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Introduction
Dune systems with wind-blown shell sands occur
at regular intervals along the whole of the west
coast of Britain. Archaeological remains have often
been found within these sands, associated with wellpreserved
assemblages of land snails. In the Western
Isles and the Northern Isles of Scotland, these dune
systems, known as the machair, were the subject
of a number of informative studies of sub-fossil
molluscan assemblages in the 1970s and 1980s,
notably by John Evans (1971, 1979, 2004), Penny
Spencer (1975), Michael Vaughan (1976), Annie
Milles (1991), and Nigel Thew (2003), from which
a number of themes began to emerge. John Evans’
work also had a considerable influence on subsequent
investigations by Paul Davies and Martin Bell
of land snails in coastal deposits in South Wales and
South West England (e.g., Bell and Johnson 1990,
Davies 2008). Since the 1990s, however, work on
assemblages of terrestrial molluscs from Scottish
dune contexts has largely ceased. John Evans did return
briefly to working on material from Sligeanach
in the Outer Hebrides in 2003, which was included
in an important paper on the relationship between
two common taxa (Evans 2004); sadly, the complete
study was only recently published posthumously
(Evans et al. 2012).
This paper is intended to reawaken an interest in
the analysis of sub-fossil land snails associated with
archaeological sites in the sand dunes of northern
Britain, by revisiting and highlighting some of the
patterns that emerged from the earlier work, proposing
new lines of enquiry, and assessing how these
ideas might be applied to Hebridean archaeology
today. Themes discussed will include the use of land
snail assemblages as a biostratigraphic tool that can
help in the relative dating of layers and structure infills,
and information that molluscan faunas can provide
about the local environment. Indeed, the analysis
of series of snail assemblages from a number of
contexts that represent both site stratigraphy and
spatial variation across a site, allows conclusions
to be drawn about changes in the local environment
and land use through time and about site-formation
processes. In turn, the comparison of data from a
number of sites scattered across the Western Isles
should help to develop a larger picture of environmental
change (Fig. 1). Land snails thus represent
an excellent palaeoenvironmental indicator that can
provide high-quality information for archaeological
investigations of machair sites.
The Taphonomy of Hebridean Snail Assemblages
Before the remains of terrestrial snails from any
context can be interpreted, the taphonomic processes
that have transformed populations of live molluscs
into assemblages of sub-fossil shells have to be
understood. Land snails from machair contexts can
be considered to be more or less autochthonous
(Evans 1972, 1979). This determination depends
upon whether they come from buried soils indicative
of surface stability, in which case the sub-fossil
molluscan faunas will represent the immediate local
environment, or from deposits of wind-blown sand
indicative of unstable surfaces, where the shells
could come from a much wider catchment area.
Rapidly accumulating wind-blown deposits may
also be subject to a high degree of temporal mixing,
as shells can be eroded from much older deposits
during blowouts. In addition, some allochthonous
shells may be introduced into machair deposits in
plant material brought in for thatching, fodder, or
manuring or carried in by flood waters from nearby
freshwater marshes and lakes (Smith 1994, Thew
2003).
Land Snails, Sand Dunes, and Archaeology in the Outer Hebrides
Matt Law1,* and Nigel Thew2
Abstract - Although the Western Isles have been subject to a number of recent archaeological investigations, there has been
limited recent work on molluscan assemblages, despite the very good degree of preservation to be expected in a number
of deposits and the significant work in the 1970s and 1980s on a number of sites such as Northton, Baleshare, and Hornish
Point. In the meantime, land snail analysis has flourished in southern England and elsewhere in Europe, with the development
of new techniques of numerical analysis such as the taxocene framework, the use of land snail assemblages in climate
reconstruction, and recent refinements in amino acid dating. This paper provides a brief summary of the work to date on
Hebridean snail assemblages, and presents preliminary results from work in progress, exploring aspects of site-formation
processes, middening, land use, and relative dating that can be explored using land snails.
Special Volume 9:125–133
2010 Hebridean Archaeology Forum
Journal of the North Atlantic
1School of History, Archaeology, and Religion, Cardiff University, John Percival Building, Colum Drive, Cardiff CF10
3EU, UK. 2Neuchâtel, Switzerland. *Corresponding author - LawMJ@cf.ac.uk.
2015
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M. Law and N. Thew
2015 Special Volume 9
Another problem with the interpretation of snail
assemblages is the relationship between the shells
and the layers in which they are found. Thus, layers
interpreted as ploughsoils may include shells
indicative of stable, shaded conditions, because the
non-intensive nature of ploughing with ards until
post-Viking times left pre-existing shells reasonably
intact. Even in modern times, ploughs in the Outer
Hebrides are often set to shallow to preserve the root
mat and maintain soil stability in the face of frequent
wind erosion (Barber 2011:50). Furthermore, the
basal part of wind-blown sands often includes shells
indicative of more stable conditions eroded from the
underlying layers, while the summit of wind-blown
sand deposits that are succeeded by a buried soil, often
include shells brought down from the overlying
soil by earthworms.
In practice, however, most published profiles
show a reasonably coherent ecological sequence,
which suggests that many of these potential problems
do not impair the integrity of land snail data
sets (Davies 2008:131). Furthermore, taking series
of spatially separated sample columns and spot samples
across a site will reduce problems of mixing or
redeposition by making aberrations in the molluscan
sequence easier to detect (Thew 2003).
Biostratigraphy and Relative Dating
Studies of blown-sand deposits have revealed
that a number of land snail species have arrived in
western Britain during the course of the last part of
the Holocene, since the start of the Neolithic (Davies
2008:130, Evans 1979:20, Thew 2003:163). These
include Cernuella virgata, Cornu aspersum (Helix
aspersa), Cochlicella acuta, and Helicella itala.
Recent arrivals in coastal dune systems include Candidula
intersecta, Cochlicella barbara, and Theba
pisana.
In the Western and Northern Isles, the arrival
and then the expansion of Cochlicella acuta and Helicella
itala provide clear biostratigraphic markers
that can act as a relative dating tool (Thew 2003).
The precise dating of the arrival and expansion of
these two species is still rather uncertain, though
a number of radiocarbon dates from sites such as
Northton, Hornish Point, and Baleshare (Fig. 1)
seem to suggest that Helicella itala appeared near
the end of the Late Bronze Age or early in the
Iron Age, and Cochlicella acuta arrived during the
earlier part of the Iron Age. Both species became
widespread and much more abundant at some stage
during the Iron Age. This relative dating tool will be
improved as more assemblages from sites with 14C
dates are studied. The expansion of Helicella itala
and Cochlicella acuta seems to have caused a major
decline in several other species (including Pupilla
muscorum, Cochlicopa spp., Vallonia spp. [especially
Vallonia costata], and Cepaea hortensis) by
competing with them for available ecological niches
(Thew 2003:167).
The use of amino acid (racemization) dating
might also be used to establish relative chronologies
with a good resolution, especially in sites or layers
where there is little material available for 14C dating,
although this method still needs to be developed in
Figure 1. Map showing key sites mentioned in the text. the context of machair sites.
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Site Formation Processes: Stability, Flux, and
Grazing
As machair systems develop, intervals of instability
cause wind-blown sands to bury earlier land
surfaces. Thus, profiles through machair deposits
typically include layers of clean wind-blown sand
over more organic, buried soil surfaces, with ironstained
deposits beneath these organic horizons that
have also been affected by pedogenetic processes
(Fig. 2). Erosion surfaces are often present, which
coincide with hiatuses in the depositional sequence.
Most of the numerical variations within molluscan
assemblages are thus attributable to differences in
the original populations coupled with the rate of
layer accumulation. Sand dunes and blown sand are,
by their nature, rather unstable habitats, which tend
to have restricted molluscan faunas. Interpretation
of the land snails therefore depends upon discerning
variations among faunas of low diversity that
indicate environments with a greater or lesser degree
of stability, herbaceous cover, moisture, and sand
accumulation (Thew 2003:163). Differences could
also be due to human activities such as ploughing,
grazing, the spread of fertilizer (including seaweed),
or the deposition of rubbish (middening). Ancient
ploughsoils tend to be thick, rather homogenous,
fairly organic layers, caused by physical mixing together
with the addition of organic material. Associated
snail assemblages have a tendency to resemble
the more varied faunas found in buried soils, but
with higher frequencies of taxa favored by middening
(Thew 2003:169-171).
Most specimens from dune deposits have lost
their periostracum, which is the colored protein
layer that covers the surface of the shell (Evans
1972:22). In rare cases, however, the periostracum
may be preserved where rapid burial has taken place
and the sandy deposits have remained both dry and
undisturbed, as in some of the sample assemblages
from Mound 2A at Bornais (M. Law and N. Thew,
unpubl. data).
Following its arrival in the Outer Hebrides, probably
during the Early Iron Age, Cochlicella acuta
came to dominate the more unstable contexts on the
machair, as they were able to flourish among tall,
deep-rooted marram grass (Ammophila arenaria)
Figure 2. Buried organic layer with cultural material sealed by wind-blown sand, Baleshare, September 2010.
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that is adapted to dune habitats with accumulating
sand (Thew 2003:167). When strong winds cause
the sand to drift, they climb the stems of the marram
grass and wait till conditions improve. Cochlicella
acuta, together with Helicella itala, has also become
the principal species of short-turf grazed machair
grassland in the Western Isles (Thew 2003:167).
Before the expansion of Cochlicella acuta and
Helicella itala, blown-sand contexts were often
dominated by low frequencies of Pupilla muscorum,
which seems to have been able to survive in
the rather dry, exposed, unstable conditions, where
most other species could not (Thew 2003:170). It
was much more abundant, however, in relatively
stable conditions associated with short-sward grazed
grassland, accompanied by Vallonia excentrica and
Vallonia costata (Thew 2003:170). Greater surface
stability is associated with greater biodiversity,
with snail populations being influenced by different
grazing regimes, which include varying intensities
of grazing by sheep and/or by cattle. In especially
stable, rather short-turf, fairly moist, sheep-grazed
grassland, for example, peaks in Vallonia excentrica
are accompanied by a number of less common
species, including Vertigo pygmaea, Punctum pygmaeum,
Euconulus fulvus, Vitrina pellucida, Vitrea
crystallina, Nesovitrea hammonis, and Oxychilus
alliarius (Thew 2003:170). In modern dry grassland
habitats, Pupilla muscorum prefers open sward
lacking in litter accumulation, whereas Vallonia
excentrica, Vallonia pulchella and Vallonia costata
favor a denser sward and the presence of leaf litter,
conditions that allow Vertigo pygmaea, Punctum
pygmaeum and Euconulus fulvus to also be present
(Jakupec 1997).
Vallonia costata, Cochlicopa spp., and Lauria
cylindracea are pioneer species that are able to flourish
in situations where the biotope is changing. Such
suitable conditions include both situations where
there is moderate surface instability, with broken
ground, as long as there is some humidity, pioneer
vegetation, and limited sand accumulation, and
also environments where there is increasing shade
due to the development of denser vegetation such
as tall herbs and possibly scattered shrubs (Thew
2003:170). These three species can sometimes take
advantage of middens and the presence of walled
structures too (Fig. 3). Peaks in these taxa often
Figure 3. Lauria cylindracea on an upturned fence post, South Uist, September 2010.
Journal of the North Atlantic
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coincide, sometimes in conjunction with Vitrina
pellucida (Thew 2003:170).
Relationships between certain key species may
thus be especially revealing. In a review of studies
from wind-blown sand contexts from throughout the
British Isles, for example, Evans (2004) suggested
that the relationship between Pupilla muscorum and
Lauria cylindracea may provide a useful indicator
of prehistoric farming practices. Pupilla was encouraged
by the relative stability of short-turf grazed
grassland, while Lauria, by contrast, was favored by
changing conditions, such as the instability associated
with broken ground caused by more intensive
land use, by more shaded conditions after land was
abandoned or left fallow, and by the effects of middening
or the formation of “plaggen” soils. Unfortunately,
over the last 2000 to 1000 years, Lauria has
extended its ecological range into more open grazed
grassland and dune habitats in much of northern and
northwestern Britain, replacing Pupilla muscorum to
a considerable extent and thus making environmental
reconstruction more difficult. In the Orkneys, for
example, which were never colonized by Cochlicella
acuta, Lauria has replaced Pupilla as the main
snail species of machair grassland (Cameron 2002,
Evans 2004:371–372).
To summarize, variations in snail assemblages
reflect differences in surface stability, vegetation
cover, and dampness in the terrestrial environment
surrounding habitation sites on the machair, which
in turn reflect differences in grazing regimes, precipitation,
wind activity, and the altitude above the
water table. In the area immediately surrounding the
walled structures of habitation sites, new species associations
occur that reflect human activities such as
middening or the spreading of seaweed, in addition
to rupestral habitats offered by walled structures.
These associations will be discussed below.
Work by Evans (1971) at Northton, South Harris,
also revealed evidence for previously existing woodland
in the Outer Hebrides, which despite partial
Neolithic and Bronze Age clearance, persisted into
the Iron Age before finally disappearing. This persistence
has been confirmed by pollen analyses across
the Western Isles, which showed that birch-hazel
woodland, with smaller frequencies of pine, oak, and
elm must have been locally present in many parts of
the islands at the start of the Neolithic (Brayshay and
Edwards 1996). As Thew (2003) has noted, phases
with shaded conditions, either open woodland or
tall herbs mixed with scattered shrubs, are usually
associated with species assemblages consisting of
at least 15 non-marsh land snail species that include
true shade-demanding taxa. At Northton, the woodland
phases are marked by the presence of up to 22
non-marsh species (Evans 1971), and at Buckquoy
in Orkney by up to 20 non-marsh taxa (Evans and
Spencer 1977). In comparison, counts of non-marsh
species at Baleshare, Hornish Point, Newtonferry,
and Balelone are lower than 15 in all samples, suggesting
rather open environments (Thew 2003).
Similarly, the highest species count for non-marsh
taxa for Mound 2A at Bornais, is 12 (M. Law and N.
Thew, unpubl. data).
Low frequencies of marsh and freshwater aquatic
species were present at Baleshare, Hornish Point,
Newtonferry, Balelone, and also at Bornais (M. Law
and N. Thew, unpubl. data; Thew 2003). These results
are probably the result of winter flooding from
nearby marshes and freshwater lakes due to rising
water tables (Ritchie 1979), though as the water
table at these sites is fairly high, patches of marshy
ground may well have persisted in hollows between
the dunes. The evidence for episodic winter flooding
at these sites raises questions about site location and
site-formation processes. Some of these sites may
only have been used seasonally, while strategies
may have had to be developed to cope with seasonal
or sporadic flooding at other permanently occupied
locations. Evans (2004) has speculated, following
ideas advanced by Pollard (1996), that midden sites
in the Scottish islands may have been created deliberately
as visible monuments. The reality of the
situation may have been far more prosaic, however,
with the build up of mounds being encouraged to
avoid winter flooding.
Site-Formation Processes: Middening, Walled
Structures, and the Spreading of Seaweed
At Baleshare, Hornish Point, Balelone, and
Newtonferry, Thew (2003:170) reported peaks
(from ~10% to 20% of total molluscs) in the relative
frequency of Oxychilus alliarius in contexts
where domestic organic refuse had been dumped,
although several stratigraphic sub-blocks with
concentrations of archaeological refuse and a fairly
high organic content had no concomitant rise in
the numbers of Oxychilus at Baleshare. Although
Oxychilus alliarius can be found in low numbers in
stable machair grassland habitats (Paul 1976), high
frequencies of this species, often associated with
Vallonia costata, were noted from the island of Flat
Holm (Bristol Channel) in habitats with a high organic
content and the presence of much decomposing
flesh (Young and Evans 1991). It seems likely,
therefore, that peaks in Oxychilus alliarius may
reflect the deposition of fresh domestic waste near
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2015 Special Volume 9
to the walls of the byre. The byre and an abandoned
kiln also revealed shells from species such as Lauria
cylindracea, Cochlicopa spp., Punctum pygmaeum,
and Nesovitrea hammonis, which had taken advantage
of the locally shaded conditions.
Climate Data from Land Snail Assemblages
Although there have been a number of significant
climatic changes over the last 6000 years, since the
beginning of the Neolithic (Magny 1995), none of
these seem to have influenced the presence or absence
of molluscan species in the Outer Hebrides.
This lack of correlation with climate can be attributed
to the fact that all of the snail taxa recovered from
the machair deposits have geographical ranges that
extend well beyond these islands, so the limited temperature
changes that took place during the second
half of the Holocene seem unlikely to have affected
their presence or absence. Instead, the occurrence of
non-marsh land snails has been largely determined
by environmental changes linked with human activities.
Nevertheless, as Dawson et al. (2011) have
shown, increased deposition of wind-blown sand
can be correlated with periods of climatic downturn,
such as the Little Ice Age; thus, the frequency and
thickness of deposits of wind-blown sand in sites
across the Western Isles may give indications of
intervals with a poorer climate.
Moreover, as Evans (2004) has pointed out, molluscan
evidence of freshwater flooding, caused by
fluctuations in the height of the water table, or of
significantly damper surface conditions, represents
a good indirect indicator of climatic changes that
affected the precipitation regime.
Numerical Methods for Modelling Ecological
Change
The application of numerical techniques to subfossil
land snail assemblages has not been fully
explored, although two particular types of technique
are reasonably common in snail reports, and are thus
worthy of comment here. The first of these is the use
of diversity indices. The diversity of an assemblage
can be measured simply by counting the total number
of taxa present; revealing patterns do emerge
from these simple counts, especially if used in conjunction
with indicator species such as the presence
of true shade-demanding taxa. More sophisticated
is the use of mathematical diversity indices, which
assess the evenness of spread of species throughout
a series of assemblages. The use of these indices is
suspect, however, because in addition to the diversity
the center of a site, while an absence of such peaks
in contexts with concentrations of archaeological
material and a relatively high soil organic content
might represent rubbish being removed from near
the center of a site and redeposited nearer the edge
of a settlement. Alternatively, if fresh waste was
rapidly buried by new refuse or by wind-blown
sand, Oxychilus may not have had time to colonize.
As Vallonia costata is also well known from gardens
and rubbish-midden locations (Evans 1972),
sudden peaks in this species might also, in some
cases, indicate the dumping of domestic rubbish.
Although Clausilia bidentata can occasionally
be found in relatively damp, tall, stable, grassland,
or grassland over rocks, this species is normally rupestral
and as such probably reflects the nearby presence
of standing walls or perhaps isolated bushes
or trees (Paul 1976, 1992). Some peaks in Lauria
cylindracea, which often lives in rupestral habitats
(Paul 1976, 1992), may also be linked with the
nearby presence of stone built structures, as seems
possible for the northern stratigraphic blocks at Iron
Age Baleshare (Thew 2003:170). This association
becomes more likely if there are no concomitant
peaks in the pioneer species Vallonia costata or Cochlicopa
spp.
The use of seaweed for animal fodder and as a
fertilizer that also stabilizes soil surfaces is wellattested
historically, including in the Outer Hebrides
(Smith 1994). Marine snails are often introduced
with the seaweed (Bell 1981:121), as seems to have
been the case at Hornish Point, Baleshare, Balelone,
and Newtonferry (Pain and Thew 2003), as well as
at Bostadh Beach (Cerón-Carrasco 2005:32) and
Bornais (M. Law and N. Thew, unpubl. data). The
seaweed snails at Newtonferry were often most frequent
in contexts with low numbers of land snails
indicative of middening, suggesting that seaweed
was primarily brought in as a fertilizer away from
the center of the site, possibly in areas that were
cultivated.
Even in locations where snails may not be expected
to be conserved in the archaeological record,
such as the acidic “blacklands” (peaty areas behind
the machair), certain socioeconomic practices may
create conditions suitable for their preservation.
This was the case at Howmore, South Uist, where
Smith (1994) attributed the presence in the byre of
snail species typical of machair grassland, to the
cutting of turves from the machair for use as animal
bedding, although turves could equally well have
been used for roofing material. She also attributed
shells of the synanthropic Oxychilus cellarius to the
presence of organic material and of rupestral taxa
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tive associations of snails in sub-fossil assemblages,
and provide a method for identifying patterns in the
land snail data without resorting to analogies drawn
from the modern ecology of the species present. For
obvious reasons, taxocenes can only be deduced
from contexts dominated by autochthonous shells.
The taxocene framework has not so far been applied
to Hebridean snail assemblages, primarily because
of a relative lack of data from appropriate contexts,
such as buried soils. Moreover, it is unlikely that the
same taxocenes identified in southern Britain will
occur on the islands.
Ultimately, however, as Evans (2004:366) has
stated, numerical methods often mask finer details.
This insight would suggest that the use of numerical
methods is appropriate as a means of exploring
data, but much depends on the taphonomic variables
affecting individual assemblages. Pielou (1975:34)
has argued that working back from statistical hypotheses
to ecological hypotheses is a very difficult
task. In sub-fossil assemblages, where the contextual
information are much less complete, the difficulties
of drawing palaeoenvironmental, palaeoecological,
and archaeological inferences is even greater.
Conclusions
Land snails are sensitive indicators of site histories
and of changes in land-management regimes.
The ways they respond to changes in the vegetation
cover, to agricultural practices such as grazing
and ploughing, and to changes in surface humidity
linked to climate change are recurrent from site
to site. Land snails can also act as indicators for
archaeological site-formation processes, such as
middening, the spread of seaweed, or the presence
of nearby walled structures. Within such structures,
the snail faunas can give some idea of their previous
function and possibly of construction materials such
as the use of turves from the machair for roofing.
Although land snail assemblages can be affected
by a number of taphonomic processes, particularly
in the unstable context of sand dunes, analyzing
multiple samples distributed spatially across a site,
the application of statistical techniques, and an understanding
of the changes that occur in molluscan
sequences in response to different depositional environments,
can help to distinguish allochthonous and
autochthonous elements in the snail assemblages
and to thus interpret faunal variations in terms of
archaeoenvironmental events.
Further work is needed to establish a precise
chronology for the arrival and spread of new snail
species in the Western Isles in order to strengthen
and density of live populations, linked to habitat
suitability, assemblages are also subject to a number
of taphonomic variables, including the concentration
or dispersal of shells during transport, the rate
of sediment accumulation, and differential preservation
of shell material linked to physical and chemical
conditions post deposition. In a review of the use of
diversity indices in archaeology, Ringrose (1993)
states that their use is not appropriate for inter-site
comparisons where there are likely to be taphonomic
differences, and Ken Thomas (Institute of Archaeology,
University College London, UK, pers. comm.)
adds that the same is true for intra-site comparisons
where taphonomic biases are suspected. Diversity
indices are thus most useful for highlighting taphonomic
issues, but not for drawing palaeoecological
conclusions. In his analysis of the infills of two treethrow
hollows from Ascott-under-Wychwood, for
example, Evans (2005) used the Shannon diversity
index to show that the higher number of species in
the upper fill was at least partially a reflection of a
greater frequency of shells. By using a combination
of rank-order curves of species abundance and
Fisher’s alpha diversity measures, Kenward (1978)
has been able to distinguish between autochthonous
and allochthonous insect taxa in urban assemblages,
with important consequences for the interpretation
of the contexts from which they derived. This same
technique could be used for land snail assemblages.
The second major type of numerical analysis applied
to sub-fossil terrestrial molluscs consists of ordination
techniques, which allow data to be explored
on the basis of the relative abundance of species in a
set of samples. These are descriptive rather than explanatory
techniques, so their effectiveness is dependent
on subjective interpretations. Detrended correspondence
analysis has been the most commonly
used for molluscan assemblages, as seen in work by
Bush (1988), Davies (1998), and Peacock and Gerber
(2008). This technique organizes the data into a
set of axes which account for different percentages
of variation between samples. This variation is usually
explained in terms of environmental factors
such as moisture, pH, or shade. The influences of
taphonomy, however, should also be considered,
as should biogeography in the case of assemblages
from islands. Detrended correspondence analysis
also can be used to identify taxocenes, although this
can be done more simply but subjectively by examining
molluscan results tables and diagrams.
The concept of taxocenes was applied to dryground
and wet-ground assemblages in central
southern England in the 1990s by Evans (1991).
Taxocenes are used to define recurring and distincJournal
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2015 Special Volume 9
their use as a relative dating tool and to detect
changes in the representation of pre-existing taxa in
response to adventive species.
Gathering snail data from larger numbers of
archaeological excavations in the machair area will
enable further refinements in the degree of sensitivity
with which molluscan responses to environmental
and anthropogenic changes can be interpreted.
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