Dig Report 3
Interim Dig Report 3
Trench 14 ‘Test Pit’
Plate 01: West Glebe Field, horseshoe shaped parch mark Credit: A. Christofides
Author: David .W. Savage BA Honours Humanities with History (Open University) MA Archaeology (University of Bristol) Certificate Humanities (Open University)
& Trench supervisor Mary Lennox BA
Email :- Glebefield@outlook.com
Dig Report 3 is to collate all material relating to Trench 14 across a Glebe Field parch mark.
Contents Page
1. Summary 3
2. Trench 4
3. Stratification 8
4. Conclusions 9
5. Future Objectives 9
6. Acknowledgements 10
7. Appendix 10
Plates. Page
01. Vertical view of parch mark 1
02. Trench 14 test pit 3
03. Trench 14 4
03. Context03 Soil sample 5
05. ‘Fiddle Key’ horseshoe nail 5
06. Bloomery Slag 6
07.Medieval potsherds 6
08.18th to early 20th century finds 7
Figures
01 Horseshoe parch mark superimposed on Res. Survey 2
02 Trench 14 position upon parch mark. 3
03 Nitrate level samples 12
1. Summary
During the 2022 dig season due to the extremely dry and hot summer
conditions new features appeared. Perhaps the most visually striking was at the
northeast edge of the West Glebe Field; see Plate 01 above. The horseshoe shape mimics a Romano British
Nymphaeum footprint and springs do periodically appear in this area of the
field.
However, after investigations and soil tests this feature has possibly
been caused by historic fungal activity weakening the soils capacity (see
appendix 1). As part of the investigation a .500 x 2mt. test pit was excavated
as positioned in figures 01 & 02.
Figure 01: Horseshoe feature position superimposed on Res. Survey
2. Trench Test Pit
Figure 02: Outline of parch mark feature and trench position.
Plate: 02: TR14 test pit viewed east to west
Plate 03: TR14 test pit viewed west to east. Peg denotes F.K. nail find
The base of the trench is 74.302 mts. ‘above sea level’ and is approximately 410mm below surrounding ground level. Datum no. 3(D3) on the north boundary of the Glebe Field is 76.322mts a s l, the base of TR14 was measured at 2.020 mts. below D3.
Context 03 finished at D3 minus 2.020mts. At this level dense grey/green clay with limestone gravel was found. It is thought this is the natural stratum but a further trench may be dug in the future to confirm this. There is a small element of doubt as also at this level a Type 1’Fiddle Key’ nail was found, see plate 05 below. This is a significant finds and can be dated to the Saxo Norman period.
Plate 04: Context 03 Soil sample TR14, grey green clay with limestone gravel
Plate 05: Context 03 Fiddle Key Nail (Clark Type 1)
Context 02 at D3 minus 1.980mts to D3 minus
1.750mts morphologically altered sub heavy loam to clay soil with both medieval
and post medieval artifacts was found. Bloomery slag was found at this level,
see plate 06 below and medieval potsherd see Plate 07 below.
Plate 06: Context 02. Bloomery slag
Plate 07: Context 02. Incised and vesicular potsherds neither are particular good quality with soft/ partly oxidised surfaces.
Context 01 The surface layer of turf and topsoil at D3 minus 1.610mts. to the sub soil at D3 minus 1.750mts. has the familiar field dressing layer of bonfire damaged 18th to early 20th century ceramics and iron nails also charcoal and small fragments of coal and of very thin glass and grey roofing slate. With the exception of a pre historic to medieval potsherd, see below.
Plate 08: Context 01 artifacts;
possible pre-historic to medieval potsherd on card
3. Stratification
This can be defined as three
layers in accord with the above contexts.
4. Conclusion
The horseshoe parch mark became visible after an exceptionally hot
and dry summer. The main impact of these investigations appears to indicate no
foundation or building remains. However, a chemical imbalance caused by
historic fungal or animal keeping activity may have lowered nitrates levels
thus weaker grass growth in this area (see Appendix 1). Other fungal rings are
regularly visible nearby in the west part of the Glebe Field (GF). Within Trench 14 a natural clay layer was
reached with no indication of any structure or disturbance. Stratification is
also similar to other trenches however, limestone both in its natural bed or
redeposited were not evident. This does align with geological contour maps; the
test pit coincides with the Stiff Clay and Rhaetic boundary. The finds
including medieval potsherds and a Saxo Norman equestrian horseshoe nail are
typical of finds in the GF’s more southerly trenches. This possibly indicates
medieval activity in this area of the field as these were found on the cusp of
contexts02 and 03 also typical of finds in other GF trenches.
5.
Future Objectives
1.
Maintain a watching brief in this area.
2. Possibly dig another test pit to confirm results.
6. Acknowledgements
- Alex Christofides
alexchristofides@hotmail.com
Drone imagery and digital 3D modelling
- Community Archaeology on the Mendip Plateau (CAMP)
Contact: Pip Osbourne https://www.camp-plateau.co.uk/home
Services: Pottery typology,
artifact dating
- Mary Lennox
for supervising and dig report.
- David Lambie and Mary Knight for Geophys. surveying
Service: Geophysical
Surveying.
- Shared Past
Contact: Penny Lock https://sharedpast.org/
Services: Community group
organisation and technical advice.
.
7. Appendix 1
The Horseshoe parch mark,
northern edge of the Glebe Field
At the end of August 2022, aerial
photographs (Fig 1) taken by drone unexpectedly revealed a very clear
horseshoe-shaped parch mark about 10m in diameter on the northern edge of the
Glebe which had not previously been noticed. Subsequent inspection showed that
it was in fact visible from ground level. The weather at that point had been
exceptionally warm and dry for several weeks, so it was assumed that the parch
mark was caused by the foundations of a circular wall or building not far below
the ground surface. A small hole approximately 30x30x30 centimetres was dug
over the western arc of the parch mark in the expectation of encountering stone
or rubble. Nothing of significance was encountered, so it was decided that a
more thorough investigation should be undertaken.
Clear aerial photographs of the
Glebe field are available on Google Earth dating back some 20 years, but none
show any trace of the parch mark.
It was then decided to undertake
a magnetrometry survey over two 20m by 20m grids centred on the parch mark, in
case it was caused by an unknown metal object such as a tank or pipe, though
this was considered highly unlikely. These two grids were not superimposed exactly
over the resistivity grids for practical reasons – there is a wire fence nearby
which would have interferred with the magnetometry readings – but the grids
were placed symetrically and centrally over the parch mark and geolocated so
they could be re-established if necessary. Again, the service pipe in the
eastern corner of the survey area showed very clearly (Fig 3) and somewhat
overwhelmed the readings from the rest of the survey area. Removing these
anomalous readings (by substituting dummy readings) left a data set which could
be reliably analysed (Fig 4). Again there was no evidence that there was any
archaeological reason for the parch mark.
In an attempt to determine what
was causing the parch mark, a mini-trench or elongated test pit was sited over
the eastern “foot” of the horseshoe shape. The test pit running in an east/west
orientation, was 2m long and 50 cm wide (Fig 5). The trench was deturfed to a
depth of approximately 10-15 cm, and was then troweled to a depth of about 25
cm ????? below ground level. A few artefacts were found (Fig 6), namely sherds
of coarse pot and a fiddle key nail, but there was no archaeology which would
explain the parch mark. At this stage, the soil having become a very hard,
dense clay, it was decided to create a spade-dug sondage in the centre of the
trench to a total depth of about 50 cm, well below the root zone of the grass.
It was apparent that whatever was causing the stressed growing conditions for
the grass, and hence the resulting parch mark, was not archaeological in origin
and it was concluded that some chemical variation in the soil must be
responsible, presumably stemming from a natural process or a previous
agricultural practice.
To explore the soil chemistry
further in an economical fashion, a commercially available soil testing kit
(marketed for garden use) was used to analyse representative soil samples for
pH, and nitrate, phosphate and potassium levels. Five soil samples were taken
from the northern trench wall just below the depth to which the roots extended
(about 10-15 cm below the surface) at 50cm intervals and including both ends of
the trench. Measuring from the western end of the trench, they are named 0.0,
0.5, 1.0, 1.5, and 2.0 with the central sample at 1m from either end being
directly below the most pronounced part of the parch mark. The results can be
summarised as follows;
1. pH – all the samples were on the alkaline side of neutral, with samples 0.0 and 0.5 (those were the samples within the horseshoe) slightly more alkaline than the others.
2. Nitrate – these samples showed
the most variation with the samples at 1.0 and 1.5 (most directly associated
with the parch mark) showing very low levels of nitrate while those at 0.5 and
2.0 were markedly higher (Fig.03). Information in the leaflet that came with
the kit states; nitrogen is essential for the growth of vegetation, especially
grass and leafy plants. The right amount of nitrogen will affect the plants and
allow for healthy growth.
3. Phosphorus – there was very
little variation in the phosphorus levels between the samples and all were very
low. Phosphorus is necessary for strong root growth, and a lack of it will
stunt the growth of a plant.
4. Potassium – again, the samples
had very similar potassium levels to each other and were all medium or perhaps
slightly lower than medium. Potassium is important for flower production.
As a final exercise, soil colour
tests were undertaken on the remaining unused soil samples using Munsell colour
cards.
The best explanation for the horseshoe parch mark is that a variation in soil chemistry especially nitrogen levels has produced a ring of weaker growth which the dry weather exacerbated. Nitrogen levels are affected by soil compaction, by micro-organisms in the soil and by water levels, so maybe there was at one time an animal feeder or something similar, or fungal activity or a transient spring which has caused soil changes which resulted in the parch mark. At this stage, it appears that there is little advantage to be gained from further investigation of this feature.
Figure 03: Nitrates levels see Item 2 above