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The Caer Alyn Archaeological and Heritage Project: Methodology

Our initial site survey at Caer Alyn comprised of the following geophysical resistance survey:-


Alan Brown, Annabelle Rogers, Alan Bunney and Phil Cox

This survey was requested by The Caer Alyn Archaeological and Heritage Project (Director of Archaeology Mr. Tony Hanna) and CADW (Welsh Heritage). The survey has been carried out under license to CADW.

The purpose of the survey was to locate possible archaeological remains within the fort defences. No archaeological investigation has ever take place on the fort; consequently the historical background of the fort is unknown.

The site is located in North East Wales. The fort is located on a promontory overlooking a hairpin bend in the river Alyn. The promontory has been formed by the action of the river cutting a deep valley into the glacial overburden, producing the hairpin bend. The land on the opposite side of the valley is at approximately the same level as the fort.

The fort is an elongated enclosure approximately 178m north south and 62m east west. The boundaries of the enclosure are defined by three banks and ditches to the north, double ramparts to the east and steep slopes to the west. The southern end of the enclosure has a steep slope leading up to an entrance. The Western boundary is marked by a section of Wat’s Dyke running north south along the top of the slope. The south end of the enclosure is extensively wooded while the northern section is partially clear with a grassed area.

The survey area is located at the northern end of the fort enclosure. The survey was limited to approximately 1600 square metres due to the wooded areas within the fort. (Survey grids located within the grassed area).

The solid geology of the area consists of Carboniferous (Westphalian) Erbistock formations overlaying the Coed-Yr-Allt sandstones. The overlaying drift is of glacial origin and consists of sands and gravels. The drift deposits in this area are quite substantial and may be as much as 19m thick. An exposure of the Erbistock formation is visible on the riverbank at the base of the promontory at the south end. (British Geological Survey, Wrexham Sheet 121, 1:50 000. ISBN 0 7518 2811 4 Folded).


A Geoscan RM 4 was used with a 0.5m probe separation. The remote probes were set at 0.5m separation, approximately 15m from the nearest sample point. Readings were taken at 1m intervals across traverses 1m wide in a zigzag pattern. All data was hand-recorded and input by hand to computer. The data was processed using the Geoplot 3 software package. Various filters were used as follows: de-spike low-pass filter, edge match and interpolation. The clipping parametres used was Minimum -2, Maximum 2, Contrast 1 and Units set to standard deviation. High contact resistance (HCR) mode set to “on”. All grid plots were compiled into one composite plot and presented in gray-scale and raw data plots.

Results and interpretation

Grids 1 & 5

grid 1 and 5

Grid 1

The results from grid 1 were affected by the near surface geology (drift deposits), which was reflected across the entire survey. The sand and gravel deposits have created a complex mixture of resistance readings. This kind of response can in some cases mask the archaeological features; however the use of various filtering processes - i.e. de-spike, high pass filter and interpolation (for smoothing) - revealed the most exiting feature of the whole survey. In the upper section of grid one (north east) there appears to be a number of low resistance readings, propagating north-east to south-west, forming a rectangular feature measuring about 5m wide by about 16m long. The north-east section of this feature seems to have a number of medium low readings forming a semi circular anomaly. These readings may indicate the foundation ditches of a building with an apse.

Rectangular feature in Grid 1

The eastern section (bottom) of grid 1 appears to have an area of medium high readings forming a semi circular anomaly, this is also seen in grid 5, western section (top), and this feature is approximately 10m in diameter.

The Western section of grid 1 is dominated by the high resistance readings in the southeast corner. This is contrasted by the area of very low readings in the south western section of grid 1. This may reflect the drift geology of the site.

Grid 5

Due to site constraints a full 20m grid was not possible. A number of dummy readings where taken during the survey of this grid. Approximately one third of this grid provided useable data. The highest resistant readings were seen in the eastern section (top) and coincide with the high readings seen in grid 1 (semi-circular feature). During the survey it was noted that the mobile probes were coming into contact with rubble just below the surface.

Grids 2 & 6

Grid 2 and 6

Grid 2

The results for grid 2 are quite complex. Again the near surface geology produced a confusing array of readings, but with filtering a number of anomalies became apparent.
The high resistance readings in the south east section (top right) of grid 2 may need investigation, as two similar anomalies appear in two other grids (grids 1 & 3) and appear to be equidistant apart. Just below theses readings are a set of medium high readings propagating northeast to southwest crossing into grid 3 forming a semi circle (see grid 3).

Approximately half way down the southern edge of grid 2 there appear to be a number of medium high resistance readings forming a rectangular anomaly (two lines propagating north south approximately 4 metres in length, connected by a line running east west, approximately 7 metres long). The east west section of this feature (approximately half way up) has a number of medium high responses running north-south which in turn connect to another set of medium high responses propagating east-west, forming a semi circular feature about 4 metres in length. Given the possible uses of the fort this may indicate structural deposits.

Grid 6

As with grid 5 this grid produced a number of dummy readings, which may indicate near surface rubble or drift geology. Again, as with grid 5, the size of the grid was dictated by site constraints.

The lower left hand section of this grid (within the usable data) produced a number of high resistance readings, which seem to be running north northeast to south-southwest. It is difficult to interpret this anomaly due to the lack of usable data.

This grid is dominated by a large number of low resistance readings concentrated in the centre of the eastern section (top). The only other feature in this grid is the high readings centered on the southeast section (top right).

Grid 3 & 7

Grid 3 and 7

Grid 3

This grid produced a very confusing data set; this may be the result of successive periods of disturbance. In order to make sense of this grid we will break it down into top, middle and bottom.

Top section

Top section

The highest readings in this section are found in the southeast section (starting approximately 3 metres in from the southern edge) forming a circular anomaly. Similar readings have been noted in grids 1 and 2. Given that these features are equidistant apart; this may indicate an intentional placement. Directly to the right (propagating west) of the high readings is a concentration of very low readings; the relationship between these two anomalies requires investigation.

Directly west of the circular features is a line of high readings propagating north south (indicated by the red arrow) for approximately 5 metres culminating in a right angle, with the readings continuing for approximately 2 metres eastwards. It is possible that this anomaly has some relationship with the high and low readings above.

Moving along the eastern edge of the top section we encounter a number of very low readings forming yet again a semi circular feature with medium high readings at its centre. This anomaly is approximately 5 metres in diameter. Due to the nature of the subsoil’s this may be a natural feature, but will need investigating in order to be certain. Just below this feature (bottom left) is a group of medium high readings propagating southeast to northwest intersecting grid 2 at its southern edge (this was noted above in the grid 2 interpretation). The semi circle formed by these readings may be structural.

Middle section

Middle section

The most outstanding feature of the middle section is the high readings found on its northern edge; these reading do not appear to cross into grid 2 but do intersect the high readings (semi circular feature noted in grid 2 and grid 3 top section) in the top section grid 3. The high resistance responses are also noted at the bottom of the middle section (left side). The complexity of these high resistance readings may indicate structural deposits.

The lowest readings in this section appear at the middle of the bottom edge and propagate west into the bottom section. These readings have no defined shape so may be natural.

Bottom section

Bottom section

The blue section of this grid indicates the position of a small tree, making the taking of readings impossible. Dummy readings were inserted at this point.

The low resistance readings at the top of the section were noted in the middle section interpretation.
The high responses in the south of the section have no specific shape and bear no relationship to the other high readings in grid 3. Investigations will be necessary to ascertain their origins.

The high readings (indicated by the red arrow) may be part of the complex set of high responses noted in grid 2 and the middle section of grid 3.

Grid 7

As with grids 5 and 6, site constraints dictated its size. Due to near surface rubble the amount of usable data was restricted, and interpretation of the grid is therefore impossible.

Grid 4

Grid 4

(Due to heavy undergrowth the top right hand section in this grid was not surveyed, as indicated by the blue shading.)

This grid produced a very promising set of responses. The highest readings are concentrated in the southern half (right hand side) of the plot. The high responses just below the blue shading roughly form a rectangle with high readings propagating west (for roughly 3 metres) from its lower right hand corner culminating in an undefined are of medium high responses. Slightly to the left of this set of readings appears to be a line of high responses (starting at the blue shading) and running west for approximately 20 metres. This line is broken in two places as it propagates west. The western end of this line culminates in a circular area of high readings approximately 3 metres in diameter. This concentration of high readings may be structural.

The high readings in the top left hand corner of the plot may be related to the high resistance concentration described above.

The northern section of the grid is dominated by medium-low and low responses starting approximately 8 metres down from the eastern edge. The area at the centre of the northern edge has very low readings with no defined shape, but just below this feature is a cluster of low to medium-low readings with the medium-low forming a grid pattern with one line east-west and two lines north-south indicating possible structural deposits. This may be related to the high responses noted in the southern section of the grid.


Despite the problems with the underling drift geology the survey was a resounding success. It is at this stage of the investigation impossible to make predictions for the underlying archaeological deposits, but in my opinion the results were extremely encouraging. This is a well-defended fort and would have been occupied in the past. The periods of occupation at this stage of the investigation is unknown but the range of features present in the survey results may indicate a number of occupation periods.


In order to understand the features present in this resistance survey a number of trenches would be required. A limited excavation of the features encountered during the survey would provide valuable information about the fort’s historical past. Also, a limited excavation of the fort’s banks and ditches (not surveyed) would provide information on the construction of the defences.

We recommend a minimum of six 5m x 2m trenches on the area covered by the survey, and one trench per bank and ditch (trench size to be decided by the site director). This would be a limited excavation digging down to the archaeology but not into it. This limited excavation would evaluate the site for possible full-scale investigation (with the permission of the relevant governing bodies).
It is also recommended that a further resistance survey be carried out to the north of the fort (beyond the banks and ditches) and at the southern end of the fort, on the circular enclosure near the southern entrance. A further survey (magnetometry) is recommended to compliment this resistance survey
The survey north of the fort (beyond the banks and ditches) is now underway and is producing very encouraging results. (Report to follow).


The project would like to thank the following for their help with this survey.

Ken White (land owner)
Russell Evans (land owner)
Neil Adams
CADW (for licensing the survey)
Ian Kirkman (Daresbury Laboratory)

  Attachment Size Date Added
      Grid 1 and 5.jpg   9.82 KB   11/07/2008 2:25 pm

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