Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.
Last Sunday we undertook a small survey in the parish of Ivinghoe Aston, Buckinghamshire. Earlier in the year, following a find by a metal detectorist, the Buckinghamshire Archaeological Society’s Active Archaeology Group undertook a small excavation and recovered a Roman cremation burial. As is always the way, tucked into one corner was the edge of a second burial. This was left in situ and we were asked if any of the survey techniques might detect cremations. Of the techniques we use, magnetometry is the most likely to be able to find cremations but given how small they are, the standard 0.5m spacing between sensors might just be too big. We can configure our cart to have a 0.25cm spacing between sensors (Fig. 1) at the cost of having to walk twice as far. In this case, we were not aiming to cover a large area and so the trade-off seemed reasonable. Jim West (CVAHS) Pauline Hey (LBDAHS), Peter Alley (WAS) and myself (WAS), along with Jean Bluck (CVAHS and BASAAG), Rhian Morgan (CVAHS and BASAAG) and Piotr Sobisz (the person who found the site) surveyed a 80 x 80m area at 40 readings per meter. Peter Alley also undertook a survey with his UAV to map the lumps and bumps in the field.
The basic results are shown in Figure 2. This image has had the readings clipped to +/- 2.5nT.
The first and most obvious things are the two strong linear features across the bottom and top left hand corners. There are also some fainter linear features. I have marked these in Fig. 3.
The lower feature marked with yellow arrows clearly lines up with a hedge in the next field and is almost certainly a grubbed-out hedge line. The upper line marked with yellow arrows is less certain. It isn’t that strong, and it isn’t that continuous. I still have my doubts, however. It may be an old field drain? There are many subtle linear features marked with red arrows. These do not make a coherent pattern and may well be earlier drainage or other agricultural work, but some could be something more interesting archaeologically.
But what about the grave? Fig. 4. marks the position of the excavation trench.
You can just see the little red square in the middle of the plot. (It might be worth clicking on the image to see it in more detail, and then clicking on that version of the image to zoom it to maximum size.) Fig. 5 shows the area in more detail (again you might like to click on it).
It is interesting to see that the excavation does not show in the data at all. The second, unexcavated cremation was in the SW corner. As can be seen from these plots, there are lots of “little blobs”, some of them are stronger than others. Little blobs which are strongly magnetic, shown with the positive pole in black and the negative in white, are likely to be metal, especially when the negative (white) part is not north of the black part. In order to see the subtle stuff, we have clipped the image to +/- 2.5 nT (which isn’t much!) but this makes it hard to see the difference between strong and moderately magnetic “anomalies”. To make this clearer, I have created an image where the strong values (greater or less than 5nT) are in red and green (Fig. 6).
This allows us to discount a number of features, but there are still lots of possible blobs. If we look at Fig. 5 closely there is a small blob on the SW corner, just about where the second cremation is likely to be. Using TerraSurveyor, we can measure the magnetic profile of the feature and draw a graph of it (Fig. 7).
From this graph we can see that the burial, if that is what it is, has a “signature” about 75cm across and ranging from -1nT to a maximum of about 5nT. From this we can suggest some possible extra burials. I have marked just a few in Fig. 8.
There are clearly quite a few more in the survey, I have just marked five. Unfortunately, the law of equifinality comes into play, i.e., very different things can have very similar magnetic signatures. All sorts of things might make small, subtle magnetic features, like old animal burrows for example. There are also a small number of larger features which might be interesting, for example the one marked in blue. This is bigger than a single cremation, being 2m north-south and 1.5m east-west but has a similar magnetic range (i.e., -1 to -5nT). The reason why I am curious about it is that cremation cemeteries sometimes also preserve evidence for the pyre site. Rarely, we also get what are called bustum burials where the pyre is built over a small pit and the pyre remains, along with possibly extra grave goods, and placed in the pit. Hopefully, the AAG might be able to test a few of these features to “ground truth” the results.
All in all we had a grand day, especially Kai (Fig. 9)! We have several possible surveys in the pipeline. If you want to be put on the mailing list, please email us.