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Submarine Neolithic Stone Rows near Carnac (Morbihan), France:: preliminary results from acoustic and underwater survey

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  • University of Nantes (IGARUN)

Abstract and Figures

Carnac is internationally recognized as the best example of Neolithic rows of standing stones in western France. To better understand the significance of such alignments, an inventory of similar sites in the Carnac area was undertaken in 2003. Specifically, the alignment of ‘Le Moulin’, a site to the south of Carnac, which consists of five parallel lines of stelae inside the town of Saint-Pierre-Quiberon, was investigated. The extension of these stelae lines has been recognized to the east along the coast on an intertidal platform named Kerbougnec. The investigation aimed to assess whether this symbolic architecture continued offshore, which would give to this site a size comparable to that of Carnac. To answer to this question, several side-scan surveys have been conducted on the submerged part of the Kerbougnec platform, and these were validated by divers’ observations on the identified acoustic anomalies. The concentration of acoustic anomalies forms a consistent extension of the architectural structure identified on land, and the orientation of the combined structure (on land and under the sea) is identical to the topological patterns recorded at Carnac.
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9
Submarine Neolithic Stone Rows near Carnac
(Morbihan), France: preliminary results from
acoustic and underwater survey
Serge Cassen, Agnès Baltzer, André Lorin, Jérôme Fournier
and Dominique Sellier
Carnac is internationally recognized as the best example of Neolithic rows of standing stones
in western France. To better understand the significance of such alignments, an inventory
of similar sites in the Carnac area was undertaken in 2003. Specifically, the alignment of
‘Le Moulin’, a site to the south of Carnac, which consists of five parallel lines of stelae inside
the town of Saint-Pierre-Quiberon, was investigated. e extension of these stelae lines has
been recognized to the east along the coast on an intertidal platform named Kerbougnec. e
investigation aimed to assess whether this symbolic architecture continued offshore, which
would give to this site a size comparable to that of Carnac. To answer to this question, several
side-scan surveys have been conducted on the submerged part of the Kerbougnec platform,
and these were validated by diversobservations on the identified acoustic anomalies. e
concentration of acoustic anomalies forms a consistent extension of the architectural structure
identified on land, and the orientation of the combined structure (on land and under the
sea) is identical to the topological patterns recorded at Carnac.
Keywords: Neolithic, stelae, acoustic survey, underwater archaeology, western France
Introduction
e archaeology of the Neolithic rows of standing
stones (straight or curvilinear, uninterrupted, or
discontinuous) is a distinct feature of western
France where such architecture is numerous
and dense (Bailloud et al. 1995). Because of the
lack of an appropriate conceptual framework,
the interpretation of these sites is difficult,
through both the lack of associated evidence
and contextual information, and by confusion
with persistence of ideas, opinions, and clichés
developed from the beginning of the nineteenth
century. Carnac epitomizes the problem. In this
specific coastal zone of Brittany (Fig. 9.1), one
of the unresolved problems remains the extent
of this archaeological site, which spreads over
several kilometres. It is not known where exactly
it begins or ends (Boujot and Pinet 2007).
e main objective of the current research is
to create an inventory of sites similar to Carnac,
which might help in the general interpretation
of standing stone alignments. e discovery
of a significant site submerged in the Bay
of Quiberon, near Carnac, also provided an
opportunity to test the hypothesis formulated
by Boujot et al. (1995) and Cassen (2009a). In
this chapter we present the preliminary results
of two side-scan sonar surveys validated by diver
surveys, and will focus on the methods used to
record the monoliths under the sea.
Serge Cassen, Agnès Baltzer, André Lorin, Jérôme Fournier and Dominique Sellier100
e question
Since the beginning of the nineteenth century,
Carnac has generally been interpreted as a
‘temple’ (Cambry 1805; Mohen 2000), a
rather vague term that is too poorly defined to
apply accurately to such a cryptic architectural
structure. In the second half of the twentieth
century research on astronomic alignments
and pseudo-scientific metrology was conducted
on a restored (up to 80%) monument to
determine if Carnac functioned as a lunar–solar
observatory, earlier proposed by Gaillard (1897)
and Devoir (1917), and to establish the existence
of a prehistoric megalithic yard’ (om 1955;
Thom and Thom 1978). The result was to
make obsolete any conclusion founded on such
measurements, essentially based on alignments
between imprecise points, and thus likely to fit
to any geometric situation.
We suggest that the ‘verticalization of a
monolithic object (a stele in this case) at the
beginning of the Neolithic period may be
regarded as a symbolic threshold between
two dimensions, two spaces, two worlds, as a
doorway-stele or a doorstep-stele. Furthermore,
according to the definition of the anthropological
concept of limit, the repetition of these stelae
in a given space gives the impression of raising
a barrier to prevent any intrusions, physical or
virtual. We further propose the hypothesis that
these stone rows acted as a cognitive barrier
(Cassen 2009a). ey should be considered
as a mineral fence that could stop, impede, or
filter movement or passage. In this way, it is
fundamental to define the topographical position
and location of those rows (Cassen 2009b).
We have been researching other sites, similar
to Carnac, to develop a model that could help
answer these questions.
At Saint-Pierre-Quiberon the alignment of
Le Moulin (several parallel lines of stelae Fig.
9.2), was restored in the nineteenth century,
and constitutes a small, preserved archaeological
site inside a modern housing development. e
opportunity to follow the extension of these
lines of stelae on the intertidal platform called
Kerbougnec (or Kerbourgnec, but originally
Kerbonnec in Breton) was a success, and groups
of stelae were first mapped on the tidal flat by
Cassen and Vaquero Lastre in 2003. Naturally,
the question was raised, does this symbolic
architecture continue under the modern sea
level, which would confer to this site a size and
importance similar to that of Carnac?
Figure 9.1: Top: e study area in Brittany. Bottom: location of Carnac north
of the Bay of Quiberon (coloured composition of a Landsat ETM image dated
16.04.2003; topography by IGN–Institut Géographique National, using BD
ALTI digital elevation model)
101
Submarine Neolithic Stone Rows near Carnac, Morbihan, France
e exploration of the Kerbougnec site
(Quiberon Bay)
Graphically recorded and georeferenced in
2002 by DGPS (Differential Global Positioning
System: Cassen and Vaquero Lastres 2003), the
monoliths discovered on the rocky platform of
Kerbougnec appear to be organized in parallel
rows in the sectors that are most protected from
swell and wave action (in comparison to the
high-energy environment of the ‘Côte sauvage
on the west part of the peninsula).
At the eastern base of the biggest block of a
broken granite slab (Grande Stèle no. 1), a buried
axe-head of Alpine jadeitite was discovered in
2003 (Fig. 9.3; petrographic determination by
M. Errera, of the Agence Nationale de Recherche
sponsored ‘Programme Jade’, directed by Pierre
Pétrequin). is axe-head is a curiosity because
of the distant provenance of the rock (Monte
Viso, Italy). It is, however, not exceptional as four
similar specimens apparently non-functional
axes have been discovered at the site of the
Petit Rohu, c. 100 m further to the south. ese
polished blades seem to have been symbolically
powerful objects rather than everyday tools.
e presence of these emblematic objects in
Kerbougnec confirmed the importance of this
underwater site and encouraged us to extend
our exploration toward the open sea.
Side-scan sonar surveys
The difficulty of prospecting in a marine
environment with our usual archaeological
methods encouraged us to collaborate with
geographers and geologists. e terrestrial part of
the archaeological and geomorphological survey
was made on the rocky intertidal platform during
Figure 9.2: Two
pictures of the high
part of Le Moulin
Neolithic stone rows
(Photos: Z. Le Rouzic
1908, S. Cassen 2002)
Figure 9.3: A–B Row
of stelae discovered
at low tide on the
intertidal platform
of Le Rohu, between
Quiberon and Saint-
Pierre-Quiberon. C–D
Polished axe-heads
found near the stelae
at the positions marked
‘23’ and ‘21(After
Cassen et al. 2010)
Serge Cassen, Agnès Baltzer, André Lorin, Jérôme Fournier and Dominique Sellier102
very low tides by traditional terrestrial and shallow
water survey methods. For the marine part (2–10
m water depth), a geophysical survey of the
possible submerged architecture was performed
using side-scan sonar and diver observations.
e side-scan sonar is an acoustic geophysical
instrument that, typically, is towed in the
water behind the survey vessel. e method is
based on recording echoes of an acoustic wave
issued artificially and reflected by the different
interfaces (Hobbs et al. 1994), the seafloor, and
the boundaries between sedimentary layers. e
results of a side-scan sonar survey are a map or
image of the seafloor acoustic properties that
can be interpreted for different objects on the
seafloor or different seafloor types. e signal
frequency and speed of the boat determine the
image resolution: the higher the frequency, the
higher the resolution, and the slower the boat
speed the higher the resolution. From the side-
scan image features such as dunes, sand waves,
ripples, and rocky outcrops are readily identified
(Augris et al. 1996). With the sonar towed c.
5–10 m above the seafloor an image (a band)
50–100 m wide on each side of the sonar can
typically be recorded. e acquisition of several
juxtaposed bands allows the realization of a
sonar mosaic’, which constitutes a very precise
picture (with a resolution of c. 20 cm) of the
seafloor. The resulting acoustic map can be
geographically referenced (Bonnot-Courtois et
al. 2005; Ehrhold et al. 2007, 2008; Fournier
et al. 2009) to allow interpretation of different
acoustic facies. Isis Sonar software and a Trimble
Pathfinder Pro XRS GPS positioning system were
used for data acquisition. To build mosaics we
employed Isis Sonar and Deph Map software.
Two sonars have been tested on the site:
• an Edgetech 272 TD was used with a frequency
of 100 kHz. It is characterized by a maximum
signal penetration of 1 cm into the sediment
and a horizontal resolution of 30 cm. is sonar
was used for initial reconnaissance to provide
wide coverage in profiles up to 200 m wide;
• a dual-channel SH1 (devised by Sture Hultqvist)
was used with a frequency of 500 kHz allowing
a horizontal resolution of about 10 cm. is
system is commonly used in the exploration
of wrecks (Cazenave de la Roche 2009) and
allowed us to acquire two smaller mosaicswith
profiles 25 m wide on each side.
In processing the data, the first step was
to recognize the different types of acoustic
anomalies that could indicate the presence
of monoliths beneath shallow coastal waters.
Potentially there is a risk of misinterpretation
between monoliths and objects such as concrete
blocks that served as moorings for yachts and are
now covered by oysters and seaweed (cf. Atallah
et al. 2005). e second step was to confirm the
archaeological or autochthonous nature of these
anomalies by diving.
Results from the acoustic surveys
e methodology proposed for the study, and
in particular the ability to recognize the stelae as
significant archaeological features was tested on
the site of ‘Le Petit Rohu’, situated in the south
of Kerbougnec (Fig. 9.3). is site revealed an
extraordinary find in 2007: four polished axes
made in Alpine jadeitite recovered within a
submerged alignment of 26 monoliths, at 3.5 m
below MSL (Cassen et al. 2008). Figure 9.4
shows the position of the sonar track acquired on
this structure. e survey yielded sonar records of
variable quality, as shown in Figure 9.5 for record
Mos028bis. For each discrete anomaly identified
from the sonar record, a corresponding anomaly
was mapped by the archaeologists with DGPS
(Cassen et al. 2008). e DGPS records were
obtained during periods of very low tide (for
a complete plan of the architecture, see Cassen
et al. 2010). e sonar image (Fig. 9.5) clearly
shows the group of fallen stelae (dimensions
around 1 m), in spite of their relative burial in
the sandy gravel seafloor.
At the site of Kerbougnec (further north),
several of the sonar records acquired in 2009 also
reveal a continuation of the monoliths recorded
in 2002 onto the tidal flat (Fig. 9.6). In this
area, the bathymetry already gives an idea of the
ancient topography of the site before its invasion
by the sea – a gently elevated platform. One can
easily distinguish the details of the rocky platform
under the sea on the photograph, as the site is
characterized by an unusually high transparency
of the water.
A number of profiles were acquired with
the SH1 sonar between the buoy of L’Ours de
Kerbougnec (‘e Bear’, which in this case refers
to a reef or shallow waters area, and the rocky
intertidal platform exposed at low tide. Figure
9.7 shows the acoustic anomalies (marked by
arrows) on a homogeneous sedimentary floor
and the rocky platform (top, left) on which it is
difficult to distinguish naturally deposited granite
blocks from prehistoric monoliths placed on this
103Submarine Neolithic Stone Rows near Carnac, Morbihan, France
Figure 9.4: Petit Rohu. Location of the sonogram Mos028 above the Neolithic stone row, the positions of the monoliths detected by
side-scan sonar, and the position of the polished axe deposit (Photo: IGN 2000)
Serge Cassen, Agnès Baltzer, André Lorin, Jérôme Fournier and Dominique Sellier104
Figure 9.5: Petit Rohu. Comparison of the archaeological plan of the Neolithic stone row (After Cassen et al. 2010) and the side-scan
sonar image of the structure (freq. 500 kHz)
105Submarine Neolithic Stone Rows near Carnac, Morbihan, France
Figure 9.6: Kerbougnec. Side-scan sonogram mosaic between the rocky intertidal platform and the Ours shoal (Photo: IGN 2000)
Serge Cassen, Agnès Baltzer, André Lorin, Jérôme Fournier and Dominique Sellier106
site reveals the performance of each side-scan
sonar system. e highest frequency was very
helpful for discerning blocks of 1–2 m length,
and sometimes down to 0.5 m, upstanding by
0.5–1 m on the textured sandy floor and by
only 0.2 m on a relatively homogeneous bottom.
However, when flora and fauna cover the blocks
on the rocky platform, the detection of acoustic
anomalies is more challenging. Among all the
sonar records, ten sites were chosen because of
their quality in terms of resolution, repetition,
and concordance of anomalies. From these, a
preliminary (but not exhaustive) list of anomalies
was selected for diving targets.
Results from the diving surveys
Surveys by divers were conducted in order to
identify the nature of the acoustic anomalies.
In this way it was shown that the majority of
the anomalies correspond to granite monoliths.
eir location on a sandy gravel seafloor was the
first indication of their allochthonous origin.
e divers conducted excavations around the
base of each of the stones, to check that they
were not connected with the subjacent substrate.
Subsequent removal of seaweed, shells, etc., from
a dozen of the blocks allowed us to observe the
diversity of the monoliths’ surfaces.
On some of
them, sharp edges have been noticed (Fig. 9.9D).
Others have blunted edges belonging to ancient
surfaces of the outcrop. If some marks indicate
the original extraction face from the substrate
substrate. Figure 9.8 synthesizes the anomalies
extracted from six sonar records. ere is an
absence of features to both the north and south
(for 100 m) from the area linking the beach with
the reef of Kerbougnec.
e combined approach undertaken at this
Figure 9.7:
Kerbougnec. Zoom on
sonograms Mos034A,
Mos031A and
Mos031B. Localization
of the anomalies
Figure 9.8:
Kerbougnec. Synthesis
between Neolithic
monoliths recorded
by DGPS on the
rocky intertidal
platform (left) and the
anomalies recorded by
side-scan sonar (right).
Inset: the jadeite
polished axe discovered
at the base of Grande
Stèle no. 1 (After
Cassen et al. 2010)
107Submarine Neolithic Stone Rows near Carnac, Morbihan, France
(the usual forms of weathering of the granite),
the sharp, angular section of the monolith
sometimes indicates a secondary position on the
floor (Fig. 9.9C); in other words, extraction and
displacement. e questions surrounding these
features remained focused on the anthropogenic
nature of the observed phenomena. erefore,
our main effort is concerned with surfaces that
are consistent with typical forms of weathering
of granite described by geomorphologists (Fig.
9.9B face of extraction uppermost, sharp
edges; Fig. 9.9A rock with an older weathering
pattern known as dome’ and pedestal’), and
comparable to the Neolithic standing stones of
Carnac used as a reference (Sellier 1995, 1997).
After validation by divers, several arguments can
be made to assert the anthropogenic character
of these features:
1. In spite of the obvious disorganization of the
original architectonic structure owing to the
force of the ocean, a regular pattern can be
drawn in the plan at Kerbougnec, which shows
straight and curvilinear alignments of granite
stones.
2. e observation of the slab surfaces allows us
to conclude that the majority of the stelae were
extracted from a substrate different from the
surface on which they presently stand. ese
blocks are also marked with forms of weathering
of the granite attesting to the fracture of some
of them and confirming an extraction from
an outcrop before the sea invaded the area.
ese fractures, testifying to the movements of
the blocks, could be explained by erosion and
disruption caused by the ocean (Fichaut and
Suanez 2006). However, such an explanation
would be very difficult to defend as the
blocks are situated in a protected area inside
Quiberon Bay (Stephan 2009). Moreover, the
concentration of anomalies forms a consistent
extension of the already confirmed architectural
structure, and does not continue to the north
or the south, supporting the interpretation.
3. Regarding the formation of the features we
note an observable change of direction in
Kerbougnec (Fig. 9.10): two main lines,
comprising around 30 monoliths, appear to
bend toward the southern base of the natural
outcrop known as L’Ours, following the curves
of the underwater relief (3 m and 4 m below
MSL). is change of axis is comparable to a
similar phenomenon noticed on the submarine
Neolithic stone row at Kerdual (La Trinité-sur-
Figure 9.9:
Kerbougnec. Variations
in the surface condition
of the monoliths
recorded, and forms
of weathering of the
granite (Photos: T.
Abiven and A. Lorin)
Serge Cassen, Agnès Baltzer, André Lorin, Jérôme Fournier and Dominique Sellier108
Mer), which is also centred on a natural rock
outcrop (Cassen and Vaquero Lastres 2003;
Cassen 2009b).
4. Finally, the direction of the structure is identical
to the pattern noticed in Carnac, from Menec to
Le Petit Menec, through Kermario, Manio, and
Kerlescan, not in terms of strict topographical
rules, or astronomic situation, but topological
pattern which prevents movement in a given
space (see Cassen 2009a).
Conclusion
is chapter has focused on the use of side-
scan sonar to identify prehistoric monoliths
in a marine context, in water depths of 2–5 m
below MSL. During the research programme
(20052009), two different side-scan sonar
systems were tested with distinct frequencies
(100 and 500 kHz). e instruments proved to
be complementary and both useful for this type
of fieldwork. For the identification of discrete
targets the highest frequency sonar (500 kHz)
is best suited, whereas the lower frequency sonar
is most useful for site contextualization. On
both sites, exceptional archaeological objects
(polished axe-heads made from Alpine jadeitite
and Iberian fibrolitite) confirm the age of these
architectural structures as c. 4500 cal BC. e
resulting maps of the structures showed the
direction of stelae alignment to be identical to
the pattern observed at Carnac, from Menec
to Le Petit Menec, through Kermario, Manio,
and Kerlescan in terms of topological pattern,
a feature that prevents movement in a given
space. We therefore suggest that such Neolithic
architectural features be described as a ‘barrier
of stelae’.
Acknowledgments
We should like to thank L. Menanteau, CNRS,
Nantes, for the work on the Kerbougnec site.
We would also like to express our warmest
thanks to the Editors for their kind invitation
to contribute to this volume, and for their help
with the English revision of the text.
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the megalithic sites in Britain. Journal of the
Royal Statistical Society. Series A (General)
118: 275–295.
om, A. and om, A. S. 1978. Megalithic
Remains in Britain and Brittany. Oxford,
Oxford University Press.
... Stopping continuing inundation was both something that many Aboriginal groups saw as a practical challenge-some erected artificial barriers-but also as a "spiritual" one, as might be represented by the stone arrangements along the Australian coast likely to be "associated with ritual control of extreme tidal regimes" (McNiven 2008, 155). Similar interpretations are used to explain coastal stone arrangements in northwest France and elsewhere (Cassen et al. 2011;Nunn 2021). ...
... Some are currently submerged or overlaid by sand dunes (Cassen et al., 2010(Cassen et al., , 2019Giot, 1998;Giot & Morzadec, 1992), highlighting significant coastal paleogeographic changes since the Neolithic in response to relative sea-level (RSL) rise. However, megalithic monuments studies that integrate reconstructions of paleocoastal environments are relatively new, such as those on the Molène Archipelago (Northwestern Brittany, Pailler et al., 2011;Stéphan et al., 2019) and Quiberon Bay (Southern Brittany, Baltzer et al., 2015;Cassen et al., 2012). Paleoenvironmental reconstructions provide additional evidence for understanding the reasoning used by Neolithic people for their site selection and the organisation of their coastal territories. ...
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