Botanical Journal of the Linnean Society (1988), 96: 57-68. With 6 figures
An assessment of lichenometry as a method for
dating recent stone movements in two stone
circles in Cumbria and Oxfordshire
VANESSA WINCHESTER
School of Geography, University of Oxford, Mansfield Road, Oxford OXJ 3TB
Received February 1987
WINCHESTER, V., 1988. An assess01ent oflicheno01etry as a 01ethod for dating recent
stone 01ove01ents in two stone circles in Cutnbria and Oxfordshire. Lichenometry uses the
size/age relationship of lichens to date stone surfaces. A study of neolithic stone circles at Rollright
in Oxfordshire and Castlerigg in Cumbria showed that stone movements in the recent past may be
deduced from the varying sizes of lichen populations on different surfaces and approximate dates
may be suggested for periods of change. Thalli of Aspicilia calcarea dating to around 1366 and of
Rhi:;.ocarpon geographicum to 1523 are reported. Problems in past work have mainly concerned the
effects on growth of environmental variation, anomalous growth, colonization rates, aspect
preferences, variable lifespans and an inconsistent methodology. Where a consistent methodology
can be applied the technique shows considerable potential not only for dating in glacial
environments, its major use to date, but also for assessing the detailed treatment over the recent
centuries of historic or prehistoric structures.
ADDITIONAL KEY WORDS:-Colonization - environment- growth - lichens- variation weathering.
CONTENTS
Introduction
Methods
Results.
Castlerigg
Rollright
Conclusions .
Acknowledgements
References .
57
58
59
59
61
66
68
68
INTRODUCTION
Lichenometry is a technique used to find either a relative or an absolute date
for a surface. Absolute dating depends on accurate construction of growth
curves from which growth rates can be estimated and age/size correlations
defined. Relative dating is achieved using the different size ranges of
neighbouring lichen populations to distinguish between surfaces exposed at
different times. A review of the technique, including some criticism, is provided
by Webber & Andrews ( 1973). Other useful commentaries are Locke, Andrews
& Webber (1979) and Innes (1985).
0024-4074/88/010057 + 12 $03.00/0
57
©
1988 The Linnean Society of London
i8
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I ER
The dating range depends on species specific and environmental factors: in
moSt temperate environments the effective range is probably unlikely to exceed
500 bears. uhile in harsh conditions rock weathering rates could determine
lifespan so that the maximum range in such circumstances may depend on
these. Ab5olute dating is based on the oldest surviving lichens but, because
man) factors affect lichen survival, surfaces may be exposed for much longer so
onl) minimum dating estimates can be established. Therefore, references to
specific dates here must be taken as minimum approximations only.
Lichenometr). it must be remembered, can only date episodes of lichen
clearmce and subsequent recolonization. Lichen community changes may occur
for a number of reasons including human and animal interference, competition
bet\\ cen lichens, \ egetation changes, weathering and other geomorphological
processes.
The Historic Buildings and Monuments Commission for England wished to
determine if lichenometry could provide evidence for stone movement in the
recent past at t w o stone circles; Rollright, near Chipping Norton, Oxfordshire
‘Sntional grid reference S P 296308, altitude 220 m ) and at Castlerigg, above
Kesuick, Cumbria ( N Y 292236, altitude 215 ml.
In this study lichenometry was tested in contrasting but limited and uniform
enLironments. Rollright stands on the crest of a Cotswold ridge, and the
limestone megaliths support a community specific to alkaline rock in southern
England, uhile at Castlerigg the megaliths carry a community specific to acidic
rocks and the climate is more severe.
Periods when the megalith surfaces were colonized by lichens could be
correlated with possible stone movements over the last 150 years at Castlerigg
and 400 years at Rollright. There is substantial historical evidence in the form of
illuqtrations of the circles which, although of variable quality, when taken
together with the lichen dating confirm that the circles are (especially at
Rollright “onl) a Ler) damaged remnant of the original design” (G. Lambrick,
perwnal communication 1.
S l t 1 HODS
It is essential that methods used for sampling, growth curve construction and
dating are standardiied and measurement is best carried out by a single
operator to avoid between-operator errors (Innes, 1985). Growth curves were
constructed by plotting single, largest diameter measurements of selected species
on memorials i n local churchyards, adding 6-9 months to allow for stone
emplacement, and enclosing the maxima €or each species with an envelope
curce.
Confidence in dating with this approach is limited since growth curves will be
based on the growth of very €e\v specimens. However, the alternative approach,
in which a curve is traced through the mean of the five largest growths on each
dated surface providing maximum diameters through time, suffers from the
disadvantage that as the population ages, individuals remaining from the
original colonization period are likely to become scarce and it may become
increasingly difficult to find the required fi\re specimens on the limited surface
areas of older tombstones.
T w o churchyards were selected for growth curve construction for each site
LICHENOMETRY FOR DATING STONE MOVEMENTS
59
and the curves were compared to determine how much environmental variation
might be affecting growth within each region. The churchyards near Castlerigg
were: Crosthwaite (National grid reference NY 244258, altitude 80 m) and St
John’s in the Vale (NY 225306, altitude 220 m); and near Rollright they were
Chipping Norton (SP 277306, altitude 150 m) and Long Compton (SP 330287,
altitude 105 m). T h e measurement method was that of Winchester (1985): an
acetate film inscribed with concentric circles at 5 mm intervals was placed over
each growth, a ‘best fit’ circle was selected and largest diameter measurements
were estimated to the nearest 1 mm. The error term was defined by comparison
with a photographic approach, using a Swift CAD drafting package on a Prime
computer to analyse the data.
The largest lichen diameters on all surfaces of the megaliths were measured,
noting the angle, aspect and height of each growth. The shape and dimensions
of the stones were also recorded to show the size of the surfaces available for
lichen colonization.
At Castlerigg, only two species had suitably circular growth forms: the
crustose Rhizocarpon geographicurn subsp. prospectans (Ras.) D. Hawksw. & Sowter
and Lecanora gangaleoides Nyl. At Rollright, four species were measured: the
crustose, slow colonizer Aspicilia calcarea (L.) Korber; the pioneer Verrucaria
nigrescens Pers. and two placodioid species, Caloplaca javescens (Huds.) Laundon
and Caloplaca aurantia (Pers.) Hellbom. This last species was used for dating with
caution since its growth curve was based on limited data; dates using this species
have been included on the plan in parentheses. The stones in each circle were
given identification numbers on the plans; those at Castlerigg agree with the
numbering shown on previous plans but at Rollright the numbering is novel.
T o verify dating, it was possible to use two, or occasionally three, species
growth rates to provide collaborative evidence for approximate dates. However,
this support was limited to 135 years, since at Rollright only A . calcarea survived
over a longer period.
RESULTS
Castlerigg
Examination of the data focused on two questions: is there any evidence for
the movement or re-erection of stones in the circle and, is the stone rectangle
within the circumference part of the original circle? (Fig. 1 ) .
The lichen cover on many of the 51 megaliths was either degraded by tourist
abrasion or formed mosaics which made measurement difficult. Nevertheless,
there was consistency in the size of the largest diameters measured and certain
episodes stand out during which a number of stones were colonized.
Figure 2 shows the numbers of surfaces dated by largest growths ranked in
20-year intervals. A peak of 51 surfaces appears between 1910 and 1939, with
22 of these dated between 1920 and 1929. There is no record of events to
account for this peak, although the possibility cannot be excluded that the
stones were cleaned on several occasions after the site was bought by the
National Trust in 1913.
One feature of the data as a whole is that ten out of the 13 19th century
growths are on the sides of stones (these are included on the plan in brackets
and blacked in on the histogram). Since neither animals nor crop burning
\" \\'!:'\CHESTER
60
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exposure dates suggested bv lichenometrv. The inset shows the dating in the stone rectangle. Thalli
on side:' of stones are shown in parentheses.
would discriminate in such a way, it may be that these side surfaces have
received less human attention.
Only seven growths in the circles date before 1882; in that year W. Davis
carried out a small excavation in the stone rectangle. Seven more growths
appear before 1900 possibly related to the excavation or to increasing public
interest. The lichens on the stones in the rectangle indicate dates similar to those
in the rest of the circle, except for one fragmentary arc of R. geographicum subsp.
prospectans on stone no. 51 dating to 1766. This date is supported by an
illustration of the circle showing the stone rectangle in 1725. Without this
evidence the 1766 date would have to be treated with caution because of (a) its
LICHENOMETRY FOR DATING STONE MOVEMENTS
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Figure 2. Histogram showing the periods of surface exposure of the stones at Castlerigg.
isolation from other dates in the circles and (b) the fragment might be part of a
composite growth.
The almost complete absence of growths before 1850 is evidently not due to
this being the maximum lifespan for the species in the region. On Crosthwaite
church roof at Keswick (3.5 km away) there are several large Rhizocarpon
growths: one measuring 202 mm diameter is the largest example of this species
yet found in England and dates to around 1520, the year the church received its
first slate roof (Wilson, 1970).
Relative dating of the megalith surfaces at Castlerigg is limited by the high
levels of lichen recolonization between 1915 and 1939. In all except three cases
the relative difference between inner and outer surfaces is less than 40 years.
On 12 stones, both species provided independent evidence for recolonization
episodes with a difference in dating of less than six years between species. They
also gave supportive nineteenth century dates for stones, no. 15 and 35.
The circle today is a key tourist attraction and lichens are being abraded by
people climbing on the stones. The circle has certainly been a focal point of
interest through the centuries; there is an unconfirmed report that sheep fairs
have been held in the circle, and another report indicates that when
Wordsworth and Coleridge visited it in 1799 the stones were painted white
(Lefebure, 1970: 53). Hutchinson ( 1794: 192) reported that "the stones are 50 in
number"; this included the inner rectangle, so it is evident that their numbers
have changed little since the late 16th century: although Hutchinson's
illustration of the stones shows that many of them were fallen in 1794, especially
in the western quadrant (Fig. 3). However, because of stone cleaning, the
lichens give no clear information on any movement or re-erection of stones
before 1915.
Rollright
At Rollright (Fig. 4), the 72 megaliths are mostly smaller than those at
Castlerigg, and the degree of weathering is such that probably none of the stones
could carry lichens from any original population. The dated surfaces cover a
much longer period than those at Castlerigg, although only the Aspicilia survives
ti2
Y. WINCHESTER
Figure 3. Castlerigg stone circle as illustrated by Hutchinson ( 1794: 192).
fC>r perhaps 800 years and the thalli of the three other species disintegrate after
about 135 years.
Lam brick (personal communication) suggests that changes in stone positions
over the last 300 years can be deduced by comparing present stone positions
with those shown in illustrations and surveys of the circle. Lichen dating of the
stone surfaces agreed closely with these deductions.
A histogram of the data (Fig. 5) suggests that during the surveys, and
f()llowing major publications, many of the megaliths may have been either reerected or cleared of vegetation so that they were available for lichen
colonization. Increased public interest may have resulted from the publications
and contributed to surface clearances.
Isolated colonization events recorded on the histogram in the more remote
past may not reflect stone movements. There is likely to be some 'background'
level of recolonization as a result of normal mortality amongst the lichens and
weathering processes on the stones. The five A. calcarea thalli appearing at
intervals between 1195 and 1450 mav be the result of incidental colonization
episodes of this nature.
New thalli begin to appear with greater frequency between 1450 and 1586
when W. Camden first described the circle. From 1607 onwards peaks on the
histogram correlate with periods of interest. Lichen dating of the stones agrees in
all except six instances with the stones marked as in situ in the illustrations since
at least 1677. Four of these carry lichen dates that are 'too young': nos II, 27
and 28, with dates between 1882 and 1897, and no. 23, dated to 1865; these
stones are severely \veathered. There is little conclusive documentary evidence
f()r the positions of stones no. 54 and 61 until the illustration of 1895; but
according to the lichen dating, these may have been re-erected in the 17th
centurv.
LICHENOMETRY FOR DATING STONE MOVEMENTS
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63
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the exposure dates suggested by lichenometry. C. aurantia thalli are shown in parentheses.
Vegetation changes may lead to alterations in a lichen community. Eighteen
larch trees were planted about 1.5 m within the circle probably in the 1850s: six
were felled between 1882 and 1895 and dense shading and disturbance caused
by the felling may explain why five inner stone-surfaces carry growths dating
between 1890 and 1897. The remaining trees were felled in 1941, and there are
five more inner surfaces dating to this period.
Major changes to the circle occurred in 1882 when the owner replaced all the
fallen stones in what he thought was "their original foundation". It is clear from
Number of surfaces doted withtn each tntervol
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LICHENOMETRY FOR DATING STONE MOVEMENTS
65
an evidently fairly accurate plan by Lockyer (Ashmolean Museum) showing
the circle in 1873 and 1905, that up to a third of the stones had been repositioned or re-erected between those dates. However, many of these stones
carry more recent dates, perhaps due to tree shading and mowing within the
circle at the beginning of this century: but a comparison of size differences
between populations on opposed surfaces was found to provide accurate relative
dating in terms of which stones, according to the illustrations, had been reerected from either fallen or leaning positions in 1882.
An example of how information can be drawn together to provide evidence
for events affecting a stone can be given by considering stone no. 36.
( 1) Relative dating. There are distinct differences between the lichen
populations on the inner and outer faces, with small growths on the inner and
very large ones outside.
(2) Cross-reference dating using different species. The mean of the dates provided by
the two species on the inner surface suggests a re-erection date in 1928.
Although this coincides with T. H. Ravenhill's work of 1926-1932, there is no
record that he moved this stone; it was probably set upright from a forwardleaning position in 1882.
Unfortunately, the nearly 800 year date of the Aspicilia growth of 240 mm on
the outer surface is only supported by one other example of the species, of
200 mm, dating to 1366. However, the existence of two similar-sized specimens
increases the probability of an early exposure date at least since 1366: this makes
these thalli the oldest examples of this species so far recognized in Britain.
(3) Competition. Dating may be questioned when specimens are degraded or of
an irregular shape so that they could be made up from several thalli. The
smaller of the two large Aspicilia thalli is nearly circular although the larger is
fragmented and plainly deteriorating.
(4) Habitat preferences. On the inner face, Verrucaria nigrescens and Caloplaca
jlavescens occur; both are rapid colonizers which do well (or can tolerate) damp,
less well-lit, vertical sites; while on the outer surface facing south, Aspicilia
calcarea prefers growth in near horizontal, well-lit positions. The two large
specimens are sited vertically on the middle to lower section of the stone, this
makes it seem likely that they were established when the stone was leaning
inwards with the surface more exposed to light.
(5) Weathering. The significance for the megaliths of the different weathering
processes is that the smooth, more rounded surfaces formed by solution can be
assumed to have lain on the soil over a long period, either before the circle was
constructed or afterwards. Sharper features will have been formed by sub-aerial
mechanical action. The weathering characteristics of stone no. 36 are
compatible with an inward-leaning position over a long period. The crown of
the stone carries a number of rounded pits typical of sub-aerial weathering and
the outer surface is fractured and has rather deeper pits which suggest that it has
been more exposed.
(6) Historical evidence. One of the oldest drawings of the circle, published by
R. Plot in 1677 (fig. 6), appears to be particularly accurate since it shows stone
shapes closely similar to those of today. In the illustration, stone no. 36 is
leaning sharply forwards; it continues to be shown in this position in all the
subsequent illustrations until 1895 when it appears upright.
f)(i
\ ·. \\'!:'\CHESTER
Figure 6. R ollright Stones as illustra ted bv Plot i 1677 ).
The historical evidence produced by Lambrick (personal communication)
suggests that onl y one-third of the 71 stones standing tod ay has remained
upright si nce th e 17th century. H owever. the lichen evidence suggests that only
a bout six stones ha ve rem ained erec t over this period , a ltho ugh vegetation
changes may ha,·e accounted for surface clearances on some otherwise
undisturbed stones. The high lc,·eJs of disturbance indicated by both the lichen
dating and the historical evidence, m a ke it clear tha t a ny theories f(Jr this circle
concerning megalithic dimensions or astronomical sighting directions should be
treated with the grea test caution.
CO:\CLCS IO?\S
The results from the two circles show that minimum dating is reliable over
the time spa ns covered . There was no evidence for over-estimation in d a ting at
R ollright but under-estim a tion may be common , especially if rocks a re severely
weathered. The study shmved tha t a number of problems previously thought to
limit the technique's effectiveness may be less serious than supposed.
At Castlerigg, although there •vas li ttle historical evidence to support dating,
there v.:as conside rable confidence in its accuracy. The growth curves of the
Rhi::.owrpon species were almost identical in the two environmentally different
churchy ard s, so it seems tha t the species was unaffected b y environmental
,·aria tion, and the accuracy of the dating of the K eswick thallus was
1
remarkabl e: it had to be accurate to 0.01 mm year - to coincide with the
origi nal roofing date of the church .
The growth curves produced from the churchyards near Rollright were less
well-founded du e to limited data but they were supported both by documentary
LICHENOMETRY FOR DATING STONE MOVEMENTS
67
evidence and the use of several species for cross-reference dating. The
extrapolation of growth curves beyond the recorded data here was justified
using the evidence provided by the correspondence of the lichen dating with the
historical records.
Where margins of originally separate thalli of the same species have become
indistinguishable as a result of fusion, there is a possibility that composite
growths may result in over-estimates in dating. None of the lichen dating
directly disagreed with the dated evidence of the illustrations. The use of
concentric circles to find 'best fit' largest diameters is of value in the detection of
anomalies and assisted in measuring growths on irregular surfaces.
Colonization rates should not be a problem when growth rates between sites
can be correlated since this period is inherent in growth curve construction and
will be the same at the dating site. Pollution and climatic change (especially
that covered by the so-called Little Ice Age) was found not to have altered
colonization or growth rates for the species studied in these areas.
Stone circles are ideal for the study of lichen aspect preferences. These
preferences must be recorded; otherwise, especially if only a single species is
measured, age differences between surfaces may be falsely assumed. The
problem is less acute if a number of species adapted to different habitats and
with similar lifespans can be used so that all surfaces can be dated using species
growing in optimum situations.
The limiting effects of competition on growth is a normal occurrence and is
likely to be included in growth curves during their construction. As a stone
becomes fully colonized and species continue to grow, competition will be
unavoidable and will be one of the keys to lichen growth and succession in each
environment.
Assigning a suitable 'background' level of survival to long-lived species is
difficult. It might be expected that with time fewer examples of a species would
survive from any original colonization period so that the size of the area needed
for sampling both for dating and for growth curve construction will increase; it
is likely that as dating becomes more remote under-estimation of dates will
therefore also increase. In the long term, growth rates may appear to be
reduced, but the apparent reduction will derive from the mistaken measurement
of younger secondary growths (in the absence of original colonizers). However,
the results in this study suggest that there was a constant linear growth rate over
the time period covered for at least the Rhizocarpon species measured at
Crosthwaite.
Wherever lichenometry is used it is most important that the areas searched for
growth curve construction and dating should be equivalent. Areas up to 500 m 2
have been found to yield growths of increasing diameter in relation to size of
area (Innes, 1985). Equivalence may be difficult to assess but the dating of
monuments would seem to be particularly suited to match the scale of search
possible in graveyards.
One of the advantages in using the technique in temperate environments is
that adequate data can be obtained for growth curve construction in local
churchyards and archival support may also be available to provide a reference
framework.
This study conclusively demonstrates that if used with sufficient rigour,
lichenometry has considerable potential for assessing the detailed treatment over
6R
\'. WINCHESTER
the recent centuries of prehistoric or historic structures. The method merits
greater attention by historians than it has hitherto received.
ACK:\OWLEDGntENTS
This paper is based on work funded by the Historic Buildings and
Monuments Commission for England, and thanks are due to the Commission's
Inspector Dr D. Fraser for his interest and support of the work at Castlerigg. I
am most grateful for the background research on the history of Castlerigg
carried out by Major M. Fisher; and at Rollright the work could not have been
completed without the help and collaboration of Mr G. Lambrick, of the
Oxford Archaeological Unit in charge of investigations in the circle. Thanks are
also due to the owner of the Rollright Stones, Miss P. Flick, for allowing me
access to study the lichens. Finally, my gratitude to my supervisors Professor
D. L. Hawksworth and Dr ~f. M. Sweeting for their comments on the text and
overall support.
REfERENCES
Ht:TCHINSO:\, \\"., 1794. The Histo~)' and Antiquities of Cumberland. 2. Carlisle: F. Jollie.
!:'\:'liES, .J., 1985. Lichenometry. Progress in PkJ'Sical Geop,rapkl'- 9: !87 -295.
LAMBRICK. G., 1983. 1he Rollright Stone.s. Oxford: Oxfc>rd .-\rchaeological Unit.
LEf'EBt'RE. :\1., 1970. Cumberland Heritage. London: Victor Gollanrz.
LOCKE. W. \\'., ANDREWS, J. T. & WEBBER. P. J., 197'l. A manual for lichenometry. British
GPOmorphological Research Group Technical Bulletin, 26: 1-47.
PLOT. R .. I677. The .\'a lura/ Histo~y ~l O.~[ord.1hire.
WEBBER. 1'. J. & A:'\DRE\\'S, J. '1' .. 1973. Lichenometry: a commentary. Arctic and Alpine Research, 5:
295-302.
WILSO;\;, T .. 1970. Histo~)' r!f'Crosthu·aite Parish Church. Krswirk: G.\\'. ~fcKane & Son.
\VINCHESTER, \'., 1984. A proposal for new approa(·h to lichenometry. British Geomorphological Research
Group Trrhniral Bulletin. 33: 3-20.