Fighting the wind

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

The weather forecast was for a windy day, and it was correct! At one point, the wind was making the flag on the cathedral look as stiff as a board, but the sun made the the yellow cross glow. Even from the far side of the Roman town it was quite striking.

Logistics led to an unusual format for the day.  Jim, Ellen, Pauline and Dave continued to expand the mag survey in Prae Wood Field.  Meanwhile, Kris, Mike, Anne and Julia firstly finished yesterday’s block of GPR data, curtailed because of rain, and the went on to complete six Earth Resistance survey blocks.

Figure 1 shows the whole of the mag survey in Prae Wood field, and Figure 2 a detail of the eastern end.

Figure 1: Mag survey after day 9.

Figure 2: Eastern end of the mag survey after day 2.

You may well ask why we have a funny diagonal edge to the survey at the eastern end.  This is because there is an electric fence creating a paddock for horses.  As we have gone a little further than we intended, we will just take what we can get.  The big new find is a ditch running across the end of the field.  I have marked this in Figure 3 with red arrows.

Figure 3: Mag survey with arrows (see text).

We have no way of knowing what date this feature is.  The first thing I will have to do is check the historic maps.  It does, however, look like much more than a field boundary.  It is 2 to 3.5m wide.  The blue arrow in Figure 3 indicates a much slighter feature than runs at a right-angle to the big ditch.  Just to the west of the ditch is a strong magnetic feature that I have marked with a yellow arrow.  The form of the feature (a bigger blob next to a smaller blob) is reminiscent of the pottery kilns we have found on the south side of the town.  The magnetic values (c. -8nT to +130nT) is also in the right sort of range for pottery kilns.  Some work I have published previously shows kilns with a maximum range of about -27nT to +180nT.  It is typical that the area where we have started finding potentially interesting features is also where there is the most modern interference from services and so on.

The GPR survey just completed the block left over from yesterday.  A quick look at the data showed no surprises.  The good news, however, is we are just one day’s worth of 1m transects from joining-up with the survey to the north.  The survey is shown in Figure 4.

Figure 4: the GPR survey after day 9. The red box marks the block completed on days 8 and 9.

The bare strip to the west of the GPR data in Figure 4 is what is left to be surveyed. That to the east has been done (I just haven’t loaded them onto the GE image).  The eagle-eyed amongst you may have noticed that the GPR blocks do not quite match-up as well as they used to.  This is a bit complicated but is basically because the OS have up-dated their guidelines for converting OS National Grid coordinates to lat and long.  I work in the National Grid, but Google Earth works in lat/long.  I’ve changed to using the OS’s official conversion webpages, partly because I can up-load them in bulk saving me a great deal of cutting-and-pasting.  My crude use of GE to display the results, however, involves dozens, if not hundreds of image over-lays.  As a result, I have a great deal of work to do to update all the slices from all the GPR blocks to the new coordinate conversions.  Arrgghhh.  The joys of doing a survey over five seasons.

The entire Earth Resistance survey is shown in Figure 5.

Figure 5: The entire Earth Resistance survey after day 9.

As can be seen, we have covered an impressive area now, about 9ha in total.  The edges in the image are the different years which have been processed slightly differently and crudely put together in Google Earth.  I will be joining them all together soon and trying to make a more seamless image.  Figure 6 shows the western 2019 block.

Figure 6: the 2019 western block of res data (the lighter strip to the west).

Most of what we managed today is the open dry valley between the buildings on on Street 23 (seen in the latest data) and Street 25 which we have yet to survey.  Tomorrow, however, we will start hitting the buildings on the latter street, and especially the cluster of buildings which lie on the junction of Streets 25 and 11 on the western corner of Insula XXXI.  In today’s data (the upper six blocks on the 2019 strip), we picked up the western half of a large building which lies back from the road with a wing running up to the road with a sequence of smaller rooms.  Fascinating stuff.

Tomorrow we are going to start off with GPR and Earth Resistance giving me time to lay-in more grids for the ever-efficient mag team.

Many thanks to everyone who braved the gales today.  We got some great results despite the weather.

 

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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!

 

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.

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 aqueduct

After the last post about the parch marks visible at Verulamium in the new Google Earth imagery, Mike Smith had a good look at what is now available. Just to the north of the city he spotted a crop mark which seems very likely to be the aqueduct heading up the valley.

The aqueduct.

Great spot Mike!

There must be lots of crop marks showing in the new Google Earth imagery.  Archaeology from your arm chair!