Tag Archives: Foerster

Ivinghoe Aston

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

Last Sunday we undertook a small survey in the parish of Ivinghoe Aston, Buckinghamshire. Earlier in the year, following a find by a metal detectorist, the Buckinghamshire Archaeological Society’s Active Archaeology Group undertook a small excavation and recovered a Roman cremation burial.  As is always the way, tucked into one corner was the edge of a second burial.  This was left in situ and we were asked if any of the survey techniques might detect cremations.  Of the techniques we use, magnetometry is the most likely to be able to find cremations but given how small they are, the standard 0.5m spacing between sensors might just be too big.  We can configure our cart to have a 0.25cm spacing between sensors (Fig. 1) at the cost of having to walk twice as far.  In this case, we were not aiming to cover a large area and so the trade-off seemed reasonable.  Jim West (CVAHS)  Pauline Hey (LBDAHS), Peter Alley (WAS) and myself (WAS), along with Jean Bluck (CVAHS and BASAAG), Rhian Morgan (CVAHS and BASAAG) and Piotr Sobisz (the person who found the site) surveyed a 80 x 80m area at 40 readings per meter.  Peter Alley also undertook a survey with his UAV to map the lumps and bumps in the field.

Fig. 1: Jim West operating the mag with a 25cm sensor spacing. Fergus is supervising closely.

The basic results are shown in Figure 2.  This image has had the readings clipped to +/- 2.5nT.

Fig. 2: the magnetometry survey results.

The first and most obvious things are the two strong linear features across the bottom and top left hand corners.  There are also some fainter linear features.  I have marked these in Fig. 3.

Fig. 3: the results with the linear features marked. Yellow: fence lines etc.; red: more subtle linears, possibly archaeology.

The lower feature marked with yellow arrows clearly lines up with a hedge in the next field and is almost certainly a grubbed-out hedge line.  The upper line marked with yellow arrows is less  certain.  It isn’t that strong, and it isn’t that continuous.  I still have my doubts, however.  It may be an old field drain?  There are many subtle linear features marked with red arrows.  These do not make a coherent pattern and may well be earlier drainage or other agricultural work, but some could be something more interesting archaeologically.

But what about the grave?  Fig. 4. marks the position of the excavation trench.

Fig. 4: the results with the excavation trench marked.

You can just see the little red square in the middle of the plot. (It might be worth clicking on the image to see it in more detail, and then clicking on that version of the image to zoom it to maximum size.)  Fig. 5 shows the area in more detail (again you might like to click on it).

Fig. 5: zoomed in view of the area of the trench.

It is interesting to see that the excavation does not show in the data at all.  The second, unexcavated cremation was in the SW corner.  As can be seen from these plots, there are lots of “little blobs”, some of them are stronger than others.  Little blobs which are strongly magnetic, shown with the positive pole in black and the negative in white, are likely to be metal, especially when the negative (white) part is not north of the black part.  In order to see the subtle stuff, we have clipped the image to +/- 2.5 nT (which isn’t much!) but this makes it hard to see the difference between strong and moderately magnetic “anomalies”.  To make this clearer, I have created an image where the strong values (greater or less than 5nT) are in red and green (Fig. 6).

Fig. 6: strong positive and negative magnetic values shown in red and green.

This allows us to discount a number of features, but there are still lots of possible blobs.  If we look at Fig. 5 closely there is a small blob on the SW corner, just about where the second cremation is likely to be.  Using TerraSurveyor, we can measure the magnetic profile of the feature and draw a graph of it (Fig. 7).

Fig. 7: Readings across the feature to the SW of the trench.

From this graph we can see that the burial, if that is what it is, has a “signature” about 75cm across and ranging from -1nT to a maximum of about 5nT.  From this we can suggest some possible extra burials.  I have marked just a few in Fig. 8.

Fig. 8: Red arrow: likely cremation next to the trench; yellow arrows: some possible cremations; blue arrow: larger feature.

There are clearly quite a few more in the survey, I have just marked five.  Unfortunately, the law of equifinality comes into play, i.e., very different things can have very similar magnetic signatures.  All sorts of things might make small, subtle magnetic features, like old animal burrows for example.  There are also a small number of larger features which might be interesting, for example the one marked in blue.  This is bigger than a single cremation, being 2m north-south and 1.5m east-west but has a similar magnetic range (i.e., -1 to -5nT).  The reason why I am curious about it is that cremation cemeteries sometimes also preserve evidence for the pyre site.  Rarely, we also get what are called bustum burials where the pyre is built over a small pit and the pyre remains, along with possibly extra grave goods, and placed in the pit.  Hopefully, the AAG might be able to test a few of these features to “ground truth” the results.

All in all we had a grand day, especially Kai (Fig. 9)!  We have several possible surveys in the pipeline.  If you want to be put on the mailing list, please email us.

Fig. 9: Kai being spoilt.

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February fun

Partly as a training exercise for UCL students, and partly just to extend our surveys at Verulamium, members of CAGG were out in Verulamium Park and Abbey Orchard last Thursday and Friday.

The magnetometer was out in Abbey Orchard on Thursday in the hopes of completing that small area.  Unfortunately, I think it is jinxed.  On Thursday we had battery problems, and on Friday the odometer refused to work properly.  The sum total of a day and a half’s work?  Two partial grid squares…  Oh well, just chalk that one up to experience.  Here is the image of the slightly extended area, such as it is.

The Abbey Orchard survey.

The Abbey Orchard survey.

Having given-up in the mag, Jim West, Pauline Hey and I decided to use the Earth Resistance meter on Friday afternoon.  I had singled out an area where there is a clear building in the mag data, but some ferrous noise masked the western end of the building.  The weather was glorious for a mid-February day.

The Earth Resistance survey underway.

The Earth Resistance survey underway.

We managed four grid squares at the usual 0.5m spacing, not too bad for one afternoon’s work. As you can see in the next image, the mag survey shows a lovely building as white lines representing low magnetism.

The mag results in the area of the Earth Resistance survey.

The mag results in the area of the Earth Resistance survey.

The wide dark line coming from the SW corner is the ‘1955 ditch’, the first century boundary of the town first excavated by Frere in 1955.  The two parallel lighter lines which run just to the north of the building are part of a road.

Unfortunately, the Earth Resistance survey showed nothing of the building at all.

The Earth Resistance survey results.

The Earth Resistance survey results.

It shows the edge of the road beautifully, and a high-resistance feature parallel to the 1955 ditch.  Even the ditch itself shows as a wide band of low resistance.  Of the building, however, nothing!  It may be simply that the soil is so wet at the moment there is no contrast between the building and the surrounding soil matrix.  Alternatively, the building may have been robbed out.  We will have to run the GPR over it one day.

Unlike last November, the GPR suffered no glitches, and Mike Smith, John Dent and Graeme Spurway completed an area 160m by 40.  Added to the same sized area completed in November, we now have a nice block of GPR data 160m by 80m to look at.

The data were sliced using Larry Conyer’s system in 3ns slices.  I’ll go through these from the top down.  There are three areas of GPR survey shown.  The top half of the large block is the latest survey, the bottom half that undertaken last November.  The detached block to the west was undertaken by Ralph Potter in 2014.  Remember that this is a rather crude “mash-up” in Google Earth so the edges do not match very well.  As always the GPR data are deserving of a much more detailed analysis.

GPR slice 3, 10.5 to 13.5ns.

GPR slice 3, 10.5 to 13.5ns.

Slice 3 (above) mainly shows modern features surviving in the topsoil, especially broad cultivation marks running NNW–SSE across this field.  There are some hints of the archaeology just starting to show through.

GPR slice 4, 13.5 to 16.5ns.

GPR slice 4, 13.5 to 16.5ns.

Slice 4 clearly shows the upper levels of the archaeology.  Watling Street, which is running roughly north-south in the eastern half of the main area, has a big hole in it.  It has been severely robbed for building stone.  There is a minor road running SW-NE with a square building alongside it to the north with a small room on the western wall and what looks like a courtyard on the eastern side.

GPR slice 5, 16.5 to 19.5ns.

GPR slice 5, 16.5 to 19.5ns.

The main addition in slice 5 is the complex of buildings in the NW corner of the main area. These are probably associated with the pottery kiln which we have just clipped (the feature that looks like Mickey Mouse’s ears in the underlying mag data).

GPR slice 6, 19.5 to 22.5ns.

GPR slice 6, 19.5 to 22.5ns.

In slice 6, the spur road is clearer, especially near to Watling Street.  It looks as though there is a shallow valley running parallel to Watling Street which is now filled with a greater depth of topsoil which means that the archaeology does not show until the deeper slices.  It is also noticeable that the centre of Watling Street has fewer reflections than in the upper slices.  I guess that we are getting below the surface of the road, and the reflections either side of the road may be the ditches that Wheeler found filled with rubble.  The building complex just to the north of the spur road has hints of two more small buildings.

GPR slice 7, 22.5 to 25.5ns.

GPR slice 7, 22.5 to 25.5ns.

In this final slice we can see the two small buildings north of the spur road in more detail. There is also a long linear feature running N-S between Watling Street and the modern path.  It looks like a modern utility to me, but there isn’t one indicated on the map I have been given.

The GPR results are excellent, and it will be worth continuing to expand this area.

I’ve had a busy time speaking to various groups about CAGG’s work recently.  One of the lectures was as part of the Society of Antiquaries public engagement lecture series held on a Tuesday lunchtime once a month.  They video the talks and put them online, so if you would like to hear me talking about Verulamium once more, here is the link.

As always, many thanks to Ruth Halliwell, Peter Alley, Jim West, Mike Smith, Pauline Hey, John Dent,  and Graeme Spurway, as well as my students from UCL, for turning out in mid-February, although we were extremely lucky with the weather,

 

Watling Street

Firstly, apologies to those who turned-up and found themselves unemployed. With ground conditions difficult for the resistance meter, we just couldn’t run all three machines. Thank you for being so understanding.  Looks like it will rain most of tomorrow, so hopefully we can press-on with the resistance survey on Saturday.

The mag survey in the second field, which I am going to call the macellum field for convenience (it isn’t the field’s real name) is going well.  Firstly, an image of the whole town:

The mag survey of Verulamium.

The mag survey of Verulamium.

My long-held dream of having a complete survey of Verulamium is getting close to becoming a reality!  Thank you everyone.

Looking at the new area in more detail:

The area surveyed on days 29 and 30 in the macellum field.

The area surveyed on days 29 and 30 in the macellum field.

Hopefully, everyone can see the clear linear feature running from the SE to the NW.  This is Watling Street, the main road from London to Chester.  As one might expect just near to the gate (which was robbed and then excavated, and lies in the trees), there appear to be lots of little buildings along the road which show as white lines against the mid-grey background. What is more curious is the rather different look of that background: much less even and more noisy. I’m not sure why, yet.  The field certainly feels different: flatter, slightly different vegetation, and is obviously closer to the river.  Perhaps we have moved from chalk to the river gravels?  I must check the geology map…

The GPR has been doing lots of bitty blocks around the edge of last year’s survey area.  Rather than hold-up the posting of the mag data, I thought I would do the GPR later when I have dropped all the different areas on to Google Earth.

Thanks to everyone who helped today, especially Graeme for acting as my driver and go-fer as I limped around the field  (don’t ask!), and Ruth for helping transport all the equipment.

 

Bring me sunshine, bring me smiles…

Magging (c) Mike Smith 2015.

Magging (c) Mike Smith 2015.

For a change, or so it seems, it didn’t rain.  It got very threatening at the end of the day so we were pretty efficient packing up, but on the whole it was a lovely day with white fluffy clouds, We were a slightly smaller team than usual, but we still managed to run the mag and the GPR today.

Following yesterdays problems with the mag, Ellen implemented a high-tech solution…

The hi-tech solution.

The hi-tech solution.

The flowerpot came from our garden but sadly Homebase didn’t stock duct tape so we had to make do with “duck” tape.  What do ducks do with tape, I wonder?  When the season is over I think the machine is going for a service.

The mag started by re-doing one of yesterday’s dodgy squares, then complete the transect with its final partial, before starting the next transect.  Here is the overall view.

The mag survey after day 16.

The mag survey after day 16.

Sadly, I don’t think we’ll get the whole next transect done in the next three days.  The area we did today is an interesting contrast to earlier days.

The area surveyed on day 16.

The area surveyed on day 16.

There are quite a few pits, and a ditch which seems to continue the line of pits seen yesterday to the WSW of the “uber magnetic building”.  One of tomorrow’s grid squares should show us how this new ditch and the “sinuous ditch” meet.  No sign of the 1955 ditch’s northern arm yet, however.

Mike Langton, of Mala, kindly processed one block of GPR data — that surveyed on day 12 — for us using Reflex-W.  The results are very good. I have (a) a lot to learn and (b) a lot to do!

Time-slices of the dat 12 data by Mike Langton.

Time-slices of the dat 12 data by Mike Langton.

The eight slices start at 10ns and are 2ns thick.

Mike and I completed a grid square with the GPR in the morning, tidying up the block of grid squares so far surveyed.  Unfortunately, there doesn’t appear to be very much in it.

The centre block is the data collected on days 15 and 16.

The centre block is the data collected on days 15 and 16.

The same area without the GPR data for comparison.

The same area without the GPR data for comparison.

I decided that I couldn’t wait a year to find out more about the “uber magnetic” building, especially after the repeated failures of the res meter on that area.  Mike and Julia valiant struggled up and down the slope, occasionally assisted by myself.  Surprisingly, given the strength of the magnetic signal, the GPR showed very little apart from one corner.

GPR results over the "uber magnetic" building.

GPR results over the “uber magnetic” building.

The same area as the previous image for comparison.

The same area as the previous image for comparison.

It seems unlikely that a building like a bath house would show so poorly in the GPR data and so it seems more likely we are dealing with a burnt down timber structure.

We are on the last leg of this year’s survey.  Despite hiccups, rain and technical failures we already have some stunning results.  Let’s hope for a few more in the last days.

Many thanks to everyone who turned up today and did such sterling work.

Heading south

Today we just about managed to run the mag, the GPR and the resistance meter.  The latter was principally to see if my attempts to get rid of the striping problem had been successful. I fear the answer is ‘no’.

Mike asked me if we were as far on as I hoped after the first week.  I have been pleased with progress, but this prompted me to look at the bigger picture.

The entire survey area after the day 6 of the 2015 season.

The entire survey area after the day 6 of the 2015 season.

As you can see, we have already completed a big area of the Gorhambury side of the Roman town, but there is still a great deal to do, even if we ignore areas like ‘The Fosse’ which lies outside the town walls.  Obviously, we will not complete it all this year.

Today’s mag survey worked its way south towards the hedge line in the middle of the ‘Theatre Field’.  Tomorrow we will finish the bits to the north of that hedge line, then start working our way further south.

The survey after day 6.

The survey after day 6.

The main finds today were buildings along Streets 11 and 25 (as defined by Niblett and Thompson 2005).

Buildings found during day 6.

Buildings found during day 6.

Building Insula XXX, No. 6 faces north east onto Street 11 and appears to have a series of rooms backed by a long corridor.  This building was seen in an aerial photograph taken in 1959 although we have revealed many more details than seen previously.

One great aspect of working in this area is the lack of previous excavations, water pipes, gas pipes, park benches, goal posts and cricket pitches messing up the geophysical data!  We’ll be out again tomorrow, watch this space.

Two days in the Park…

… Page’s Park, Leighton Buzzard, this time! CAGG teamed up with members of the Leighton Buzzard and District Archaeological and Historical Society to undertake a survey in Page’s Park which is next to the light railway.  Why here?  There is an early reference to the find of a “Roman well” in the Park which is marked on the OS maps.  Here is the description from the Bedfordshire HER:

A supposed Roman well was found at Stonhill (Page’s Park) in the mid 19th century. The structure was lined with sandstone. Red deer antlers were found in the well, but no datable finds. Other depressions visible on air photos of Page’s Park are likely to be the results of quarrying.

We spent two days in the Park and managed to survey nine grid squares on the first day and twelve on the second.  Quite a feat to cover so much ground.  It was a challenge for me too as I was trying to lay out the 40m grid using  UCL’s nice new shiny dGPS but not aligned on the OS grid this time. The layout of the park made it more sensible to rotate the grid about 30 degrees anticlockwise.  I’m glad to say it worked, but I am painfully aware I need to learn how to use QGIS for the data processing.  It helped that there was a good ‘phone signal for a change, but the nice mature trees were a problem around the edges.  The Park also has a surprising amount of topography. The NE side where the cricket pitch lies is quite flat but the SW side is surprisingly hilly.  Even the football pitch lies in a hollow.

Pauline Hey heads for the goal.

Pauline Hey heads for the goal.

As one might expect, there was considerable modern interference: two goal mouths, a mast for a CCTV camera, manhole covers indicating the presence of utilities…

Page's Park magnetometry survey.

Page’s Park magnetometry survey.

A first glance at the results of the survey shows these features quite clearly. The vast majority of strong readings, shown as black or white in the plot, are modern features: the gas pipe which runs along the northern edge of the park, the utilities for the cricket pavilion, as well as some other unknown ferrous items.  I have plotted some of these in the next figure.

Some of the modern features in the Park.

Some of the modern features in the Park.

Two of the questions which arose from looking at the plot of the first day’s work (the area to the left of the line of trees) were: what are the two curving lines in the top-left corner and more centrally (both marked as ‘edge of slope’ above); and why does the depression near the approximate location of the well have such a strange mixed but strong signal?

To answer the first question I decided to use the new dGPS to undertake a rapid topographic survey of the area to the west of the trees (another first for me!).  The next three images show the results: firstly the topo survey, secondly the mag survey of that area and lastly the two overlain but with the topo survey made moderately transparent.

Topographic survey of the Park. Green is low, white is high.

Topographic survey of the Park. Green is low, white is high.

The western half of the mag survey.

The western half of the mag survey.

The topographic survey overlain on the magnetic survey.

The topographic survey overlain on the magnetic survey.

As you can clearly see, some of the curving features are clearly the result of the topography.  Most probably they are the result of the magnetically enhanced topsoil accumulating at the bottom of the slopes.

What about the funny “depression”?  This could be clearly seen on the surface.  I have boosted the colour and contrast in the next image to make it clearer on the photograph.

The depression in the park. The colours and contrast have been boosted.

The depression in the park. The colours and contrast have been boosted.

The depression can be clearly seen in the topo data, and on the ground.  It is near the approximate site of the well as far as we can tell from the 19th century records.  This could be the well, but why does the survey show such a busy strong pattern?  Towards the end of the first day some of the workmen stopped to chat.  One told us they had dumped two foot or so of material from the old car park in the hole!  This could still be the well, but the results from the mag are more modern noise.

Having discounted slopes, pipes, rubble and goal posts, did we find anything?  There are a number of features which merit more attention.  I have marked some of them in the next image.

Possible archaeological features in the survey data.

Possible archaeological features in the survey data.

These are only a few of the possible features.  There are also a number of potential pits.  Sorting this out in detail will require much more time to go through the data carefully.  Of course, we cannot date these features and further interpretation will require a careful look at any historical or map data that might be available, or even the excavation of some carefully targeted test trenches.  The value of a survey such as this is that any test trenches can be placed precisely in order to examine the potential archaeology, rather than being placed randomly with the hope of hitting something interesting!

Many thanks to all that helped including Pauline Hey, Bernard Jones, Richard Gleave; Trudi Ball, Miranda McGarry and Jeff Langdown.

Muddy day at Watton

Last Friday, Jim West, Ellen and I spent a cold and muddy morning trying to complete the survey at Broomhall Farm, Watton-at-Stone.  This survey has a long history.  It started as a training exercise for UCL students in 2008 when we surveyed a small area of a piece of pasture using a variety of instruments.  We found two ditches, one of which proved to be Roman and one from the cusp of the Iron Age/Roman periods.  Two years later I had to teach another geophysics course and so we returned.  More interesting features were revealed and so the Welwyn Archaeological Society surveyed the majority of the pasture with the Bartington.  In addition to the original two ditches we found two sides of a possible enclosure and a horseshoe shaped feature in the middle.  Again, they all were very late Iron Age / early Roman.  (Many thanks to Isobel Thompson for looking at the ceramics for WAS.)  The finds included a Dressel 1b amphora handle.

Fast-forward to May 2013 and we had our training day with the Foerster cart system in the pasture, and the first “real site” we surveyed was in the arable field called Cartway to the east of the original pasture.  Unfortunately we ran out of time due to the crops and so we have now returned with the aim of completing the survey of this exciting site.

Unfortunately, the mud hindered the operation of the cart!

The muddy wheels.

The muddy wheels.

We did, however, manage to complete four partial squares before the weather and the mud finally drove us away,

The magnetometer surveys at Six Acres/Cartway.

The magnetometer surveys at Six Acres/Cartway.

The image of the results is a rather crude mash-up of the Bartington and Foerster surveys as I have to convert our floating grid coordinates into real-world coordinates before I can plot them accurately,  The survey to the west inside the field boundaries is the Bartington, that to the east the Foerster,  In the Foerster data it appears that we have picked up a series of ditches which are probably field and enclosure ditches surrounding a small Roman period farmstead.  It is probable that the western enclosure had gone out of use in the early Roman period but the Roman farmstead continued to be occupied.

I will be adding some more pages to this blog with descriptions of some of the sites we are working on, and of the techniques we have been using.

We could do with some more help finishing these sites.  I realise that working in the mud and cold is not as attractive as in the sunshine in Verulamium Park!