Tag Archives: archaeology

Just two more days

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

I did consider using antepenultimate again, but I thought you might think me pretentious…

Everyone worked extremely hard today.  Mike and Jim on the GPR finished yesterday’s block and managed another 40x40m grid square.  No easy task over the long grass and thistles.  Ruth, Dave and Julia completed five earth resistance squares, including two that had to be done in two parts and joined together in the software later due to an inconvenient hedge!  Good job everybody, and many thanks.

Here is the Earth Resistance survey, both normal, high-pass filtered, and the magnetometry data from the same area.

Fig. 1: the Earth Resistance data at the end of day 16.

Fig. 2: the Earth Resistance data at the end of day 16, high pass filtered.

Fig. 3: the magnetometry data for the area shown in Figs. 1 and 2.

I hate to say it, but our five squares, including the two annoying partials, appear to lie between the buildings along the road to the north of the hedge line, and to the north of the buildings we found yesterday. We do, however, have a nice tidy area surveyed now.  We couldn’t have left quite such a silly hole in our survey data.  Tomorrow we head north to survey along the northern edge of the block we did last year.

The first block of GPR data from today was a continuation of yesterday’s

Fig. 4: the GPR time slices from the block completed on day 16.

Nothing very exciting jumps out from the plots, although there are some things to check out.  Slice 6 (second from top on left) has a strange upside-down M shaped feature (in black) and slice 8 (bottom-left) has something semi- or sub-circular near the northern edge.

How about the second block?

Fig. 5: the GPR time slices from the second block completed on Day 16.

Not a great deal there either.  Sorry guys!

Tomorrow sees the GPR crew filling-in an odd gap between last year’s survey and this years.  The plus side is that the mag shows lots of buildings, so tomorrow’s results ought to be much more interesting!

Many thanks to everyone who worked so hard in the sun today.  Just two more days.

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A picture is worth a 1000 words

The Verulamium magnetometry survey.

The Verulamium magnetometry survey.

I suppose I cannot really get away with that simple a post.

First of all, CONGRATULATIONS to everyone, it is a fantastic achievement and I am so proud of all of you.  Secondly, a big thanks to everyone who turned out for an extra day on Bank Holiday Monday to complete the Macellum field.

How about some numbers?  Well, Verulamium is the third biggest Roman town in Britain, after London and Cirencester.  It is, however, the largest Roman town in Britain which doesn’t have a modern settlement built over most of it.  We have surveyed 64.5ha of the total area of 81ha.  It has taken us 83 working days starting in the summer of 2013, but we didn’t do much at Verulamium in 2014.  It took 12,900,400 readings to cover those 64.5 ha.   That, of course, doesn’t include the grids we did twice because of frozen sensors or other problems. People pushing the cart walked about 322km, not including having to go back to the start for partials, getting to the squares in the first place, or laying in the tapes and strings.

Let us look in more detail at the last bit surveyed in the Macellum Field.

The area surveyed during day 37) high contrast).

The area surveyed during day 37) high contrast).

Several things come to mind.  Firstly, there is very little there!  Towards the NE and along the western side there may be a ditch feature, although it is quite faint.  Other than that, the main (and annoying) thing are the strong magnetic anomalies along the edge of the field.  Some of you may remember the 12″ gas main which runs across the Park… well here it is again.  What I do not entirely understand why there are differences between the negative and positive readings along our grid lines.  Jim and I spent some time making sure I put the composite together correctly, and we are sure it isn’t a survey error.

This end of the field is know to contain two Romano-Celtic temples.  These are known from aerial photographs taken in the hot summer of 1976.  I wonder if this area of the town was kept clear of encroaching buildings, pits, ditches and the like deliberately?  If we turn the contrast down (i.e., clip the image at +/- 40nT instead of +/- 7.5nT, we can see one of the temples close to the hedge as a faint white line.

Low contrast version of the area surveyed on day 37.

Low contrast version of the area surveyed on day 37.

Yet another target for the GPR next year!

Some of the team (many thanks Ellen, Mike and Jim!) helped re-do a number of areas of the res survey, plus one extra bonus square.  The biblical deluge of Sunday night (Lamer Lane was flooded once more) was not ideal.  This is the final area completed in 2016:

The 2016 resistance survey.

The 2016 resistance survey.

It is a pretty good result.  There is almost no use of the “edge match” feature of the software to get the various grids to join neatly.  It could be improved.  The very high contrast of the temple rather makes the buildings faint, but either the creation of selective composites (i.e., processing bit of the survey separately), or use of a high pass filter, would improve that.  The survey is quite big for a res survey: 2.5739ha according to TS (or 2.6ha to sane people who round numbers), which equals about 103,000 resistance readings.  That, of course, doesn’t include the large numbers of squares we re-did due to the dry conditions.

There is a great deal more to do in terms of data processing and interpretation, but I think we all deserve a well-earned rest.  Well, at least until Thursday…!

Almost round the bend

Today saw a slightly smaller team than we have had, but we still managed a good area of magnetometry survey and GPR, and even one small square of Earth Resistance survey.

First, the mag survey.  The team are starting to work their way north along the western edge of our survey area filling in between what we have already surveyed and the third century town wall which is hidden in the trees in the Google Earth image.

The magnetometry survey up to the end of day 24.

The magnetometry survey up to the end of day 24.

Looking at the area surveyed today in more detail, we can see the beginnings of the corner of the “1955 ditch”.

The area surveyed on day 24.

The area surveyed on day 24.

There is surprisingly little of anything much showing inside or outside the ditch in this corner.  The ditch is, however, slightly narrowing and bowing.  How curious!  Tomorrow should, fingers crossed, see us pick up the rest of the corner.

The GPR team, way down the hill near the drive, completed another 40x80m block.  Here are four 3ns thick time-slices.

Day 24 GPR, time slice 2 (9.5 to 12.5ns).

Day 24 GPR, time slice 2 (9.5 to 12.5ns).

Day 24 GPR, time slice 3 (12.5 to 15.5ns).

Day 24 GPR, time slice 3 (12.5 to 15.5ns).

Day 24 GPR, time slice 4 (15.5 to 18.5ns).

Day 24 GPR, time slice 4 (15.5 to 18.5ns).

Day 24 GPR, time slice 5 (18.5 to 21.5ns).

Day 24 GPR, time slice 5 (18.5 to 21.5ns).

The top image (time slice 2) just shows the noise in the ploughsoil.  The second image, however, shows a lovely little building 16.5m by 10m in size, aligned with the “1955 ditch”.  I’m not certain what this building is, it seems an unusual plan for a domestic structure.  The third image (time slice 4) shows this building, but also a very strong reflection from a wall on the eastern side.  Presumably this is part of a building which has been robbed out more thoroughly.  In the last time slice the signal has “attenuated” and we are only getting the strongest reflections showing.

The overall image gives some idea of how much we have now covered.

The area covered by the GPR at the end of day 24.

The area covered by the GPR at the end of day 24.

This image is a bit of a mismatched mishmash as the data was collected at different times and the time slices are somewhat variable as I have learnt to process the data over the last year.  At some point all the data will need to be reworked systematically, but that is beyond me while we are out collecting yet more data every day!

Many thanks to everyone who helped, and welcome to the people who have recently joined the team.  Your efforts are producing spectacular results.

 

Back to Gorhambury

When I woke at about 6am I could hear the pitter-pat of rain outside the window.  I groaned. Today was to see the start of the second season of work at Gorhambury.  Rain!  Just what we didn’t need.  I shouldn’t have worried as the rain soon stopped, and by midday the sun was out and the weather was warm.

My other worry was that a week ago we had no equipment.  The mag was being repaired, the GPR was somewhere in Oxford and the Institute of Archaeology’s nice new RM85 Resistance meter had yet to be delivered.  On Sunday we tested the mag… all seemed well.  Yesterday Ellen drove to Oxford to collect the GPR (she’s a star!), and UPS delivered the res kit.  Yay.  All ready to go.

We had a large team today consisting of a mixture of regular members and some new faces, mainly from the St Albans and Hertfordshire Architectural and Archaeological Society.  It was excellent to have such a large team allowing us to run all three instruments.  Many thanks to everyone who helped.

The new resistance meter has the ability to take two readings side-by-side, and does so very quickly.  I was very impressed with the first day we used it.  We managed five 20m x 20m squares at four readings per meter, even with a delay as we set it up.

The new resistance meter in action with St Albans Abbey in the background.

The new resistance meter in action with St Albans Abbey in the background.

We also completed eight 40m x 40m mag grids and two GPR grids, one 40 x 40m and one with a ragged edge.  The next image shows the mag survey of the entire town so far.

The town as surveyed so far.

The town as surveyed so far.

I’m starting to believe we might actually get the whole town, well the area available within the town, surveyed!  Zooming into the area we did today, we can see that we have picked up some more of the “1955 ditch”, the first-century boundary of the town first excavated by Frere in 1955.  It is the wide, straight dark linear feature which runs diagonally from the SW to the NE in the image below.

The area surveyed on day 19.

The area surveyed on day 19.

The annoying sensor-freeze struck again in the penultimate square.  Luckily it was only one line (the funny stripes are a result of the data processing) and we can re-do just that tiny bit and merge the data into the square in the software.  We mainly found ditches and pits (the dark blobs and lines), and are just getting to the busy area near the road.  There is a faint suggestion of a road behind the 1955 ditch, as we found in the southern area.

The resistance survey was a great success!  We did an area to the WSW of the theatre.

Location of the resistance survey.

Location of the resistance survey.

As I hoped, it picked up the road very clearly, but also picked up some of the details of the buildings.  The next two images show the resistance data and the mag data for comparison.

The resistance survey.

The resistance survey.

The mag survey for the same area as the previous image.

The mag survey for the same area as the previous image.

The GPR survey is just to the south of the res survey.  I have only processed the north block as it is too late to process the jagged-edged one too.  A number of walls and buildings are evident in the third and fourth time-slices.

Day 19 GPR survey, north block, slice 3 (14 to 18 ns).

Day 19 GPR survey, north block, slice 3 (14 to 18 ns).

Day 19 GPR survey, north block, slice 4 (18 to 22 ns).

Day 19 GPR survey, north block, slice 4 (18 to 22 ns).

The comparison with the resistance survey is also very interesting.  The GPR time-slices tend to show the walls better, but the resistance survey tends to show the road better.

Resistance survey overlain on the GPR and mag surveys.

Resistance survey overlain on the GPR and mag surveys.

GPR data from the same grids as the res survey in the previous image.

GPR data from the same grids as the res survey in the previous image.

Short and sweet posting this evening as we have another busy day tomorrow, but at least this gives you a feel for what we are now finding.

Bank Holiday Monday

The August Bank Holiday lived up to its reputation.  It rained, pretty much all day.  According to our rain gauge we had another centimeter and the roads were all flooded once more.  So, folks, that it is for Gorhambury for the next 11 months.  Plenty to do, however, with data processing, analysis and interpretation.

Today Ellen and I went back to site to collect the last remaining pegs.  Ellen organised us into walking back and forth across the field systematically to see if we could find my missing survey book.  On our second pass…

A very damp notebook!

A very damp notebook!

It was very soggy, but thanks to being a “rite-in-the-rain” product it looks OK and is now drying in the airing cupboard.  Yay!  Inevitably, as we were walking around picking up pegs and finding the book, it started to rain and we were soaked by the time we left.

On Sunday, I had some fun making timelapse videos.  Here is a condensed version of surveying three squares.

I did a poor job of setting it up for the GPR, but here it is anyway:

I have quickly processed the GPR data from using the large 160mhz antenna. We surveyed a block 20x40m which went over the “sinuous ditch” in the hopes we might be able to see it more clearly. We have picked up the walls that we saw with the 450mhz antenna.  In the radargrams (the vertical slices which is how the data is collected) I think I can see the ditch… but I need to check with someone more experienced at reading them.  Here are the time slices.  Note that we used a 1m spacing between lines which leads to a cruder picture.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

I thought it would be useful to show one radargram from the day 15/16 block.  This is line 1893 which was surveyed from north to south, i.e., from the top of a block (as I usually present them) to the bottom.  Here is a screen grab from “radexplorer” showing unprocessed data.

An unprocessed radargram of line 1893.

An unprocessed radargram of line 1893.

It is very difficult to see much.  This is because most of the signal is the loud “noise” near the surface.  The first thing to do is tell the software where ground level is, i.e., the start of the first ‘loud’ reflection shown as black band at the top of the radargram.  Then we need to apply ‘gain’.  This is simply amplifying the lower reflections which are much weaker than the ones near the surface.

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

We can see much more now, but there is still a great deal of banding.  The bands are, essentially, echos and can be removed with the “background removal” function.

The processed radargram of line 1893.

The processed radargram of line 1893.

There is a great deal going on in this line of data.  Note, however, how the vertical column of echos under where I have put the label “wall” starts down into the radargram where as the column of echos where I have put “ring” starts at the top.  The “ring” is a small incipient mushroom ring which I noticed as we pushed the GPR over it.

A small incipient mushroom ring.

A small incipient mushroom ring.

If we look at the upper time slices we can see the ring:

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

For the wall, however, we need to look at a deeper slice.

Slice 3, 14-18ns.

Slice 3, 14-18ns.

In this slice we can see a small rectangular building showing faintly in the data. This building explains some of the things we can see in the radargram.  Remember that north is to the left of this radargram.

The arc we can see at the bottom of the top slice starting at about 27m west and going to 34m west is the northern edge of a very big mushroom ring.

A large mushroom ring.

A large mushroom ring.

The interaction between the fungus and the grass is quite complex.  The rings are showing in the radargrams probably because they are retaining water.  This can make the grass grow lush, or in extreme circumstances kill the grass.

A mushroom ring killing the grass.

A mushroom ring killing the grass.

Some of the rings merge and become quite complex.

A complex mushroom ring.

A complex mushroom ring.

And some of them cause mad photographers to get to their knees.

yummy circle

Yet another mushroom ring.

Yes, I became a bit obsessed.

Although we have finished for this season, there are lots of other things on the horizon, as well as working through all this great data.

I know I have said it before, but it is worth saying again.  Many thanks to everyone who contributed.  We collected great data, and we have found some really intriguing things.

And so it begins

Today saw a fair sized group for the start of the survey of the Gorhambury side of Verulamium.  Having completed the Park side of the Roman town in January 2014 we were keen to do the other half.  Our thanks are due to the Earl of Verulam for not only allowing us to work on his land, but for acting as intermediary on our behalf between the various ‘stakeholders.’

We are going to be running the magnetometer mainly, and will aim to do as much of the town as we can.  The first day was a little bitty and the rain didn’t help, but we have already got some nice results.

Day 1: the area south of the theatre.

Day 1: the area south of the theatre.

The temple which lies to the SW of the theatre shows in the mag data to the west of our blocks, but only faintly,  The large black ‘blob’ is very interesting.  It is about 8m across and ranges from -7 to 24nT.  A big pit, maybe?  The dark linear feature clearly runs parallel to the walls of the temple, but again, what is it? It is about 10m long and is quite magnetic with a range of -11 to 36nT.  Near the middle of our plot are a series of white lines representing buildings with flint footings cut through more magnetic material, either burnt or very organic.  These are probably the footings of buildings already known, although seem here in more detail (M451–454 in Niblett and Thompson Albans Buried Towns). Not bad for the first day!

As well as the magnetometer, we have the use of SEAHA‘s Mala Ground Penetrating Radar and will be running that alongside the mag.  It is much slower as it is only collecting one line of data for each pass, not four, but as we have seen before, combining the mag and GPR data can be very useful.  We collected 128 lines of GPR data today, but I haven’t had time to process them yet.  Watch this space.  Finally, we are also collecting some resistance data but that is even slower than the mag.  It is good, however, to be able to combine multiple data sets.

Watch this space over the next three weeks to see how far we get!

What a difference a day makes

Our second day was much nicer than our first. We even ended up taking our coats off!

Grahame Spurway, Stuart Henderson, Dave Minty and Jim West push the cart at Great Buttway.

Grahame Spurway, Stuart Henderson, Dave Minty and Jim West push the cart at Great Buttway.

We had another very successful day completing 11 grids of which five were partials and, more importantly, completing the area I was hoping to cover.  We have now covered all the area where we knew there were archaeological remains as shown by Mark Noel’s previous survey.  Many thanks to Ivor, Nigel, Dave, Grahame, Jim, Ellen, Stuart and Peter for an excellent couple of day’s work.  Even bigger thanks go to Sam Sheppard for allowing us access to the land once more.

The Ashwell End / Great Buttway survey at the end of day 8.

The Ashwell End / Great Buttway survey at the end of day 8.

The final area we surveyed is very quiet indeed, lying within the curved feature, possibly a ditch, that comes across the field from the east and then turns north until it hits the road.  Outside of this feature there are stripes which are clearly cultivation marks either running east-west, or north-south.  I had been ignoring these assuming them to be modern, but now I am wondering if we have the remnants of ridge-and-furrow, especially the north-south marks to the west of the clear north-south ditch.  I decided to look at old field boundaries and had a look at the 1888 first edition map.  Unfortunately, I cannot post it here, but I did trace off the field boundaries using Google Earth, as well as adding in the modern footpath.

The Great Buttway survey showing field boundaries from the 1888 map in orange, and the modern footpath in blue.

The Great Buttway survey showing field boundaries from the 1888 map in orange, and the modern footpath in blue.

The precise location of the boundaries is not totally accurate due to the way the OS first series maps were made, but we can see that the east-west part of the curving ditch follows the boundary until it turns north.  There is no marked boundary where the ditch can be seen clearly on the plot.  I need to look-up the tithe map. The boundary which heads south from the dog-leg in the road matches the curves in the data.  We do, therefore, seem to have some remnant of the earlier cultivation pattern.  I’m surprised that modern ploughing has not obliterated the evidence for this.  The footpath originally followed the line of the long east-west field boundary and has clearly migrated south onto the straight line across the field which it now follows.

The process of coming up with a detailed interpretation of all the features in this field is going to be long as there are a great many of them.  Just as a taster, here is a detail of some of the features in the Roman part of the site.

Detail of the Roman site in the NW corner of Great Buttway.

Detail of the Roman site in the NW corner of Great Buttway.

We will take a break from this site for now.  It would be good to return once the silage has been cut.  It is unlikely we’ll get into the field to the NE — Prycem’s — this year as it is going to be planted with oil-seed rape once the wheat has been harvested.  Next year, however, we should have a good window of opportunity to extend the survey into that field in which Roman buildings are known.