We held a very successful Geophysics Field School at Cwmffwrn Farm in early October, lead by Dr Ian Brooks.
We used two techniques – resistivity and magnetometry – to look at various humps and bumps in the landscape.
Both techniques detect very small changes in soil characteristics up to 0.5m of the ground surface and are used as reconnaissance methods to look for indicators of features of archaeological interest.
Over the course of the 3 days of the Field School we investigated 4 locations within the farm including two building platforms, a circular bank and a possible Bronze Age cairn.
This relies on variations in soil magnetic susceptibility and magnetic remanence which often result from past human activities. Using a Fluxgate Gradiometer these variations can be mapped. This technique is particularly useful for locating features where the fill is different from the surrounding material. It is also useful for locating areas of burning.
This relies on variations in the electrical conductivity of the soil and subsoil which in general is related to soil moisture levels. As such, results can be seasonally dependant. Slower than Magnetometry this technique is best suited to locating positive features such as buried walls that give rise to high resistance anomalies. Large features such as ditches or pits where moisture is retained in the fill can also be located.
Both techniques used rely on taking a series of readings on a regular grid pattern. The results are recorded automatically within the machine and at the end of the day, they are downloaded onto a laptop computer and processed so that we can see the initial results.
This Field School was supported by a grant from Wales Council for Voluntary Action.
The participant’s handbook for this field school is given here Geophysics Field School Handbook 2021
We are have some initial results for the two building platforms. At both sites the results were very interesting and may well lead to further investigations in future.
An enclosure and platform has been found on the upper reached of the sheepwalk close to its highest point. The enclosure, formed by low earth banks, is rectilinear with one side, the narrowest, open. The south east side dips to a platform. The whole is set above a natural spring, the outflow of which has eroded the side of the hill.
We surveyed this site during our Landscape Archaeology Field School in September 2020. Phil Olivant used that survey to layout the grid for the geophysics, as shown.
The image below is from a drone survey conducted by Julian Ravest. Please note the different orientation.
The results for the geophysics at this site are shown below.
This site was also surveyed during the Landscape Archaeology Field School in September 2020.
This structure and its surrounding field system appear to be an abandoned farmstead. It is set next to a small stream which is now silted up and overgrown. The vegetation of the stream has encroached on the lower part of the building. The stream appears to split into two as it enters the Cwm Hir brook. One of the branches is manmade and cuts through a pre-existing enclosure. This may have been made to improve drainage near the building.
This is Phil Olivant’s grid for the geophysics, based upon the previous survey.
The image below is from a drone survey by Julian Ravest.
The results for the geophysics for this site are shown below.
The results presented here are being interpreted further by Ian Brooks and more findings will be placed here when available.
Thank you for your comments Tony. I will pass them to Ian Brooks for his response.
Very interesting. We have the aerial image, the magnetometry and resistivity, but did we record the simple observation of ground condition, in particular, moisture content. From memory, this was very quaggy in places (If top of page is north, very wet in northeast and south margin.) and dry in others. I would expect this to have a considerable effect on the resistivity. I notice that just pushing the pegs in required more or less effort in different places. If one did a simple plot of the penetration resistance, would it show up similar features to a resistivity plot?
Just a thought.
The combination of photogrammetry to map the surface and geophysics to map what may lie beneath is clearly a very powerful tool. This summary of the data is going to assist our interpretation of the marks in the landscape on this farm whose origins probably go back to prehistoric times.