Tag Archives: Roman road

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.

Filling in the gaps

Thanks to Stuart Gray of Strutt and Parker, I was able to contact Pete and Fiona Letanka and obtain permission for us to survey the paddock of Darrowfield House which lies immediately behind Verulamium Museum. It is the last large-ish area of Verulamium left that has not been totally surveyed using magnetometry. Members of CAGG are, therefore, spending this week filling in the gap.  Our mag was away being repaired, but Pat Johnson of Foerster kindly agreed to bring it down to us today.  We started, therefore, with Earth Resistance and GPR, but after lunch the poor old res meter was left abandoned on the grass while the team got started with the mag survey.

The paddock ought to be exciting.  It lies just behind the line of the Roman wall, and there is a road shown in publications running SW-NE across it.  The first mag results are shown in Fig. 1.

Fig. 1: the mag survey.

They are surprisingly unexciting.  The diagonal lines running SW-NE look like cultivation marks of some sort.  There is a quiet area behind the supposed line of the wall, running NW-SE, and then one high-magnetic curved feature which may match a bank we can see in the field.  No real sign of the wall, or of the road,

The GPR survey has been processed in 3ns thick slices.  As usual, I’ll work my way down from the top.

Fig. 2: GPR survey, time slice 3, from 10.5ns to 13.5ns.

Fig. 3: GPR survey, time slice 4, from 13.5ns to 16.5ns.

The third slice shown in Fig. 3 really shows surprisingly little. I’m guessing we are only looking into the topsoil here.  The fourth slice (Fig. 3) starts to show some diagonal striping similar to what we can see in the mag data.  Again, this is probably agricultural.

Fig. 4: GPR survey, time slice 5, from 16.5ns to 19.5ns.

Fig. 5: GPR survey, time slice 6, from 19.5ns to 22.5ns.

The fifth slice (Fig. 4) shows the striping very strongly, with two alignments at right angles.  I think these must be land drainage.  They look too straight for “ridge and furrow” and do not have a headland. What is also interesting — or is that worrying? — is that the wheel-ruts which show so clearly on the Google Earth image in the background, also show in the GPR data at this depth.

The sixth slice (Fig 5) faintly shows echoes of the drainage.  To the north are some black squiggly features.  Yes, folks, we have mapped the badger setts!  Nothing very archaeological, so far.

Fig. 6: GPR survey, time slice 7, from 22.5ns to 25.5ns.

In the final slice (Fig. 6), we still have the setts in the north, but we have a very strong linear feature to the south running SW-NE.  Comparing this to the proposed line of the road in Fig. 7, we can see that this would appear to be one edge of it.

Fig. 7: GPR survey, time slice 7, from 22.5ns to 25.5ns, with overlain map of Verulamium.

Right at the edge of the GPR’s signal depth where the returns are very attenuated, we are starting to see the archaeology. Unfortunately, I cannot yet easily convert the time (22.5ns–25.5ns) into physical depth.  It would seem, however, that the archaeology in the lower area may well be at the limit of what we can pick up.  If we look at the radargrams, however, we can see a big feature running across our vertical slices and I would guess this is the road surface.

Fig. 8: an example radargram showing the possible road.

Many thanks to everyone who turned out today and enjoyed the beautiful sunshine.  Also, big thanks to Pat for bringing the mag back for us, and Pete and Fiona for arranging for us to be able to survey the Paddock.

There probably won’t be a posting tomorrow, but I hope on Weds to catch-up.