Thorness Bay Fossil Hunting

c.1859–1880s – A’Court Smith’s collecting established the Insect Limestone record
James A’Court Smith collected extensively along the coast between Cowes and the Newtown River, including the east side of Thorness Bay, building the core fossil plant and insect collections that later made the bay one of Britain’s classic Palaeogene localities.

1878–1879 – Fossil insects and associated arthropods were formally reported
Early descriptions drew attention to the rich insect-bearing freshwater limestone at the Thorness–Gurnard section, including branchiopod and isopod remains with numerous insect fossils, confirming the bed as an exceptional Tertiary insect deposit.

1883–1888 – Early fossil plant records showed the botanical importance of the bay
Work on the fossil flora from the Insect Limestone recorded a diverse assemblage of fruits, seeds, leaves and aquatic plants, setting up Thorness Bay as a key site for understanding late Eocene vegetation on the Isle of Wight.

1926 – The Bembridge Flora was catalogued
The main Thorness Bay collections were used in the first major study of the Bembridge Flora, naming and describing many plants from the Insect Limestone and making the bay the type locality for numerous fossil plant species.

1963–1964 – The flora was revised and reinterpreted
A major revision of the Lower Tertiary flora reassessed many Thorness Bay plant fossils, recognising a rich assemblage dominated by aquatic and wetland plants, with rarer remains from nearby woodland habitats.

1966 – Thorness Bay was notified as an SSSI
The bay was protected for its biological and geological interest, including the coastal section from Saltmead Ledge to Gurnard Ledge, the Bembridge Limestone, Bembridge Marls and the fossil-rich Insect Limestone.

1971 – Intra-Palaeogene folding was demonstrated
Detailed work on the relationship between the Bembridge Limestone and overlying Bembridge Marls showed an unconformable contact and revealed previously unrecognised folding within the Palaeogene succession at Thorness Bay.

1972–1973 – The Bembridge Marls were tied to changing lagoon and estuary conditions
Studies of the Thorness Bay succession and its macroinvertebrate assemblages showed that the lower Bembridge Marls record brackish, quiet-water environments with changing salinity and periodic marine influence.

1975 – Whole fertile plants of Azolla prisca were described
Material from the Insect Limestone included exceptionally preserved water fern remains, among the few known whole fertile fossil examples of Azolla and the only such record then recognised from the English Tertiary.

1980 – The fossil insect fauna was comprehensively reviewed
A major modern study of Bembridge Marls insects confirmed the Thorness–Gurnard Insect Limestone as the largest British Tertiary insect assemblage, with termites, ants, flies, bugs and other groups indicating a warm climate.

1993–1995 – Plant fossils helped reconstruct habitats near the Eocene–Oligocene boundary
Work on plants, mammals and geochemistry used Thorness Bay material to distinguish the Bembridge Limestone pond or lake setting from the marshier Bembridge Marls environment, helping refine climate and habitat change across the boundary interval.

1998 – Insect preservation was investigated
Taphonomic work on the Insect Limestone examined why the bay’s insects are so finely preserved, helping explain the unusual survival of delicate arthropod remains in thin limestone beds.

1999–2000 – Rare seed-plume and rodent-gnawing evidence was recognised
Thorness Bay seeds were shown to preserve rare wind-dispersal structures and gnaw marks on Stratiotes seeds, providing evidence for seed predation by glirid rodents in the Bembridge Limestone environment.

2014 – The Insect Limestone was reviewed in a major thematic study
A new synthesis brought together the history, stratigraphy, sedimentology, molluscs, plants and arthropods of the Insect Limestone, reaffirming Thorness Bay as a nationally important late Eocene site for combined plant and insect evidence.

2019 – Vertebrates were recorded from the Insect Limestone
A small but important vertebrate assemblage was documented, including fish, lizards, birds and the rodent Isoptychus, with the lizard record including one of the youngest British scincoid occurrences.

2019–2024 – New insect studies expanded the fauna
Further work on the Insect Limestone added or revised records of cockroaches, mantises, earwigs, bugs, flies, lacewings and dragonflies, showing that Thorness Bay continues to yield important information on Britain’s late Eocene insect fauna.

2023 – Trace fossils supported brief marine incursions
Study of burrow systems in the Bembridge Limestone strengthened the interpretation that the mainly freshwater to brackish Thorness Bay succession was periodically affected by short-lived marine flooding events.

SOLENT GROUP

Headon Hill Formation (Upper Eocene / Priabonian)

Osborne Member

Bed TB1 — Upper Osborne Member (context interval)

Green, olive and red calcareous clays and lime-rich muds with discontinuous concretionary limestones are exposed intermittently south-westwards from Burnt Wood and form the highest visible part of the Headon Hill Formation in the Thorness Bay section. These beds are not the main fossil attraction here, but they are important because the overlying Bembridge Limestone rests sharply on them across much of the north-west coast. They represent shallow, fluctuating lacustrine to marginal-lagoonal mud accumulation with repeated carbonate concentration and local pedogenic modification before establishment of the more persistent Bembridge carbonate system.

Bembridge Limestone Formation (Upper Eocene / Priabonian)

The Thorness Bay–Gurnard Bembridge Limestone is not normally described with a widely used formal local bed-number scheme. The TB numbers used below are practical site-use divisions following the published threefold lithological organisation at Gurnard Ledge: a lower carbonate unit, a central muddy interval, and an upper carbonate unit. This arrangement is critical because it makes the locality much more mud-dominated than the better-known Whitecliff Bay succession.

Bed TB2 — Lower Bembridge Limestone Lower Carbonate Unit

Buff to pale grey fossiliferous limestones and marls form the lower ledges of the Bembridge Limestone at Gurnard Ledge. The rocks are mainly biomicritic and locally porous or vuggy, with rhizolith-like structures, casts of freshwater to low-salinity gastropods and bivalves, charophytes and occasional foraminiferal material. Near Gurnard Ledge, the uppermost limestone bed of this lower carbonate unit contains ferruginous chert concretions with silica pseudomorphs after gypsum, a distinctive feature apparently unique to this locality. Compared with the land-gastropod-rich limestones of places such as Prospect Quarry, the Thorness–Gurnard lower carbonate unit appears more offshore within the lacustrine–lagoonal system and less strongly palustrine.

Bed TB3 — Middle Muddy Interval (c. 3.3 m)

About 3.3 m of lime-rich muds, marls and clays separate the lower and upper carbonate units. This muddy interval is the main reason the Thorness Bay Bembridge Limestone is thinner and less limestone-dominated than the type section at Whitecliff Bay. The lowest cliff sections at Gurnard have yielded fruits and seeds from these Bembridge Limestone muds, and the broader flora of the formation here includes Azolla prisca, Sparganium multiloculare, Stratiotes neglectus, Brasenia spinosa, Potamogeton pygmaeus and Aldrovanda intermedia, together with charophytes in the upper limestone and rare rodent gnaw marks on Stratiotes seeds. This interval represents low-energy lime-rich mud accumulation in calcareous ponds or lakes with open water and emergent vegetation.

Bed TB4 — Upper Bembridge Limestone Upper Carbonate Unit And Erosive Top

Buff limestones and marls form the upper carbonate unit, but its lithology varies laterally south-westwards, in part because of contemporaneous erosion before deposition of the overlying marls. The top of the Bembridge Limestone here is therefore not simply a neat conformable bedding plane: at Thorness Bay the contact with the Bembridge Marls is locally unconformable and records intra-Palaeogene uplift and warping along the Porchfield Anticline. The upper unit completes a 6.7 m formation at Gurnard Ledge and provides the irregular, locally omission-prone substrate on which the Bembridge Marls accumulated.

Total Thickness Of Bembridge Limestone Formation At Gurnard Ledge: 6.7 Metres

Bouldnor Formation (Latest Eocene To Earliest Oligocene)

Bembridge Marls Member

Published local horizon codes such as BW I, BW IV, THOR III and GUR IX are retained below where they are geologically important. The TB numbers used alongside them are broader site-use identifiers for practical website description and are not intended to replace the formal local horizon terminology developed for Thorness Bay and Gurnard. At Gurnard Ledge, the member is 21.5 m thick and is composed mainly of black, grey and green muds and marls with shell seams, lime-mudstones and the famous Bembridge Insect Bed low in the succession.

Bed TB5 — Basal Shell-Band Interval And BW I Algal Horizon

The Bembridge Marls rest on an erosional and omission-prone surface cut into the Bembridge Limestone. Unlike Whitecliff Bay, there is no thick basal sandy Bembridge Oyster Bed here, but two oyster-bearing shell bands occur near the base. At Burnt Wood, the bottom bed of the member, BW I, has yielded small stromatolitic or oncolitic algae, apparently the only such fossils recorded from the Palaeogene of southern England. Lithologically the interval consists of black, grey and green muds and marls with shell seams and local lime-mudstone, recording the first flooding of the irregular limestone surface by shallow brackish water, still quiet enough for delicate sedimentation but intermittently agitated enough to concentrate shells.

Bed TB6 — Lower Brackish Marl And Life-Position Bivalve Interval, Including THOR III

Grey and blue-green clays with shell seams dominate the lower part of the member above the basal shell bands. Numerous shallow-burrowing bivalves occur in life position towards the base of the succession, showing that much of the substrate remained undisturbed for long periods. On the southern limb of the syncline, west of Pilgrims Park, bed THOR III yielded the shell-encrusting alga Epivalvia edwardsii. Shell preservation varies strongly with lithology: the best material comes from grey clays, whereas shells in blue-green clays are commonly chalky, friable or decalcified. Environmentally this interval represents a quiet brackish to freshwater lagoonal and marsh-waterbody setting with fluctuating salinity.

Bed TB7 — Bembridge Insect Bed / GUR IV = BW IV (whole bed commonly c. 0.65–0.85 m; tabular limestone locally c. 0.1–0.6 m)

This is the celebrated Thorness–Gurnard insect and plant horizon, lying about 4 m above the base of the member. It is better treated as a broader Bembridge Insect Bed than as a single “Insect Limestone”, because the bed is predominantly argillaceous and includes discontinuous concretionary limestones and hard marls as well as a more or less tabular micritic limestone. At Gurnard the whole bed is commonly about 0.65–0.75 m thick and consists of a lower, more or less continuous limestone with upper discontinuous concretions; at Saltmead Ledge on the opposite limb it is about 0.65–0.85 m thick and divided into four calcareous courses. Fresh rock is blue-grey and exceptionally fine grained, weathering with a brownish rind, and contains thin silty laminae rich in carbonaceous detritus, ostracods, plant fragments—especially Typha—and insect debris. This is one of the richest units in the British Tertiary for its thickness: more than 200 insect species, spiders, anostracan and isopod crustaceans, ostracods, rare bird feathers and the main source of the Bembridge flora have all come from this level. Molluscs include freshwater hard-water forms such as Lymnaea (Galba) longiscata, Planorbarius discus, Gyraulus similis, Hippeutis headonensis, Viviparus lentus and V. angulosus, together with the more brackish Tarebia acuta and Polymesoda obovata. The flora includes delicate fruits and seeds with wings and plumes, Typha, aquatic plants and whole Azolla plants. Taken together, the sedimentology and fauna suggest an exceptionally quiet shallow depocentre within a marsh–lagoon complex, probably fresh to slightly brackish, into which some shells and plant fragments were washed.

Bed TB8 — Lower–Middle Bembridge Marls With The Serpulid Marker GUR IX

Above the Insect Bed, black, grey and green marls and muds continue with thin shell seams and locally calcareous bands. Bed GUR IX is a very thin serpulid-rich marker horizon that can be traced widely wherever the lower part of the member is visible, including at localities many kilometres away. The interval as a whole shows how even very thin faunal beds could become regionally persistent. Some coquinas and convex-upward bivalve concentrations indicate short episodes of water movement, but the dominant impression is of prolonged quiescence, supported by varve-like lamination and undisturbed mud accumulation.

Bed TB9 — Middle–Upper Shell-Bearing Marls, Including Corbicula And Nystia Horizons

The middle to upper part of the Bembridge Marls at Gurnard Ledge includes additional shell-rich intervals recognised in the classic local log, among them a corbiculid-rich bed in the middle to upper member and a higher Nystia-bearing band close to the top. Grey clays again provide the best shell preservation. These beds record repeated oscillations between brackish lagoon, fresher marsh pool and shallow standing-water conditions. Outside the main Insect Bed, other plant-bearing horizons in the member have yielded aquatic fruits and seeds such as Rhamnospermum bilobatum, Stratiotes neglectus and Sabrenia chandlerae, showing that open-water and marsh vegetation remained widespread through much of the succession.

Bed TB10 — Upper Bembridge Marls And Rooted Top Surface

The upper part of the member is commonly obscured by vegetation and cliff deterioration, but the classical succession continues upward through varicoloured muds and silts with shell seams into a rooted top surface below the Black Band. Mud cracks and pyrite “pins” indicate very shallow water and rooted herbaceous vegetation, and the broader Thorness Bay palaeobotanical evidence suggests extensive persistent marshland with some open waters, some fluvial influence and locally small wooded “tree islands”. The uppermost Bembridge Marls pass into an omission surface and palaeosol rather than a simple uninterrupted bedding transition.

Total Thickness Of Bembridge Marls Member At Gurnard Ledge: 21.5 Metres

Hamstead Member

Bed TB11 — Black Band And Basal Hamstead Omission Surface (c. 0.4 m; poorly exposed)

Poor exposures of the Black Band occur just south of Gurnard Ledge. Where visible, it is a dark brown to black shelly organic-rich clay about 0.4 m thick, locally with scattered angular flint pebbles, resting on a rooted palaeosol with calcrete-like concretions developed at the top of the Bembridge Marls. It contains a freshwater fauna and flora and forms the defining basal marker of the Hamstead Member. At Thorness Bay it is more important as a stratigraphic datum than as a productive fossil bed because exposures are usually poor and intermittent.

Structural Style And Composite Nature

Thorness Bay must be treated as a composite section assembled from Burnt Wood, Saltmead Ledge, the southern limb west of Pilgrims Park, the cliffs south of Gurnard and Gurnard Ledge. The beds occur on both limbs of the shallow Thorness Bay Syncline and are not exposed as one simple uninterrupted face. The locally unconformable base of the Bembridge Marls on the Bembridge Limestone, together with the south-westward variation and partial erosion of the upper limestone unit, was one of the key pieces of evidence used to demonstrate intra-Palaeogene warping along the Porchfield Anticline. In field terms, that means marker beds are more reliable than any attempt to force the locality into a single uniform vertical log.

Depositional Environment

The upper Osborne Member represents calcareous mud accumulation in shallow lagoonal to lacustrine settings. The Bembridge Limestone formed in warm, clear, calcareous ponds or lakes with open water and emergent vegetation in relatively dry surroundings. The Bembridge Marls then record a broader and more persistent brackish-to-freshwater marshland and lagoon system with sluggish rivers, open pools and salinity fluctuations; the Insect Bed formed in one exceptionally fine-grained quiet-water depocentre within that system; and the Black Band marks renewed organic accumulation above a rooted omission surface at the base of the Hamstead Member.

Stratigraphic And Palaeontological Significance

Thorness Bay is one of the most important British late Eocene coastal sites because it combines the main source of the Bembridge flora with the classic Bembridge Insect Bed. The flora exceeds 100 species, many unique to the site, and the insect assemblage is the largest known from the British Tertiary. Together they provide one of the best windows into ecological and climatic change around the Eocene–Oligocene transition in north-west Europe, while the sedimentary relationships between the Bembridge Limestone and Bembridge Marls also make the locality important for understanding intra-Palaeogene earth movements on the Isle of Wight.

Total Thickness Covered Here

The composite exposed succession is less than 30 m thick, comprising upper Osborne Member context beds, 6.7 m of Bembridge Limestone at Gurnard Ledge, 21.5 m of Bembridge Marls Member, and a thin but significant Black Band at the base of the Hamstead Member. Because the key beds are distributed across both limbs of the syncline and are variably obscured by vegetation, slip and beach cover, Thorness Bay is best understood as a composite coast section rather than as one single continuous cliff log.

References

Reid, C. & Chandler, M.E.J. (1926). The Bembridge Flora.
Daley, B. (1972, 1973, 1974, 1989, 1999) on the Thorness Bay and Gurnard stratigraphy, Bembridge Marls faunas, algal horizons and Isle of Wight Palaeogene sections.
Daley, B. & Edwards, N. (1971, 1990) on intra-Palaeogene warping and the Bembridge Limestone succession.
Insole, A. & Daley, B. (1985). Revision of the late Eocene and early Oligocene lithostratigraphy of the Hampshire Basin.
Collinson, M.E., Hooker, J.J. and co-authors (1983, 1987, 1993, 2000) on the Bembridge flora, palaeoecology and rodent gnaw marks.
Jarzembowski, E.A. (1980a, 1980b) on the Bembridge Insect Bed insect fauna and field sections.
Munt, M.C. (2014). Molluscs from the Insect Limestone of the Bembridge Marls Member.
British Geological Survey memoir: Geology of the Isle of Wight, and BGS Lexicon entries for Osborne Member, Bembridge Limestone Formation, Bembridge Marls Member and Hamstead Member.
JNCC Geological Conservation Review accounts for Thorness Bay and for Thorness Bay and Gurnard.