Caer Alyn Archaeological and
Heritage Project
THE CAER ALYN FORT
NORTH OF BANKS AND DITCHES
(Lodge Field)
GEOPHYSICAL REPORT
(RESISTANCE)
Alan Brown, Annabelle Rogers
This survey was undertaken by Alyn-Geoprospecting
on behalf of The Caer Alyn Archaeological and
Heritage Project
Ref CAAHP/ft/Serv2
Alyn- Geoprospecting
Introduction
The survey was requested to evaluate the area directly north of the Iron Age banks and
ditches of the Caer Alyn fort. This area is known to the project team as Lodge Field. This
survey will also compliment the survey carried out in the interior of the fort
(CAAHP/ft/Serv1, A. Brown et al.).
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.
Survey Location
The survey area is located directly north of the fort and is bounded in the south by the Iron
Age Banks and ditches that form the forts northern defences. The western boundary is
marked by the remains of Wats Dyke which runs along the ridge above the River Alyn. The
eastern boundary is marked by a fence between the Lodge Field and Stable Field (see site
map Appendix B). The northern boundary is approximately 80m from the Iron Age banks
and ditches (southern boundary) and runs east-west.
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
(For grid location and tie-in see Appendix B.)
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 parameters used were 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. (For raw
data plots see Appendix A.)
Results and Interpretation
(For interpretation plot see Appendix D)
Due to the size and complexity of the features observed the results will be presented using the
full gray scale composite plot (see Appendix A for false colour plot).
Grid, 1A and 1B
The most significant feature in Grid 1A is the area of low resistance reading propagating
from the top left hand corner (South) to the bottom right hand corner (North). The readings
appear to form a broad arc, then carry on into Grid 2B and stop in the top left hand corner of
Grid 2C. This may indicate the foundation ditch of a defensive feature 1, (possibly a palisade).
The ditch is approximately 5 meters wide.
A number of low readings starting in the top left hand corner of Grid 1B (and propagating
south westerly through Grids 2C, top left hand corner of 3C, 3D and 4D) appear to follow the
line of Watts Dike (Wats Dyke runs along the western edge of the land overlooking the
river). This feature may be contemporary with the dyke and may indicate a ditch. (See fig 1)
The ditch appears to cut the feature (reported above) in Grid 1B
Fig 1
Red line indicates the direction of the possible
ditch running parallel to Wats Dyke
1
False culler plot indicates varying levels of resistance (Low Res) equally spaced, indicating possible post
holes (see Appendix A)
Grid 2A
Grid 2A is dominated by a number of low resistance readings running from the top left hand
corner to the bottom right hand corner. The readings continue into Grids 3A and 3B
terminating in Grid 3C. This may indicate a very large ditch (probably defensive). At its
widest the ditch is approximately 8m in width and 60m long. Given its proximity to the ditch
feature reported above (Grids 1A, 2B and 2C), this may indicate that the fort had formidable
defences at its northern approaches.
Grid 3B
The high resistance features at the bottom of this grid have no definite shape. This may
indicate geology or a rubble spread from a building. These readings continue into Grid 4B.
Excavation will be needed to identify the feature.
Grids 3C , 4B , 4C , 3D and 4D
The low resistance reading (almost centre) in this grid may indicate a pit. Low resistance
readings propagating from the bottom left hand corner of Grin 4B, running south westerly
into Grid 4C through Grid 3C, and terminating in Grid 3D before running north into Grid 4D
are the remnants of the old school football pitch (see Appendix C)
Conclusion
Apart from the visible banks and ditches, this survey has provided further evidence for the
very substantial defences of the Caer Alyn Fort.
It is possible that defensive ditches in this survey may be earlier than the visible Iron Age
bank and ditch defences; this may indicate prolonged occupation of the fort.
Recommendations
Excavation to ascertain the depth and age of the ditches; it is recommended that two trenches
be excavated, one on each of the two cross ditches.
Appendix A
Raw Data Plot
Fort North of banks and ditches (raw data), showing grids of interest.
False Culler Plot
Appendix B
Site Map and grid location
Grids
Appendix C
Interpretation and Information Plot
Geophysics Plot for the Caer Alyn Fort Northern entrance
The Iron Age banks and ditches are marked by
the Red Line. The northern entrance to the fort
is marked by the blue arrow.
The dark areas are high resistance readings, the
lighter areas are low readings (see plot info
below).
Low readings showing possible cross ditches.
Wat’s Dyke
Wat’s Dyke ditch (possible)
Possible building (approximately 10m x 10m)
Old football pitch ( the lime used to mark the
pitch lines helps break down the soil, making it
less dense and more able to carry water, thus
giving it a low resistance reading)
Light blue areas indicate no readings
Appendix D
A
1
2
3
4
B
C
D