Tag Archives: community archaeology

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…!

The ditch continues

Today we ran both the magnetometer and the GPR.  We have some problems ‘compensating’ the mag which may have been due to the geomagnetic storm which hit Earth at 9am BST but we still managed to survey a good number of grids.  We have one case of “frozen sensors” (see the lines near the top of today’s plot) but we can just redo a few lines and merge the data.

The magnetometer survey after day five.

The magnetometer survey after day five.

Two things stand out from today’s survey.  Firstly, the long sinuous ditch we first found on Wednesday continues, but with a curious dog-leg around a strongly magnetic feature.  Working out to which period this ditch belongs is going to be a challenge, but it doesn’t really seem to fit the town?  Answers on a postcard, please…

The second feature is the very strongly magnetic building on the westernmost edge of our survey.  It is aligned on the street grid, but is very magnetic.  Is it a building which burnt down?  Or perhaps it is something like a bath house?  Hard to say right now, but it is clearly a candidate for further investigation with GPR and/or resistance survey.

Talking of the other techniques, we managed a further 80x40m block of GPR data.  I’m going to look at that shortly.  We have also attempted further repairs to the cable of the resistance meter.  Fingers crossed we get good data with it next time.

The plan now is to work southwards until we hit the edge of the town.  The mystery of the sinuous ditch will have to wait a few days!

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!

Rain stopped play

Today we headed out to Verulamium for our second day of survey.  We got off to a good start with eleven grids laid out and ready to go.  Before lunch, all went well and we re-did grid 2 from last week and did grids 6 and 7.  We stopped for lunch but by the time we started again it was drizzling.  Then it poured.  Yuck. Although the machine is shower-proof, the rain seemed just a bit too heavy to carry on so we abandoned play for today. What a shame.

The results looked good, however, and are posted below.  Interpretation of the results will be easier when we have the GIS data of what is already known about the town.  We can then overlay our results on the known plan.

day two results

Results of the second day of survey

As always, remember these are a crude overlay in Google Earth.  We’ll get more accurate positioning when the data is included in our project GIS.