Headon Hill and Totland Bay

[SZ 305 857]–[SZ 323 875]

Introduction

Plant fossils have been found at various stratigraphical levels through this classic section of the late Eocene succession on the Isle of Wight, from the upper Barton Clay Formation to the Bembridge Limestone Formation. They provide information on the vegetation and their habitats for this crucial part of the late Eocene, when climatic change was having a significant impact on this part of Britain.

Reid and Strahan (1889) and Chandler (1963a, 1964) have recorded plant fossils from here, but no single horizon has so far yielded a significant flora. However, the extensive exposures and the associated mammalian faunas allow the palaeoecology to be investigated (Hooker et al., 1995). Specimens from here were used in a study of the chemistry of water-plant seed coats (van Bergen et al., 1994a).

Description

Stratigraphy

Exposed along this part of the Isle of Wight coast is an almost unbroken succession of late Eocene strata, including the Barton Clay, Headon Hill and Bembridge Limestone Formations (Figure 9.12), (Figure 9.13) and (Figure 9.17). Of particular interest are the lignite bands that occur at several levels through the succession, the thickest being the Hatherwood lignite (Figure 9.16), some 8 m above the Chara Bed. Keeping and Tawney (1881) and Insole and Daley (1985) document  the stratigraphy of this site. Daley (in Daley and Balson, 1999) deals with its broader significance.

Palaeobotany

Plant fossils occur here in the Totland Bay Member (lower Headon Hill Formation; (Figure 9.14) and (Figure 9.18)). Fruits and seeds have never been described in the literature, but come from the same level (Limnocarpus band) as those described by Chandler (1961c) from nearby Colwell Bay and Hordle (see (Table 8.2)). The flora is exclusively aquatic, including pondweeds (Limnocarpus), water lilies (Sabrenia) and water soldiers (Stratiotes) (Collinson, pers. obs.).

Coniferous tree stumps from here with Glyptostroboxylon wood, together with those at Hordle, are the only in-situ tree stumps in the Tertiary deposits of the Isle of Wight (Fowler et al., 1973).

Reid and Strahan (1889) reported the presence here of a Leaf Bed from the Fishbourne Member (upper Headon Hill Formation). No fossils have ever been described from the Leaf Bed at this locality but the flora is likely to be similar to that found at Chapel Corner on the east side of the island (see GCR site report).

Chandler (1963a) reported calcite casts of Celtis edwardsii Chandler from the lower part of the Bembridge Limestone Formation. It was the only species found at this level at this locality and has not been found since. Hooker et al. (1995) reported other, mainly aquatic, plant remains including charophytes, and fruits of Stratiotes (Figure 9.19) and Caricoidea. Other significant fossils include leech cocoons and pellets of wood-feeding termites (Hooker et al., 1995).

Charophytes were described by Feist-Castel (1977) from several horizons through the Headon Hill Formation along this stretch of coast. The sequence demonstrated the difference between what Feist-Castel referred to as the Verzenay and Bembridge Charophyte Zones. The former (lower Headon Hill Formation) included characterisic forms such as Stephanochara edwardsii Grambast, Psilochara polita (Reid and Groves) Grambast, P. bitruncata (Reid and Groves) Feist-Castel and the early form of Harrisichara vasiformis (Reid and Groves) Grambast. These taxa were missing from the higher horizons, and instead Chara  attenuata Grambast, Grambastichara tornata (Reid and Groves) Horn aff. Rantzien and the  transitional forms of Harrisichara vasiformis–H. tuberculata were found.

Interpretation

There have been few detailed palaeobotanical studies along this stretch of coast on the Isle of Wight and its full potential has therefore still to be established. However, nowhere else in Britain are there such good exposures of plant-bearing strata through the upper Eocene, and the site holds considerable potential for further collecting, as well for palaeoecological study.

Significant work here has been on the palaeoecology of the Bembridge Limestone Formation (Figure 9.15) by Hooker et al. (1995), where the macropalaeobotany was integrated with palynological, mammal, invertebrate, organic geochemistry and stable isotope evidence. This revealed apparently conflicting results. The mammal evidence strongly indicated that the freshwater lake in which these deposits were formed was surrounded by a forest or closed woodland, with no more than a narrow belt of open habitat around the shore. However, the other lines of evidence all indicate that there was an extensive zone of marshland around the margins of the lake. The only evidence of trees was the calcified endocarps of Celtis, and bisaccate pollen, both of which were capable of being transported over considerable distances. There is clearly much potential here for similar work, which may help reconcile these apparently conflicting lines of evidence.

The exposures near Totland and on Headon Hill are important for their aquatic angiosperm flora, found in the Totland Bay Member. The deposits here are more freshwater-dominated than those seen at the Hordle Cliffs site, which has yielded the classic flora from the lower Headon Hill Formation. If they sample only local vegetation this may explain the absence of fruits and seeds of forest plants. Paddy's Gap may represent a similar situation.

Headon Hill is the best section in Britain for the study of late Eocene charophyte biostratigraphy. Nowhere else in this country has it been possible to document the succession of charophytes through the rocks of this age, and it is the only site in Europe where they can be seen in a continuously exposed succession.

Conclusions

Headon Hill has the best-exposed sequence of plant-bearing rocks of late Eocene age in Britain. It provides excellent opportunities for studying the vegetational habitats of Britain as they were 35–37 Ma ago. The section is especially valuable because of the association of fossil plants with the remains of mammals and other tetrapods, enabling the study of their co-evolution.

References