Is that a magnetometer on the horizon?

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

Figure 1: Ruth and the magnetometer on the horizon.

The mag team were in wheel spinning mode today. In other words, it was partial madness.  For logistical reasons I don’t have the mag this evening to download the data, so it’ll be a magfest on Sunday.

The GPR team completed its usual 80 x 40m block today, although the steep slope made it harder work than usual.  Figure 2 shows the time slices.

Figure 2: GPR time slices from day 12.

Although we don’t have any clear exciting buildings we have picked-up the line of two roads running at right angles.  We’ll get the junction tomorrow.  the NW–SE road shows best in slice 6 (Figure 2, second row, first slice), and the SW–NE road in slices 4 and 5.   Figures 3 and 4 show these two slices in context.

Figure 3: GPR time slice 4 with the line of Street 26 (as numbered by Niblett and Thompson) indicated.

Figure 4: GPR time slice 6 with the line of Street 11 (as numbered by Niblett and Thompson) indicated.

The 1955 ditch also seems to show in some slices, e.g., in Figure 3.  The western end of the aqueduct as seen in Wednesday’s slice clearly has a complicated relationship with the 1955 ditch.  At some point I need to look at the radargrams too.

Tomorrow’s block across the road junction will be interesting to see.  Junctions are usually prime locations for structures, but there is nothing much showing in the mag data.  Fingers crossed!

The next update will be on Sunday as Ellen and I will be at a family wedding tomorrow.

Figure 5: One of these things is not like the others.

 

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End of week two, part 1

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

Despite the unpromising weather forecast (and the unpromising weather at about 8.30am), we managed a whole day of survey including mag, GPR and a little bit of earth resistance. There was some drizzle around about lunchtime, but other than that is was an OK day, if a little windy at times.

The GPR team were working on “sawtooth” edges to the theatre field.  As we have a couple of days off, I will post the results of their efforts tomorrow.

The mag team completed twelve grids, including two partials and despite one case of “sensor freeze”; a super effort (Figure 1).

Figure 1: the mag crew, consisting of Ruth Halliwell, Jim West and Dave Minty, operate the Foerster in Mobbs Hole.

Figure 2 shows all the areas surveyed so far at Verulamium.

Figure 2: all the mag survey so far.

This really shows that the strength of the Foerster cart system is when one has large open fields.  So far, according to TerraSurveyor, we have completed 9.8ha of Mobbs Hole over ten days, but remember we only had part of an afternoon on day 1 and lost parts of two days this week to rain.  Compare that to surveying in Verulamium Park with all its trees, hedge lines, park benches and so forth.  There we surveyed just under 30ha in 45 days.

Figure 3 zooms into the area surveyed over the last few days.

Figure 3: mag survey after day 10.

So far the results are curious.  There are linear features associated with the line of the Fosse, plough scars running NW–SE down the slope, a few large “blobby” anomalies (pits, maybe?) and some bits of old iron.  Very little which could be interpreted as structures has been found.  Why build an enormous ditch and bank around nothing?  Ploughing may have removed some superficial features, but there is very little that could be seen as pits or ditches either.  The Fosse remains a mystery.

Figure 4 shows where the Fosse enters the wood.  The dip in the fence marks the ditch.  Once inside the wood the Fosse does a sharp left turn and heads southwards.

Figure 4: the dip in the line of the fence shows where the Fosse enters the woodland.

Tomorrow I will report on the earth resistance and GPR surveys in the theatre field.

Many thanks to everyone who has helped this season.  We are already half way through.  Doesn’t time fly?

 

One hundred and fifty

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

We managed a full day today, and I’m just about keeping up! The mag team completed two grids yesterday, half of one in the aforementioned deluge. Today they completed 11 grids: three partials and eight complete ones. Way to go! Well done everyone. Figure 1 shows the survey so far.

Figure 1: the mag survey after day 9.

One really does wonder if that break in the mag data is an entrance.  It doesn’t seem like it on the ground.  I have downloaded the LiDAR data but haven’t had a chance to process it yet.

The GPR crew finished their 80x40m block, and then did some of the next “sawtooth” section, another 14m worth.  Figure 2 shows the time slices.

Figure 2: day 9, time slices 3 to 6.

Nothing jumps out at one, although there are some curious “light” lines in the fourth slice (top-right) which are parallel to the aqueduct.  Figure 3 shows that slice in context.

Figure 3: GPR survey after day 9, slice 4.

After all the rain I thought it would be worth trying the Earth Resistance survey (Fig. 4).  I spent the morning laying in grids for the mag, but managed some survey in the afternoon.

Figure 4: Earth Resistance survey in action.

Although the rain has softened the surface, it won’t have penetrated 50cm yet, and I was concerned that there would be no contrast at that depth.  I decided to survey a grid where we knew there was a building.  Fig. 5 shows the comparison between the GPR survey and the two squares of res I managed to complete (thanks Anne!).

Figure 5: Earth resistance survey compared to GPR results.

Given the drought, the results are pretty good.  It would be interesting to compare these to results from a normal English summer!

Tomorrow isn’t looking great.  We might get some work done in the morning.  Fingers crossed.

Many thanks to everyone who helped out today.  Especially big thanks to Mike, Ellen, Jim and Ruth who take on the responsibility of shipping the equipment back and forth.

By the way, this is the 150th blog post…

7.2mm

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

That is the quantity of rain which fell on Lamer Park today up to when I started writing this posting.  It is now at 10.3mm! No wonder we gave-up at lunch time all feeling a bit cold and damp.

The mag team did manage three grid squares before lunch (Fig. 1).

Figure 1: mag survey after day 7.

There is a interesting possibility that we have detected a break in the Fosse, maybe some form of entrance.  I have noticed that some of the maps of the Fosse show a break, and some do not.  Perhaps we may be able to resolve this.

The GPR crew managed their usual 40x40m block before the rain drove us away at lunch time.  Figure 2 shows four time slices.

Figure 2: GPR data from day 7.

There is a quite substantial feature showing in slices 3 and 4 (top row) to the west.  This “big blob” is on the edge of the aqueduct, but I am not sure what it is.  Finally, however, we have some clearer looking buildings, best seen on the east side of slice 4 (top-right of Figure 2).  Figures 3 and 4 show this slice in context.

Figure 3: GPR from day 7 in context, slice 4.

Figure 4: the mag data for the same area as Figure 3.

As we have seen before, the buildings in this part of town show in the GPR data but not in the mag data.

On an unrelated note, this week’s Herts Advertiser (dated August 9th) carries a short letter from me about the project.  It also carries the news that the school at Batford has been given the go-ahead (see https://wp.me/p3AAk9-gy for our survey there).

Getting from the Theatre Field to Mobbs Hole is a bit of a problem because some pesky Romans built a wall between them.  There is, however, a gate we can use which is moderately convenient, which we have dubbed “The Manikin Gate”.

Figure 5: the Manikin Gate.

The weather tomorrow morning looks OK according to the Met Office, but we may be rained off again in the afternoon.  On the bright side, it might be possible to break out the earth resistance meter soon!

 

 

 

A light wind swept over the corn, and all nature laughed in the sunshine.

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

Not much sign of corn, but the wind was blowing over the parched grass.  It was a welcome relief to be working in cooler conditions, although the weather was still beautiful.

Figure 1: St Albans Abbey.

The mag team completed an excellent eight grids in Mobbs Hole (Fig. 2).

Figure 2: the mag survey after day 6.

As before I have overlain the survey on the 2006 imagery in Google Earth which shows the Fosse most clearly.  The hints of a line along the inner edge.  Maybe this is the remains of a palisade trench?  Disappointingly little otherwise.  One thing to note is that the “noise” from random ferrous trash is more prevalent to the south of the old fence line than to the north.  I wonder if the NW corner of this field was pasture previously?

The GPR crew completed their two grids.  Figure 3 shows some time slices.

Figure 3: Day 6 GPR results.

Not a great deal showing apart from in slice 4 (in the top-right corner of Figure 3) which clearly shows the aqueduct.  This is about as clear as I have ever seen it in GPR data.  Awkwardly, the direction of the transects is close to the direction of the aqueduct.  Figure 4 shows the slice in context with the others we have surveyed this year.

Figure 4: GPR day 6, slice 4.

The edges do not match because I keep playing with the settings in the software.  One day, I’ll slowly process the whole lot so that we get a nice final result.  One day (more like several months…).

Barney and Becca came and helped with the GPR in the morning.  After lunch, we blew the dust off our Bartington and did a couple of squares.  I rather liked this image of Barney.

Figure 5: Barney and a Bartington.

The survey moves on and we cover more ground.  Many thanks to everyone who comes and helps expand the area we have covered.  Luckily, most of the area you can see in the last inage has already been done!

Figure 6: Ellen and Mike operate the GPR.

 

Sawtooth Saturday and other tales

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

The lack of posts doesn’t mean we haven’t been out working…

Enough of that!  The lack of posts is simply that we had a friend staying and a BBQ and beer won over working on the blog.  Sorry…  Well, not very sorry.

The mag team have been working southwards across the “Fosse field”, the proper name for which is Mobbs Hole.  The area we have covered in around Verulamium is getting pretty large (Fig. 1)!

Figure 1: total area surveyed to date.

The survey has now started to clip the edge of the Fosse itself. In Figure 2 I have used the 2006 imagery in which one can see the Fosse clearly as a soil mark.

Figure 2: the mag data on the 2006 imagery in Google Earth.

Progress is excellent for four and a half days of survey.  Figure 3 shows the survey in more detail.

Figure 3: the mag data after day 5.

The edges of the Fosse can be seen in the mag data, mainly as a lighter line.  This is because the topsoil will be thinner over the lip of the ditch as soil has eroded down into the fill.  There is a line of large dark “blobs” along the lip of the Fosse.  Although these might be something interesting, I suspect they are tree-throws (i.e., the hole made by a tree being blown over).

The field system which shows in the upper half of the survey is interesting.  Jon Mein has kindly shown me the 1799 map of the parish and these boundaries do not match those mapped then.  There is a line of woodland following the line of the Fosse a little to the south of the area we have reached.  The track which runs along the northern edge of Mobbs Hole was a much more important road at that date.

Wheeler thought that the Fosse represented the “first Roman city” at Verulamium and cut several sections across it (Figures 4, 5 and 6).

Figure 4: Wheeler and Wheeler 1936, plate 109, detail showing the location of the sections excavated by the Wheelers.

Figure 5: Wheeler and Wheeler 1936, plate 18. Sections across the Fosse.

Figure 6: Wheeler and Wheeler 1936, plate 78. Sections across the Fosse.

We now know that the early Roman town was based down towards the forum where the Museum now is.  The Fosse, however, does appear from Wheeler’s finds to be first century.  But what was it for?  Hopefully, the geophysics within the line of the Fosse may give us a clue.

One aspect of the landscape I had not appreciated was that the dry valley which the aqueduct has to dog-leg across as shown in our survey (the V-shaped long linear feature within the town walls shown in Figure 2), becomes quite a major feature to the west of the walls.  The northern arm of the Fosse lies on the crest between the dry valley and the valley of the Ver, and then when it turns to form the southern arm it has to cross that valley.  Figures 7 and 8 compares the plate published by the Wheelers showing the view from the crest to a panorama I took the other day.  The hedge line shown in the Wheelers’ plate is no longer there.

Figure 7: view southward across the Fosse as published by the Wheelers in 1936.

Figure 8: Panorama of Mobbs Hole (click to see full sized).

Back inside the town walls the GPR crew completed an awkward couple of blocks on “sawtooth” Saturday and another 80 x 40m block on Sunday.  Figures 9 and 10 show some of the time slices.

Figure 9: the GPR from day 4.

Figure 10: GPR time slices from day 5.

On neither day do we have some of the beautifully clear buildings we have seen previously.  There does seem to be a small square structure showing on day 5: the NW edges showing in slice 8 (the sixth image in Figure 10) and the bottom edge in slice 10.  Figures 11 and 12 show slices 5 and 6 from days 2 to 5 in Google Earth.

Figure 11: GPR days 2 to 5, slice 5.

Figure 12: GPR days 2 to 5, slice 6.

Although we have not got lovely clear buildings, the “blobby” bits do have a generally SW — NE orientation which matches the town grid.  It is, however, very difficult to interpret.  Looking carefully, however, there is more than immediately catches the eye.  Lets look at the day 5 data more closely.

Figure 13: GPR day 5, slice 6.

It all seems rather dull.  Now plot that on top of the mag data.  Look at the point the red arrow is indicating in Figure 14.  Figure 15 just shows the mag data.

Figure 14: Day 5 GPR overlain on the mag data.

Figure 15: the mag data from the same area as Figure 14.

The feature which shows quite clearly in the mag data does show quite faintly in the GPR.  The moral: lovely clear buildings are wonderful, but sometimes there is more there than you think.

We have been enjoying our two days off, and will be back at it on Wednesday.  The forecast is for it to be a bit cooler.  Luckily, our guard dog has been keeping a close eye on the flags…

Figure 16: Guard Dog.

Many thanks to everyone involved.  The heat has made it quite hard work, but the survey looks ever more amazing with each new grid square!

 

El Scorchio

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

It was hot today. Not a cloud in the sky in the morning (Figure 1).

Figure 1: the GPR crew setting-up under a cloudless sky.

We did, however, manage to complete our target of two 40x40m GPR grids and seven mag grids, so a good day’s work all round.  I also put in enough grid pegs to keep people busy for the next two days.

The mag team of Jim West, Graham Spurway and Rhian Morgan continued working in the “Fosse Field” outside of the town walls (Figure 2).

Figure 2: Rhian Morgan (CVAHS) operates the magnetometer.

The results are encouraging.  As well as more of the very faint features we detected yesterday, we have some much clearer linear features, almost certainly ditches, as well as a couple of other more amorphous ones which may well be pits (Figure 3).

Figure 3: Magnetometry results after day 2.

The linear features look like field boundaries but they do not bear any relationship to the current boundaries. Indeed, just to the north of the area we have surveyed, past the track which can be seen in the image, is a large lynchet caused by many years of ploughing.  We would appear to be seeing something pre-dating the laying out of the park at Gorhambury.

The GPR team of Mike Smith, Dave Minty, Nigel Harper-Scott and John Ridge, having filled-in the annoying gap yesterday, have moved to the north end of the site near where we park.  Slices 3 to 8 are shown in Figure 4.

Figure 4: GPR time slices 3 to 8.

Unlike yesterday, we didn’t have quite such clear buildings.  There does, however, seem to be a rather knocked-around one in the western half of the block.  The mag isn’t all that clear in this area ether apart from a ditch running NNW–SSE.  The next two figures show two slices in context.

Figure 5: Time slice 5.

Figure 6: Time slice 6.

With the eye of faith one can just about see the ditch in the GPR data, but the large areas of high reflections are hard to interpret clearly.

By the end of the day, we did start getting some clouds.

Figure 7: Clouds and stripy GPR.

I’m lovin’ those stripes in the brown grass!