The western block, however, has some clearly recognisable Roman-style corridor houses.  Yay! Finally some buildings we can recognise!

The last two images show slice 6 in context, firstly on the mag data, and then the mag data with an outline of the location of the GPR blocks.

Figure 5: GPR data from day 16, slice 6.

Figure 6: mag data with the location of the Day 16 GPR data indicated by the red box.

The huge black and white feature in the middle of the mag plot (Figure 6) is the water trough. As you can see, some of the walls of the buildings show in the mag data, but are much clearer in the GPR data.  Some only show in the GPR.  I know I am beginning to sound like a stuck record, but that is the strength of multi-method survey.

Tomorrow is our antepenultimate day (I had to get that in once again), so fingers crossed for dry weather.

Many thanks to the whole team for their wonderful effort and commitment.