Constraining the Fremington Glaciolacustrine Clays:
OD-Referenced
Analysis of the C.H. Brannam Ltd Borehole Survey
and its implications
for the Irish Sea Ice extent.
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ABSTRACT
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This report presents the first OD-referenced analysis of BGS
borehole record SS53SW62–79 (BGS ID 703480), an 18-hole test boring campaign
drilled by C.H. Brannam Ltd in October 1972 across the Roundswell Plain,
Brynsworthy, and Claypits area of north Devon. By converting all borehole
depths to metres above Ordnance Datum using corrected collar positions, the
true spatial geometry of the Fremington potters clay — the glaciolacustrine
unit at the heart of the formation — is established for the first time from
this dataset. The analysis reveals
that only 8 of the 18 boreholes confirmed the diagnostic lacustrine
stratigraphy by reaching the basal glaciofluvial gravel. The remaining 10
holes, classified by Edmonds et al. (1985) as penetrating 'boulder clay', are
shown to have been drilling through periglacial head and solifluction
material — in three cases from collars at 41–43 m OD, some 15 metres above
the lacustrine basin. These findings directly quantify a conflation first
identified qualitatively by Edmonds himself in a note that was never followed
to its mapping consequences. The
report further demonstrates that this conflation has its root in a systematic
misreading of George Maw's founding 1864 paper, which explicitly identified
the overlying stony gravel as 'independent of the clay, and coeval with its
erosion' — a distinction that was collapsed by Stephens (1966) and
institutionalised in the BGS Sheet 293 mapping of 1982. The OD data presented
here are consistent with all subsequent field observations at the Lake
cutting (Hawkins & Hawkins 1990; Cattell 2003) and provide the subsurface
spatial model needed to quantify the true extent of the lacustrine potters
clay.
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Borehole Map from https://api.bgs.ac.uk/sobi-scans/v1/borehole/scans/items/703466
1.
Introduction
The Fremington Clay Series is a sequence of
Quaternary superficial deposits on the south shore of the Taw–Torridge estuary
in north Devon, approximately 4 km west of Barnstaple. The formation is notable
as one of the southernmost occurrences of glacigenic or glacially-influenced
sediment on the English mainland, and has been the subject of sustained
scientific debate since George Maw's first description in 1864.
At the centre of the formation is a unit of
smooth, stone-free, highly plastic clay — the potters clay — worked
commercially for centuries and described by Maw as 'perfectly homogeneous,
exceeding tough, free from the slightest grit, and as smooth and soft as
butter.' This clay is the genuine lacustrine deposit: the product of
quiet-water settling in a proglacial lake impounded by Irish Sea ice during a
Middle Pleistocene glaciation. Above it lies a geologically distinct sequence
of periglacial head and reworked stony material that is not genetically
connected to the lacustrine clay below.
The distinction between these two units — clear
in Maw's original paper and reinstated by field observations in the 1990 and
2003 road cutting investigations — was collapsed by Stephens (1966) and never
fully reinstated in the BGS mapping of the area. As a result, the published
1:50,000 geological map (Sheet 293, 1982) maps a substantially larger area as
'Boulder Clay' than is warranted by the evidence for genuine lacustrine
deposition.
This report addresses a specific and previously
unexploited dataset: the 18 test boreholes drilled by C.H. Brannam Ltd (erroneously
recorded as CH Brennam Ltd) in October 1972 across the Roundswell Plain,
Brynsworthy, and Claypits area. The raw borehole logs, archived as BGS record
SS53SW62–79, have been analysed here for the first time in terms of Ordnance
Datum (OD) elevations. This conversion — straightforward in principle but not
previously undertaken — reveals the three-dimensional geometry of the lacustrine
basin and provides direct quantitative evidence for the extent of the
head/boulder clay conflation in the BGS mapped area.
2.
Background: The Two-Unit Stratigraphy and Its Misreading
2.1 What Maw Actually Described
George Maw first inspected the Fremington
sections in 1852 and published his paper 'On a supposed deposit of boulder-clay
in North Devon' in the Quarterly Journal of the Geological Society in 1864. His
account is precise, internally consistent, and establishes a clear two-unit
stratigraphic model:
The lower unit is the potters clay proper —
smooth, homogeneous, stone-free, chocolate to brown, deposited in a quiet water
environment and worked commercially for its exceptional plastic qualities. At
the Roundswell well, sunk to the east of Fremington in 1862, Mr J. Bowden
penetrated 78 feet of clay from a surface elevation of approximately 100 feet
above sea level (~30.5 m OD). Of those 78 feet, the upper 12 feet was 'somewhat
intermixed with stones, the number of which decreased from the surface.' The lower
66 feet was perfectly homogeneous clay — the genuine potters clay — with
blackened driftwood at approximately 40 feet depth.
The upper unit is an independent stony gravel,
5 to 7 feet thick, covering the surface of the clay. On this unit Maw was
unambiguous:
'This superficial gravel is, I believe,
independent of the clay, and coeval with its erosion.'
This is not an incidental observation. Maw
devoted specific attention to it, noting that the boulders of trap-ash in the
superficial gravel 'may have been derived from the eroded clay, and left after
its denudation on the surface of what remained.' He identified the stony upper
12 feet of the Roundswell well as belonging to this independent superficial
gravel, 'commingled with the top of the clay at the time of its denudation.'
Maw's conclusion on the single authenticated
erratic within the clay is equally important. The large boulder of basaltic
trap at Combrew, found ten feet below the clay surface, is described as 'the
only authenticated example of a boulder occurring in the drift.' The other
boulders — those in the superficial gravel and at coastal locations — he did
not regard as demonstrably part of the same deposit.
2.2 The Collapse of the Two-Unit
Model
The careful two-unit model Maw established was
progressively collapsed into a single 'boulder clay' through a sequence of
publications that can be traced precisely.
Stephens (1966), working within a Wolstonian
glacial framework, described 'a calcareous, shelly boulder clay' at Fremington
and equated the entire superficial sequence from surface to bedrock with Irish
Sea till. He noted a preferred pebble orientation and equated the basal gravel
with the coastal raised beach deposits — a stratigraphic interpretation
subsequently shown to be incorrect by Kidson and Wood (1974). Critically,
Stephens did not distinguish between the upper stony head and the lower
lacustrine clay; both became 'boulder clay.'
The BGS six-inch survey of Sheet 293, conducted
by Edmonds and colleagues between 1968 and 1977, adopted the Stephensian
framework. The resulting 1:50,000 map (published 1982) and accompanying memoir
(Edmonds et al., 1985) mapped the entire superficial sequence across the valley
floor as 'Boulder Clay', with the boundary between boulder clay and pebbly
drift described in the memoir itself as 'arbitrary.'
The 1972 Brannam boreholes — the dataset
central to this report — were drilled during the survey period and their data
was available to Edmonds. He incorporated them into the memoir's discussion of
the deposit, but classified the drillers' 'dirty clay' and 'overburden' as
'boulder clay', reserving 'lake clay' or 'smooth clay' only for the potters
clay proper. This reclassification was not unreasonable, but led to a critical
observation that Edmonds recorded and then did not follow to its mapping
conclusion:
'It is noteworthy that gravel was recorded in
the bottom of only those holes which penetrated smooth clays.' — Edmonds et al.
(1985), Memoir for Sheets 277 and 293, p.56.
2.3 The Reinstatement of Maw's
Distinction
The construction of the A39 Barnstaple Bypass
in the late 1980s cut up to 9 metres through the eastern part of the deposit
near Lake and provided the most extensive sections seen since the clay pits
were active. Two papers from this cutting directly reinstated Maw's distinction
from field observation.
Hawkins and Hawkins (1990) logged both cut
faces at 20-metre intervals and produced a five-unit stratigraphy. Their
uppermost unit (Unit A) — yellow-brown silty sandy clay with many angular
clasts of local sandstone and mudstone — was explicitly identified as head in
their own conclusion: 'the stratigraphic sequence from top to bottom [is]: head
— Unit A; till — Unit B; lacustrine clay — Unit C and glaciofluvial gravel —
Unit E.' They noted no evidence to support a raised beach origin for the basal
gravels.
Cattell (2003), investigating a landslip in the
cutting west of Lake Overbridge, went further. He recorded that the base of the
lacustrine clay falls from 26 m OD at the cutting crest to 20 m OD at road
level, dipping northward at 11° — the geometry of a lake basin margin. He
stated explicitly that the overlying gravelly head deposit is 'typical of head
deposits in the area, and not genetically connected to the underlying
sequence', and placed the southern boundary of the Fremington Clay outcrop
approximately 200 m south of the cutting — well north of the BGS mapped
boundary.
3.
The 1972 Borehole Survey: Provenance and Methodology
3.1 Survey Context
The 18-hole test boring campaign was
commissioned by C.H. Brannam Ltd — the Barnstaple pottery company whose Higher
Gorse Claypits were the last commercial workings in the formation — and drilled
between 9 and 19 October 1972. The purpose was purely commercial: to determine
whether workable reserves of stone-free potters clay existed beyond the
immediate vicinity of the active pit, particularly in the Roundswell Plain and
Brynsworthy areas to the east and south-east of the claypits.
The campaign was preceded by a seismic survey
carried out by T.R. Wood of Bangor University around 1968 — the same Wood whose
1974 QRA field guide entry on the Fremington deposits is cited in all
subsequent literature, and who is referenced in the memoir as the source of
seismic data used in the BGS survey of Sheet 293. The boreholes were thus
drilled into terrain whose shallow geology had already been explored
geophysically.
The drilling method used a rotary-percussive
hammer through overburden (which was not sampled) and switched to auger on
penetrating the clay. Clay samples of approximately 24 inches in length were
extracted at 2-foot intervals and bagged for Brannam's own assessment of
pottery quality. Each hole was plugged with sand on completion. The maximum
depth was 41 feet (12.5 m), achieved in Borehole 11.
3.2 The Original NGR Coordinates
The 1972 drilling report contains pencilled
National Grid sub-grid references alongside each hole number on the log pages.
These coordinates are incorrect — as confirmed by cross-referencing the BGS
GeoIndex mapped borehole symbols against the location plan included in the
report. The discrepancy is consistent with transcription errors in the
pencilled references rather than genuine positional uncertainty. Corrected
positions, derived from the BGS mapped symbol locations, are used throughout
this report. All corrected positions are within the British National Grid
system (OSGB36 / EPSG:27700).
3.3 The OD Conversion
No surface elevations are recorded in the
original 1972 drilling report. The OD conversion was performed by
cross-referencing the corrected borehole collar positions against the OS
terrain model, using contour interpolation from the 1:25,000 mapping of the
area. All depths in the original report are in imperial feet; conversion to
metres uses the standard factor of 1 foot = 0.3048 m.
The corrected positions, surface OD values, and
complete depth records for all 18 boreholes are tabulated in Section 4.
4.
Borehole Register
Table 1 presents the full register of all 18
boreholes with corrected positions, surface OD elevations, and depth data from
the original 1972 drilling report. Imperial depths are shown alongside metric
conversions. The 'Max?' column indicates whether the borehole was terminated
within clay (the log notes 'continues below' the final depth recorded), meaning
the clay thickness figure is a minimum.
|
BH
|
BGS Ref
|
Easting (BNG)
|
Northing (BNG)
|
Surface OD (m)
|
O/B (ft)
|
Clay top (ft)
|
Clay thick (ft)
|
Max?
|
Gravel
|
Water
|
Clay quality
|
|
1
|
SS53SW62
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253410
|
131580
|
25.80
|
20
|
NIL
|
—
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N
|
no
|
no
|
No clay
|
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2
|
SS53SW63
|
253460
|
131520
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37.60
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25
|
25
|
8
|
Y
|
no
|
no
|
Dirty
|
|
3
|
SS53SW64
|
253540
|
131600
|
35.10
|
13
|
13
|
14
|
Y
|
no
|
no
|
Dirty, worsens
below 21ft
|
|
4
|
SS53SW65
|
253490
|
131700
|
32.80
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11
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11
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19
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Y
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no
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YES
|
Dirty
|
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5
|
SS53SW66
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253490
|
131880
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27.66
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16
|
16
|
16
|
Y
|
no
|
YES
|
Dirty
|
|
6
|
SS53SW67
|
253490
|
132000
|
24.00
|
13
|
13
|
12
|
N
|
YES
|
no
|
Dirty then
gravel
|
|
7
|
SS53SW68
|
253740
|
131620
|
35.20
|
13
|
13
|
12
|
Y
|
no
|
no
|
Dirty
|
|
8
|
SS53SW69
|
254030
|
131620
|
34.70
|
11
|
11
|
18
|
Y
|
no
|
YES
|
Dirty
|
|
9
|
SS53SW70
|
253980
|
131360
|
41.20
|
9
|
9
|
20
|
Y
|
no
|
YES
|
Very
dirty/earthy
|
|
10
|
SS53SW71
|
254050
|
131310
|
42.50
|
9
|
9
|
16
|
Y
|
no
|
no
|
Dirty/earthy
|
|
11
|
SS53SW72
|
254150
|
131250
|
42.80
|
9
|
9
|
32
|
Y
|
no
|
no
|
Dirty to 29ft;
clean clay at 40ft
|
|
12
|
SS53SW73
|
252960
|
131490
|
36.50
|
12
|
12
|
18
|
N
|
YES
|
no
|
Earthy
|
|
13
|
SS53SW74
|
252950
|
131530
|
35.20
|
9
|
9
|
20
|
N
|
YES
|
YES
|
Good with finey
|
|
14
|
SS53SW75
|
252900
|
131520
|
35.10
|
9
|
11
|
14
|
N
|
YES
|
no
|
Good from 13ft
|
|
15
|
SS53SW76
|
252840
|
131610
|
31.60
|
9
|
15
|
10
|
N
|
YES
|
no
|
Mainly clean
|
|
16
|
SS53SW77
|
252830
|
131520
|
35.00
|
9
|
11
|
12
|
N
|
YES
|
no
|
Sandy/short
|
|
17
|
SS53SW78
|
253020
|
131920
|
25.20
|
11
|
11
|
20
|
N
|
YES
|
no
|
Good
|
|
18
|
SS53SW79
|
252990
|
131920
|
24.70
|
7
|
7
|
24
|
N
|
YES
|
no
|
Good; horseflesh
at 17ft
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Table 1. Full borehole register. Green rows
= gravel reached (lacustrine stratigraphy confirmed). Red rows = surface OD
>38m (above lacustrine basin). Grey row = no clay found. O/B = overburden.
Max? = whether borehole terminated in clay.
5.
OD-Referenced Analysis
5.1 The Gravel-Bearing Holes:
Lacustrine Stratigraphy Confirmed
Eight of the 18 boreholes reached the basal
glaciofluvial gravel: Boreholes 6, 12, 13, 14, 15, 16, 17, and 18. These are
the only holes that confirmed the diagnostic three-tier stratigraphy of the
formation: potters clay resting on glaciofluvial gravel resting on bedrock.
Their OD elevations are presented in Table 2.
|
BH
|
BGS Ref
|
Surface OD (m)
|
Clay top OD (m)
|
Clay base OD (m)
|
Gravel top OD (m)
|
Clay thick (m)
|
Quality / Notes
|
|
6
|
SS53SW67
|
24.00
|
20.04
|
16.38
|
16.38
|
3.66
|
Gravel at 25ft.
Almost no water. Only ~9ft usable.
|
|
12
|
SS53SW73
|
36.50
|
32.84
|
27.36
|
27.36
|
5.49
|
Earthy short
clay. Gravel at 30ft. Nearly usable.
|
|
13
|
SS53SW74
|
35.20
|
32.46
|
26.36
|
26.36
|
6.10
|
Good clay with
sand layers. Gravel and water at 27ft.
|
|
14
|
SS53SW75
|
35.10
|
31.75
|
27.48
|
27.48
|
4.27
|
Dirty at 11ft;
good clay from 13ft. Gravel ~23ft.
|
|
15
|
SS53SW76
|
31.60
|
27.03
|
23.98
|
23.98
|
3.05
|
Clean clay
17–27ft. ~10ft usable. Gravel at 27ft.
|
|
16
|
SS53SW77
|
35.00
|
31.65
|
27.99
|
27.99
|
3.66
|
Sandy clay below
15ft. Gravel touched at 21ft.
|
|
17
|
SS53SW78
|
25.20
|
21.85
|
15.75
|
15.75
|
6.10
|
Good clay
11–31ft. Gravel at 31ft. Recommended.
|
|
18
|
SS53SW79
|
24.70
|
22.57
|
15.25
|
15.25
|
7.32
|
Best hole. Good
clay 7–31ft. Gravel at 31ft. Recommended.
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Table 2. Gravel-bearing boreholes with full
OD stratigraphy. Gravel top OD = base of potters clay / top of glaciofluvial
outwash.
The gravel top OD values — the base of the
potters clay — range from 15.3 m OD (BH18) to 28.0 m OD (BH16), with a mean of
approximately 21 m OD. This is consistent with the Lake cutting data from
Cattell (2003), who recorded the clay–gravel contact at 19.9 and 20.4 m OD to
the north of the A39, and at 20–26 m OD in the cutting itself (the higher
figure representing the dipping southern margin). The agreement across two
independent datasets, separated by approximately 1 km and derived from
completely different investigation methods, is strong confirmation that these
holes are sampling the same lacustrine basin floor.
Boreholes 17 and 18 — in Hollands land north of
the old pit workings — show the lowest gravel OD values (15.3 and 15.8 m OD)
and the thickest good-quality clay (6.1 and 7.3 m respectively). These are the
deepest parts of the lacustrine basin reached by this survey and were correctly
identified in the 1972 narrative as the best commercial prospects. Their clay
top OD values (21.9 and 22.6 m OD) represent the lake floor level most closely.
Boreholes 12–16 show clay tops at 27.0–32.8 m
OD and gravel at 24.0–28.0 m OD. These elevated values — compared to BH17 and
BH18 — indicate that these holes are on the southern basin margin, where the
deposit shallows toward the Carboniferous shale bedrock ridge to the south.
Borehole 12 in particular (clay top 32.8 m OD) may be on the outermost southern
rim of the basin.
5.2 The High-Elevation Holes: Above
the Lacustrine Basin
Boreholes 9, 10, and 11 — located in the
Roundswell–Brynsworthy area to the south-east of the claypits — show surface
elevations of 41.2, 42.5, and 42.8 m OD respectively. Their clay tops,
calculated from the depth records, are at 38.5, 39.8, and 40.1 m OD. Table 3
presents their OD data alongside the expected lacustrine range.
|
BH
|
Surface OD (m)
|
Clay top OD (m)
|
Expected lacustrine clay top (~20–27 m
OD)
|
Elevation above expected range (m)
|
Interpretation
|
|
9
|
41.20
|
38.46
|
20–27 m OD
|
~12–18 m ABOVE
|
Head/solifluction.
Not lacustrine clay.
|
|
10
|
42.50
|
39.76
|
20–27 m OD
|
~13–20 m ABOVE
|
Head/solifluction.
Not lacustrine clay.
|
|
11
|
42.80
|
40.06
|
20–27 m OD
|
~13–20 m ABOVE
|
Head/solifluction
dominant. 0.3 m clean clay only at 30.3 m OD (borehole base).
|
Table 3. High-elevation non-gravel
boreholes. Clay tops are 12–20 m above the expected lacustrine clay level.
The significance of these figures cannot be
overstated. The clay top in Boreholes 9, 10, and 11 sits at 38–40 m OD. The
lacustrine potters clay surface confirmed by the gravel-bearing holes and by
the Lake cutting data lies at approximately 20–27 m OD. The material
encountered in Boreholes 9–11 begins approximately 15 metres above the
lacustrine basin. It is periglacial head and solifluction: locally-derived
stony silty clay produced by frost action and downslope movement during
Pleistocene cold stages, not glaciolacustrine sediment.
This is precisely what the drillers themselves
observed. Their log notes for these holes describe material that is 'very dirty
and earthy', 'shorter and more earthy than the others', and — for Borehole 9 —
'this seemed dirtier and more earthy and sandy than the others... very wet
throughout.' These are the sensory characteristics of solifluction head, not of
the smooth, homogeneous potters clay.
Borehole 11, the deepest hole at 41 feet (12.5
m), is the most instructive case. Drilled specifically to test whether clean
potters clay existed in the Roundswell area, it encountered dirty clay from 9
feet downward, with clean clay only appearing at 40 feet — in the final foot
before the borehole was terminated. That 1-foot (0.3 m) horizon of clean clay,
at 30.3 m OD, is approaching the upper range of the expected lacustrine basin
but still above the confirmed gravel-bearing levels. The borehole was stopped
before reaching the diagnostic base; the clean clay at the base of BH11 may
represent the first appearance of the lacustrine unit, but this was not
confirmed.
5.3 The Intermediate Holes: Head
Masking the Basin
The remaining non-gravel holes — Boreholes 2,
3, 4, 5, 7, and 8 — show surface ODs of 27.7–37.6 m and clay tops at 22.8–31.4
m OD. These are less clear-cut than Boreholes 9–11, and several fall within or
close to the upper range of the expected lacustrine clay zone. However, without
the diagnostic basal gravel, the lacustrine stratigraphy cannot be confirmed in
any of them.
The clay quality descriptions for all six of
these holes are consistently negative: 'dirty', 'worsens with depth', 'very
dirty and earthy'. None produced clay considered commercially usable by
Brannam's. This is consistent with the interpretation that these holes are
penetrating the thick head sequence overlying the lacustrine basin, without
reaching the potters clay below — or are on the margins of the basin where head
is thick and the lacustrine unit thin.
Borehole 1 — surface OD 25.8 m, no clay in 20
feet — was drilled in the central part of the survey area, approximately 460 m
east-southeast of the Higher Gorse claypits. Its position at 25.8 m OD and
failure to find clay suggest it may sit on a bedrock high or at the edge of a
gap in the lacustrine deposit within the basin, rather than at the outer
western margin.
5.4 Comparison with Published OD
Datums
Table 4 compiles all confirmed Potter’s Clay
top OD values from every independent source: the 1972 Brannam boreholes, the
Lake cutting investigations, Maw’s Roundswell well datum, and the additional
A39 bypass borehole 703458. Only boreholes and sections where the lacustrine
clay was positively identified are included; the non-gravel holes (BH2–5, 7–11)
and the 12 head-dominated additional boreholes are excluded, as they did not
confirm the diagnostic lacustrine stratigraphy.
|
Source
|
Location
|
Position
|
Potter’s Clay Top OD (m)
|
Notes
|
|
Basin
floor: Potter’s Clay top ~20–23 m OD
|
|
Cattell (2003)
|
Lake cutting — N
of road
|
Basin floor
|
~20–21
|
Flat basin floor
north of A39.
|
|
This survey —
BH17
|
Hollands land, N
of old pits
|
Basin floor
|
21.85
|
Best commercial
hole. Gravel at 15.75 m OD.
|
|
This survey —
BH18
|
Hollands land, N
of old pits
|
Basin floor
|
22.57
|
Best hole
overall. Gravel at 15.25 m OD.
|
|
Additional BHs —
703458
|
~253950, 132050
(A39 bypass)
|
Basin floor
|
~20.76
|
3.05 m
Fremington Clay. Labelled by driller.
|
|
Basin
margin: Potter’s Clay top ~26–33 m OD
|
|
Maw (1864)
|
Roundswell well
(~1 km SE)
|
Basin margin
|
~26.8
|
Clean clay top
below 12 ft stony reworked material.
|
|
Cattell (2003)
|
Lake cutting — S
crest
|
Basin margin
|
~26
|
Dipping S
margin; 11° N dip. Southern limit ~200 m S.
|
|
H & H (1990)
|
Lake cutting
Ch.11000
|
Basin margin
|
~27–29
|
Unit C
(lacustrine) top, below head (A) and till (B).
|
|
This survey —
BH15
|
Nr Higher Gorse
|
Basin margin
|
27.03
|
Clean clay 17–27
ft. Gravel at 23.98 m OD.
|
|
This survey —
BH6
|
Nr Higher Gorse
|
Basin margin
|
20.04
|
Gravel at 16.38
m OD. Transitional to floor.
|
|
This survey —
BH16
|
Nr Brynsworthy
|
Basin margin
|
31.65
|
Sandy clay.
Gravel at 27.99 m OD.
|
|
This survey —
BH14
|
Nr Brynsworthy
|
Basin margin
|
31.75
|
Good clay from
13 ft. Gravel at 27.48 m OD.
|
|
This survey —
BH13
|
Nr Brynsworthy
|
Basin margin
|
32.46
|
Good clay with
sand layers. Gravel at 26.36 m OD.
|
|
This survey —
BH12
|
Nr Higher Gorse
|
Outermost rim
|
32.84
|
Earthy clay.
Gravel at 27.36 m OD. Southern limit.
|
Table 4. Confirmed Potter’s Clay top OD from
all independent sources, grouped by basin position.
The pattern across all sources is consistent.
On the basin floor, the Potter’s Clay top clusters tightly between ~20 and 23 m
OD — the level at which the lake sediment accumulated. On the basin margins,
the clay top rises to ~26–29 m OD as the deposit thins and laps onto the
bedrock slopes, with the outermost rim (BH12) at 32.8 m OD. The convergence of
the margin values from four independent datasets — Maw’s Roundswell well (~26.8
m), Cattell’s Lake cutting southern crest (~26 m), Hawkins and Hawkins’s Unit C
top (~27–29 m), and the 1972 boreholes BH12–16 (~27–33 m) — implies a maximum
lake surface at approximately 27–29 m OD. This is the level of the ice-dammed
proglacial lake in which the Potter’s Clay was deposited: above this elevation,
no lacustrine clay exists, and all fine-grained superficial material is
periglacial head.
5.5 Corroborating Evidence from
Additional Area Boreholes
Figure 2: BGS map of Fremington Area showing “Till” in light blues and
borehole locations.
The spatial pattern established by the 1972
Brannam survey is strongly corroborated by a representative sample of 13
additional boreholes from the wider area, drawn from BGS archival records
including A39 Barnstaple Bypass site investigations and other geotechnical
surveys. These holes, summarised in Table 5a, span the terrain from the bypass
corridor in the east (~255 317) through Roundswell (~253 305) to the western
approaches (~248 314), and provide independent control on the distribution of
the potter’s clay outside the 1972 survey footprint.
Of the 13 boreholes examined, 12 are dominated
entirely by head deposits: stony, mottled, gravelly clay sitting directly over
gravel or bedrock, with no trace of the diagnostic stone-free lacustrine clay.
Their descriptions — firm to stiff yellow-brown or red-brown sandy silty clay
with much subangular gravel, occasional cobbles, and variable sand content —
are characteristic of periglacial solifluction head derived from local
Carboniferous and Devonian sources. None records the smooth, homogeneous, stone-free
clay that defines the potter’s clay proper.
The single exception is borehole 703458
(SS53SW54), located at ~253950, 132050 — centrally within the basin area at a
surface OD of 22.7 m. This hole, drilled as part of the A39 Barnstaple Bypass
investigation in August–September 1982 by Norwest Holst for the Department of
Transport, records a substantive sequence of firm to stiff red-brown slightly
silty clay from 1.95 m to 5.0 m depth (20.76–17.71 m OD), explicitly labelled
“(FREMINGTON CLAY)” by the driller. This 3.05 m unit is underlain by a 0.2 m basal
transition (firm to stiff brown slightly sandy silty clay with yellow-brown
silty sand laminations and occasional sub-rounded gravel), then by soft brown
and greenish grey sandy clayey silt with much grey subangular gravel, resting
on probable siltstone bedrock at 7.5 m depth (15.21 m OD). The clay top OD of
~20.76 m and the presence of a relatively stone-free, cohesive lacustrine clay
sequence are fully consistent with the basin-floor geometry established by the
1972 gravel-bearing boreholes (BH17 clay top 21.85 m OD; BH18 clay top 22.57 m
OD) and by Cattell’s (2003) Lake cutting data (~20–21 m OD on the basin floor).
The pattern from these additional holes
reinforces the central conclusion of the OD analysis: genuine lacustrine
potter’s clay is confined to the low-elevation basin floor, and the great
majority of the surrounding terrain mapped as Boulder Clay on Sheet 293
contains only periglacial head. The commercial behaviour of C.H. Brannam Ltd is
instructive here. Brannam’s applied what might be called Sutton’s Law — they
looked where the clay was most likely to be. Guided by T.R. Wood’s seismic
survey and by generations of direct working knowledge, they targeted their 1972
boreholes on the low-elevation ground around the existing pits. They did not
bore in the ground mapped as Fremington Clay south of the Hele–Bickington
ridge. A pottery firm whose livelihood depended on finding workable clay,
guided by geophysical survey, would not have overlooked accessible reserves if
they existed. Their silence on the southern ground — Sutton’s Law applied in
reverse — is itself evidence that the mapped Fremington Clay extent in that
area reflects head and reworked boulder clay rather than the genuine lacustrine
deposit.
6.
The Edmonds Observation and the Misreading of Maw
Edmonds’ observation in the 1985 memoir — that
gravel was found only in holes that penetrated smooth clay — correctly
identified the diagnostic criterion for lacustrine stratigraphy. The OD
analysis presented in this report quantifies its implications: the 10 holes
without gravel have clay tops at 22.8–40.1 m OD, significantly above the 20–27
m OD range confirmed by the 8 gravel-bearing holes and by the Lake cutting
data. In the three highest-elevation cases (BH9–11), the clay tops are 12–15 m
above the lacustrine range. These holes were drilling through periglacial head
throughout. The mapping of all 18 holes as ‘Boulder Clay’ on Sheet 293 followed
the Stephensian framework rather than the stratigraphic logic of Edmonds’ own
borehole evidence.
6.1 The Stepwise Conflation
The path from Maw's careful description to the
BGS's consolidated 'Boulder Clay' can be traced through five steps:
Step 1: Taylor (1956) grouped 'Saunton and
Fremington erratics' together, conflating coastal boulders of sea-ice origin
with clay-embedded clasts of different transport history. This inflated the
apparent exotic component of the inland deposit and gave the impression of a
more extensively glaciated area than the evidence warranted.
Step 2: Stephens (1966) adopted the full
glacial till interpretation, treating the entire surface-to-bedrock sequence as
a single boulder clay of Wolstonian age, without distinguishing the upper head
from the lower lacustrine clay. This paper became the standard reference for
age and character of the deposit.
Step 3: Edmonds et al. (1985) surveyed Sheet
293 within the Stephensian framework, drew an admitted arbitrary boundary
between boulder clay and pebbly drift, and mapped the full valley floor
superficial as Boulder Clay. The 1972 borehole data, which contained the
diagnostic information to restrict the mapped extent, was incorporated without
the OD conversion that would have made its implications apparent.
Step 4: Croot et al. (1996), working at the
type section in the old clay pits, focused on the lacustrine unit proper and
confirmed its glaciolacustrine character with excellent sedimentological and
geotechnical data. Their work is authoritative for the lacustrine unit but does
not address the question of how much of the BGS mapped area represents genuine
lacustrine clay rather than overlying head.
Step 5: Bennett et al. (2024), the current
authoritative synthesis, accepted the BGS mapped extent as the starting
distribution for the deposit. The Hawkins & Hawkins (1990) and Cattell
(2003) findings — which directly reinstated Maw's distinction at the field
observation level — were not connected back to the subsurface data or the
mapped boundary.
6.2 What the OD Data Restore
The OD analysis presented in this report closes
the loop that Edmonds opened in 1985. By establishing that the gravel-bearing
holes cluster consistently around a potters clay base of 15–28 m OD —
consistent with all independent published datums — and that the non-gravel
holes at 38–40 m OD are clearly above the lacustrine basin, it provides the
quantitative foundation for a revised mapped extent.
The true spatial boundary of the lacustrine
potters clay is not coextensive with the BGS Boulder Clay polygon. It is
confined to the area where the three-tier stratigraphy (potters clay on
glaciofluvial gravel on bedrock) can be confirmed, which from the 1972 borehole
data is limited to the vicinity of the Higher Gorse claypits and the Hollands
land area to their north. The apparent extent of the deposit on Sheet 293
reflects the distribution of all fine-grained superficial material in the
valley — lacustrine, head, and reworked — mapped as a single undifferentiated
unit within a glacial interpretive framework that Maw himself never endorsed.
7.
Summary: OD Classification of All 18 Boreholes
Table 5 presents the complete OD classification
of all 18 boreholes, with each hole's status relative to the lacustrine basin
and its implications for the BGS mapping.
|
BH
|
Surface OD (m)
|
Clay top OD (m)
|
Gravel OD (m)
|
Status
|
BGS Sheet 293 mapping implication
|
|
1
|
25.8
|
NIL
|
—
|
Outside basin
|
Boulder
Clay mapping DOUBTFUL — no clay
|
|
2
|
37.6
|
30.0
|
—
|
Partial — base unknown
|
Boulder
Clay mapping UNCERTAIN
|
|
3
|
35.1
|
31.1
|
—
|
Elevated margin
|
Boulder
Clay mapping UNCERTAIN
|
|
4
|
32.8
|
29.4
|
—
|
Partial — base unknown
|
Boulder
Clay mapping UNCERTAIN
|
|
5
|
27.7
|
22.8
|
—
|
Partial — base unknown
|
Boulder
Clay mapping UNCERTAIN
|
|
6
|
24.0
|
20.0
|
16.4
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
7
|
35.2
|
31.2
|
—
|
Elevated margin
|
Boulder
Clay mapping UNCERTAIN
|
|
8
|
34.7
|
31.3
|
—
|
Elevated margin
|
Boulder
Clay mapping UNCERTAIN
|
|
9
|
41.2
|
38.5
|
—
|
Head — above basin
|
Boulder
Clay mapping INVALID — head only
|
|
10
|
42.5
|
39.8
|
—
|
Head — above basin
|
Boulder
Clay mapping INVALID — head only
|
|
11
|
42.8
|
40.1
|
—
|
Head — above basin
|
Boulder
Clay mapping INVALID — head only
|
|
12
|
36.5
|
32.8
|
27.4
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
13
|
35.2
|
32.5
|
26.4
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
14
|
35.1
|
31.7
|
27.5
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
15
|
31.6
|
27.0
|
24.0
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
16
|
35.0
|
31.6
|
28.0
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
17
|
25.2
|
21.8
|
15.8
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
18
|
24.7
|
22.6
|
15.3
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
Table 5. OD classification summary. Green =
confirmed lacustrine. Red = head above basin. Amber = elevated margin or
uncertain. Grey = outside basin.
8.
Conclusions
The following conclusions are drawn from the
OD-referenced analysis of the 18 Brannam boreholes (BGS SS53SW62–79, 1972):
(1) The
lacustrine potters clay — Maw's original deposit — is confirmed in 8 of 18
boreholes by the presence of the diagnostic basal glaciofluvial gravel. The
gravel top OD in these holes ranges from 15.3 to 28.0 m OD, consistent with a
basin floor at approximately 20–22 m OD and a basin margin rising to
approximately 28 m OD. This is fully consistent with Cattell's (2003) Lake
cutting data (gravel top 19.9–26 m OD) and with Maw's (1864) Roundswell well
datum (~26.8 m OD for the clean clay top).
(2)
Three boreholes (BH9, BH10, BH11) have collars at 41.2–42.8 m OD, with
clay tops at 38.5–40.1 m OD — 12 to 20 metres above the lacustrine basin. The
material encountered in these holes is periglacial head and solifluction, not
potters clay. Their classification by Edmonds et al. (1985) as 'boulder clay'
extended the mapped extent of the formation into terrain that contains no
lacustrine sediment.
(3) The
remaining 7 non-gravel holes (BH2–8) show clay tops at 22.8–31.4 m OD. Without
the diagnostic basal gravel, the presence of lacustrine clay cannot be
confirmed in any of these holes. The uniformly poor clay quality (dirty,
earthy, contaminated) is more consistent with periglacial head overlying the
lacustrine unit than with the lacustrine unit itself.
(4) The
true spatial extent of the Fremington lacustrine potters clay is confined to
the area where the three-tier stratigraphy can be confirmed. From the 1972
borehole data, this is centred on the Higher Gorse claypits area and the
Hollands land to the north (the area recommended by Brannam's for commercial
expansion), with the basin margins rising steeply to the south (BH12–16) and
south-east (BH9–11, Roundswell area). The BGS Sheet 293 Boulder Clay polygon
substantially overstates this extent.
9.
Recommendations for Further Work
The
OD-referenced analysis presented here provides the quantitative subsurface
framework needed to revise the mapped extent of the true glaciolacustrine
potter’s clay. The following targeted steps are recommended to complete the
rigorous geological map and fully resolve the head-mixing issue identified in
the literature:
- Further borehole analysis A representative
sample of the additional archival borehole records from the Barnstaple
Bypass (A39) construction and associated site investigations has now been
analysed and incorporated into this report (Section 5.5). The results
confirm the spatial model established by the 1972 Brannam dataset: genuine
lacustrine potter’s clay is confined to the low-elevation basin, with head
dominant elsewhere. The remaining unsampled archival records could be
OD-referenced for completeness, but given the consistency of the pattern
already established across multiple independent datasets, they are
unlikely to alter the conclusions materially. Priority should instead be
given to the corrected map and cross-sections (below) and to targeted
investigation of the ground south of the Hele–Bickington ridge.
- Preparation of a corrected map and
cross-sections Using the validated 1972 dataset together with the new
borehole control, a revised geological map should be produced that
separates the confirmed lacustrine potter’s clay (stoneless, gravel-based,
low-elevation basin fill) from the overlying periglacial head.
Cross-sections (e.g., Lake–Roundswell–Bickington and north–south transects
across the Hele–Bickington ridge) will illustrate the true geometry of the
glacial clays, the northward 11° basin-margin dip, and the extent of head
overprinting. These outputs will form the core of the final project map.
- Investigation of the area south of the
Hele–Bickington ridge The ground mapped as Fremington Clay south of
the Hele–Bickington ridge (currently included in the BGS Sheet 293 Boulder
Clay polygon) should be prioritised for targeted field inspection and
shallow boreholes. On the balance of probabilities this area represents
reworked fluvial boulder clay or head derived from local Carboniferous
sources rather than the glaciolacustrine potter’s clay. This
interpretation is supported by the negative evidence from the additional
area boreholes (Section 5.5), none of which found lacustrine clay south of
the ridge, and by the Sutton’s Law argument set out there: Brannam’s,
guided by seismic survey, showed no interest in this ground as a source of
workable clay. Confirmation here will further contract the true
glacial-clay extent and eliminate the unhelpful expansion introduced by
later mapping.
- Implications for understanding of glacial
intrusion into the area This reinterpretation severely limits the
inferred onshore reach of Irish Sea ice during the relevant Pleistocene
stage (Anglian/MIS 12 favoured). The potter’s clay is now shown to be
confined to a low-elevation (~15–28 m OD) proglacial lake basin rather
than a widespread till sheet. The dominant head overprint and absence of
thick, stony till across much of the mapped area indicate only marginal
ice-dammed ponding in the Taw estuary, not deep glacial overriding of the
Devon landscape. Preliminary thoughts suggest the ice front was pinned
against the Bristol Channel coast, with the lake forming in the lee of the
ice dam; this model is consistent with sparse/local erratics and the lack
of far-travelled Irish Sea indicators in the clean lacustrine facies.
- Implications for deeper glacial intrusion onto
land The revised extent and elevation data constrain any model of
deeper glacial intrusion onto land and, by extension, the viability of
long-distance glacial rafting or transport of distant erratics (including
potential bluestone sources) across Devon to Stonehenge. A thick, grounded
Irish Sea ice sheet capable of such transport is incompatible with the
restricted, low-elevation lacustrine basin documented here. Further work
on the corrected map will allow precise contouring of the maximum
ice-surface elevation in the Bristol Channel, providing a firmer limit on
onshore incursion and strengthening (or ruling out) glacial-transport
hypotheses.
References
Maw, G. (1864). On a supposed deposit of boulder-clay
in North Devon. Quarterly Journal of the Geological Society of London, 20,
445–451.
Dewey, H. (1910). Notes on some igneous rocks from
North Devon. Proceedings of the Geologists' Association, 21(4), 429–434.
Taylor, C.W. (1956). Erratics of the Saunton and
Fremington areas. Report and Transactions of the Devonshire Association, 88,
52–64.
Arber, M.A. (1964). Erratic boulders within the
Fremington Clay of North Devon. Geological Magazine, 101(3), 282–283.
Stephens, N. (1966). Some Pleistocene deposits in
North Devon. Biuletyn Peryglacjalny, 15, 103–114.
Edmonds, E.A. (1972). The Pleistocene history of the
Barnstaple area. Institute of Geological Sciences Report 72/2.
Kidson, C. & Wood, R. (1974). The Pleistocene
stratigraphy of Barnstaple Bay. Proceedings of the Geologists' Association,
85(2), 223–237.
Wood, T.R. (1974). Quaternary deposits around
Fremington. In: Straw, A. (Ed.), QRA Easter Field Meeting Handbook. Quaternary
Research Association, Exeter, pp. 30–34.
Madgett, P.A. & Inglis, E.A. (1987). A
re-appraisal of the erratic suite of the Saunton and Croyde areas. Transactions
of the Devonshire Association, 119, 135–144.
Edmonds, E.A., Whittaker, A. & Williams, B.J.
(1985). Geology of the country around Ilfracombe and Barnstaple. Memoir of the
British Geological Survey, Sheets 277 and 293 (England and Wales). HMSO,
London. ISBN 0 11 884364 8.
Hawkins, A.B. & Hawkins, S.C. (1990). Quaternary
deposits in the Lake cutting of the Barnstaple Bypass, North Devon. Proceedings
of the Ussher Society, 7, 301–303.
Croot, D.G., Gilbert, A., Griffiths, J. & van der
Meer, J.J. (1996). The character, age and depositional environments of the
Fremington Clay Series, North Devon. Quaternary Newsletter, 80, 1–15.
Cattell, A. (2003). Geological and geotechnical
aspects of a landslip in the Fremington Clay, North Devon. Geoscience in
South-West England, 10, 397–402.
Bennett, J.A., Cullingford, R.A., Gibbard, P.L.,
Hughes, P.D. & Murton, J.B. (2024). The Quaternary geology of Devon.
Proceedings of the Ussher Society, 15, 84–130.
Daw, T., Ixer, R. & Madgett, P. (2026). A review
of the Ramson Cliff erratic: evidence of high-level ice flow? Quaternary
Newsletter, 167, 13–19. DOI: 10.64926/qn.20517.
Appendix A: Unified Borehole Register
The accompanying spreadsheet
(Appendix_A_Unified_Boreholes.xlsx) presents the complete borehole register for
all 31 boreholes referenced in this report: the 18 holes of the 1972 Brannam
survey (BGS SS53SW62–79) and 13 additional boreholes from A39 Barnstaple Bypass
site investigations and other archival BGS records.
For each borehole the register includes: BGS
reference, BGS ID, corrected grid position (BNG), surface OD, overburden
thickness, clay top OD, clay thickness, clay base OD, gravel OD (where
reached), classification status, and descriptive notes. Brannam borehole depths
are converted from imperial feet using 1 ft = 0.3048 m. Surface OD values are
derived from the OS terrain model at corrected borehole positions.
Colour coding: green = lacustrine stratigraphy
confirmed (diagnostic basal gravel reached or Fremington Clay positively
identified); red = head above basin (clay top >38 m OD); amber = elevated
margin or base unknown; grey = outside basin or no clay found. Of the 31
boreholes, 9 confirm lacustrine stratigraphy (8 Brannam gravel-bearing holes
plus borehole 703458), 3 are classified as head above the basin (BH9–11), and
the remainder are either on the basin margin with unconfirmed base, or
dominated entirely by head deposits.
|
BH
|
BGS Ref
|
Easting (BNG)
|
Northing (BNG)
|
Surface OD (m)
|
O/B (ft)
|
Clay top (ft)
|
Clay thick (ft)
|
Max?
|
Gravel
|
Water
|
Clay quality
|
|
1
|
SS53SW62
|
253410
|
131580
|
25.80
|
20
|
NIL
|
—
|
N
|
no
|
no
|
No clay
|
|
2
|
SS53SW63
|
253460
|
131520
|
37.60
|
25
|
25
|
8
|
Y
|
no
|
no
|
Dirty
|
|
3
|
SS53SW64
|
253540
|
131600
|
35.10
|
13
|
13
|
14
|
Y
|
no
|
no
|
Dirty, worsens
below 21ft
|
|
4
|
SS53SW65
|
253490
|
131700
|
32.80
|
11
|
11
|
19
|
Y
|
no
|
YES
|
Dirty
|
|
5
|
SS53SW66
|
253490
|
131880
|
27.66
|
16
|
16
|
16
|
Y
|
no
|
YES
|
Dirty
|
|
6
|
SS53SW67
|
253490
|
132000
|
24.00
|
13
|
13
|
12
|
N
|
YES
|
no
|
Dirty then
gravel
|
|
7
|
SS53SW68
|
253740
|
131620
|
35.20
|
13
|
13
|
12
|
Y
|
no
|
no
|
Dirty
|
|
8
|
SS53SW69
|
254030
|
131620
|
34.70
|
11
|
11
|
18
|
Y
|
no
|
YES
|
Dirty
|
|
9
|
SS53SW70
|
253980
|
131360
|
41.20
|
9
|
9
|
20
|
Y
|
no
|
YES
|
Very
dirty/earthy
|
|
10
|
SS53SW71
|
254050
|
131310
|
42.50
|
9
|
9
|
16
|
Y
|
no
|
no
|
Dirty/earthy
|
|
11
|
SS53SW72
|
254150
|
131250
|
42.80
|
9
|
9
|
32
|
Y
|
no
|
no
|
Dirty to 29ft;
clean clay at 40ft
|
|
12
|
SS53SW73
|
252960
|
131490
|
36.50
|
12
|
12
|
18
|
N
|
YES
|
no
|
Earthy
|
|
13
|
SS53SW74
|
252950
|
131530
|
35.20
|
9
|
9
|
20
|
N
|
YES
|
YES
|
Good with finey
|
|
14
|
SS53SW75
|
252900
|
131520
|
35.10
|
9
|
11
|
14
|
N
|
YES
|
no
|
Good from 13ft
|
|
15
|
SS53SW76
|
252840
|
131610
|
31.60
|
9
|
15
|
10
|
N
|
YES
|
no
|
Mainly clean
|
|
16
|
SS53SW77
|
252830
|
131520
|
35.00
|
9
|
11
|
12
|
N
|
YES
|
no
|
Sandy/short
|
|
17
|
SS53SW78
|
253020
|
131920
|
25.20
|
11
|
11
|
20
|
N
|
YES
|
no
|
Good
|
|
18
|
SS53SW79
|
252990
|
131920
|
24.70
|
7
|
7
|
24
|
N
|
YES
|
no
|
Good; horseflesh
at 17ft
|
Table 1. Full borehole register. Green rows
= gravel reached (lacustrine stratigraphy confirmed). Red rows = surface OD
>38m (above lacustrine basin). Grey row = no clay found. O/B = overburden.
Max? = whether borehole terminated in clay.
5.
OD-Referenced Analysis
5.1 The Gravel-Bearing Holes:
Lacustrine Stratigraphy Confirmed
Eight of the 18 boreholes reached the basal
glaciofluvial gravel: Boreholes 6, 12, 13, 14, 15, 16, 17, and 18. These are
the only holes that confirmed the diagnostic three-tier stratigraphy of the
formation: potters clay resting on glaciofluvial gravel resting on bedrock.
Their OD elevations are presented in Table 2.
|
BH
|
BGS Ref
|
Surface OD (m)
|
Clay top OD (m)
|
Clay base OD (m)
|
Gravel top OD (m)
|
Clay thick (m)
|
Quality / Notes
|
|
6
|
SS53SW67
|
24.00
|
20.04
|
16.38
|
16.38
|
3.66
|
Gravel at 25ft.
Almost no water. Only ~9ft usable.
|
|
12
|
SS53SW73
|
36.50
|
32.84
|
27.36
|
27.36
|
5.49
|
Earthy short
clay. Gravel at 30ft. Nearly usable.
|
|
13
|
SS53SW74
|
35.20
|
32.46
|
26.36
|
26.36
|
6.10
|
Good clay with
sand layers. Gravel and water at 27ft.
|
|
14
|
SS53SW75
|
35.10
|
31.75
|
27.48
|
27.48
|
4.27
|
Dirty at 11ft;
good clay from 13ft. Gravel ~23ft.
|
|
15
|
SS53SW76
|
31.60
|
27.03
|
23.98
|
23.98
|
3.05
|
Clean clay
17–27ft. ~10ft usable. Gravel at 27ft.
|
|
16
|
SS53SW77
|
35.00
|
31.65
|
27.99
|
27.99
|
3.66
|
Sandy clay below
15ft. Gravel touched at 21ft.
|
|
17
|
SS53SW78
|
25.20
|
21.85
|
15.75
|
15.75
|
6.10
|
Good clay
11–31ft. Gravel at 31ft. Recommended.
|
|
18
|
SS53SW79
|
24.70
|
22.57
|
15.25
|
15.25
|
7.32
|
Best hole. Good
clay 7–31ft. Gravel at 31ft. Recommended.
|
Table 2. Gravel-bearing boreholes with full
OD stratigraphy. Gravel top OD = base of potters clay / top of glaciofluvial
outwash.
The gravel top OD values — the base of the
potters clay — range from 15.3 m OD (BH18) to 28.0 m OD (BH16), with a mean of
approximately 21 m OD. This is consistent with the Lake cutting data from
Cattell (2003), who recorded the clay–gravel contact at 19.9 and 20.4 m OD to
the north of the A39, and at 20–26 m OD in the cutting itself (the higher
figure representing the dipping southern margin). The agreement across two
independent datasets, separated by approximately 1 km and derived from
completely different investigation methods, is strong confirmation that these
holes are sampling the same lacustrine basin floor.
Boreholes 17 and 18 — in Hollands land north of
the old pit workings — show the lowest gravel OD values (15.3 and 15.8 m OD)
and the thickest good-quality clay (6.1 and 7.3 m respectively). These are the
deepest parts of the lacustrine basin reached by this survey and were correctly
identified in the 1972 narrative as the best commercial prospects. Their clay
top OD values (21.9 and 22.6 m OD) represent the lake floor level most closely.
Boreholes 12–16 show clay tops at 27.0–32.8 m
OD and gravel at 24.0–28.0 m OD. These elevated values — compared to BH17 and
BH18 — indicate that these holes are on the southern basin margin, where the
deposit shallows toward the Carboniferous shale bedrock ridge to the south.
Borehole 12 in particular (clay top 32.8 m OD) may be on the outermost southern
rim of the basin.
5.2 The High-Elevation Holes: Above
the Lacustrine Basin
Boreholes 9, 10, and 11 — located in the
Roundswell–Brynsworthy area to the south-east of the claypits — show surface
elevations of 41.2, 42.5, and 42.8 m OD respectively. Their clay tops,
calculated from the depth records, are at 38.5, 39.8, and 40.1 m OD. Table 3
presents their OD data alongside the expected lacustrine range.
|
BH
|
Surface OD (m)
|
Clay top OD (m)
|
Expected lacustrine clay top (~20–27 m
OD)
|
Elevation above expected range (m)
|
Interpretation
|
|
9
|
41.20
|
38.46
|
20–27 m OD
|
~12–18 m ABOVE
|
Head/solifluction.
Not lacustrine clay.
|
|
10
|
42.50
|
39.76
|
20–27 m OD
|
~13–20 m ABOVE
|
Head/solifluction.
Not lacustrine clay.
|
|
11
|
42.80
|
40.06
|
20–27 m OD
|
~13–20 m ABOVE
|
Head/solifluction
dominant. 0.3 m clean clay only at 30.3 m OD (borehole base).
|
Table 3. High-elevation non-gravel
boreholes. Clay tops are 12–20 m above the expected lacustrine clay level.
The significance of these figures cannot be
overstated. The clay top in Boreholes 9, 10, and 11 sits at 38–40 m OD. The
lacustrine potters clay surface confirmed by the gravel-bearing holes and by
the Lake cutting data lies at approximately 20–27 m OD. The material
encountered in Boreholes 9–11 begins approximately 15 metres above the
lacustrine basin. It is periglacial head and solifluction: locally-derived
stony silty clay produced by frost action and downslope movement during
Pleistocene cold stages, not glaciolacustrine sediment.
This is precisely what the drillers themselves
observed. Their log notes for these holes describe material that is 'very dirty
and earthy', 'shorter and more earthy than the others', and — for Borehole 9 —
'this seemed dirtier and more earthy and sandy than the others... very wet
throughout.' These are the sensory characteristics of solifluction head, not of
the smooth, homogeneous potters clay.
Borehole 11, the deepest hole at 41 feet (12.5
m), is the most instructive case. Drilled specifically to test whether clean
potters clay existed in the Roundswell area, it encountered dirty clay from 9
feet downward, with clean clay only appearing at 40 feet — in the final foot
before the borehole was terminated. That 1-foot (0.3 m) horizon of clean clay,
at 30.3 m OD, is approaching the upper range of the expected lacustrine basin
but still above the confirmed gravel-bearing levels. The borehole was stopped
before reaching the diagnostic base; the clean clay at the base of BH11 may
represent the first appearance of the lacustrine unit, but this was not
confirmed.
5.3 The Intermediate Holes: Head
Masking the Basin
The remaining non-gravel holes — Boreholes 2,
3, 4, 5, 7, and 8 — show surface ODs of 27.7–37.6 m and clay tops at 22.8–31.4
m OD. These are less clear-cut than Boreholes 9–11, and several fall within or
close to the upper range of the expected lacustrine clay zone. However, without
the diagnostic basal gravel, the lacustrine stratigraphy cannot be confirmed in
any of them.
The clay quality descriptions for all six of
these holes are consistently negative: 'dirty', 'worsens with depth', 'very
dirty and earthy'. None produced clay considered commercially usable by
Brannam's. This is consistent with the interpretation that these holes are
penetrating the thick head sequence overlying the lacustrine basin, without
reaching the potters clay below — or are on the margins of the basin where head
is thick and the lacustrine unit thin.
Borehole 1 — surface OD 25.8 m, no clay in 20
feet — was drilled in the central part of the survey area, approximately 460 m
east-southeast of the Higher Gorse claypits. Its position at 25.8 m OD and
failure to find clay suggest it may sit on a bedrock high or at the edge of a
gap in the lacustrine deposit within the basin, rather than at the outer
western margin.
5.4 Comparison with Published OD
Datums
Table 4 compiles all confirmed Potter’s Clay
top OD values from every independent source: the 1972 Brannam boreholes, the
Lake cutting investigations, Maw’s Roundswell well datum, and the additional
A39 bypass borehole 703458. Only boreholes and sections where the lacustrine
clay was positively identified are included; the non-gravel holes (BH2–5, 7–11)
and the 12 head-dominated additional boreholes are excluded, as they did not
confirm the diagnostic lacustrine stratigraphy.
|
Source
|
Location
|
Position
|
Potter’s Clay Top OD (m)
|
Notes
|
|
Basin
floor: Potter’s Clay top ~20–23 m OD
|
|
Cattell (2003)
|
Lake cutting — N
of road
|
Basin floor
|
~20–21
|
Flat basin floor
north of A39.
|
|
This survey —
BH17
|
Hollands land, N
of old pits
|
Basin floor
|
21.85
|
Best commercial
hole. Gravel at 15.75 m OD.
|
|
This survey —
BH18
|
Hollands land, N
of old pits
|
Basin floor
|
22.57
|
Best hole
overall. Gravel at 15.25 m OD.
|
|
Additional BHs —
703458
|
~253950, 132050
(A39 bypass)
|
Basin floor
|
~20.76
|
3.05 m
Fremington Clay. Labelled by driller.
|
|
Basin
margin: Potter’s Clay top ~26–33 m OD
|
|
Maw (1864)
|
Roundswell well
(~1 km SE)
|
Basin margin
|
~26.8
|
Clean clay top
below 12 ft stony reworked material.
|
|
Cattell (2003)
|
Lake cutting — S
crest
|
Basin margin
|
~26
|
Dipping S
margin; 11° N dip. Southern limit ~200 m S.
|
|
H & H (1990)
|
Lake cutting
Ch.11000
|
Basin margin
|
~27–29
|
Unit C
(lacustrine) top, below head (A) and till (B).
|
|
This survey —
BH15
|
Nr Higher Gorse
|
Basin margin
|
27.03
|
Clean clay 17–27
ft. Gravel at 23.98 m OD.
|
|
This survey —
BH6
|
Nr Higher Gorse
|
Basin margin
|
20.04
|
Gravel at 16.38
m OD. Transitional to floor.
|
|
This survey —
BH16
|
Nr Brynsworthy
|
Basin margin
|
31.65
|
Sandy clay.
Gravel at 27.99 m OD.
|
|
This survey —
BH14
|
Nr Brynsworthy
|
Basin margin
|
31.75
|
Good clay from
13 ft. Gravel at 27.48 m OD.
|
|
This survey —
BH13
|
Nr Brynsworthy
|
Basin margin
|
32.46
|
Good clay with
sand layers. Gravel at 26.36 m OD.
|
|
This survey —
BH12
|
Nr Higher Gorse
|
Outermost rim
|
32.84
|
Earthy clay.
Gravel at 27.36 m OD. Southern limit.
|
Table 4. Confirmed Potter’s Clay top OD from
all independent sources, grouped by basin position.
The pattern across all sources is consistent.
On the basin floor, the Potter’s Clay top clusters tightly between ~20 and 23 m
OD — the level at which the lake sediment accumulated. On the basin margins,
the clay top rises to ~26–29 m OD as the deposit thins and laps onto the
bedrock slopes, with the outermost rim (BH12) at 32.8 m OD. The convergence of
the margin values from four independent datasets — Maw’s Roundswell well (~26.8
m), Cattell’s Lake cutting southern crest (~26 m), Hawkins and Hawkins’s Unit C
top (~27–29 m), and the 1972 boreholes BH12–16 (~27–33 m) — implies a maximum
lake surface at approximately 27–29 m OD. This is the level of the ice-dammed
proglacial lake in which the Potter’s Clay was deposited: above this elevation,
no lacustrine clay exists, and all fine-grained superficial material is
periglacial head.
5.5 Corroborating Evidence from
Additional Area Boreholes
Figure 2: BGS map of Fremington Area showing “Till” in light blues and
borehole locations.
The spatial pattern established by the 1972
Brannam survey is strongly corroborated by a representative sample of 13
additional boreholes from the wider area, drawn from BGS archival records
including A39 Barnstaple Bypass site investigations and other geotechnical
surveys. These holes, summarised in Table 5a, span the terrain from the bypass
corridor in the east (~255 317) through Roundswell (~253 305) to the western
approaches (~248 314), and provide independent control on the distribution of
the potter’s clay outside the 1972 survey footprint.
Of the 13 boreholes examined, 12 are dominated
entirely by head deposits: stony, mottled, gravelly clay sitting directly over
gravel or bedrock, with no trace of the diagnostic stone-free lacustrine clay.
Their descriptions — firm to stiff yellow-brown or red-brown sandy silty clay
with much subangular gravel, occasional cobbles, and variable sand content —
are characteristic of periglacial solifluction head derived from local
Carboniferous and Devonian sources. None records the smooth, homogeneous, stone-free
clay that defines the potter’s clay proper.
The single exception is borehole 703458
(SS53SW54), located at ~253950, 132050 — centrally within the basin area at a
surface OD of 22.7 m. This hole, drilled as part of the A39 Barnstaple Bypass
investigation in August–September 1982 by Norwest Holst for the Department of
Transport, records a substantive sequence of firm to stiff red-brown slightly
silty clay from 1.95 m to 5.0 m depth (20.76–17.71 m OD), explicitly labelled
“(FREMINGTON CLAY)” by the driller. This 3.05 m unit is underlain by a 0.2 m basal
transition (firm to stiff brown slightly sandy silty clay with yellow-brown
silty sand laminations and occasional sub-rounded gravel), then by soft brown
and greenish grey sandy clayey silt with much grey subangular gravel, resting
on probable siltstone bedrock at 7.5 m depth (15.21 m OD). The clay top OD of
~20.76 m and the presence of a relatively stone-free, cohesive lacustrine clay
sequence are fully consistent with the basin-floor geometry established by the
1972 gravel-bearing boreholes (BH17 clay top 21.85 m OD; BH18 clay top 22.57 m
OD) and by Cattell’s (2003) Lake cutting data (~20–21 m OD on the basin floor).
The pattern from these additional holes
reinforces the central conclusion of the OD analysis: genuine lacustrine
potter’s clay is confined to the low-elevation basin floor, and the great
majority of the surrounding terrain mapped as Boulder Clay on Sheet 293
contains only periglacial head. The commercial behaviour of C.H. Brannam Ltd is
instructive here. Brannam’s applied what might be called Sutton’s Law — they
looked where the clay was most likely to be. Guided by T.R. Wood’s seismic
survey and by generations of direct working knowledge, they targeted their 1972
boreholes on the low-elevation ground around the existing pits. They did not
bore in the ground mapped as Fremington Clay south of the Hele–Bickington
ridge. A pottery firm whose livelihood depended on finding workable clay,
guided by geophysical survey, would not have overlooked accessible reserves if
they existed. Their silence on the southern ground — Sutton’s Law applied in
reverse — is itself evidence that the mapped Fremington Clay extent in that
area reflects head and reworked boulder clay rather than the genuine lacustrine
deposit.
6.
The Edmonds Observation and the Misreading of Maw
Edmonds’ observation in the 1985 memoir — that
gravel was found only in holes that penetrated smooth clay — correctly
identified the diagnostic criterion for lacustrine stratigraphy. The OD
analysis presented in this report quantifies its implications: the 10 holes
without gravel have clay tops at 22.8–40.1 m OD, significantly above the 20–27
m OD range confirmed by the 8 gravel-bearing holes and by the Lake cutting
data. In the three highest-elevation cases (BH9–11), the clay tops are 12–15 m
above the lacustrine range. These holes were drilling through periglacial head
throughout. The mapping of all 18 holes as ‘Boulder Clay’ on Sheet 293 followed
the Stephensian framework rather than the stratigraphic logic of Edmonds’ own
borehole evidence.
6.1 The Stepwise Conflation
The path from Maw's careful description to the
BGS's consolidated 'Boulder Clay' can be traced through five steps:
Step 1: Taylor (1956) grouped 'Saunton and
Fremington erratics' together, conflating coastal boulders of sea-ice origin
with clay-embedded clasts of different transport history. This inflated the
apparent exotic component of the inland deposit and gave the impression of a
more extensively glaciated area than the evidence warranted.
Step 2: Stephens (1966) adopted the full
glacial till interpretation, treating the entire surface-to-bedrock sequence as
a single boulder clay of Wolstonian age, without distinguishing the upper head
from the lower lacustrine clay. This paper became the standard reference for
age and character of the deposit.
Step 3: Edmonds et al. (1985) surveyed Sheet
293 within the Stephensian framework, drew an admitted arbitrary boundary
between boulder clay and pebbly drift, and mapped the full valley floor
superficial as Boulder Clay. The 1972 borehole data, which contained the
diagnostic information to restrict the mapped extent, was incorporated without
the OD conversion that would have made its implications apparent.
Step 4: Croot et al. (1996), working at the
type section in the old clay pits, focused on the lacustrine unit proper and
confirmed its glaciolacustrine character with excellent sedimentological and
geotechnical data. Their work is authoritative for the lacustrine unit but does
not address the question of how much of the BGS mapped area represents genuine
lacustrine clay rather than overlying head.
Step 5: Bennett et al. (2024), the current
authoritative synthesis, accepted the BGS mapped extent as the starting
distribution for the deposit. The Hawkins & Hawkins (1990) and Cattell
(2003) findings — which directly reinstated Maw's distinction at the field
observation level — were not connected back to the subsurface data or the
mapped boundary.
6.2 What the OD Data Restore
The OD analysis presented in this report closes
the loop that Edmonds opened in 1985. By establishing that the gravel-bearing
holes cluster consistently around a potters clay base of 15–28 m OD —
consistent with all independent published datums — and that the non-gravel
holes at 38–40 m OD are clearly above the lacustrine basin, it provides the
quantitative foundation for a revised mapped extent.
The true spatial boundary of the lacustrine
potters clay is not coextensive with the BGS Boulder Clay polygon. It is
confined to the area where the three-tier stratigraphy (potters clay on
glaciofluvial gravel on bedrock) can be confirmed, which from the 1972 borehole
data is limited to the vicinity of the Higher Gorse claypits and the Hollands
land area to their north. The apparent extent of the deposit on Sheet 293
reflects the distribution of all fine-grained superficial material in the
valley — lacustrine, head, and reworked — mapped as a single undifferentiated
unit within a glacial interpretive framework that Maw himself never endorsed.
7.
Summary: OD Classification of All 18 Boreholes
Table 5 presents the complete OD classification
of all 18 boreholes, with each hole's status relative to the lacustrine basin
and its implications for the BGS mapping.
|
BH
|
Surface OD (m)
|
Clay top OD (m)
|
Gravel OD (m)
|
Status
|
BGS Sheet 293 mapping implication
|
|
1
|
25.8
|
NIL
|
—
|
Outside basin
|
Boulder
Clay mapping DOUBTFUL — no clay
|
|
2
|
37.6
|
30.0
|
—
|
Partial — base unknown
|
Boulder
Clay mapping UNCERTAIN
|
|
3
|
35.1
|
31.1
|
—
|
Elevated margin
|
Boulder
Clay mapping UNCERTAIN
|
|
4
|
32.8
|
29.4
|
—
|
Partial — base unknown
|
Boulder
Clay mapping UNCERTAIN
|
|
5
|
27.7
|
22.8
|
—
|
Partial — base unknown
|
Boulder
Clay mapping UNCERTAIN
|
|
6
|
24.0
|
20.0
|
16.4
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
7
|
35.2
|
31.2
|
—
|
Elevated margin
|
Boulder
Clay mapping UNCERTAIN
|
|
8
|
34.7
|
31.3
|
—
|
Elevated margin
|
Boulder
Clay mapping UNCERTAIN
|
|
9
|
41.2
|
38.5
|
—
|
Head — above basin
|
Boulder
Clay mapping INVALID — head only
|
|
10
|
42.5
|
39.8
|
—
|
Head — above basin
|
Boulder
Clay mapping INVALID — head only
|
|
11
|
42.8
|
40.1
|
—
|
Head — above basin
|
Boulder
Clay mapping INVALID — head only
|
|
12
|
36.5
|
32.8
|
27.4
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
13
|
35.2
|
32.5
|
26.4
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
14
|
35.1
|
31.7
|
27.5
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
15
|
31.6
|
27.0
|
24.0
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
16
|
35.0
|
31.6
|
28.0
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
17
|
25.2
|
21.8
|
15.8
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
|
18
|
24.7
|
22.6
|
15.3
|
Lacustrine confirmed
|
Boulder
Clay mapping VALID
|
Table 5. OD classification summary. Green =
confirmed lacustrine. Red = head above basin. Amber = elevated margin or
uncertain. Grey = outside basin.
8.
Conclusions
The following conclusions are drawn from the
OD-referenced analysis of the 18 Brannam boreholes (BGS SS53SW62–79, 1972):
(1) The
lacustrine potters clay — Maw's original deposit — is confirmed in 8 of 18
boreholes by the presence of the diagnostic basal glaciofluvial gravel. The
gravel top OD in these holes ranges from 15.3 to 28.0 m OD, consistent with a
basin floor at approximately 20–22 m OD and a basin margin rising to
approximately 28 m OD. This is fully consistent with Cattell's (2003) Lake
cutting data (gravel top 19.9–26 m OD) and with Maw's (1864) Roundswell well
datum (~26.8 m OD for the clean clay top).
(2)
Three boreholes (BH9, BH10, BH11) have collars at 41.2–42.8 m OD, with
clay tops at 38.5–40.1 m OD — 12 to 20 metres above the lacustrine basin. The
material encountered in these holes is periglacial head and solifluction, not
potters clay. Their classification by Edmonds et al. (1985) as 'boulder clay'
extended the mapped extent of the formation into terrain that contains no
lacustrine sediment.
(3) The
remaining 7 non-gravel holes (BH2–8) show clay tops at 22.8–31.4 m OD. Without
the diagnostic basal gravel, the presence of lacustrine clay cannot be
confirmed in any of these holes. The uniformly poor clay quality (dirty,
earthy, contaminated) is more consistent with periglacial head overlying the
lacustrine unit than with the lacustrine unit itself.
(4) The
true spatial extent of the Fremington lacustrine potters clay is confined to
the area where the three-tier stratigraphy can be confirmed. From the 1972
borehole data, this is centred on the Higher Gorse claypits area and the
Hollands land to the north (the area recommended by Brannam's for commercial
expansion), with the basin margins rising steeply to the south (BH12–16) and
south-east (BH9–11, Roundswell area). The BGS Sheet 293 Boulder Clay polygon
substantially overstates this extent.
9.
Recommendations for Further Work
The
OD-referenced analysis presented here provides the quantitative subsurface
framework needed to revise the mapped extent of the true glaciolacustrine
potter’s clay. The following targeted steps are recommended to complete the
rigorous geological map and fully resolve the head-mixing issue identified in
the literature:
- Further borehole analysis A representative
sample of the additional archival borehole records from the Barnstaple
Bypass (A39) construction and associated site investigations has now been
analysed and incorporated into this report (Section 5.5). The results
confirm the spatial model established by the 1972 Brannam dataset: genuine
lacustrine potter’s clay is confined to the low-elevation basin, with head
dominant elsewhere. The remaining unsampled archival records could be
OD-referenced for completeness, but given the consistency of the pattern
already established across multiple independent datasets, they are
unlikely to alter the conclusions materially. Priority should instead be
given to the corrected map and cross-sections (below) and to targeted
investigation of the ground south of the Hele–Bickington ridge.
- Preparation of a corrected map and
cross-sections Using the validated 1972 dataset together with the new
borehole control, a revised geological map should be produced that
separates the confirmed lacustrine potter’s clay (stoneless, gravel-based,
low-elevation basin fill) from the overlying periglacial head.
Cross-sections (e.g., Lake–Roundswell–Bickington and north–south transects
across the Hele–Bickington ridge) will illustrate the true geometry of the
glacial clays, the northward 11° basin-margin dip, and the extent of head
overprinting. These outputs will form the core of the final project map.
- Investigation of the area south of the
Hele–Bickington ridge The ground mapped as Fremington Clay south of
the Hele–Bickington ridge (currently included in the BGS Sheet 293 Boulder
Clay polygon) should be prioritised for targeted field inspection and
shallow boreholes. On the balance of probabilities this area represents
reworked fluvial boulder clay or head derived from local Carboniferous
sources rather than the glaciolacustrine potter’s clay. This
interpretation is supported by the negative evidence from the additional
area boreholes (Section 5.5), none of which found lacustrine clay south of
the ridge, and by the Sutton’s Law argument set out there: Brannam’s,
guided by seismic survey, showed no interest in this ground as a source of
workable clay. Confirmation here will further contract the true
glacial-clay extent and eliminate the unhelpful expansion introduced by
later mapping.
- Implications for understanding of glacial
intrusion into the area This reinterpretation severely limits the
inferred onshore reach of Irish Sea ice during the relevant Pleistocene
stage (Anglian/MIS 12 favoured). The potter’s clay is now shown to be
confined to a low-elevation (~15–28 m OD) proglacial lake basin rather
than a widespread till sheet. The dominant head overprint and absence of
thick, stony till across much of the mapped area indicate only marginal
ice-dammed ponding in the Taw estuary, not deep glacial overriding of the
Devon landscape. Preliminary thoughts suggest the ice front was pinned
against the Bristol Channel coast, with the lake forming in the lee of the
ice dam; this model is consistent with sparse/local erratics and the lack
of far-travelled Irish Sea indicators in the clean lacustrine facies.
- Implications for deeper glacial intrusion onto
land The revised extent and elevation data constrain any model of
deeper glacial intrusion onto land and, by extension, the viability of
long-distance glacial rafting or transport of distant erratics (including
potential bluestone sources) across Devon to Stonehenge. A thick, grounded
Irish Sea ice sheet capable of such transport is incompatible with the
restricted, low-elevation lacustrine basin documented here. Further work
on the corrected map will allow precise contouring of the maximum
ice-surface elevation in the Bristol Channel, providing a firmer limit on
onshore incursion and strengthening (or ruling out) glacial-transport
hypotheses.
References
Maw, G. (1864). On a supposed deposit of boulder-clay
in North Devon. Quarterly Journal of the Geological Society of London, 20,
445–451.
Dewey, H. (1910). Notes on some igneous rocks from
North Devon. Proceedings of the Geologists' Association, 21(4), 429–434.
Taylor, C.W. (1956). Erratics of the Saunton and
Fremington areas. Report and Transactions of the Devonshire Association, 88,
52–64.
Arber, M.A. (1964). Erratic boulders within the
Fremington Clay of North Devon. Geological Magazine, 101(3), 282–283.
Stephens, N. (1966). Some Pleistocene deposits in
North Devon. Biuletyn Peryglacjalny, 15, 103–114.
Edmonds, E.A. (1972). The Pleistocene history of the
Barnstaple area. Institute of Geological Sciences Report 72/2.
Kidson, C. & Wood, R. (1974). The Pleistocene
stratigraphy of Barnstaple Bay. Proceedings of the Geologists' Association,
85(2), 223–237.
Wood, T.R. (1974). Quaternary deposits around
Fremington. In: Straw, A. (Ed.), QRA Easter Field Meeting Handbook. Quaternary
Research Association, Exeter, pp. 30–34.
Madgett, P.A. & Inglis, E.A. (1987). A
re-appraisal of the erratic suite of the Saunton and Croyde areas. Transactions
of the Devonshire Association, 119, 135–144.
Edmonds, E.A., Whittaker, A. & Williams, B.J.
(1985). Geology of the country around Ilfracombe and Barnstaple. Memoir of the
British Geological Survey, Sheets 277 and 293 (England and Wales). HMSO,
London. ISBN 0 11 884364 8.
Hawkins, A.B. & Hawkins, S.C. (1990). Quaternary
deposits in the Lake cutting of the Barnstaple Bypass, North Devon. Proceedings
of the Ussher Society, 7, 301–303.
Croot, D.G., Gilbert, A., Griffiths, J. & van der
Meer, J.J. (1996). The character, age and depositional environments of the
Fremington Clay Series, North Devon. Quaternary Newsletter, 80, 1–15.
Cattell, A. (2003). Geological and geotechnical
aspects of a landslip in the Fremington Clay, North Devon. Geoscience in
South-West England, 10, 397–402.
Bennett, J.A., Cullingford, R.A., Gibbard, P.L.,
Hughes, P.D. & Murton, J.B. (2024). The Quaternary geology of Devon.
Proceedings of the Ussher Society, 15, 84–130.
Daw, T., Ixer, R. & Madgett, P. (2026). A review
of the Ramson Cliff erratic: evidence of high-level ice flow? Quaternary
Newsletter, 167, 13–19. DOI: 10.64926/qn.20517.
Appendix A: Unified Borehole Register
The accompanying spreadsheet
(Appendix_A_Unified_Boreholes.xlsx) presents the complete borehole register for
all 31 boreholes referenced in this report: the 18 holes of the 1972 Brannam
survey (BGS SS53SW62–79) and 13 additional boreholes from A39 Barnstaple Bypass
site investigations and other archival BGS records.
For each borehole the register includes: BGS
reference, BGS ID, corrected grid position (BNG), surface OD, overburden
thickness, clay top OD, clay thickness, clay base OD, gravel OD (where
reached), classification status, and descriptive notes. Brannam borehole depths
are converted from imperial feet using 1 ft = 0.3048 m. Surface OD values are
derived from the OS terrain model at corrected borehole positions.
Colour coding: green = lacustrine stratigraphy
confirmed (diagnostic basal gravel reached or Fremington Clay positively
identified); red = head above basin (clay top >38 m OD); amber = elevated
margin or base unknown; grey = outside basin or no clay found. Of the 31
boreholes, 9 confirm lacustrine stratigraphy (8 Brannam gravel-bearing holes
plus borehole 703458), 3 are classified as head above the basin (BH9–11), and
the remainder are either on the basin margin with unconfirmed base, or
dominated entirely by head deposits.
