Caer Alyn Archaeological and Heritage
Project
Geophysical Survey (Resistance)
Sothern Enclosure
(Caer Alyn Hill Fort)
Lydia Jones, Tamsyn Fraser, Josef Roberts
Ben Regel and Alan Brown
Ref CHA/BWDS1
Introduction
The aim of this survey was to extend the area which had already been
investigated using Geophysical survey (Magnetometry, Gondek June 2008) and
so investigate the activity that occurred on the southern side of the hill fort. Two
adjoining 10x10m grids in the southern enclosure next to the excavation area
were surveyed. Resistivity survey was decided on because the high iron content
of the soil would affect the results of a magnetometry survey.
Site Location
The site is located just outside the village of Llay, North East Wales. The fort
(map ref. SJ35SW, grid ref. SJ33125370) 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 sides of the valley to both
sides of the fort is approximately the same level as the fort.
The area that this survey covers is in the southern end of the site. The land is
agro-pastoral and evidence of ridge and furrow was found during the
excavation, so we should be aware of possible disturbance to any archaeological
evidence. (For Grid location and layout see Appendix A)
Geology
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 river bank 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).
Method
A Geoscan RM 4 was used with a 0.5m probe separation. The remote probes
were set at 0.5m separation at a point from which both 10x10m grids were
accessible. Readings were taken at 0.5m intervals across traverses 0.5m wide in
a zigzag pattern. Due to the survey being carried out on a training dig different
people took the readings across both grid 1 and 2, thus the readings may not be
as accurate as they would be if one person had worked consistently across the
survey area. The soil of the surveyed area was very dry and so to combat this
problem the remote probes were dampened. The data was hand-recorded and
input by hand on to the computer. The data was processed using the Geoplot 3
software package. Various filters were used: de-spike low-pass, edge match and
interpolation. The clipping parameters used was Minimum -2.1, Maximum 2.1,
Contrast 1 and Units set to standard deviation. HCR mode was set to on. Both
plots were put in to one composite plot and presented in grayscale (see appendix
B).
Interpretation
Fig 1, Grid 1 (For Composite and
Raw data see appendix B)
Grid 1
The blue areas of the grid are the result of very high readings in comparison
with those around them and as such were considered anomalies. There are
several different areas of high resistance, with the other areas in the grid giving
medium and medium to low readings.
In the south-east of the grid there is a noticeable area of high resistance which
seems to extend up to the top of the grid and then curve round to the right.
Whilst extending southwards it seems to stay roughly rectangular and after
turning to the right has a quite straight edge jutting down in to the grid. This
could indicate foundations of a structure however it is hard to tell without
excavation. From this survey it would be advisable to excavate in the large area
of high resistance on the left hand side of the grid about half way up, also
including some of the area of medium resistance just outside it.
There are two roughly circular areas of high resistance slightly below the centre
of the grid, with one slightly more south-west than the other. Given the period
and nature of the site these could be indicative of round house structures;
however excavation would be needed in order to investigate further. A 1m test
pit on the upper and most definite of the two areas would be useful, as would a
larger excavation carried out on the lower one to determine the relationship
between that and the possible feature which will be discussed below.
Another area of high resistance is in the very bottom right of the grid, more
north than the other areas. A few areas of high resistance may indicate it is
joined to the more northerly of the two possible circular features, maybe
suggesting that it is also an archaeological feature. This area of high resistance
coincides with a roughly circular dip in the ground level.
Grid 2
Fig 2, Grid 2. (For composite
and Raw Data see Appendix B
In this grid there are more anomalous and low readings than there are in Grid 1.
The possible feature seen in the bottom right hand corner of Grid 1 cannot be
seen continuing in to Grid 2 because either it does not or the areas in blue are
covering it. However below that area there is a large patch of high and medium
readings which may be related to those at the bottom of Grid 1. Digging a 1m
test pit over this possible feature and straddling Grids 1 and 2 would hopefully
reveal if it is a feature and maybe hint at what the anomalous readings in the rest
of the survey could be indicating.
In this grid there are more anomalous and low readings than there are in Grid 1.
The possible feature seen in the bottom right hand corner of Grid 1 cannot be
seen continuing in to Grid 2 because either it does not or the areas in blue are
covering it. However below that area there is a large patch of high and medium
readings which may be related to the those at the bottom of Grid 1. Digging a
1m test pit over this possible feature and straddling Grids 1 and 2 would
hopefully reveal if it is a feature and maybe hint at what the anomalous readings
in the rest of the survey could be indicating.
The curving red lines on Fig.3 follow two areas of low resistance which
encompass an area of both high and medium
readings. This curved area of low resistance
may be a ditch or gully which could be tested
through a small excavation on the edge of the
low resistance just to the left of the centre of
Grid 2 so that the pit covers both the low and
area of high resistance in the middle of the
Fig. 3, Grid 2
possible ditches. The low resistance also
seems to carry on, if slightly disturbed with
medium readings to the bottom right hand corner of the grid. If the low
resistance does turn out to be a ditch it would be interesting to know whether it
continues to curve around and join up so containing the area of high resistance
within it. It may be a drainage ditch surrounding a round house.
Conclusion
The earth in the area being surveyed was very dry and so to get better results the
remote probes were dampened at a point during the survey of the second grid.
This could have affected the accuracy of the results however due to ground
conditions seemed the best way of producing less anomalous readings. These
conditions may account for the large amount of very high readings (the blue
areas in the plot) which has made it difficult to get a good picture of the area
surveyed. The results however are constant with the known history of the area
and the observations made at ground level. The high resistance at the bottom of
Grid 1 fits in with the dip in the ground and the circular high resistance areas in
Grid 1 could indicate a round houses. The area of curving low resistance in Grid
2 could indicate a foundation ditch and drip gully, this fits in with a possible
area of settlement. Due to the areas position near the entrance to the hill fort,
and the find of a possible late Roman coin, the disturbance of the soil in the area
may indicate settlement of the Prehistoric and Roman Periods. However, the
ridge and furrow that was shown in the excavation close to the survey grids also
indicates that the area was used through the medieval period, thus the possible
features may be much later. Only excavation can conclusively tell.
Recommendation
A more extensive survey of the area would be an advantage. This could be
completed after the completion of the present excavation
A new extended excavation into the survey area, concentrating on the areas of
high resistance in Grid 1 and both high and low resistance in Grid 2
Appendix A
Grid layout and Location
Peg 1
Peg 2
Grid 4
Grid 3
Peg 3
Peg 4
Grid 1
Excavated area
Grid 2
Peg1, N 354054.796, E 333334.528, H 62.779
Peg2, N 354059.430, E 333325.734, H 63.441
Peg3, N 354069.123, E 333313.394, H 64.190
Peg4, N 354073.670, E 333306.048, H 64.837
Appendix B
Processed data
Grid 1
Grid 2
Raw Data
Grid 1
Grid 2