Highcliffe

[SZ 195 927]–[SZ 199 928]

Introduction

Highcliffe (also sometimes known as 'Friars Cliff, Mudeford') is the type section for the Boscombe Sand Formation, which is an arenaceous deposit immediately below the Barton Clay. This is the only site known to yield abundant plant macrofossils from the Eocene Boscombe Sand Formation. Over 50 species have been reported and it is the type locality for 14 of them. For 11 species and 3 genera, this is the only known British locality. The flora includes a mixture of wetland and paratropical rain forest plants, and represents a transitional phase in the history of European Palaeogene vegetation.

Plant fossils here were noted first by Prestwich (1849), but for many years they remained unstudied, at least partly due to the difficulties of collecting significant quantities of fresh material. During the 1930s, however, the direction of outflow of the River Run changed, causing the cliff to be eroded at its base (Burton, 1931). For two years, the resulting cliff-falls allowed considerable quantities of material to be collected, but in 1935 another change in the river flow caused this to cease and since then collecting has been difficult. Most of what we know about this site comes from Chandler's (1963b) descriptions of the fossils collected then.

Description

Stratigraphy

Daley (in Daley and Balson, 1999) discusses details of the geology of this site. The exposed sequence consists of about 14 m mainly of sands, which have been interpreted as being prograding mouth-bars at the seaward end of a tidal channel (Plint, 1988). Dinoflagellate biostratigraphy (Costa et al., 1976) suggests that the Boscombe Sands are the same age as the upper Selsey Formation further east, and are thus late Lutetian in age (see also Collinson, 1996b).

Palaeobotany

Chandler (1963b) gives details of the plant fossils that were found here. These include ferns, principally Lygodium kaulfussii Heer (Schizaeaceae), and the conifers Pinus sp., Sequoia couttsiae Heer (see footnote to (Table 8.2)) and Cupressistrobus gardneri Chandler. There are also conifer shoots originally described as 'Araucarites' sternbergii Goppert but which may belong to Sequoiadendron fordii Chandler, 1978 (Chandler, 1978, p.36–40; see comments on Araucarites' in the Bracklesham GCR site report). However, the angiosperms were by far the most diverse components; those included are listed in (Table 9.1). The plant remains are mostly preserved as delicate compression fossils. Compared with some of the other Tertiary fruit and seed sites in southern England, the fossils here are sparse.

Interpretation

The Boscombe Sand flora belongs to the 'Late Middle Eocene floras' of Collinson and Hooker (1987), which they state to be poorly known. Highcliffe is the only known locality to have yielded abundant plant remains from these deposits. Lignites in the Boscombe Sands at Bournemouth have yielded some plant remains but they are poorly preserved (Chandler, 1963b). The flora is probably coeval with the basal Hengistbury Head assemblage (Chandler, 1960) and the upper Selsey Formation exposed at Bracklesham Bay (Collinson, 1996b), but these have not yielded anywhere near as diverse assemblages as Highcliffe.

Although not abundant, the plant fossils from Highcliffe are of considerable interest as they throw some light on the vegetational changes taking place in Europe during middle Palaeogene times. They represent the stratigraphically highest occurrence of several taxa of angiosperm, including Rutaspermum rugosum, Toddaliospermum ornatum, Oncoba rugosa, Thymelaeaspermum bournense and Nyssoidea eocenica. On the other hand, it yields the oldest known remains of Cladiocarya foveolata, ?Gordonia truncata, Microdiptera parva and Epacridicarpum mudense. Ten species have not been reported from other localities in the British Tertiary: Scleriocarya tribracteata, Toddalia excavata, Grewia minima, Eurya mudense, Clethra hantonense, Sambucus mudensis, Cucurbitospermum mudense and three species of Carpolithus.

Two genera are unique ^.as fossils to Highcliffe. Scleriocarya was established for fruits similar to those of the living razor-sedge (Scleria), but which differed in details of shape and surface from any living species. The record of Grewia was based on a single endocarp that is very similar to those of the fruits of the mainly tropical living genus of that name. Highcliffe is also the only British site to yield fossils of Clethra, although it has been recorded from Polish amber (Conwentz, 1886) and Danish Miocene deposits (Friis, 1985). Friis (1985) considered the inclusion of the Highcliffe species in Clethra as uncertain.

Chandler (1963b) regarded the assemblage as having more in common with the Ypresian, para-tropical rain forest vegetation than of the later Eocene and Oligocene floras. Some of the families found at Highcliffe (e.g. dogwood, flacourtia, moonseed and tea families) indeed suggest the surrounding vegetation had a tropical aspect. However, neither the mangrove palm Nypa, which forms such a characteristic element of the Ypresian fossil floras, nor 'Scirpus' lakensis occur at Highcliffe (Collinson, 2000a). Also absent are many of the characteristic families of the Ypresian paratropical forests, such as the sumac, custard apple, dogbane, frankincense, icacina, laurel and soapberry families. The presence of some sedges, pondweeds, water lilies and frog's bits indicates that freshwater wetlands were already starting to become a dominant habitat in southern England. It would seem that the Highcliffe flora represents a transition between the Ypresian paratropical mangroves and rain forests, and the late Eocene wetlands, but with, on the whole, more in common with the latter.

Conclusions

Highcliffe is the only site known to yield a significant number of fossil fruits and seeds from the Boscombe Sand Formation, about 42 Ma old. The flora includes 11 species and 3 genera that are unique in the British Tertiary deposits. It reflects the transition between the paratropical rain forests of early Eocene times and the wetlands of late Eocene times, and is thus important for understanding the vegetational and climatic changes that were occurring in Britain during that time.

(Table 9.1). Composition of the angiosperm flora from the Boscombe Sand Formation, Highcliffe. Species are described in Chandler (1963b) unless otherwise referenced. Some are also discussed by Mai and Walther (1978, 1985) and Mai (2000). The family classification listed here is summarized in Chapter 1 of the present volume.

References