Tag Archives: Hertfordshire

Being completist

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

The team are very patient with my need to be neat and tidy and do the silly little bits around the edges.  Today was perhaps an extreme example.  Due to yesterday’s little hiccup, I set-up the res kit and, with Graham’s help, surveyed about a sixth of a grid square, then packed it all up again.  Now I can sleep easy.  On a more ambitious note, I have finally put all the res grid squares into one large composite.  The survey now consists of nine hectares, which is about 360,000 individual readings.  Most Earth Resistance surveys use what is known as a twin-probe configuration.  That means that there are two mobile probes on the frame, and two stationary probes on the end of a long cable, normally about half a meter apart.  One mobile probe, and one remote probe set-up an electric circuit part of which is the soil.  The other mobile probe and the other remote probe measure the resistance.  The problem with this “standard” set-up is that when you have to move the remote probes, the readings for the same spot will change.  This leads to endless struggles to “grid-match” each set of squares.  Since 2016 I have gone over to using a pole-pole configuration.  This is basically the same except the remote probes are a long way away (I aim for about 30m or more) and a long way apart from each other (I aim for more than 20m). This helps enormously with the grid matching.  Grids completed on different days of the survey will match quite nicely, usually.  Where this is not true is when (a) there is a lot of rain in the middle of a survey and (b) when the survey is split over multiple years.  In the case of our 9ha block, this has been completed over four seasons.  Unsurprisingly, one can see the edges.  TerraSurveyor has a function called “periphery match” which will, sometimes, do an excellent job of grid matching.  In this case, it was pretty good.  Figure 1 shows the survey with a periphery match applied.

Figure 1: the res survey as of the end of Thursday.

If you click on the image and see it full size you can see the detail of many buildings.  Unfortunately, the range of values makes seeing some buildings quite hard.  A high-pass filter is a background trend removing tool that makes some buildings show more clearly (Figure 2).

Figure 2: the res survey, high-pass filtered.

There are still many features, many buildings, that one can only see when looking at smaller blocks.  With such a big area getting everything to look clear is going to be impossible, I fear.

From tomorrow the res will be working in Church Meadow where we hope it will map the remains of St Mary du Pré.

The GPR team is getting really close to finishing the theatre field (Figure 3).

Figure 3: the GPR survey so far.

The team have some fiddly bits around the edges to complete, one missed block, and one block we want to re-survey at 50cm intervals as there is a building and road in it.  Fingers crossed, two more days should do it.  Then, for a bit of last day fun, the GPR will also have a look at St Mary du Pré.

The mag team completed another eight blocks today.  Since moving to Church Meadow, they have managed to survey four hectares in three days which is very impressive, especially given that one block had to be repeated due to a sensor freeze.  Having lots of whole grids and no partials makes such a difference.  The team are now just 2.5 ha away from completing a square kilometer of mag at Verulamium.  Figure 4 shows the mag results in Church Meadow.

Figure 4: Church Meadow mag survey after day 3.

Three things are of note.  Firstly, earlier today I wondered which of the two raised areas in the field was Watling Street.  Looking at the survey results, it looks like the road splits in two near the edge of our survey.  Perhaps it is two phases?  I’m not convinced.  Secondly, we have some marked linear features showing that almost look like enclosures.  These are, however, low magnetism suggesting and might be yet more pipes, but not of metal this time?  Again, I’m not convinced.  They might well be archaeological features.  I will have to survey all the pipes I can see in the field.  Lastly, as we get closer to the town to the south, there are many more little black blobs.  Seasoned readers of this blog will know that usually little black blobs in mag data are often pits.  In this case, I wonder if we are starting to pick up the edge of the cemetery which is likely to have lined Watling Street?  In the Roman world, the richer you were, the closer you wanted to be buried to the road and the town.

Tomorrow is the antepenultimate day of the 2019 Gorhambury survey season.  The weather looks good so fingers crossed all goes well.

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Mea culpa

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

Firstly, an apology to those to whom I said “no, we’ve done that” this afternoon when we hadn’t.  Oops.

Just a quick posting tonight as it is late.  The mag team completed an excellent nine grids in Church Meadow.  They are half-way along the field already (Figure 1).

Figure 1: the mag results in Church Meadow.

Sadly, the most obvious things are the two pipelines, and the dark stripes across one grid square.  The latter is our old friend the frozen sensor.  Way back during the survey in the Park we once managed twelve grids in a day only to have to re-do a bunch of them thanks to the sensor.  Of the archaeology, we cam most clearly see the edge of Watling Street heading SSE towards the Chester Gate.  The ditch we picked-up on Sunday appears to be the SW edge of an enclosure, possibly part of the nunnery.

The GPR crew managed an excellent five grids today despite only having one battery.  Figure 2 shows the entire GPR survey in the theatre field to date.

Figure 2: the GPR survey to date.

I’d like to give a big thank you to Jimmy Adcock of Guideline Geo for going above and beyond in finding a second battery for us as one of ours has died.  As can be seen from the figure, we are within spitting distance of finishing this field, and it would be a shame if we missed that target due to a dodgy battery.

The res team made it to the fence line. We would have finished the new transect if it wasn’t for my incompetence (see the opening paragraph!).  Figure 3 shows the res survey in the theatre field.

Figure 3: the resistance survey to date.

The funny little notch in the top by the fence is my little oppsie. Figure 4 shows the northern area with a high-pass filter applied to the new data.

Figure 4: the new area (high pass filtered).

The most obvious feature in the new data is the aqueduct showing as a very low resistance feature in white cutting across the transect.  There are, however, other buildings showing very subtly in the data that are very hard to pull out in an overall plot.  They show best in the thumbnails as one is putting together the data in TerraSurveyor.  Apart from the little partial which I’ll sort out tomorrow (actually, later today!), the Earth Resistance meter will move into Church Meadow tomorrow to try and retrieve a plan of St Mary du Pre denied the mag by the pipelines.

Just four days left!

One hundred and eighty!

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

As much as I would enjoy getting three triple tops, in this case the 180 refers to the number of posts on this blog.  We have had almost 50,000 views and the text is now almost 100,000 words.  That’s more than a PhD thesis…  There are also over a thousand images in the media library.  Granted, some of those are of dogs and sunsets, but there’s nowt wrong with that!

Just a quick posting tonight as I have been struggling with some awkward data processing.  Although we lost half a day to rain, all three teams did an excellent job before the heavens opened.

The res team managed an excellent four grids today which took them over the cross roads.  The results are shown in Figure 1.

Figure 1: Today’s resistance results.

The buildings to the east of Street 11 show very nicely.  The north side of the cross-roads does not seem to show a building, and it is this spot which the mag data has a very strong feature that I have speculated previously might be a burnt down timber building.  The street running to the SE appears to have been robbed.

The mag team completed another strip of grids in Prae Wood field.  Figure 2 shows progress so far.

Figure 2: the mag survey of Prae Wood field.

An examination of the results from today does not (Figure 3), sadly, show any more enclosures.  What a shame.  Hopefully, weather and equipment willing, the field should be completed soon.

Figure 3: the mag survey at the western end of Prae Wood field.

The GPR team have been working away down the western side of the town completing sawtooth-edged grid after grid.  I can barely keep-up.  Correlating my lists of coordinates and the sketch plans of grids / line numbers takes a bit of time, but I am getting there.  A GPR report will be available soon…ish!

Many thanks to everyone who put up with the rain today.  Fingers crossed for nicer weather tomorrow.

Saving the best ’til last?

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

Unfortunately the weather forecast was accurate yesterday.  Far too much rain for us to go out and play.  Today, however, was very pleasant and at least the pegs went in very easily for a change! I’m afraid that a new toy (Figure 1) resulted in me getting a late start processing data this evening, and so I only have the mag and res to report on.  The GPR team, however, did a sterling job and completed a multitude to sawtooth-edged grids along the western side of the theatre field.  I promise I’ll process all that tomorrow!

Figure 1: Kris’ new multiformat pinhole camera being put through its paces.

The Earth Resistance meter was operated by John and Grahame. We have moved north of the hedge line and are close to the cross-roads around which both mag and GPR have shown multiple buildings.  Figure 2 shows the entire res survey, and Figure 3 a zoomed-in view of today’s area.

Figure 2: the Earth Resistance survey so far.

Figure 3: the area surveyed today and some context.

We have just clipped the edge of Street 11 (in the top-right corner of the new block) and have revealed some wonderfully clear images of the buildings to the south of that street.  These are clearer than the GPR survey of the same area, so it is really pleasing to see.  Excellent stuff!

Over in Prae Wood field, the mag team completed two more strips of grid squares.  Figure 4 shows the survey so far.

Figure 4: the Prae Wood Field survey, so far.

Two more days should, weather and equipment willing, see the team finish the field.  After Prae Wood we are heading north into Church Meadow, so-called because St Mary du Pré lies within that field.  Watling Street also runs through it so we should get some exciting results.

But, back to Prae Wood.  Let us look in more detail at the western end of the survey (Figure 5).

Figure 5: the western side of the mag survey of Prae Wood field.

The dark lines in the new area represent ditches, and it looks like we have some enclosures running across this western end of the field.  Given that the major Iron Age settlement lies in the wood just to the SW, these could be of that date, but they could be Roman or medieval as well.  Once more, we need to check out the historic map data.  Still, after many blank grid we now have features in both the western and eastern extensions of this field.  Saving the best for last?

Many thanks to everyone who turned out today.  Tomorrow looks like we might have to pack-up early due to yet more rain. This season is just flying by.

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.

 

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.