Caer Alyn Archaeological and Heritage
Project
Geophysical Survey (Resistance)
Southern Enclosure
(Caer Alyn Hill Fort)
Alan Brown, Lydia Jones, Annabelle
Rogers, Alison Doughty, Emelia Elve and
Charlotte Wilson
Ref CHA/BWDS2
Introduction
The aim of this survey was to investigate areas within the previous
magnetometry survey carried out in June 2006 (Gondek June 2008).
The survey was also intended to give guidance to excavations being
carried out during the 2010 summer school. The Survey was
completed by visiting students under the guidance of the author (Alan
Brown). This gave the students practical experience of geophysical
survey. Grid 1 & 2 of the survey are the subject of report
CHA/BWDS2 ( Lydia Jones et el).
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 on both sides of the fort is
approximately the same level as the fort.
The area that this survey covers is located at 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 southern 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, approximately 15m from the
nearest sample point. Readings were taken at 0.5m intervals across
traverses 0.5m 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: despike low-pass filter, edge match and interpolation. The clipping
parameters used were Minimum -3, Maximum 3, Contrast 1 and Units
set to standard deviation. High Contact Resistance mode (HCR) was
set to “on”. All grid plots were compiled into one composite plot and
are presented in grey-scale, and raw data plots. (See Appendix B).
Interpretation
Grid 3
Fig 1, Grid 3 (For Composite and
Raw data see Appendix B)
Grid 3 is by far the most complex grid of the survey. There is a
number of curving high resistance readings starting in the top left
hand corner of the grid (North). The roughly semi-circular
configuration may indicate the presence of a roundhouse foundation.
The feature is approximately 7m in diameter. This is surrounded by a
faint low resistance anomaly, possibly evidence for a drip gully. The
bottom right hand corner of the grid is dominated by a series of
medium high readings forming a roughly square feature with low
readings at the centre. This is possibly geological in origin (glacial
till).The recent excavation of the site has revealed medium large
stones with no particular pattern and these are possibly natural
placements. The rest if the grid is dominated by readings consistent
with natural background.
Grid 4
Fig 2, Grid 4. (For composite
and Raw Data see Appendix B
Grid 4 has two areas of high resistance readings. The first area is in
the top left hand corner of the grid with the second area just below. It
is possible that these areas are concentrations of glacial geology. The
only area of interest is the diagonal area of high resistance running
from roughly 3m down from the top of the northwest edge of the grid
and run in an easterly direction. This propagates into Grid 3. In Grid 3
the readings are slightly lower. The possible roundhouse foundations
in Grid 3 don’t continue into Grid 4, but there is a semi circular area
of medium low readings (in grid 4) completing the circle. This is hard
to explain and can only be solved by excavation.
Conclusion
The survey revealed a number of interesting features. The results are
consistent with the possible use of the site as a prehistoric settlement
or farming area. Some features may indicate later use, possibly
medieval. The ridge and furrow detected during excavation does not
show up on the survey plot. This may be due to the nature of the top
and sub-soils. The site is free draining due to the sand and gravel subsoils; this produces extensive drying of the soil making resistance
surveying very difficult.
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 3 and 4
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 4
Grid 3
Raw Data
Grid 4
Grid 3