Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.
Well, possibly not. The mag team’s first grid this season was a 1.5m x 40m grid. Why? Well, in the penultimate grid of the very last day last year we had one single frozen sensor for one line of data. For the last 11 months this has annoyed me every time I saw it. Finally, I have been able to fix that grid! Yay. Having completed that line, the team went on to complete another seven grids of data. Well done team (Figure 1)!
Figure 1: Jim West and the mag.
Figure 2 shows the whole of Prae Wood Field and the survey completed so far (but without the dodgy line!), and Figure 3 zooms into the area completed on Sunday.
Figure 2: the mag survey of Prae Wood Field.
Figure 3: detail of the mag survey of Prae Wood field.
The overall impression one gets from both Figures 2 and 3 is a whole lot of nothing. There are the occasional strong magnetic responses from iron objects, and on Sunday we picked-up two pipelines (shown in Figure 3 with yellow arrows). The larger area of noisy magnetic data near the southern edge of the survey might be an historic structure. In the new area, there is a very faint line (as indicated by the blue arrows) which might be an old fence line, or might be my imagination. There are some “monopolar positive” features (i.e., ones which are mainly positive but with a slight negative response on the north side) which could well be pits.
What makes this all fascinating is that the Urban Archaeological Database (the UAD), suggests that the field is within a “rectilinear enclosure”, Monument Number M27. At the moment, I’m not sure where this idea comes from, but at the moment it seems as though it is an enclosure around not very much! This field, and part of Prae Wood itself, are within the area mapped by the Environment Agency using LiDAR (Figure 4).
Figure 4: LiDAR image of Verulamium. Data from the Environment Agency, image courtesy Mike Smith.
The Fosse, running through the woodland along the NE edge of the field shows nicely (Figure 4, right-hand red arrow). The little fragment of Prae Wood itself shows a mass of features in the woodland, some of which are parts of the Iron Age settlement (blue arrow). Our field shows the faint hint of the ploughed-in Fosse (left-hand and central red arrow), and a whole lot of not-much-else. How very curious!
Despite the very dry ground surface and the small team, we did manage a further three res squares on Sunday. Many thanks to Pauline for putting-up with my cursing as we did the work. Figure 5 shows the results. The edge between the earlier survey to the east and the current block of eight squares is due to my processing differing between the two seasons.
Figure 5: the Earth Resistance survey after day 4.
As can be seen, we have a line of buildings along the SW–NE road. This road, Street 23 in Niblett and Thompson’s Alban’s Buried Towns, shows very poorly on all three survey techniques. In the res survey, it almost looks like an eroded channel, and I have often been a bit confused as to the where the dry undulation (seems a bit grand to call it a valley) lies. It lies, however, behind these buildings and those that face onto Street 25 to the west. I have indicated the valley in question with a yellow arrow in Figure 4.
A further source of data is the work undertaken by the Oxford Archaeological Unit in January of 2000. They excavated 379 1.6m x 1m test pits using a mechanical excavator to strip the topsoil. As they were investigating plough damage, they did not excavate the features revealed. The distribution of test pits is shown in Figure 6.
Figure 6: test pits excavated in 2000 by the OAU.
If we zoom into the area we have surveyed using res over the weekend we can see which test pits are relevant (Figure 7).
Figure 7: test pits and the res survey (click on the image to see a larger version).
Test-pit 268 is described as showing a possible floor and a wall foundation of chalk. Judging by its position in the middle of a small building that seems appropriate. Test pit 258 is described as having “?floors” and “?Fill of wall trench”, whereas pit 257 which lies either on, or more probably just outside the wall of the long building, just has a “layer”. In all three test pits the topsoil was between 27 and 32cm deep. I have yet to see the plans of these trenches, but clearly combining the evidence of the trenches with the geophysics data is going to be very informative.
The GPR team jumped a few grids to complete a block next to the one that they completed on the last day last season. We intend to swap to 1m transects soon, and I wanted to catch the details of the buildings that clearly intruded into this block. Figure 8 shows the location (the bit sticking out to the west).
Figure 8: the GPR survey showing the location of the block surveyed on Sunday.
As can been seen in Figure 8, we have managed to complete the building which lies over the grid edges, but there doesn’t appear to be much more. Lets look at the first 12 slices (Figure 9).
Figure 9: the first 12 time slices from Sunday’s survey.
Slices 1 and 2 are basically showing the top surface and the topsoil. In slice 3 we can start to see the building and a long, wide, linear feature. These show in slices 4 and 5 too. By slice 6, we are already in the natural and/or where the signal has started to attenuate. Slices 7 to 12 are basically a few deeper things and echoes / attenuated signal. The only thing of technical interest is the semicircle which shows on the eastern edge of slices 7 to 12, and also shows in slices 1 and 2. This is an “airwave” caused by the radar signal bouncing off the underside of the tree canopy.
GPR slice comes with a plethora of palettes for display time slices. Figures 10 and 11 show slices 4 and 5 in the first 12 palettes.
Figure 10: slice 4 shown in 12 different colour palettes.
Figure 11: slice 5 shown in 12 different colour palettes.
The building along the eastern edge shows well in palette four, and the big linear thing running across the plot shows well in slice 4, palette 11. So what is the big linear thing? Figure 12 has a clue…
Figure 12: the GPR data with the line of the 1955 ditch indicated.
Yes, you’ve guessed it (or at least read the caption), the “long linear thing” is our old friend, the 1955 ditch.
Well I think that is enough for now. We will be back on site again tomorrow, hopefully running all three machines if the long promised rain actually happens this evening. Many thanks to everyone who has contributed this week. It was a great beginning to the 2019 season!