Journal of Archaeological Science 1976, 3,267-270

A Neolithic Moss Flora from Silbury Hill, Wiltshire D. Williams”

The 1968-1969 excavation at Silbury Hill, Wiltshire (map reference SU 100685), unearthed a considerable amount of well-preserved biological material. This unique discovery was important not only in providing evidence of the localized flora and fauna of the site at the time of the mound’s construction, but also in broader terms, of the vegetational history of a region in which direct botanical evidence is scarce. One of the best indications of the prehistoric environment was provided by the mosses because of their relatively specific habitat requirements. These plants were recovered in appreciable quantities from a heap of stacked turves at the centre of the mound (see Atkinson, 1967), where they were firmly attached to the surface of the turves, evidently having grown in situ. The turves exhibited a rendzina soil profile and had not been derived from the land surface under the mound, which had a soil of a brown earth type, developed in a local patch of clay-with-flints. Evans (1972) suggests that this calcareous soil had been formed from material of periglacial origin; similar chalk meltwater deposits are commonly found at the base of slopes in valleys and coombes (Evans, 1968) and it seems reasonable to assume that these turves had originated from somewhere in the immediate vicinity of the mound. Species of moss identified are listed in Table 1.

Table I. Moss species recovered from Silbury Hill, Wiltshire

Species o/O occurrence

by volume

Pseuaiwcleropodium purum (Hedw.) Fleisch. Rhytidiadelphus squarrosus’ (Hedw.) Wamst. Acrocladium cuspidatum (Hedw.) Lindb. Neckera complanata (Hedw.) Huben. Mnium longirostrum Brid

45+ 30 15

5

Mnium sp~.~ (cf. M. afine Bland. or possibly M. seligeri (Lindb.) Limpr.)

Mnium punctatum Hedw.

I

<5

Brachythecium rutabulum (Hedw.) B., S. & G. Thuidium SP.~ (cf. T. philibertii Limpr.)

“Possibly some Hylocomium splendens (Hedw.) B., S. 8z G. also present. bInsufficient material for positive identification.

The majority of species listed in the table are those of mature chalk grassland. Although no precise calculations have been made, the relative proportions of the species found are

“Environmental Archaeology Unit, University of York, York, England. 261

268 D. WILLIAMS

interesting and informative. From studies of chalk grassland (Cornish, 1954; Hope- Simpson, 1941), it is evident that Pseudoscleropodium purum is the dominant moss species in such habitats, being common and constant in all areas studied. Watson (1960), work- ing on a small area of the South Downs and using a numerical approach, was able to demonstrate that Pseudoscleropodium purum was three times as abundant as its nearest rival, Rhytidiudelphus squarrosus. Although this 3 : 1 ratio has not been demonstrated here there is no doubt that these two mosses were the dominant species. In several of the published lists of the commoner mosses of chalk grasslands (e.g. Cornish, 1954; Hope- Simpson, 1941; Tansley, 1939) another two of the species recovered here are consistently recorded, namely Acrocladium cuspidatum and Hylocomium splendens.

The majority of the remaining mosses listed in Table 1 have also been recorded from chalk grasslands. Differences in field records for these species can be attributed to changing conditions from area to area; compare Hope-Simpson’s (1941) findings on the South Downs with Cornish’s (1954) results from a study of the North Downs. The dominant species of a particular habitat are generally constant, but a fully comprehensive list of species is difficult to formulate. Of the other species listed here, the following may on the whole be regarded as associated with calcareous grasslands, although of only scattered and infrequent occurrence, viz:-Brachythecium rutabulum, Mnium a&Se, Mnium longirostrum, Neckera complanata and Thuidium philibertii. For example, Watson (1968) states that Mnium Zongirostrum occurs in calcareous pasture where the scattered stems can be easily overlooked amidst the grass. Even if growing luxuriantly it does not form a closely matted, intricately branching system.

Although most of the species recovered could have formed part of a calcareous grass- land sward, some of those found in small quantities are more characteristic of shaded, moister habitats and their presence does not seem entirely consistent with the dry conditions of a chalk grassland habitat. This is particularly true of Mnium punctatum and fragments of this species probably originated in a neighbouring habitat more conducive to its growth, e.g. woodland or marsh. This is corroborated by the fragmentary nature of these specimens and the small amounts recovered, especially of Mnium afine, in contrast to the complete plants of the species characteristic of calcareous turf. The occurrence of Brachythecium rutabulum, a nitrophilous species, could be due to the presence of man on the site with consequent localized nitrogenous enrichment of the soil.

Moss was very abundant in some samples, forming prominent tussocks, indicative of vigorous growth. This could be attributable to lack of competition, perhaps on bare ground in the grassland community. However, Pseudoscleropodium purum and Rhyti- diadelphussquarrosus were the most abundant of the species recovered and it is known that these flourish exceptionally well in dense swards, particularly in favourable “niches”. There is also a tendency for these mosses to flourish on and around ant hills and these were probably present; ant remains were identified in the turves by Dr M. Speight (Dimbleby, personal communication).

Watson (1960) points out that bryophytes are sensitive indicators of microclimate and soil surface conditions. Thus from present day ecological knowledge of these species we can obtain some indication of conditions in the neolithic sward. Two factors seem to be of major importance : (i) Angle and direction of slope. Rhytidiadelphus squarrosus and Acrocladium cuspidatum

are almost totally confined to north-facing slopes. Neckera complanata on the other hand, favours south-facing slopes. Pseudoscleropodium purum does not seem to have any such preferences and this may account for its wide occurrence.

(ii) Turf characteristics. A fescue sward is usually far richer in bryophytes than tussocky communities dominated by Zerna erecta (Huds.) Gray. A high proportion of rosette

NEOLITHIC MOSS FLORA FROM SILBURY HILL 269

weeds, such as might occur in heavily-grazed turf, is detrimental to moss growth. Tall herbaceous vegetation is not favourable for mosses and neither is a very short sward, particularly on south-facing slopes; the most favourable height of grass is in the region of 3-8 inches.

The moss remains indicate that moderately grazed mature chalk grassland existed in the vicinity of Silbury Hill at the time of the mound’s construction c. 2145f95 BC (I-4136). The moss flora was similar in composition to that of present day pasture on the South Downs. Pseudoscleropodium purum was the dominant moss, with RhytidiadeIphus squarrosus and Acrocladium cuspidatum also common, suggesting that the turves may have originated from a north-facing slope.

Our knowledge of the vegetational history of this region is limited due to poor preserv- ation of plant remains, particularly pollen, in aerobic calcareous deposits. This is unfor- tunate since this area was amongst the most heavily settled by neolithic man. It has been suggested by Turner (1970), that the downs, unlike the rest of the country, had been extensively cleared of woodland by the Bronze Age. This statement is based chiefly on the pollen work of Godwin (1962) at Wingham in Kent. Turner’s statement refers mainly to the south-east of England but there is even less evidence of the prehistoric vegetation for the rest of the English chalklands. She suggests that in view of their similar density neolithic and Bronze Age settlements, these areas would also have been cleared to a considerable extent by the Bronze Age. Such uncertainties have been partly resolved in latter years by the study of mollusc shells from sites on the chalk (Evans, 1972). Several of the sites studied, namely Beckhampton, South Street and Avebury are in close proximity to Silbury Hill, around the headwaters of the River Kennet, and indicate that dry open grassland conditions were widespread even in the third millennium BC.

The bryological evidence presented here would accord with this, and it is likely that extensive chalk grassland existed on slopes in the neighbourhood of the mound. The evidence for calcareous grassland is not directly relatable to the site of the mound itself which, as has been said, is on clay-with-flints, but it seems Iikely, on the basis of other evidence (Williams, 1975), that clearance was also extensive here too, with non- calcareous, weedy grassland and hazel scrub predominating.

Acknowledgements This paper is part of an unpublished M.Sc. thesis presented to the University of Bradford, 1975. I would like to take this opportunity to thank my supervisor Dr D. J. Hambler for all his help and encouragement during my years’ study at Bradford. I am also indebted to Professor G. W. Dimbleby for making available the Silbury Hill material for a research project, Professor R. J. C. Atkinson for providing information on the excavation as well as further material from Cardiff. Finally, I am grateful to Dr M. R. D. Seaward for confirming and identifying some of the more critical moss material.

References Atkinson, R. J. C. (1967). Silbury Hill. Antiquity 41, 259-262. Cornish, M. W. (1954). The origin and structure of grassland types of the central North

Downs. Journal of Ecology 42, 359-374. Evans, J. G. (1968). Periglacial deposits on the chalk of Wiltshire. Wiltshire Archaeological and

Natural History Magazine 63, 12-26. Evans, J. G. (1972). Landsnails in Archaeology. London: Seminar Press. Godwin, H. (1962). Vegetational history of the Kentish chalk downs as seen at Wingham and

Frogholt. Ver@entIichungen des Geobotanischen Institutes Riibel in Ziirich 37, 83-99.

270 D. WILLIAMS

Hope-Simpson, J. F. (1941). Studies in the vegetation of the English chalk. VII. Bryophytes and lichens in chalk grassland, with a comparison of their occurrence in other calcareous grasslands. Journal of Ecology 29, 107-l 16.

Tansley, A. G. (1939). The British Isles and their Vegetation. Cambridge: The University Press.

Turner, J. (1970). Post-Neolithic disturbance of British vegetation. In (D. Walker & R. G. West, Eds) Studies in the Vegetational History of the British Isles. Cambridge: The University Press, pp. 96-116.

Watson, E. V. (1960). A quantitative study of the bryophytes of chalk grassland. Journal of Ecology 48, 397414.

Watson, E. V. (1968). British Mosses and Liverworts 2nd ed. Cambridge: The University Press.

Williams, D. (1975) The Neolithic Flora of the Silbury Hill Site. Unpublished MSc. Thesis, University of Bradford.