Tag Archives: Roman town

The beast lives on

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

Yesterday saw Kris, Pauline and Jim wrestling with the beast once more. It really is something one only does once in a while simply because one would die of boredom to spend too much time on it. The tick-tick-tick-tick-tick-tick-cherble cannot go fast enough. We are going to do one more day so that we have covered the whole of the house on the hill. Here are the results so far:

Fig. 1: the multiplexed Earth Resistance survey, day 2.

As we saw yesterday, the downslope wall seems to have much deeper foundations than the upslope ones.  I’ll do a more detailed write-up when I have the last day’s worth of data and I can compare the images to the GPR slices.

The GPR team of Mike, Nigel, Adrian and Dave completed yet another 80x40m block. This time they headed south.  The block crosses the ‘1955 ditch’, the late first century boundary of the town. Figure 2 shows a composite of time slices.

Fig. 2: Composite of GPR slices (top row, then second row etc.).

As can be seen, there is a rectangular building in the top-middle of the plot. This seems to be a simple building with four downstairs rooms, the thin one of which is probably a staircase to an upper floor.  It lies parallel to the ‘1955 ditch’ which shows as a lighter band running across some of the slices from about 0m 30 to 40m 0.

The new software allows me to undertake an “overlay” analysis which produces a composite image from different time-slices.  This should help see the whole of a structure when different parts of it are at different depths.

Fig. 3: Overlay analysis of the GPR results from day 8.

The overlay shown in Fig. 3 certainly shows part of the 1955 ditch more clearly.

Fig. 4: the mag data in this area. The yellow box represents the area surveyed on day 8 shown in Fig. 3.

One curious thing about the 1955 ditch is how varied its response in the magnetic data is.  Here, the ditch shows as a strong black and white feature running across Fig. 4 from the NW corner.  There is, however, quite a sudden change just at the edge of the GPR grid shown in yellow. Clearly, the ditch must have had quite a complex history and that the story of “dug c.AD 80, out of use c.AD 125″ is probably too simple.  In the southern part of the town, we found a later building constructed over the line of the ditch, but here the ditch seems to have remained clear of buildings, at least for some of its length.  There are some quite large circular black blobs in the magnetic data (“circular magnetic anomalies” in the jargon) which are probably large pits.  A few of those show in the GPR data when one compares the two data sets carefully, but you wouldn’t notice them otherwise.  Although some things show in both data sets, such as the building we found in the latest GPR results, some only show in one or other.  It is definitely worth doing both.

On Wednesday we will be back at Gorhambury, one team using “the beast” and one using the GPR.  We’ll be back to “normal” Earth Resistance survey on Thursday.

Many thanks to everyone who helped.

An amazing effort

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

A quick post on yesterday’s results. The GPR crew consisting of Mike, Nigel and Dave managed an amazing 100x40m of GPR, 20m finishing off Thursday’s two grids and two more. Good job! The Earth Resistance team of Jim, Julia and Kris finished a series of fiddly partials and a couple of whole blocks. A quick look at the results.

Fig. 1: slice 4 of the GPR results.

Figure 1 shows the main 80x40m block.  A series of rooms can be seen running NW-SE across the SW corner.  The walls have been robbed and are showing a light lines, but the floors survive and are the dark block blobs. On the original image, there may be signs of a hypocaust showing.

Fig. 2: slice 5 of the GPR results.

The next slice down shows the courtyard building to the north of the previous one in better detail, and a long two-roomed building showing clearly lying along the line of the dry valley.

Fig. 3: slice 6 of the GPR results.

In Figure 3 we are already getting below the level of the buildings, but a few more walls are showing-up.  Today we will be looking at the block to the north, and so should pick up more of the courtyard building.

Fig. 4: the Earth Resistance survey.

The Earth Resistance survey shows another building quite clearly near the fence.  One of the dark black blobs is the result of the oak tree sucking water out of the ground.  The other one is the line of the Roman road.

Fig. 5: the Earth Resistance survey, high-pass filtered.

The high-pass filtered version (Fig. 5) shows the walls more clearly and gets rid of the some of the impact of the tree and the road.  Some more details can be seen, such as a double line along the edge of the road.  Drains, maybe?

Fig. 6: the mag data in the same area as the Earth Resistance survey.

Fig. 6 shows the mag data in the same area as Figs 4–5.  Some of the building lines can be seen, nut not as clearly as in the Earth Resistance data.

Fig. 7: total area covered by the Earth Resistance survey.

The last image just shows the area covered by the Resistance survey so far.

Many thanks to everyone who helped.

Today we are going to try the six-probe multiple depth resistance survey, otherwise known as “the beast”!

A postdiluvian survey

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

Yesterday’s survey was called off due to rain. Not the usual English wet weekend in September sort of rain, but real rain. We had over 20mm at home, and I guess St Albans was much the same.  Many thanks to Anne, Julia, Pauline, Mike, Grahame, Joe and Dave for braving the damp conditions. I think it says a great deal as to how dry the preceding months have been that it still didn’t seem particularly muddy underfoot.  We managed a good six 20×20 grids with the Earth Resistance meter, including one awkward one with a big oak tree in the middle, and a 40x60m block of GPR.

Fig. 1: the Earth Resistance survey after three days.

As can be seen in Fig. 1, we have picked-up a nice courtyard building to the east near the road.  This isn’t listed in Niblett and Thompson or is it on the UAD, so this is a nice addition to the corpus.  We can see it in the mag data as well.

Fig. 2: the magnetometry data for the same area as Fig. 1.

It is a little clearer in the resistance data. It shows in the GPR too, much some features more clearly and some less.  A combination of techniques should give us a nice result.  We have covered quite large area with the resistance meter now (Fig. 3).  The difference between last year’s survey and this is simply in the processing.  I have applied a high-pass filter to this year’s data.  Once we have joined the two areas up, I can apply the same processing to the whole survey.

Fig. 3: the total area surveyed with Earth Resistance.

The GPR undertook the two grids to the west of the “House on the Hill”.  Here are four time slices, each just over 3ns thick.

Fig. 4: time slices of the area surveyed today.

As can be seen, we have multiple bits of buildings, but nothing (yet) that forms a coherent plan.  The curious empty SW-NE band persists.  This is in a small dry valley running downhill to the Ver.  Was it empty in the Roman period?  Has the archaeology eroded away, or has the archaeology been buried by alluvium?  Impossible to say at the moment.

The next three figures give time slices 5 and 6 with the other GPR data from this field (all just one slice).  The greyer (is that a real word?) images to the SE are this year’s survey, the brighter images to the NE are surveys from 2015 and 2016.  The detached block is today’s survey.  (We will complete the block to join it to the House on the Hill tomorrow.)

Fig. 5: time slice 5 with previous survey results.

Fig. 6: time slice 6 with previous survey results.

Fig 7: the magnetometry data underlying the GPR surveys.

Tomorrow we will be back, extending both the Earth Resistance and GPR surveys yet further.

Working in the rain

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

Today’s weather was “variable”, to put it mildly.  I ended-up with slightly sun burnt legs, but at lunch time we were shivering under a tree as the hailstones hammered down.  Nevertheless we got quite a bit done, and so many thanks to Dave, Anne, Ellen, Pauline, Mike, Jim, Nigel and Graham for persisting despite the dark clouds and the occasional flash of lightning and thunderclap (thankfully a long way off!).

Some surveys we get results where a great deal of squinting is required and some optimistic mental joining of dots is needed.  Today wasn’t one of those days!  Let us start with the Earth Resistance survey.  We decided it would be interesting to run the meter over the nice house on the hill for which we have mag and two GPR data sets.  The first image shows the magnetometry data from two years ago, the second today’s resistance data.

Fig. 1: the magnetometry data of the area with the “house on the hill”.

Fig. 2: the Earth Resistance data of the “house on the hill”.

The Earth Resistance data has been high-pass filtered to bring out the building nicely, but I think you’ll agree it is pretty clear.  It is interesting that the rooms on the western end of the house are showing much more clearly than they did in the GPR survey that we did two days ago (see this post).

Having wiped the GPR’s memory, we completed the grid square from yesterday and then managed another grid and three-quarters further south. Tomorrow we have to do the jaggedy bits along the edge of the field.  First of all, lets look at the mag data.

Fig. 3: the magnetometry data of the area covered by the GPR survey.

The strong linear feature running diagonally across the lower part of the image is the “1955” ditch, the 1st century boundary of the town which had a section cut across it by Sheppard Frere in 1955. The “house on the hill” can be seen in the middle towards the top.

Now for the GPR.

Fig. 4: the GPR survey at the end of day 3.

The grid below the house on the hill is mighty dull.  There is the faint building we saw yesterday, and one or two other possible features, but nothing too exciting.  The grid towards the bottom though shows as lovely town house facing SE.  The next image zooms in a little.

Fig. 5: the town house in the south of today’s GPR survey.

It looks like it lies on the road which runs along the inside of the 1955 ditch. Sweet!

The new software has a variety of palettes.  You have seen the BW one, and the default colour scheme No. 7.  The software can create a composite of all the palettes.

Fig. 6: a plethora of palettes.

Which do you prefer?  Answers on a postcard, please…

The weather forecast is better for tomorrow.  Fingers crossed!

9999 is the number…

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

….not of the beast, but of the number of transects we have surveyed with the GPR.

Fig. 1: Nigel completing transect 9999.

Fig. 2: line 9999 completed.

The completion of line 9999 is of interest because (9999 x 40) / 1000 = 399.96km.  That’s a lot of GPR data…!  Secondly, we were wondering what happens after line 9999.  Does the machine reset to line 0?  Does it go on to line 10,000? Or, millennium bug-like, does the machine die horribly?  Well, on the next line the machine went on to line 10,000.  Yay!  OK!

But…  yes, there had to be a but…  Sadly, the next line was also 10,000, as was the next one, and the next one!  We needed to reset the clock back to zero, but the only way to do that is to download the data, then delete all the files. Unfortunately, I hadn’t thought to bring a USB stick, so we had to head home early.  Well, Friday was always known as POETS day, wasn’t it?

The one good piece of news is that I managed to get the grid in the right place.  The next two images are the underlying mag data, and then yesterday and today’s GPR timeslices.

Fig. 3: the underlying magnetometry data.

Fig. 4: The GPR data.

As you can see, the lower block of GPR data, which we surveyed today, seems remarkable unexciting.  The slices largely reflected cultivation patterns.  I was playing with the new software and tried other colour palettes.  How about traditional black and white?

Fig 5: the less colourful version of the GPR data.

It is faint, but at the northern edge near the southernmost corner of the townhouse we surveyed yesterday, is a faint rectangular building.  In the colour image it is impossible to see it, but in the BW it is faint, but clear. Lesson to self, the “boring” BW palette actually works quite well!

Thanks to Mike and Nigel for coming today.  Tomorrow we soldier-on with the GPR’s memory freshly-wiped.



Gorhambury once more

Today saw a small but dedicated team of Kris, Graham and Mike brave the poor weather to survey at Gorhambury. Luckily the weather improved during the day and was pleasant if windy in the afternoon.

I am going to have to start the 2017 season with two apologies.  The first apology is to Graham who agreed to come and collect some equipment from my house.  Unfortunately, road works and unhelpful workmen led Graham on a mystery tour of Hertfordshire.  The second apology is that I am utterly incompetent.  With one thing and another we only managed a single grid today, but unfortunately it was a grid we had surveyed in 2015.  Aaaaarrrrghhhh.  I’ll be grovelling for a while I fear.  It does, however, give us a chance to compare the old survey with the today’s, processed with the new software.

Fig. 1: the old survey data from 2015.

Fig. 4: today’s data. 17.55-21.46ns (c. 79 to 97cm below surface).

Fig. 4: today’s data. 14.76-18.66ns (c. 66 to 84cm below surface).

Fig. 3: today’s data. 11.96-15.87ns (c. 54 to 71cm below surface).

Fig. 5: today’s data. 9.17-13.07ns (c. 41 to 59cm below surface).

There are some interesting comparisons.  For example, in Fig. 4 we can clearly see a square block in the middle of the room towards the east.  Not sure what that is… but it is very clear.  On the other hand, the rooms to the west show better on the old plot than they do on the new one.  At the moment I do not know if that is a function of the the data being collected on a different day (it was pretty wet on the surface today which may have limited depth penetration), or a factor of my use of the new software!  I’ll have to investigate some more.

Tomorrow we will start something new.  I promise…!

Pipe dreams

Yes, I know, blog posts are like busses…

Some of you may have seen a recent BBC News item about the holes for the gas pipe in Verulamium Park.

Fig. 1: BBC News item about Verulamium Park.

I must admit to being a little peeved that a few small roughly dug holes to look at a gas pipe got more media attention than CAGG’s extensive surveys in Verulamium!  The gas pipe has always been, however, the bane of our lives as it’s magnetic field is so strong it masks a 30m wide strip of the archaeology in our magnetometer survey.  That field, however, has no impact on the GPR and I have been intending to extend the GPR survey to cover the areas affected by the pipe. This, and the forthcoming meeting of the Roman Finds Group in Verulamium this Friday, prompted me to see if anyone could come out at short notice and survey around one of the pipe holes.

Fig. 2: Surveying the pipeline (photo: Mike Smith).

So, Mike Smith and I spent a day surveying around the site of the hole in which the traces of a Roman building were discovered.  It got a little fiddly at times, especially as the pipe had to be dragged along 11m every 30 minutes as they joined the lengths together.  The location of our new survey is given in Fig. 3.

Fig. 3: the location of the latest GPR survey shown in red.

As usual, we surveyed in 40m lengths (where possible) at 0.5m transect spacing.  The data were sliced using GPR Process and the images produced using Surfer v. 8, and imported into Google Earth via Adobe Photoshop.  The last step is needed to make the missing bits transparent. Unusually, the deeper slices (nos. 8 and 9) still showed useful information.

One annoying thing is that Google Earth have recently updated the satellite image for Verulamium.  It is a bit dark to my eyes, but more seriously it is in the wrong place.  In the time slices I could see the relationship between the path and the image was clearly wrong.  I spent some time checking my notes, but when I checked the historical imagery, I found the 2009 images matched the plots much better.  I have, therefore, used the older imagery for the figures below.

Fig 4: the magnetometer survey in the area of the new GPR survey.

In Fig. 4 we can see the magnetometer survey for the area in question.  The pipe line, running under the path and then diagonally across the football pitch, dominates the plot.  We can however, see the road running clearly SW–NE across the map (Niblett and Thompson’s Streets 16 and 17 crossing Street 12), and bit of buildings showing as the white lines against the grey and black.  As well as the gas main, there are three pipelines running from the cafe in the NE corner.

Fig. 5: third time slice, 10.5 to 13.5ns.

In Fig. 5, the topmost time slice in the sequence, we can already see the road surface running SW–NE, and we can also see signs of the pipe running across the plot although this might be a marker in the upper levels as the pipe itself shows more clearly lower down.

Fig. 6: fourth time slice, 13.5 to 16.5ns.

The next slice (Fig. 6) shows the road more clearly, and some hints of buildings.  I am unsure what the area of high reflections is to the south-east and parallel to the road is.  It is almost as if the road line was changed a little at some point.

Fig. 7: fifth time slice, 16.5 to 19.5ns.

Fig. 7 is starting to show the buildings up near the path much more clearly.  These are buildings that front onto Street 12 to the north.  Street 12 eventually leads into the forum and is probably part of Watling Street.  At the southern edge of the survey there are some white areas which may be robbed walls, with perhaps a surviving floor surface.

Fig. 8: sixth time slice, 19.5 to 22.5ns.

In Fig 8. we can now see two very clear lines either side of the road.  These maybe stone conduits or roadside drains?  The buildings near the path are starting to show more clearly, and now we can see the gas pipe quite clearer.  The fact that is disappears to the west is probably due to topography.  Ideally, time slices would be topographically corrected.

Fig. 9: seventh time slice, 22.5 to 25.5ns.

Fig. 9 shows the gas main beautifully, and more of the buildings near the path.  Those to the east of Street 16 and south of Street 12 are labelled “baths” by Niblett and Thompson.  The GPR data would make that interpretation a little more tentative, I think.

Fig. 10: eighth time slice, 25.5 to 28.5ns.

Fig. 11: ninth time slice, 28.5 to 31.5ns.

Figs 10 and 11 show some of the deeper features.  Some hints of more buildings are showing, especially in the NW corner of the plot.

Personally, I think this is much more interesting and informative than the gas pipe trench!

Fig. 12: the gas pipe.

The one thing that will be useful, is that by knowing how deep the gas pipe is below the surface, we can calibrate the speed of the GPR signal and convert nanoseconds into depth.

I am hoping to continue to expand the GPR survey when I can over the coming months.

Many thanks to Mike for coming out for the day in the cold sunshine.