PRESELI DOLERITE BLUESTONES: AXE-HEADS, STONEHENGE MONOLITHS, AND OUTCROP SOURCES
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
PRESELI DOLERITE BLUESTONES: AXE-HEADS,
STONEHENGE MONOLITHS, AND OUTCROP SOURCES
Summary. Chemical compositions and magnetic susceptibility data were
compared for 12 dolerite bluestone implements including axes, axe-hammers
and battle-axes, 11 Stonehenge monoliths (chemical data only), and potential
source outcrops in Preseli, South Wales. Most of the studied artefacts are of
spotted dolerite, a small number being unspotted dolerite. Bivariate graphs,
discriminant analysis and t-tests were used singly and in combination to show,
respectively, that the implements found at sites in England are mainly similar
to Stonehenge monoliths, while the implements found in Wales have a variety
of compositions and are much less similar to Stonehenge monoliths. The
dichotomy between English and Welsh dolerite bluestone implements could be
explained by exploitation of different Preseli outcrops or erratic assemblages
derived from them. A small number of spotted dolerite implements have
previously been shown to have chemical compositions atypical of and marginal
to Preseli, suggesting the possibility of a source of spotted dolerite outside
Preseli. Previously published analytical data in combination with the new
implement/outcrop comparisons presented in this paper support derivation of
the majority of analysed Stonehenge monoliths at one particular outcrop within
the group of four identified by Thorpe et al. 15 years ago. Analysis of all the
extant bluestone monoliths at Stonehenge (now possible using non-destructive
methods) would allow progress in identifying monolith outcrop sources, and in
understanding the links with the bluestone axe trade.
introduction and background
Archaeological ‘bluestone’ studies have traditionally focused on the stones used for the
Stonehenge monoliths, but more recently attention has turned to the polished axe-heads and
other implements manufactured from bluestone (Williams-Thorpe et al. 1999; Jones and
Williams-Thorpe 2001; Williams-Thorpe et al. 2004).
In this paper, we bring together information on the sources of dolerite bluestone
implements, and of the Stonehenge monoliths, in order to comment on the relationship between
these two groups of artefacts. In particular, we examine the question of whether the monoliths
and the implements originated at the same sources within Preseli, and what this tells us about
the links between the procurement of bluestone for Stonehenge, and the bluestone employed in
England and Wales for implements.
OXFORD JOURNAL OF ARCHAEOLOGY 25(1) 29–46 2006
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd., 9600 Garsington Road, Oxford OX4 2DQ, UK
and 350 Main Street Malden, MA 02148, USA. 29PRESELI DOLERITE BLUESTONES
Recent non-destructive geochemical and magnetic studies of spotted dolerite
implements (including axes, axe-hammers, battle-axes and maces) showed that fewer than a
dozen can be reliably assigned to the Preseli source area (Williams-Thorpe et al. 2004). That
work resulted in a revised, and much reduced, distribution of these implements (which are also
referred to as ‘Group XIII’, following the terminology of the British Implement Petrology
Committee (IPC); Clough and Cummins (1988)).
Within the dataset for the implements confirmed as Preseli by Williams-Thorpe et al.
(op. cit.), there appeared to be small variations in chemical and magnetic characteristics,
suggesting that not all originated at the same outcrop source(s) within Preseli. This observation
prompted us to investigate whether these small differences were statistically significant, and to
assess the feasibility of provenance determinations at outcrop level. In particular, we wished to
investigate the relationship between outcrop origins and archaeological aspects such as find-
location and morphology of the implements.
Eleven (out of 27 remaining) Stonehenge monoliths of dolerite (nine spotted, two
unspotted) were analysed by Thorpe et al. (1991). They were all confirmed as Preseli in origin,
and most were linked to a fairly general source area of eastern Preseli (four outcrops, including
Carnmenyn which is the outcrop traditionally regarded as the Stonehenge bluestone source).
table 1
Chemical compositions and magnetic susceptibility of Preseli dolerite implements, and chemical compositions of
selected Stonehenge monoliths
Reference SH33 Ha41 Wi118 Wi109 Wi108 SH44* Wi302*
Find location Stonehenge Bankes Stockton Stonehenge Stonehenge Stonehenge Wilsford
Heath, Earthworks south
Bournemouth barrow 54
Artefact type monolith axe axe miscellaneous miscellaneous monolith battle-axe
artefact artefact
fragment fragment
K2O wt % 0.18 0.54 0.42 0.31 0.36 0.42 0.72
CaO 11.30 9.96 11.11 10.49 12.83 9.06 13.29
TiO2 1.15 0.96 0.74 1.01 0.71 1.30 1.29
MnO 0.16 0.10 0.11 0.15 0.13 0.19 0.12
Fe2O3 9.27 9.28 9.05 9.52 9.94 11.80 8.88
Ba ppm 120 100 173 122 144 225 207
Nb 6 4 1 3 2 7 4
Rb 10 5 2 −1 2 12 8
Sr 225 215 199 239 173 235 239
V 210 159 220 203 220 240 257
Y 21 13 16 14 17 23 24
Zr 67 59 57 55 61 71 73
PXRF n 6 5 5 5 5
Mean magnetic 0.52 0.44 0.47 0.43 0.61
susceptibility
(10−3 SI)
s.d. 0.04 0.04 0.04 0.03 0.05
Mag sus. n 10 6 10 6 10
Notes:
Data from Thorpe et al. (1991) (monoliths), and Jones and Williams-Thorpe 2001 and Williams-Thorpe et al. 2004 (implements; Pb omitted
(large s.d.) and V added to support groupings (new data, rsd 20–50 %)); analytical details in those papers.
Major and minor elements are given as weight % (iron as Fe2O3 total iron), trace elements as ppm. Negative values are retained, so means can
be zero.
The implements are grouped according to their chemical compositions (cf. text discussions).
All artefacts and monoliths are spotted dolerite except those marked*, which are unspotted dolerite with Preseli compositions.
Artefact types after Thorpe et al. op. cit. and Williams-Thorpe et al. op. cit. table 4.
Abbreviations: An Anglesey, Car Carmarthenshire, Dev Devon, Do Dorset, Ha Hampshire, M Monmouthshire, Me Merionethshire, Ra
Radnorshire, Wi Wiltshire; SH Stonehenge.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
30 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
The chemical database given by Thorpe et al. (op. cit.) allows us to comment on the likelihood
of these 11 monoliths, and Preseli dolerite implements, coming from the same outcrop sources,
or from different outcrop sources. This information will help us to interpret the putative links
between the selection of raw material for axes and other implements, and for Stonehenge
monoliths (cf. e.g. Atkinson 1979; Green 1997).
To support the present paper, we will draw on our recent in situ characterization of
outcrops of the Preseli spotted dolerite area using portable X-ray fluorescence (PXRF) (Jones
et al. 2005) to extend the available information on the chemical composition of potential source
outcrops.
the axes and other implements
Twelve implements that were linked to a Preseli source by Williams-Thorpe et al.
(2004) are considered in the present paper, and compositions of these implements are listed in
Table 1 (together with other analytical data, which are discussed later). Eight artefacts in Table
1 are of Preseli spotted dolerite, and two (Wi302 and An10) are of Preseli unspotted dolerite
(strictly, therefore, members of IPC Group XXIIIb rather than XIII; cf. Clough and Cummins
Do41 Dev1 Car23 Ra4 M8 Me8 An10* SH42
Maiden Castle High Peak, Trelech a’r Clap yr Arian, St Brides, Bwlch-Gwyn Llanfaethlu Stonehenge
Sidmouth Bettws Llansantffraed- Netherwent Farm, near
Cwmdeuddwr Arthog
reworked axe mace battle-axe battle-axe axe axe-hammer axe-hammer monolith
0.72 0.62 1.76 0.75 0.99 0.99 0.36 0.15
11.61 9.67 8.07 9.68 9.68 9.31 5.91 11.53
1.12 0.56 1.12 1.07 0.73 0.70 0.96 1.37
0.17 0.13 0.14 0.12 0.15
9.04 9.49 9.06 9.43 10.94 8.75 8.54 9.17
222 129 115 98 132 192 23 123
6 4 4 4 0 −6 −5 7
3 −2 10 −3 −5 32 −1 8
225 210 230 256 329 518 246 235
249 267 231 245 213
22 14 14 19 11 10 60 31
72 50 64 81 72 94 98 101
5 5 5 5 5 1 1
0.43 0.44 0.40 0.59 0.44 0.52 0.54
0.03 0.02 0.05 0.05 0.02 0.05 0.02
6 5 21 11 5 6 5
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 31PRESELI DOLERITE BLUESTONES
1988). M8 and Me8 have spotted dolerite mineralogy, but chemical compositions that are
marginal to Preseli (cf. Williams-Thorpe et al. op. cit., 372).
The dating evidence and styles of the implements were noted by Williams-Thorpe et
al. (2004). In summary, they include Neolithic as well as Bronze Age types, and rare round
barrow and Beaker associations. Three battle-axes – Car23, Ra4 and Wi302 – stand out as more
developed in style, and are the most carefully finished and highly polished of the pieces studied.
The artefacts were found variously in southern England and in Wales, and the find locations are
shown on Figure 1.
the preseli source outcrops, and characterizing data available
Spotted dolerites crop out within a small (c.5 × 2 km) area of eastern Preseli (Fig. 2).
They form one part of the much larger south-west Wales Lower Palaeozoic volcanic and
intrusive complex (Bevins et al. 1989), within which spotted dolerite rocks are reported only
within the area shown on Figure 2 (Evans 1945; cf. Thorpe et al. 1991).
Previous analytical provenancing studies of spotted dolerite artefacts have relied mainly
on comparative source data drawn from conventional laboratory X-ray fluorescence analyses of
rock samples (Bevins et al. 1989; Thorpe et al. 1991; summary in Jones and Williams-Thorpe
2001). These data represent very good coverage of the Preseli spotted dolerite area as a whole
(they include 45 samples from 14 outcrops), but several of the outcrops are represented by the
compositions of only one or two rock samples each.
Jones et al. (2005) used a new PXRF survey of Preseli dolerite outcrops to show that
the geochemical variance (distinguished from analytical and sampling variances) within a single
outcrop often exceeds the variance derived from the limited sample analysis that is traditionally
used to characterize sources for geological work. Jones et al. also considered some of the Preseli
outcrops, in particular Carnmenyn, Carnbreseb and Cerrigmarchogion, to be geochemically
heterogeneous for some elements.
Bearing in mind this intra-outcrop variation, for our investigation of outcrop-specific
provenancing of artefacts we will use the PXRF database reported by Jones et al., which
provides more detail for individual outcrops. The dataset comprises 122 PXRF measurements
made at 55 locations dispersed over eight outcrops representing good geographical coverage of
the Preseli spotted dolerite area (Fig. 2).
In addition to the geochemical data, a recent survey of magnetic suceptibility of Preseli
outcrops, reported in summary by Williams-Thorpe et al. (2004), offers useful characterizing
data. The magnetic susceptibility dataset comprises 853 measurements dispersed over 15
outcrops or discrete exposures. Mean measurements for these 15 areas are given in Table 2 and
their geographical distribution is shown in Figure 2 above.
While Williams-Thorpe et al. considered the magnetic susceptibility in terms of a range
for the whole of the Preseli spotted dolerites (op. cit., 366), in the present paper we shall
investigate these data in terms of individual outcrops in order to identify any distinctions that
might be useful in provenancing artefacts.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
32 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
Figure 1
Map of part of Britain showing find locations of 12 axes and other implements that have been linked to a Preseli
source by previous work, glacial systems (directions and limits, shown respectively as arrowed lines and dotted lines),
and the locations of places mentioned in the text. The broken line in southern England encloses four implements that
are compositionally similar to each other and to the majority of analysed Stonehenge monoliths. Me8 and M8 (with
question marks) are spotted dolerite implements with chemical compositions marginal to Preseli (cf. text). Glacial
information is mainly taken from Bowen (1991) and Charlesworth (1957), with additions after Briggs (1994) and John
(1984) (small arrows near Preseli); the dot-dash arrowed line represents the controversial proposed further extension
of Pliocene ice towards Salisbury Plain as suggested by Kellaway (2002).
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 33PRESELI DOLERITE BLUESTONES
0m
25
Garn Ddu Fach
Carngoedog 0.49
0m
25
*Carnbreseb 0.59 Carn Ddafad-Ias 0.49
33
y d d Carn Gwr 0.43 B 0.70
y n A 0.59 Sheepfold East 0.78
M
e l i smaller 0.54
e s Sheepfold 0.58
P r Carngyfrwy 0.61
Mynydd-bach
Carnbica 0.47 Carnmenyn
*Cerrigmarchogion 0.55 main 0.48
m
Carnarthur 0.46 250
Carnsiân 0.50
32
11 12 Craig Talfynydd
(unspotted)
250 31
m
Mynachlog-ddu
Area of
30
main map
13 14 15
Figure 2
Sketch map of eastern Preseli in South Wales showing the locations of dolerite outcrops, magnetic susceptibility data
(means for each of 15 exposures, cf. text and Table 2), and high Zr outcrops (indicated by *). Outcrops measured by
PXRF are named in bold italics.
investigating links between implements and with the outcrops
In order to compare the implements with each other, and with outcrop sources, a number
of different approaches were investigated, with varying success. We will describe first the tests
based on geochemical characteristics, and then those using the magnetic data.
The geochemical data
In earlier work, Jones et al. (2005) identified some distinctions between Preseli outcrops
using the PXRF dataset. Using Discriminant Analysis (DA) in the standard SPSS software, Jones
et al. (op. cit.) measured the separation in terms of SPSS’s cross-validation estimate (that is, the
percentage of data points, treated in turn as unknowns, for which SPSS correctly predicts the
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
34 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
table 2
Magnetic susceptibility data for Preseli spotted dolerite exposures
Outcrop Magnetic susceptibility (×10−3 SI)
mean n relative s.e.
of mean (%)
Carnmenyn ‘smaller’ 0.54 119 1.6
Carnmenyn ‘main’ 0.48 33 1.6
Carn Ddafad-las 0.49 69 0.9
Carnbreseb 0.59 73 1.8
Carn Sian 0.50 30 2.6
Carn Arthur 0.46 44 1.4
Cerrigmarchogion 0.55 133 1.4
Carngoedog 0.49 71 1.4
Carnbica 0.47 71 1.6
Carn Gwr 0.43 44 1.4
Carngwfrwy 0.61 52 8.4
Sheepfold 0.58 53 3.4
Sheepfold East 0.78 36 5.0
Outcrop ‘A’ 0.59 12 5.4
Outcrop ‘B’ 0.70 13 5.2
Total n 853
group, or outcrop, membership), obtaining reclassification rates (for various element
combinations) of up to 72 per cent. Encouraged by this, we decided to extend the DA to
investigate the relationship of our 12 Preseli-related implements with the outcrop sources, and
with each other. However, there is an important difference between the treatment of the PXRF
data required for Jones et al.’s work, and for the present paper.
PXRF outcrop data may be affected by weathering processes that can significantly alter
the chemical composition of the (measured) surface layer from that of the bulk rock. However,
the 12 implements discussed here were measured on generally unweathered surfaces, and so
their PXRF analyses are not similarly affected. Therefore, in order to compare outcrops with
implements, the outcrop data must be adjusted to compensate for weathering effects.
Weathering correction factors were derived by comparing (mean) PXRF analyses for
eight outcrops (total n = 110) with (mean) laboratory XRF analyses (total n = 23) of rock samples
from the same outcrops. Regression analysis from SPSS provided estimates of slopes and their
standard errors for each element in turn. The correction factors derived from these slopes for
the elements used in this paper (and with 95 per cent confidence limits that reflect variations in
the behaviour of individual elements and rocks) are as follows: K 0.78 ± 0.17, Ca 2.30 ± 0.72,
Fe 1.72 ± 0.36, Ba 1.28 ± 0.23, and Sr 1.47 ± 0.31.
Support for these correction factors comes from the work of Potts et al. (2006) who
evaluated analogous factors using the ratio of the surface (weathered) concentration of Preseli
dolerite samples, to the corresponding (fresh rock) concentration below the altered layer. The
corrections derived by Potts et al. (op. cit.) agree well with ours except for Fe and Sr for which
our corrections are larger but nevertheless agree with Potts et al. within the 95 per cent
confidence limits reported, respectively, by Potts et al. and above. Potts et al.’s factors form part
of a detailed study of weathering behaviour in four samples from three outcrops, while the
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 35PRESELI DOLERITE BLUESTONES
PXRF/laboratory XRF comparison is based on measurements at a much larger number of
outcrops and locations within outcrops. Corrections from the latter dataset may better reflect the
variations in weathering behaviour in Preseli, and, therefore, are used in the present paper. (Note
that, since all the corrections were derived from Preseli dolerites, they are not directly applicable
to other areas or rock types.)
In Figures 3(a) and 3(b), then, we use DA to compare the 12 implements (treated as
unknowns by the program) with the (weathering corrected) Preseli PXRF outcrop dataset. The
choice of elements largely follows that of Jones et al. (2005). Thus, we use Fe, Ba, Sr, Ca and
K, but omit Zr (a relatively minor contributor to the discrimination noted by Jones et al.) because
it showed some slight but systematic depletion in comparisons with laboratory XRF discussed
below. K is added separately, in Figure 3(b), in order to illustrate the effect this addition has on
the position of, in particular, Car23, on the plots.
A noticeable feature of these figures is that the implements which were found in England
(‘English’ implements) all lie in the upper portion of the plots, while those found in Wales
(‘Welsh’ implements) lie in the lower portion. Me8 is a compositionally marginal Preseli spotted
dolerite (Williams-Thorpe et al. 2004, 372), whose outlying position on the plots may be partly
a function of its unusually high (single) Sr determination (cf. Table 1). M8 is also marginal
Preseli composition, though its differences are less apparent on the DA plots.
Our initial (by eye) examination of the data had suggested close similarities between
certain implements (particularly within the English finds), with putative groupings consisting
of: Ha41, Wi118, Wi109 and Wi108; and Wi302 and Do41 (cf. Table 1). The similarities between
Wi302 and Do41 are reflected in Figures 3(a) and 3(b), but the distinction of these two from
the other English artefacts is not supported by this DA treatment.
The assigning of artefacts to specific outcrops is much more difficult. Firstly, the
outcrops are closely related geologically (and geographically) and are all fairly similar in
composition, so it is unsurprising that many outcrop fields overlap in Figure 3. Secondly, the
weathering corrections applied to the PXRF outcrop dataset have large uncertainties (cf. above).
While these uncertainties do not affect the relative positions of the outcrops in Figure 3, they
can affect the positions of the implements in relation to the outcrops (because the implements
do not require the same weathering corrections as the outcrops). In effect, each implement
plotted on Figure 3 has an uncertainty on its position relative to the outcrops, an uncertainty
that can be represented by very approximately ± 1 unit on the Function 1 scales, and ± 0.5 units
on the Function 2 scales.
Given these limitations, DA can at present only give the most tentative indications of
likely, and unlikely, outcrop sources for implements. However, from Figure 3, it is notable that
most of the Carnmenyn and Carn Ddafad-las samples are drawn off by the DA to the left-hand
side of the diagrams, away from the majority of the implements. In Figure 3(b) the implement
Car23 is placed near to Carnbica and Cerrigmarchogion reflecting higher K content in Car23
and in these outcrops (cf. Table 1; PXRF Preseli data are given in full in Jones et al. 2005,
Appendix and cf. table 2).
The magnetic susceptibility data
Eight hundred and fifty-three measurements of magnetic susceptibility were considered
in terms of individual outcrops. The division of the data into outcrops is less easy than might
be expected, because some outcrops that are named separately on Ordnance Survey maps are
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
36 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
FeBaSrCa (a)
English implements
Welsh implements
Me8
FeBaSrCaK (b)
English implements
Welsh implements
Me8
Figure 3
Discriminant analysis plots for eight Preseli spotted dolerite outcrops (n = 122) and for 12 implements (means; n =
typically 5 per object, one each for An10 and Me8), using concentrations of (a) Fe, Ba, Sr and Ca, and (b) Fe, Ba, Sr,
Ca and K. Element concentrations were all determined by PXRF, and the outcrop analyses have been adjusted for
weathering effects using corrections described in the text. The implements are named on the figures. The dashed lines
(which have no statistical significance) are drawn between implements found in England, and those found in Wales.
Car23 is strongly affected by the addition of K (Fig. 3(b)) because this implement has a relatively high concentration
of K (cf. discussion in text).
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 37PRESELI DOLERITE BLUESTONES
nearly contiguous, while others bear only one name but comprise more than one discrete
exposure. For this exercise, we divided the data into 15 areas, each one a discrete exposure on
the ground, and all marked on Figure 2 above (note that Carnmenyn was divided into two areas,
called by us ‘main’ and ‘smaller’ respectively).
The magnetic susceptibility readings for outcrops and for implements are illustrated on
Figure 4. Most outcrops have overlapping measurements, but some, such as Carngwr and Carn
Ddafad-las, have relatively restricted ranges. The magnetic susceptibility ranges of outcrops are
not simply related to the outcrop size. Sheepfold East, for example, is one of the smallest
outcrops, yet it has a larger magnetic susceptibility range than Cerrigmarchogion, an outcrop
that extends over some 500 m in length. The implements plot between about 0.40 and 0.60 ×
10−3 SI on Figure 4.
T-tests were used to compare each of the implements to each outcrop in turn, following
the procedures established by Williams-Thorpe et al. (1996). Given the overlapping outcrop
ranges shown on Figure 4, we did not expect to identify specific outcrop sources, rather to use
the tests to investigate similar, or dissimilar, behaviour of the artefacts.
The interpretation of t-tests rests heavily on the selected level of significance (called
‘alpha’) at which outcrops are ‘allowed’ to be similar to an artefact. A level of significance might
be determined by empirical factors (such as the level at which some already-known provenance
is ‘allowed’ by the procedure, cf. Williams-Thorpe et al. 1996), or a commonly used significance
level of, say, 95 per cent might be selected (equivalent to alpha of 0.95). In our present case, a
much higher alpha of 0.999 was indicated. The key to use of such a high alpha is to note that
3.00
Carnmenyn smaller
Carnmenyn main
Carn Ddafad-las
Carnbreseb
Carn Sian
Carn Arthur
Cerrigmarchogion
Carngoedog
Carnbica
Carn Gwr
Carngwfrwy
Sheepfold
Sheepfold East
Outcrop A
Outcrop B
Implements
2.50
Magnetic susceptibility (S.I.x10-3)
2.00
1.50
1.00
0.50
0.00
Figure 4
Chart showing 853 magnetic susceptibility measurements for 15 spotted dolerite exposures in eastern Preseli, together
with average magnetic susceptibilities for each of the 12 implements discussed (data from Table 1). Measurement
precision is 5 per cent relative or better (Williams-Thorpe and Thorpe 1993). Note that the plotted positions of the
implements overlap on the figure.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
38 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
t-tests are tests for equality of means of source and artefact values, while we are really interested
in whether artefact values lie within the distribution of source values. T-tests with high alpha
are therefore being used as proxies for the latter, determining that an artefact did not come from
a source only if their respective means are considerably different. This increase in alpha value
‘allows’ a larger number of outcrops to be treated as the potential source for each artefact, thus
realistically reflecting the overlaps between outcrops.
For most of the artefacts, the t-test selects a large proportion of the outcrops – six to
13 – effectively allowing most of the source area. However, there are differences between the
test responses. Of the English artefacts, six have between seven and 13 selected outcrops; Devon
1 appears to be a little different, with only four selected outcrops. The Welsh axes are also
variable: Car23 has only one selected outcrop, and M8, four. The other Welsh artefacts, An10,
Ra4, and Me8, have six, seven and 13 outcrops respectively.
Magnetic susceptibility data for the artefacts may also be usefully combined with
chemical data (Fig. 5), choosing Zr, a frequently used geochemical discriminator (e.g. Rollinson
1993), to represent trace elements.
Figure 5, like the DA plots, suggests some differences between the Welsh and the
English implements, with three of the former (Ra4, An10 and Me8) having higher Zr, and
generally higher magnetic susceptibility, than the latter.
investigating links between the implements and the stonehenge monoliths
Only 11 (out of 27) Stonehenge dolerite monoliths have yet been analysed for chemical
compositions, and there are no magnetic susceptibility data for any of the monoliths. They are
0.65
Wi 302
Magnetic susceptibility (S.I. x 10-3)
0.60
Ra 4
0.55 An 10
Ha 41
0.50 Me 8
Wi 109
0.45
Wi 108 M8
Dev 1 Wi 118 Do 41
0.40
Car 23
0.35 English implements
Welsh implements
0.30
40 50 60 70 80 90 100 110
Zr (ppm)
Figure 5
Chart showing magnetic susceptibility plotted against Zr concentrations (PXRF analyses) for the studied implements,
distinguishing the implements found in England (diamonds) and those found in Wales (squares). The error bar shows
a typical standard error of the mean.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 39PRESELI DOLERITE BLUESTONES
therefore not included in the statistical comparisons described above. Nevertheless, some
observations concerning the relationship of the monoliths with the Preseli dolerite implements
can be made using the limited monolith dataset in Thorpe et al. (1991).
Figures 6(a), (b) and (c) show the concentrations of selected chemical elements in the
monoliths, and in the axes and other implements included in this work. Note that here we are
comparing laboratory (wavelength-dispersive and energy-dispersive) X-ray fluorescence
analyses for the monoliths with PXRF analyses (not corrected for weathering) for the
implements.
Thorpe et al. (1991) separated the 11 monoliths into three compositional groups, and
these groups are reflected in the data shown in Figure 6: firstly, a group of eight monoliths that
may be conveniently named after the stone that has the identifier ‘SH (Stonehenge) 33’;
secondly, SH44 and SH45; and finally, SH42. SH62 (named on Figure 6) was included by
Thorpe et al. in the SH33 grouping on the basis of similar concentrations of geochemically
immobile elements. Nevertheless, it has some compositional differences (cf. Figures 6(a) and
6(b)), and was also subsequently proved to be distinctive in its opaque mineral assemblage (Ixer
1997).
Figure 6 also suggests some compositional similarities between the monoliths and some
of the implements, mainly the English implements. Five implements (Wi108, Wi109, Wi118
and Ha41 all found in England, Car23 found in Wales) consistently plot close to the SH33 group.
Wi302 and Do41 plot nearer to SH44 and 45. Examination of the full range of analysed elements
(Table 1) best supports the parallel of four English implements with the SH33 group. We noted
that Zr is slightly and systematically lower in the implements than in the monoliths (cf. Figure
6(c)); this could be a result either of analytical factors (remembering that we are comparing here
PXRF of implements with laboratory XRF of monoliths), or slight weathering-related depletion
of Zr in the implement surfaces measured. The unweathered appearance of measured surfaces
argues for an analytical explanation.
Figure 6 shows variability of composition within the Welsh implements, but little
consistent similarity of these Welsh implements to monolith groupings. An10 plots near to the
monolith SH42 on Figure 6(c) (only), and in respect of this SH42 is included in Table 1 to allow
a fuller comparison. Car23 plots within the SH33 group on Figure 6, but other elements differ,
and also its separation from the English implements was underlined by the DA. Overall, the
Welsh artefacts appear compositionally unlike the monoliths.
These observations carry the implication that some of the implements could have
originated at the same outcrop sources as some monoliths. And, equally importantly, some of
the implements do not appear to be from sources that provided monoliths.
summary of findings, and discussion
The axes and other implements
The investigations described above consistently support compositional groupings
within the studied implements, and a dichotomy between implements found in England, and
those found in Wales. Such groupings and distinctions are likely to reflect derivation of the raw
material from different outcrops or areas of the Preseli spotted dolerite region. It should be noted
that the two unspotted implements that have Preseli chemical and magnetic characteristics (An10
and Wi302) could potentially be derived from ‘spotted’ outcrops, since these frequently contain
unspotted parts (cf. Williams-Thorpe et al. 2004).
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
40 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
600
(a) English implements
Me 8
500 Welsh implements
Stonehenge monoliths
400
Sr (ppm)
M8
300 Wi SH 45
Ra 4 Wi
An 10 109 SH 302 SH 62
Car 23 42 SH 44
Do
200 Ha
41 Dev 1 41
Wi
Wi 118
SH 33 group
108
100
0
0 100 200 300 400 500 600
Ba (ppm)
13
(b)
12
SH 44
M8
11
Fe2O3 (wt.%)
Wi 108 SH 45
10
Wi 109 SH 62
Ra 4 Do
Dev 1 Wi 118
Ha 41 41
9
Car 23 Wi
Me 8
An 10 302
8
SH 33 group
7
6
0 100 200 300 400 500
Ba (ppm)
(c) Me 8
500
400
Sr (ppm)
M8
300 SH 62 SH 44 SH 45
Ra 4 An 10
Wi 109 Car 23
Dev 1 Ha 41 SH 42
200 Wi 118 Wi 302
Wi 108 Do 41
SH 33 group
100
40 60 80 100 120
Zr (ppm)
Figure 6
Graphs of (a) Sr (ppm) vs. Ba (ppm), (b) Fe2O3 (wt %) vs. Ba (ppm) and (c) Sr (ppm) vs. Zr (ppm), showing the
studied implements (diamonds and squares) and 11 Stonehenge dolerite monoliths (triangles). Lines are drawn round
the monolith groupings proposed by Thorpe et al. (1991) and have no statistical significance. The error bars show
typical standard errors of the means for implements, and analytical uncertainties for monoliths (not shown if within
symbol size). The elements shown were selected in order to best reflect monolith groupings based on Thorpe et al.
(op. cit.) and further information in Ixer (1997), and to illustrate the relationship of the implements with monolith
groupings. The data are taken from Thorpe et al. (op. cit.; monoliths) and Williams-Thorpe et al. (2004; implements).
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 41PRESELI DOLERITE BLUESTONES
The implements that were found in England (all found in the Stonehenge area or in the
south-west, cf. Figure 1) are also those which show the most obvious compositional similarities
to Stonehenge monoliths. Three implements that were found at or very near to the site of
Stonehenge (Wi108, 109, 118), together with Ha41 (found on the south coast), are most similar
to the main monolith group (the SH33 group). Support for this link comes from Ixer (1997),
who made the observation (Ixer op. cit., 16) that a particular opaque mineral assemblage
characterizes the SH33 Stonehenge monolith group, and also appears to be ‘very significant
numerically’ within the Group XIII examples in the South-West England Museums’ implement
collection. That collection includes all four of our most SH33-like artefacts – Ha41, Wi118,
Wi108 and Wi109.
The implements found in Wales not only differ from those found in England, but also
lack the compositional similarities to Stonehenge monoliths that we observed in the English
implements.
A weakness of our observations is that they are based on small numbers of artefacts:
that is, the 12 Preseli-related implements; and only 11 of the 27 remaining dolerite Stonehenge
monoliths. However, a reasonable interpretation of the available evidence is that Preseli
implements found in England are generally derived from the same outcrops as the monoliths,
while the implements found in Wales are not. This adds weight to the possibility that the
‘English’ implements might have been manufactured from the same rock assemblage, whether
outcrop or erratic material (or waste material from the monoliths themselves), that was exploited
to build the bluestone settings at Stonehenge. These implements may then have been distributed,
perhaps traded, to other parts of south-west England, within the framework of Beaker period or
Early Bronze Age activities.
The compositional distinction of the Welsh implements, on the other hand, suggests
a separate resource procurement strategy involving exploitation of several different Preseli
outcrops, either at primary (outcrop) sources, or at secondary (e.g. glacially or river-transported)
sources. The extent of human distribution of the Welsh implements cannot be inferred in the
same way as for those found in south-west England, because of the opportunity for glacial
dispersion of rocks in Wales. While the main ice flow directions across Preseli were broadly
from north-west to south-east (Fig. 1), more local, northward moving, tongues of Devensian ice
(Briggs 1994) and also westward movement related to the course of the Gwaun Valley (e.g. John
1984) could have provided mechanisms for more complex natural dispersal of Preseli rocks.
Curiously, all three implements that were found at sites north of Preseli (An10, Ra4,
Me8) have markedly higher Zr levels (between 81 and 94 ppm) than the implements that were
found at sites south of Preseli (Zr between 50 and 73 ppm) (Table 1; the higher Zr is generally
correlated with higher magnetic susceptibility (cf. Figure 5) giving us confidence that the
differences in Zr are not simply weathering effects). The highest Zr source outcrops (with
average Zr of >80 ppm from published laboratory XRF analyses, again correlated with relatively
high magnetic susceptibility) are Carnbreseb and Cerrigmarchogion. These outcrops are located,
respectively, on the northern and western edges of the Preseli range (Fig. 2), positions from
which natural boulder transport northward, as well as westward, is feasible. Exploitation of
varied secondary source material dispersed by a number of different mechanisms may offer an
explanation for the compositional variation seen within the Welsh artefacts.
An alternative explanation for the compositional variations observed within both
English and Welsh implements is that all these implements may have been selected randomly
from a single, mixed (secondary) assemblage within Preseli. However, this explanation is made
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
42 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
unlikely by the compositional correlation between the English artefacts (but not the Welsh
artefacts) and Stonehenge monoliths.
We were not able to identify correlations between implement style and composition or
outcrop source (though shaft-hole implements are more prevalent among the Welsh studied
artefacts). The three stylistically developed battle-axes, although extremely alike in form (Car23
and Ra4 are Roe’s Type IIa, and Wi302, Type IIc; Roe 1966, 1979), are of three different
compositions and probably, therefore, from three different Preseli outcrops. And conversely,
material that could be from the same outcrop was used for axes of different styles – for example,
the small reworked axe from Maiden Castle (Do41) is of very similar composition to the Wi302
battle-axe. Williams-Thorpe et al. (2004) noted that some battle-axes of analogous style to
Wi302 were made from non-Preseli rocks, for example Whin Sill dolerites (cf. Williams-Thorpe
et al. 2003).
Comments on bluestone outcrop sources
The new chemical (PXRF) data, analysed above using DA, prompt us to consider that
the Carnmenyn outcrop, traditionally regarded as the most important source both of monoliths
and of axe-heads, may not be the source of any of the implements that we studied. If that is the
case, neither is it likely to be the source of the most numerous monolith group, the SH33 group.
Thorpe et al. (1991), using the available (laboratory XRF) analytical data on Preseli
outcrops, identified the SH33 group with ‘East Preseli, Carnmenyn-Carngyfrwy or
Cerrigmarchogion-Carngoedog’. Two monoliths were likened to Carn Ddafad-las, and one to
Carnbreseb (Thorpe et al. op. cit., 139–40), but in general Thorpe et al. remained cautious about
identifying specific outcrops because of the compositional similarity of many of these outcrops.
Nevertheless, their published data do contain some clues that may inform the search
for further supporting evidence. It is apparent, for example, that the (very spotted) Carngoedog
outcrop contains dolerites that are compositionally closer to the SH33 monoliths than are those
of Carnmenyn (cf. Thorpe et al. op. cit., 130–2, 143).
Interestingly, in his comparison of opaque mineralogy of monoliths and Preseli
outcrops, Ixer (1997) suggested the exclusion of several outcrops and narrowed the likely source
of most of the monoliths to either Carngoedog or Carnmenyn.
Analysis of all the Stonehenge dolerite monoliths using PXRF (a possibility which we
are investigating) would allow progress in this debate.
An alternative spotted dolerite source?
One further question arises from our work, a question that is both worrying and
intriguing. Is there another source of spotted dolerite outside Preseli, a source that contains rocks
of similar appearance to those of Preseli, but with a different chemical composition?
The uniqueness of Preseli spotted dolerites has, of course, already been considered
during the long bluestone debate, with the consensus that eastern Preseli is the only relevant
potential source area for monoliths (Thomas 1923, 250; Thorpe et al. 1991, 119). It is, indeed,
most unlikely that further outcrops or exposures of spotted dolerite large enough to provide the
Stonehenge monoliths have escaped the scrutiny of geologists. Smaller artefacts such as axes
and axe-hammers, on the other hand, could feasibly be produced from rock exposures of much
more limited extent.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 43PRESELI DOLERITE BLUESTONES
Williams-Thorpe et al. (2004) identified an axe (Sx178; found in Newhaven, Sussex;
cf. Figure 1) which has mineralogical characteristics analogous to Preseli spotted dolerite, but
a chemical composition clearly unlike the available Preseli data, suggesting the existence of a
source not represented within that dataset. Further, M8 and Me8, discussed in the present paper,
are spotted dolerites but have marginal Preseli compositions. These anomalous data suggest that
a reconsideration of alternative spotted dolerite sources is now appropriate.
Scores of rock outcrops and areas have been discussed as potential bluestone sources
(cf. summary in Thorpe et al. op. cit., 120 table 5), or are described as ‘spotted’ in geological
literature. In many cases the spots are mineralogical features, such as crystals, that are not the
same as the metamorphic spots in Preseli (compare, for example, Elsden (1905, 594) and Cox
(1915, 311) both describing rocks in north Pembrokeshire).
One case, however, merits further investigation. This is the ‘insignificant outcrop of
similar rock [similar, that is, to Preseli spotted dolerite; present authors’ insertion] in the Cader
Idris district’ (Thomas 1923, 250, quoting a personal communication from Professor A.H. Cox,
University of Wales). While this outcrop was ‘disregarded as a possible source’ of monoliths
(Thomas, op. cit.; presumably because of its small size), it may be relevant to axe/implement
provenance. Cader Idris lies within the Aran igneous province (Dunkley 1979) which together
with the Rhobell complex (Kokelaar 1986) constitutes the Ordovician volcanics and intrusions
of north-central Wales (general locations are on Figure 1 above). Dolerites within both the Aran
and Rhobell provinces have undergone similar metamorphic alteration to those of Preseli (cf.
Dunkley op. cit., 600; Kokelaar op. cit., 892) and thus might resemble, mineralogically, those
of Preseli. In this context, it may be more than coincidence that one of the spotted axes with
rather anomalous chemistry (Me8) was found at Arthog, on the north-western margins of Cader
Idris.
conclusions
The bluestone spotted dolerite implement group (Group XIII) does not comprise a
single compositional assemblage, but, rather, a mixed grouping that probably originated at
several outcrops mainly in Preseli and perhaps at another (unidentified) spotted dolerite source.
Spotted dolerite axes and other implements that have been found in England are mainly
similar in chemical composition to analysed Stonehenge spotted dolerite monoliths.
Most of the spotted dolerite artefacts found at Stonehenge and in the south and south-
west of England could have been manufactured from the same assemblage of outcrop rocks or
erratics that was used for the Stonehenge monoliths.
Spotted dolerite implements found in Wales have a variety of chemical signatures and
show little or no similarity to analysed Stonehenge monoliths.
The information on outcrop level provenancing implies that resource procurement
strategies for the spotted dolerite used in England were different from those used in Wales.
Geochemical evidence points, overall, to Carngoedog as the most similar outcrop to the
largest group of analysed Stonehenge monoliths, and to the geochemically analogous (English)
implements.
An alternative source of spotted dolerite rocks, perhaps within the Aran or Rhobell
complexes of north-central Wales, may have been exploited to provide material for
spotted dolerite implements that have compositions different from, or only marginal to, Preseli
dolerites.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
44 Journal compilation © 2006 Blackwell Publishing Ltd.OLWEN WILLIAMS-THORPE, M.C. JONES, P.J. POTTS AND P.C. WEBB
The complete picture of dolerite bluestone implement provenance and its relationship
with Stonehenge monoliths will be understood much better in the light of analysis of all the
Stonehenge dolerite monoliths. Clarification of the possibility of alternative spotted dolerite
source(s) is also needed.
Acknowledgements
We are grateful to Sid Howells of the Countryside Council for Wales for advice and permissions
for work in Preseli (with particular reference to the magnetic susceptibility survey for which full results
are given for the first time here). The paper stems from earlier studies of dolerite implements and outcrops,
funded by The Leverhulme Trust (grant no. F/00269/C). We thank John Taylor (Figs. 1, 3–6) and Andrew
Lloyd (Fig. 2) for cartography.
(OWT, PJP, PCW) Department of Earth Sciences
(MCJ) Department of Statistics
The Open University
Milton Keynes MK7 6AA, UK
references
atkinson, r.j.c. 1979: Stonehenge: archaeology and interpretation (3rd ed.) (Harmondsworth).
bevins, r.e., lees, g.j. and roach, r.a. 1989: Ordovician intrusions of the Strumble Head-Mynydd Preseli
region, Wales: lateral extensions of the Fishguard Volcanic Complex. Journal of the Geological Society
of London 146, 113–23.
bowen, d.q. 1991: Time and space in the glacial sediment systems of the British Isles. In Ehlers, J.,
Gibbard, P.L. and Rose, J. (eds.), Glacial Deposits in Great Britain and Ireland (Rotterdam), 3–11.
briggs, s. 1994: The Bronze Age. In Davis, J.L. and Kirby, D.P. (eds.), Cardiganshire County History
Volume I (Cardiff, Cardiganshire Antiquarian Society and Royal Commission on the Ancient and Historical
Monuments of Wales), 169.
charlesworth, j.k. 1957: The Quaternary Era (London).
clough, t.h.mck. and cummins, w.a. (eds.) 1988: Stone Axe Studies Volume 2 (London, CBA Research
Report No. 67).
cox, a.h. 1915: The geology of the district between Abereiddy and Abercastle (Pembrokeshire). Quarterly
Journal of the Geological Society of London 71, 273–340.
dunkley, p.n. 1979: Ordovician volcanicity of the SE Harlech Dome. In Harris, A.L., Holland, C.H. and
Leake, B.E. (eds.), The Caledonides of the British Isles – reviewed (London, Special Publication of the
Geological Society of London 8), 597–601.
elsden, j.v. 1905: On the igneous rocks occurring between St. David’s Head and Strumble Head
(Pembrokeshire). Quarterly Journal of the Geological Society of London 61, 579–607.
evans, w.d. 1945: The geology of the Prescelly Hills, north Pembrokeshire. Quarterly Journal of the
Geological Society of London 101, 89–110.
green, c.p. 1997: Stonehenge: geology and prehistory. Proceedings of the Geologists’ Association 108,
1–10.
ixer, r.a. 1997: Detailed provenancing of the Stonehenge dolerites using reflected light petrography. In
Sinclair, A., Slater, E. and Gowlett, J. (eds.), Archaeological Sciences 1995 (Oxford, Oxbow Monograph
64), 11–7.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
Journal compilation © 2006 Blackwell Publishing Ltd. 45PRESELI DOLERITE BLUESTONES
john, b.s. 1984: Pembrokeshire (Newport).
jones, m.c. and williams-thorpe, o. 2001: An illustration of the use of an atypicality index in
provenancing British stone axes. Archaeometry 43, 1–18.
jones, m.c., williams-thorpe, o., potts, p.j. and webb, p.c. 2005: Using field-portable XRF to assess
geochemical variations within and between dolerite outcrops of Preseli, South Wales. Geostandards and
Geoanalytical Research 29, 251–69.
kellaway, g.a. 2002: Glacial and tectonic factors in the emplacement of the bluestones of Salisbury
Plain. In Chapman, M. and Holland, E. (eds.), The Survey of Bath and District No. 17 (Bath), 57–71.
kokelaar, p. 1986: Petrology and geochemistry of the Rhobell volcanic complex: amphibole-dominated
fractionation at an early Ordovician arc volcano in North Wales. Journal of Petrology 27, 887–914.
potts, p.j., bernardini, f., jones, m.c., williams-thorpe, o. and webb, p.c. 2006: Effects of weathering
on in situ portable X-ray fluorescence analyses of geological outcrops: dolerite and rhyolite outcrops from
the Preseli Mountains, South Wales. X-Ray Spectrometry 35.
roe, f.e.s. 1966: The battle-axe series in Britain. Proceedings of the Prehistoric Society 32, 199–245.
roe, f.e.s. 1979: Typology of implements with shaft-holes. In Clough, T.H.McK. and Cummins, W.A.
(eds.), Stone Axe Studies (London, CBA Research Report 23), 23–47.
rollinson, h. 1993: Using geochemical data (Harlow).
thomas, h.h. 1923: The sources of the stones of Stonehenge. The Antiquaries Journal 3, 239–60.
thorpe, r.s., williams-thorpe, o., jenkins, d.g. and watson, j.s. with contributions by ixer, r.a. and
thomas, r.g. 1991: The geological sources and transport of the bluestones of Stonehenge, Wiltshire, UK.
Proceedings of the Prehistoric Society 57, 103–57.
williams-thorpe, o. and thorpe, r.s. 1993: Magnetic susceptibility used in non-destructive
provenancing of Roman granite columns. Archaeometry 35, 185–95.
williams-thorpe, o., jones, m.c., tindle, a.g. and thorpe, r.s. 1996: Magnetic susceptibility variations
at Mons Claudianus and in Roman columns: a method of provenancing to within a single quarry.
Archaeometry 38, 15–41.
williams-thorpe, o., potts, p.j. and webb, p.c. 1999: Field-portable non-destructive analysis of lithic
archaeological samples by X-ray fluorescence instrumentation using a mercury iodide detector:
comparison with wavelength-dispersive XRF and a case study in British stone axe provenancing. Journal
of Archaeological Science 26, 215–37.
williams-thorpe, o., webb, p.c. and jones, m.c. 2003: Non-destructive geochemical and magnetic
characterisation of Group XVIII dolerite stone axes and shaft-hole implements from England. Journal of
Archaeological Science 30, 1237–67.
williams-thorpe, o., potts, p.j. and jones, m.c. 2004: Non-destructive provenancing of bluestone
axe-heads in Britain. Antiquity 78, 359–79.
OXFORD JOURNAL OF ARCHAEOLOGY
© 2006 The Authors
46 Journal compilation © 2006 Blackwell Publishing Ltd.You can also read
