Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.
Gil Burleigh asked us if we could survey a field in Ashwell. The field is quite small at just 1ha. Some nice finds had come from this field including a nice scabbard chape (Fig. 1) in the 1970s. The field was ploughed for a short period in the 1980s, and North Herts Museums undertook a fieldwalking survey in 1986 which retrieved pottery from many periods with concentrations of Roman, Medieval and post-Medieval material and a thinner scatter of Iron Age and Anglo-Saxon pottery.
Figure 1: scabbard chape © North Herts Museums.
Members of North Herts Archaeological Society excavated three small test trenches over the weekend of the 16/17th March and found archaeological features in two of them. Soil test trenches has also revealed archaeological features in test pits SA01 and SA02 (although they were misinterpreted as just deeper topsoil! Figure 2).
Figure 2: Location of the NHAS test pits (in red) and the soil test pits (SA01 to SA05). © CAGG.
As the site is pasture over chalk, it seemed likely we would get a good result. Magnetometry was the obvious technique to try (Fig. 3).
Figure 3: Nigel “compensates” the magnetometer. Photo © Mike Smith.
We managed to complete the whole field in a day with the mag, which was excellent progress. I was exciting to see what we had detected and processed the data that evening (Fig. 4).
Figure 4: the mag survey © CAGG.
My initial reaction was “ugh”. Why was the data so horribly noisy, like some sort of giant waffle maker? Although we can see some features, especially on the western edge, we cannot see all that much going on, and nothing around where the trenches had revealed archaeological features. Can we explain this?
Peter Alley flew the site with his UAV. The photos from his flight can be used in two ways: they can be stitched together to produce an orthomosaic (basically an aerial photograph corrected for distortions) or to map the topography. The orthomosaic is fun, but doesn’t help us solve our problem (Fig. 5).
Figure 5: Orthomosaic from the aerial survey © Peter Alley.
Mapping the topography shows the basic form of the site, basically a flat field on a slope (Fig. 6)!
Figure 6: the topographic survey derived from the UAV images. © Peter Alley.
If we use a hill-shade, however, we can see the micro-topography of the field. Lighting the field from the SW creates Figure 7.
Figure 7: topography lit from the south west. © Peter Alley.
We can see from Figure 7 that even though the field has not been ploughed since the late 1980s, the plough scars remain running up and down the slope. If we move the light to shine from the NW we get Figure 8.
Figure 8: topography lit from the NW. © Peter Alley.
Now we can see plough scars running across the slope! This is, in part, the origin of the noisy data: the cross cutting ploughing has left a surprisingly uneven surface, like the proverbial giant waffle iron. We should assume this is mirrored by scars in the surface of the underlying chalk and archaeology. One further possible cause of the problem are thirty years worth of ant hills (Fig. 9).
Figure 9: ant hills at Ashwell. © Kris Lockyear.
These aspects of the site, in part, explain the noisy looking data but do not completely explain the lack of contrast between the fills of the features and the chalk. Gil reported that the metal dectectorists were also having problems with large numbers of responses in this field, so the mystery isn’t completely solved. A plane did crash in this field during World War Two, but I cannot see how that has created this problem unless the whole field went up in flames. Ideally, we would take some samples and test the magnetic susceptibility of them.
There are, however, some features in the data, despite the noise. I have outlined the ones I can see in Figure 10.
Figure 10: magnetometry survey results with interpretation shown in yellow (cf. Fig. 4). © CAGG.
Two of the possible features run down and across the slope in a manner similar to the plough scars, and we must be cautious in their interpretation as a result. The corner that is protruding from the western field boundary is much more interesting. It looks like we may have clipped one side of an enclosure. (We seem to specialise in enclosures that don’t enclose things.) It looks like it probably runs under Ashwell House next door. In the middle is a “dark blob”. Initially, I thought this was likely to be metal as it is so close to the edge of the field where the fencing creates a strong response. Checking the values in TerraSurveyor, the range of values towards the east of the blob is from -2nT to +12nT. If one goes right to the edge where one can see the impact of the fence on either side, the values jump to about -4nT to +26nT. Contrary to my initial thought, this might well be an archaeological feature in the enclosure.
On the Saturday we completed a 40x40m block of Earth Resistance survey in the NE corner of the field. Not a great deal showed! We decided to persist on the Sunday so that we could include the area of the “enclosure”. We had some unusual help (Fig. 11).
Figure 11: Unusual help with the Earth Resistance survey. © Kris Lockyear.
As has become our standard method, we used the Earth Resistance meter on the 1+2 setting. In other words, every time we stick the mobile probes in the ground the machine takes three readings. One uses the two outer probes which are a meter apart to measure down to about a meter or so. The other two readings use the inner probe to take two side-by-side readings with a 0.5m probe separation, looking about 50–70cm into the ground (Fig. 12). Given the topsoil is only about 30cm deep, this should be fine.
Figure 12: the Earth Resistance survey underway. Photo © Gil Burleigh.
The results, were, underwhelming… (Figs 13 and 14).
Figure 13: Earth Resistance survey with 0.5m mobile probe spacing.
Figure 14: the Earth Resistance survey, 1.0m mobile probe spacing.
I can only conclude that the difference in moisture retention between the features and the chalk was minimal. Peter Alley made the excellent suggestion that the banding we can see is possibly related to the layers within the chalk geology.
We have one last source of data to examine. In 1986 Keith Fitzpatrick-Matthews undertook a field walking survey here for North Herts Museum Service. Obviously, he didn’t have the luxury of a GPS to log his finds or lay in his grid. We can, however, roughly position the results on the map with a bit of careful editing in a drawing package. Figures 15–17 show the three most common types of pottery: Romano-British, Medieval and post-Medieval.
Figure 15: Romano-British pottery distribution © North Herts Museum Service.
Figure 16: Medieval pottery distribution © North Herts Museum Service.
Figure 17: Post-Medieval pottery distribution © North Herts Museum Service.
The post-Med distribution doesn’t seem to fit our survey results. Both the Roman and Medieval ones, however, are temptingly close to the feature on the western side. By making the distributions transparent we can see how these match (Figs. 18 and 19).
Figure 18: Romano-British pottery overlain on the mag data © North Herts Museum Service and CAGG.
Figure 19: Medieval pottery overlain on the mag data © North Herts Museum Service and CAGG.
Both the Romano-British and Medieval pottery distributions lie just to the north of the “enclosure”, down slope from it. This is what one would expect if the material collected on the surfaces derives from the fills of the features. We can only be sure of the date of this feature by further excavation.
Although this survey has not produced the beautifully clear results we have had from other sites such as Kelshall, it has shown some interesting features. It is also a good example of the value of combining different data sets, in this case field walking, aerial photogrammetry and magnetometry.
Many thanks to everyone who helped with the survey and fetched and carried equipment. Also, thanks for Gil for suggesting we have a look at this site and David Short for allowing us access to the site. As 19 images seems an odd number to finish on, I thought I would sign off with another photograph of one of David’s magnificent sheep (Figure 20)!
Figure 20: A sheep. Photo © Kris Lockyear.