Gurnard Ledge Fossil Hunting

Mid 19th century – Gurnard began attracting geological attention for its fossil-bearing Bembridge beds
Gurnard has attracted the attention of geologists since the middle of the nineteenth century, especially for the fossil-rich horizon now known as the Bembridge Insect Limestone. This marks the beginning of the site’s scientific importance.

c. 1850s–1870s – James A’Court Smith built the classic Gurnard fossil collections
Much of the knowledge of the Gurnard fossil flora and insect fauna comes from the collecting of the local amateur and retired sailor James A’Court Smith, who spent around thirty years building up an important collection from these beds. His work was fundamental to the later scientific study of the site.

1874 – James A’Court Smith published the early records of the insect fossils
Early references to the Gurnard insect fossils were published by A’Court Smith in 1874. This was one of the first major published milestones for the site and helped establish the Gurnard Insect Limestone as an important British Tertiary insect locality.

1878 – Brodie recorded fossil insects and associated fossils from the Isle of Wight beds including Gurnard
Work by P. B. Brodie in 1878 added to the early published history of the Gurnard insect beds and also forms part of the record of rare fossil bird feathers from the locality. These early studies helped show that the site yielded more than just isolated insect fragments.

1879 – Branchipodites vectensis described from the Gurnard insect-bearing beds
The fairy shrimp Branchipodites vectensis was one of the early named invertebrates from the Gurnard fauna. It remains one of the characteristic fossils of the Insect Limestone assemblage.

1888 – Eoatypus woodwardii described from the Gurnard Insect Limestone
The spider Eoatypus woodwardii was described from the Gurnard Insect Limestone in 1888. It became the classic early spider fossil from the site and was later re-studied in the light of better material.

1913 – Mastotermes anglicus recorded from Gurnard
The termite Mastotermes anglicus is one of the notable insects from Gurnard and helped demonstrate the unusual diversity of the fauna. Its presence adds to the wider palaeoenvironmental importance of the site.

1915 – several important insect fossils were named by Cockerell
Important Gurnard insects described by T. D. A. Cockerell in 1915 include Carsidarina booleyi, Sciara lacoei, Odontomyia brodiei and Oecophylla perdita. These names mark one of the major phases in the formal scientific description of the Gurnard insect fauna.

1917 – Aeolothrips brodiei added to the known Gurnard insect fauna
The naming of Aeolothrips brodiei further expanded the range of insect groups known from Gurnard and helped show how taxonomically varied the site really is.

1921 – Cockerell published a major account of Oligocene insects from Gurnard Bay
Cockerell’s 1921 paper on Oligocene insects from Gurnard Bay was one of the major milestones in the study of the locality. It included species such as Pterotriamescaptor primus, Psocus acourti, Pterostichus gurnetensis, Bittacus veternus and Beraeodes anglica.

1925–1926 – Chandler and Reid described important plant fossils from the Gurnard Insect Limestone
Much of the classic fossil flora from the Gurnard Insect Limestone was described in the 1920s by Reid and Chandler, largely from the A’Court Smith collection. These studies established the site as a major British late Eocene plant locality as well as an insect site.

1972 – Martini provided a modern account of the Gurnard isopods
Martini’s work on the isopods was part of the twentieth-century revival of scientific attention on the Gurnard fauna. It showed that the site still had much to contribute beyond the older historic collections.

1976 and 1980 – Jarzembowski produced major modern work on the Gurnard insect fauna
R. A. Jarzembowski’s studies in the late 1970s and 1980 were among the most important modern contributions to understanding the Gurnard insects. His work helped establish the fauna as the largest known insect assemblage in the British Tertiary.

1980 – new named fossils such as Paratriaxomasia solentensis and Megalomus tinctus were described
Jarzembowski’s 1980 work added further named insect fossils to the Gurnard fauna, helping expand the already impressive list of recorded species from the site.

2001 – Vectaranaeus yulei described and the classic spider fauna revised
Selden described the spider Vectaranaeus yulei in 2001 and re-described Eoatypus woodwardii. This was an important milestone because it showed that even the best-known parts of the Gurnard fauna could still yield important new discoveries.

2005 – the Gurnard insect fauna was shown to represent about 167 known families
By 2005 the known insect fauna of Gurnard had expanded so much that around 167 families were recognised, with new finds including an earwig and a praying mantis. This underlined the extraordinary diversity of the site.

Modern understanding – Gurnard is recognised as one of Britain’s most important late Eocene insect and plant fossil sites
Today Gurnard is regarded as a classic and internationally important site for the Bembridge Marls Insect Limestone, with about 220 insect species, several spiders, over 100 plant species, rare bird feathers, crustaceans, gastropods, bivalves and even a fragment of lizard skin recorded from the beds. Its importance lies not only in the diversity of the fossils, but in the exceptional preservation of delicate tissues and structures close to the Eocene–Oligocene transition.

Section Architecture

This locality is not a single simple cliff log. The useful section is a composite of low cliff and foreshore exposures around and immediately south of Gurnard Ledge on the northern limb of the Thorness Bay Syncline, together with equivalent beds in east Thorness Bay on the southern limb, especially north-west of Pilgrim’s Park. Exposures are generally best near the base; the higher Bembridge Marls are commonly vegetated, slipped or beach-masked, and the Black Band is only poorly exposed just south of Gurnard Ledge.

Local Stratigraphic Note

The sequence exposed in the Gurnard–east Thorness tract extends from the Bembridge Limestone Formation into the lowermost Hamstead Member of the Bouldnor Formation and totals less than 30 metres. The stratigraphy is formally documented, but the Gurnard page is best written using real lithological packages and named local horizons rather than pretending that every part of the succession is continuously visible in one face. The key published local horizon names retained here are GUR IV for the main Insect Bed at Gurnard Ledge and THOR III for a lower Bembridge Marls horizon in east Thorness Bay.

SOLENT GROUP

Bembridge Limestone Formation (Upper Eocene)

Bed GL1 — Lower Carbonate Unit Of The Bembridge Limestone

The lower part of the Bembridge Limestone at Gurnard is a buff freshwater limestone and marl package forming the main rock platform of Gurnard Ledge. It belongs to the distinctive palustrine–lacustrine Bembridge Limestone facies of the north Isle of Wight, deposited in very shallow carbonate-rich ponds and lakes subject to repeated pedogenic overprint. Ferruginous chert concretions occur in the uppermost limestone bed of this lower carbonate unit near Gurnard Ledge, and these are locally notable for silica pseudomorphs after microlenticular gypsum, showing evaporitic and pedogenic modification within the limestone. Typical fossils are freshwater gastropods and plant remains, but land-snail faunas such as those known from Prospect Quarry are not a characteristic feature of this Gurnard section.

Bed GL2 — Middle Muddy Sequence / Gurnard Bembridge Limestone Muds (c. 3.3 m)

A muddy sequence 3.3 m thick separates the lower and upper carbonate units at Gurnard. These muds and marls form the soft middle part of the local Bembridge Limestone succession and are important because the lowest parts of the Gurnard cliffs have yielded fruits and seeds from this interval. In regional terms this muddy package probably corresponds in part to the “middle muds” recognized elsewhere in the Bembridge Limestone, but the Gurnard summaries do not support a finer formal local subdivision. Depositional conditions were quieter and muddier than in the limestone packages above and below, representing shallow standing water on a low-relief carbonate plain with intermittent marsh or pond development.

Bed GL3 — Upper Carbonate Unit Of The Bembridge Limestone

The upper Bembridge Limestone at Gurnard is again composed of buff limestones and marls, but the lithology varies laterally toward the south-west and this variation has been interpreted in part as the result of contemporaneous erosion before deposition of the Bembridge Marls. The top of the unit is therefore important structurally as well as stratigraphically. Like the lower carbonate unit, it represents very shallow lacustrine to palustrine carbonate sedimentation with repeated emergence and soil-forming modification. The upper surface is locally erosional, and at Thorness Bay this contact relationship was one of the key pieces of evidence used to demonstrate previously unrecognized intra-Palaeogene warping on the Isle of Wight.

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

Bouldnor Formation (Latest Eocene To Early Oligocene)

Bembridge Marls Member

Bed GL4 — Basal Oyster-Bearing Shell-Band Interval (lowest c. 4 m of the member)

At Gurnard and east Thorness Bay the base of the Bembridge Marls Member rests on an erosion surface cut into the Bembridge Limestone. Unlike Whitecliff Bay, there is no well-developed basal sandy oyster bed here, but oysters do occur in two shell bands at the base of the succession, and the lower part broadly represents the local equivalent of the traditional lower “oyster-bed” interval. The lithology is mainly grey, blue-green and green mud with marlier beds low in the sequence. Numerous burrowing bivalves occur in life position toward the base, and shell preservation varies strongly with lithology: shells are best preserved in grey clays, whereas those from blue-green clays may be friable, chalky or completely decalcified. This basal package records the initial brackish to hyposaline flooding of the Gurnard carbonate surface in a quiet, sluggish estuarine or lagoonal setting.

Bed GL5 — Bembridge Insect Bed / “Insect Limestone” / GUR IV

The Bembridge Insect Bed lies about 4 m above the base of the Bembridge Marls Member and is the most important single horizon in the whole Gurnard section. Although traditionally called the “Insect Limestone”, it is not one simple limestone bed; at Gurnard it consists of a predominantly argillaceous horizon with discontinuous concretionary limestones, hard marls and, in places, a more tabular micritic limestone commonly around 10 cm thick, though the bed-status unit as a whole may reach up to about 0.6 m. Fossils include an exceptionally rich insect fauna, spiders, fairy shrimp, isopods, ostracods, rare bird feathers, a fragment of lizard skin, freshwater gastropods such as Galba, the brackish bivalve Polymesoda, and abundant plant remains including Typha, Potamogeton, Limnocarpus and seeds of Stratiotes. Preservation is famously exquisite, with three-dimensional insect and spider tissues mineralized in calcite. The associated fairy shrimp, the scarcity of fish, and pseudomorphs after halite in the bed and gypsum in adjacent marls indicate episodes of abnormal, probably hypersaline, water chemistry within an otherwise brackish to freshwater marsh-lagoon system.

Bed GL6 — Lower To Middle Bembridge Marls Above The Insect Bed, Including THOR III On The Southern Limb

Above the Insect Bed the Bembridge Marls continue as black, grey and green muds with scattered marls and shell-bearing levels. These beds contain good brackish to freshwater molluscan assemblages and show the strongly fluctuating salinity typical of the member as a whole. In east Thorness Bay, north-west of Pilgrim’s Park, a lower horizon in this part of the succession is represented by THOR III, from which the shell-encrusting alga Epivalvia edwardsii was described. That horizon demonstrates that the lower marls on the southern limb were still quiet-water shell-bearing lagoonal or marsh deposits, with calcareous encrustation developing on mollusc shells in situ. The marls above Gurnard Ledge and their southern-limb equivalents were deposited in shallow, low-energy waters with extensive marshland and intermittent open water.

Bed GL7 — Upper Bembridge Marls Varicoloured Mud Sequence

The upper part of the Bembridge Marls at Gurnard is dominated by black, grey and green muds and silts with shell seams and local ironstone nodules. Some plant remains described historically from the Gurnard section came from these higher Bembridge Marls rather than from the Insect Bed itself, and the ironstone nodules are mainly associated with freshwater gastropod-bearing levels. This part of the succession is commonly obscured by vegetation and slip, so it is safer to treat it as a single upper varicoloured mud sequence rather than to invent unsupported local bed divisions. Regionally, the Nystia Band lies near the top of the member elsewhere on the Isle of Wight, but the higher Gurnard and east Thorness exposures are not sufficiently clear to isolate that band confidently here. The overall trend is regressive, with fluctuating salinity but increasing freshwater influence upward through sluggish marsh, lagoon and low-energy fluvial-margin environments.

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

Hamstead Member

Bed GL8 — Black Band (c. 0.4 m; Poorly Exposed Just South Of Gurnard Ledge)

The Black Band is the basal marker bed of the Hamstead Member and is only poorly exposed in this locality, just south of Gurnard Ledge. Where recognizable it is a dark brown to black shelly organic-rich clay with scattered angular flint pebbles, lying above an omission surface and a rooted palaeosol with calcrete-like concretions developed at the top of the Bembridge Marls. It contains a freshwater fauna and flora and marks a sharp environmental change from the varicoloured brackish to freshwater Bembridge Marls below into the more fully continental lower Hamstead regime above. Because it is usually obscured here, it is best treated as a marker horizon rather than as the start of a fuller Hamstead log.

Depositional Environment

The Gurnard Ledge–east Thorness Bay tract records repeated shifts across a low-relief late Eocene to earliest Oligocene marginal basin. The Bembridge Limestone formed in very shallow carbonate-rich lakes, ponds and palustrine flats under relatively arid conditions, with pedogenic modification, local evaporitic gypsum growth and repeated emergence. The overlying Bembridge Marls record a broad regressive brackish to freshwater succession deposited in a sluggish estuary or lagoon with extensive marshland, some open water, nearby woodland and intermittent fluvial influence. Salinity fluctuated repeatedly: the basal shell-band interval records the initial flooding of the Bembridge Limestone surface, the Insect Bed reflects a very quiet but chemically unusual pool or marsh-water setting with exceptional fossil preservation and local hypersaline episodes, and the higher marls record continued marsh, lagoonal and low-energy freshwater deposition leading up to the palaeosol and Black Band at the base of the Hamstead Member. The erosional and locally unconformable junction between the Bembridge Limestone and Bembridge Marls shows that sedimentation here was influenced by intra-Palaeogene warping as well as by simple facies change.

Total Thickness Covered Here: Less Than 30 Metres In All, Comprising 6.7 Metres Of Bembridge Limestone Formation, 21.5 Metres Of Bembridge Marls Member, And The Poorly Exposed Black Band At The Base Of The Hamstead Member

References

Hopson, P.M. & Farrant, A.R. (2015). Geology of the Isle of Wight, British Geological Survey sheet explanation.
British Geological Survey Lexicon of Named Rock Units: Bembridge Limestone Formation, Bembridge Marls Member and Hamstead Member.
Daley, B. (1972a, 1972b, 1972c, 1973a, 1974) on the Bembridge Marls succession, macroinvertebrate assemblages, deformation structures and algal remains of Thorness Bay and Gurnard.
Daley, B. & Balson, P. (1999). British Tertiary Stratigraphy, Geological Conservation Review accounts for Thorness Bay and Gurnard.
Daley, B. & Edwards, N. (1971). Palaeogene warping in the Isle of Wight.
Daley, B. & Edwards, N. (1990). Stratigraphical revision of the Bembridge Limestone, Isle of Wight.
Insole, A. & Daley, B. (1985). Revision of the late Eocene and early Oligocene lithostratigraphy of the Hampshire Basin.
Armenteros, I., Daley, B. & García, E. (1997). Lacustrine and palustrine facies in the Bembridge Limestone of the Isle of Wight.
Jarzembowski, E.A. (1980a, 1980b) on the insect fauna of the Bembridge Insect Bed.
McCobb, L.M.E., Jarzembowski, E.A. and co-authors (1998) on the lithology and taphonomy of the Insect Bed.
Collinson, M.E. and co-authors on the Bembridge Flora and the contrasting floras of the Bembridge Limestone and Bembridge Marls at Gurnard.