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, Mansjeld Road, Oxford O X 1 3TB

Received Februay 1987

WINCHESTER, V., 1988. An assessment of lichenometry as a method for dating recent stone movements in two stone circles in Cumbria and Oxfordshire. Lichenometry uses the size/age relationship of lichens to date stone surfaces. A study of neolithic stone circles a t Rollright in Oxfordshire and Castlerigg in Cumbria showed that stone movements in the recent past may he 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 Rhzzocarpon geographicurn 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.

Introduction . Methods . . Results. . .

Castlerigg . Rollright .

Conclusions . . Acknowledgements References . .

CONTENTS

. . . . . . . . . . . . . . 5 7

. . . . . . . . . . . . . . 58

. . . . . . . . . . . . . . 5 9

. . . . . . . . . . . . . . 5 9

. . . . . . . . . . . . . . 61

. . . . . . . . . . . . . . 66

. . . . . . . . . . . . . . 68

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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).

57 0024-4074/88/010057 + 12 $03.00/0 0 1988 The Linnean Society of London

i 8 \ \\ IhC’HtS 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 SP 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.

Two churchyards were selected for growth curve construction for each site

LICHENOMETRY FOR DATING STONE MOVEMENTS

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). The 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.

To 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.

59

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

0 4 0 I2 16 20 rn c * 8 u '2 * ' L

Figurc I . Plan of Chstlcrigg stone circle showing the numbers allocated ti) thr stimps and thc c.xpoaui-c dates suggrsted by lichenomrtry. The inset shows the dating in the stone rectangle. 'Ihalli o n aitler of stoiirs arc sh(ir\n i n parentheses.

bvould discriminate in such a way, i t 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 o u t a small excavation in the stone rectangle. Seven more growths appear before 1900 possibly related to the excavation or to increasing public interest. T h e 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. geographicurn subsp. prospcctans 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 e\.idence the 1766 date would have to be treated with caution because of (a) its

LICHENOMETRY FOR DATING STONE MOVEMENTS 61

LL Growths on sides of stones E - 2 - I

? 0s

6.r

5 2 0 -

E 16- z u s

3 14- @A :: 91 12- - 9

2 8 -

O 4- F E 2 2 - 5 0 - , -

- 0 c o = 0 : 18-

Q VI 10 - L nI-

a, 5 6 - L

1960 1900 I800 1760 Colonizotlon of wrfoces, ranked in 20 year intervols

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

Fiqure 3 . Cistlrrigg stone circle as illustrated by Hutchinson !1794: 192)

for perhaps 800 vears and the thalli of the three other species disintegrate after about 135 )ears.

Larnbrick I personal communication) suggests that changes in stone positions o\er the last 300 )ears can be deduced b) comparing present stone pocitions with thow sho\+ri in illustrations and surveys of the circle. Lichen dating of the stone surfaces agreed closely u ith these deductions. '4 histogram of the data [Fig. 5) suggests that during the surveys, and

f o l h ing major publications, many of the megaliths may have been either re- crected 01 cleared of vegetation so that they were available for lichen ( olonkatiori. 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 ma) not reflect stone movements. There is likely to be some 'background' le\el of recolonization as a result of normal mortality amongst the lichens and \\ eathering processes on the stones. The five A . calcnrea thalli appearing at Intervals between 1195 and 1450 ma) be the result of incidental colonization episodes of this nature.

Xc\z thalli begin to appear with greater frequency between 1450 and 1586 \\hen 12'. Camden first described the circle. From 1607 onwards peaks on the hi5togram correlate with periods of interest. Lichen dating of the stones agrees in all except six instances with the stones marked as zn Jztu in the illustrations since at least 1677. Four of these carr) lichen dates that are 'too young': nos 1 1 , 27 nnd 28, \zith dates betMeen 1882 and 1897, and no. 23, dated to 1865; these stones arc se\rerely ucathered. There is little conclusive documentary evidence lor the pmitionc of stones no. 54 and 61 until the illustration of 1895; but accordinq to the lichen dating. these may have been re-erected in thc 17th enturb.

€9

c c c

c

LICHENOMETRY FOR DATING STONE MOVEMENI’S 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 re- positioned 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 re- erected 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 dzfferent 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 forward- leaning 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 jz’avescens 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.

Fiqurc 6 . Rollright Stones a s illustratrd 11) Plot 1677:

‘The historical evidence produced by Lambrick (personal communication) suggests that only one-third of the 71 stones standing today has remained upright since the 17th century. Hokvever, the lichen evidence suggests that only about six stones ha\.e remained erect over this period, although vegetation changes may have accounted for surface clearances on some otherwise undisturbed stones. ‘I’he high levels of disturbance indicated by both the lichen dating and the historical evidence, make i t clear that any theories for this circle concerning megalithic dimensions or astronomical sighting directions should be trcated with the greatest caution.

cosc:LusIosS ‘I‘hr results from the two circles shokv that minimum dating is reliable over

the time spans covered. There was no evidence for over-estimation in dating at Rollright but under-estimation may be common, especially if rocks are severely weathered. The study showed that a number of problems previously thought to limit the technique’s effectiveness may be less serious than supposed.

:It Castlerigg, although there was little historical evidence to support dating, there was considerable confidence in its accuracy. The growth curves of the Rhi:occirpon species were almost identical in the two environmentally different churchyards, so it seems that the species was unaffected by environmental \.ariation, and the accuracy of the dating of the Keswick thallus was remarkable: it had to be accurate to 0.01 mm year-’ to coincide with the original roofing date of the church.

The growth curves produced from the churchyards near Rollright were less \wll-fourided due 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. I n 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

h8 1' IVIXCHESTER

the recent centuries of prehistoric or historic structures. The method merits greater attention by historians than it has hitherto received.

ACKSOiYLEDGESlENTS

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 M r 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. I,. Haw ksworth and Dr M. M. Sweeting for their comments on the text and overall support.

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PL,CYI'. K.. f t i77. The .Vaturn/ Hirioy of Oyforhhire. \VEBBER. 1'. J. ti AXDREiVS. J. ' I , . . 1973. Licliertornetry: a comrncntary. -4rclic and Alpine Hesearch, 5:

Gmmorpholo~giral Rrsearrh Croup Technical Bulletin. -%: 1-47.

295 -302. 1970. Histocy of C'roslhu~ait~ Parish L'hurrh. Krswirk: G. \V. XlcKanc & Son. R. 1. .. 1984. .-I proposal for new approach to lichenornetry. British Geomorphological Research

Group Terhnirol Builttin. .?3: 5-20.