Porchfield Fossil Hunting

c.1859 onwards – A’Court Smith’s collecting ground established
E.J. A’Court Smith began collecting fossils from the north-west Isle of Wight coast between West Cowes and Newtown River by the mid-19th century. Many older fossils from the Bembridge Insect Bed were later labelled Gurnet Bay or Gurnard Bay, so exact find-spots can be difficult to separate, but this collecting coast includes the Burnt Wood and Saltmead Ledge exposures now often treated locally as the Porchfield stretch.

1874 – first published arthropod record
A’Court Smith’s notice of fossil plants and insects from the Insect Bed near Cowes marked the beginning of the published scientific record for this delicate fossil horizon. It showed that the fine limestone and marl near the base of the Bembridge Marls could preserve fragile terrestrial arthropods as well as plant remains.

1878–1879 – insects and crustaceans brought into early study
Early work on the Bembridge Insect Bed recorded a varied assemblage of insects and aquatic crustaceans. Important early names included Branchipodites vectensis and an isopod first described as Eosphaeroma fluviatile, later treated within the Eosphaeroma margarum record, helping establish that the bed preserved both land-derived insects and shallow-water life.

1883–1888 – Bembridge Flora recognised from the Insect Limestone
The Insect Limestone became known as one of the main sources of the Bembridge Flora. Fruits, seeds, fern remains, conifers and aquatic plants showed that the insect-bearing bed was also a major plant-fossil horizon, preserving wetland and woodland vegetation from around the Eocene–Oligocene transition.

1888–1889 – spider, ostracod and moth records added
The fossil spider Eoatypus woodwardii, the ostracod Potamocypris brodiei and the moth Lithopsyche antiqua were added to the Insect Bed record. These tiny and delicate fossils helped show why the horizon is exceptional: it can preserve organisms that are normally absent from ordinary coastal fossil sites.

1913–1923 – classic insect naming phase
Early 20th-century work greatly expanded the named insect fauna, including termites, ants, flies and other groups from older Insect Bed collections. Fossils such as Mastotermes anglicus and Oecophylla perdita helped demonstrate that the fauna was diverse, warm-climate in character and far richer than a normal British Palaeogene insect assemblage.

1926 – Bembridge Flora described in detail
A major study of the Bembridge Flora used A’Court Smith’s old collections and established the Insect Limestone as a key plant horizon. The flora included aquatic and wetland plants, conifers and many angiosperm fruits and seeds, making the Burnt Wood–Saltmead part of the wider Insect Bed coast important for both insects and plants.

1966 – Thorness Bay SSSI protection
The wider Thorness Bay coast, including the Saltmead Ledge and Burnt Wood side of the section, was notified as a Site of Special Scientific Interest for its biological and geological value. This protected the cliff and foreshore setting of the Bembridge Insect Bed and recognised the importance of the locality beyond casual collecting.

1971 – Porchfield Anticline movement recognised
Work on the beds around Burnt Wood and Saltmead Ledge showed that the Bembridge Marls rest on an eroded and locally warped Bembridge Limestone surface. This evidence for early movement on the Porchfield Anticline made the locality important for understanding Palaeogene earth movements as well as fossil preservation.

1972 – Insect Bed isopods revised
The fossil isopods from the Insect Bed were re-examined, clarifying the record of Eosphaeroma margarum. This strengthened the interpretation of the bed as a quiet, shallow, fresh to brackish-water setting where aquatic crustaceans, insects, plants and occasional vertebrate traces could accumulate together.

1975 – whole fertile Azolla recorded
Whole fertile plants of Azolla prisca were recognised from the Insect Limestone. These tiny floating water ferns are unusually complete and are important because they preserve delicate reproductive structures, adding rare botanical detail to the site’s wetland environment.

1976–1977 – Saltmead and Burnt Wood section logged
Fieldwork on the coast west of Burnt Wood recorded the Insect Bed before the western end of Saltmead Ledge and traced it eastwards in the lower cliff and shore blocks. The logged bed was less than a metre thick, with several calcareous courses, showing how a very thin limestone and marl unit could produce one of Britain’s most important fossil insect faunas.

1980 – major insect fauna review
A full review of the Bembridge Marls insects confirmed the Insect Bed as the only sizeable British Tertiary insect fauna above the lower Eocene. Thousands of specimens were assessed, with Hymenoptera, Diptera and Coleoptera especially abundant, and the Burnt Wood–Saltmead exposures became part of the key field evidence for understanding the horizon.

1998 – exceptional preservation explained
Study of the Insect Limestone showed that some fossils are preserved in three dimensions within very fine micritic limestone. Cuticle, colour patterns and even internal structures can survive, explaining why this thin bed can preserve insects, spiders and other delicate fossils in a way sometimes compared with opaque amber.

1999–2000 – rare plant structures and rodent gnaw marks recorded
Further work on the Insect Limestone flora recognised delicate pappus-bearing seeds and feeding marks on seeds attributed to small rodents. These records are important because they add direct evidence of plant dispersal structures and animal behaviour to the Porchfield–Thorness Insect Bed story.

2001 – water-spider anatomy preserved
The fossil spider Vectaraneus yulei was described from the Bembridge Marls Insect Bed, with respiratory structures preserved by calcium carbonate replacement. Alongside the older Eoatypus woodwardii record, it showed that the Insect Bed could preserve details useful for spider evolution and palaeoecology, not just outlines of bodies.

2014 – flora and Hymenoptera reassessed
Modern studies re-examined the Insect Limestone flora and hymenopteran insects. The plant work refined which old records truly came from the Insect Limestone and highlighted wetland plants such as Typha, Acrostichum, Azolla, Potamogeton and Sparganium, while the Hymenoptera work showed an exceptionally diverse ant, wasp and bee-related fauna from a well-wooded late Eocene landscape.

2019 – rare vertebrates recorded from Saltmead Ledge
Rare vertebrate material from the Insect Limestone was reviewed, including a rodent incisor of Isoptychus sp. from Saltmead Ledge. The same work brought together fish, lizard, bird and skin or feather records, showing that very occasional land and shoreline vertebrate remains also entered the insect-bearing lagoonal deposits.

2019 – new lacewings and wider arthropod revisions
New lacewings from the coastal outcrop west of Burnt Wood behind Saltmead Ledge included Sympherobius yulei, adding to the delicate insect record from the exact Porchfield-side exposure. The same modern phase of work also revised cockroaches, true flies, crustaceans and other arthropods from the Insect Limestone, including the clam shrimp Paraleptestheria mitchelli.

2024 – new dragonflies from Saltmead and Sticelett ledges
Three fossil dragonfly wings from Saltmead and Sticelett ledges were described from the Insect Bed. The finds added to the known Odonata fauna and confirmed that the Burnt Wood–Saltmead part of the coast continues to produce scientifically useful insect fossils when the thin limestone and marl are exposed.

Locality Scope And Published Names

“Porchfield” is an informal local and website name for this particular stretch of coast rather than a formally defined geological locality. In the scientific literature, the fossil-bearing section is usually described as the western end of Saltmead Ledge, the section west of Burnt Wood, or the southern side of the wider Thorness Bay–Gurnard outcrop. The principal published Bembridge Insect Bed exposure begins near National Grid Reference SZ 433 927, shortly before the western end of Saltmead Ledge, and was traced eastwards for about 500 m to approximately SZ 438 928. This page is restricted to that western section and should not be interpreted as a repetition of the full Thorness Bay or Gurnard Ledge succession.

Bed Numbering Used Below

The BW bed codes are the published Burnt Wood bed designations used by Daley and later authors for the basal Bembridge Marls succession. Bed BW IV is the formal local horizon containing the Bembridge Insect Bed, while Beds BW III and BW V are the shell-bearing marker horizons immediately below and above it. The PC numbers used for the underlying Osborne Member, Bembridge Limestone and incompletely exposed beds above BW V are practical site-use identifiers only; they are not formal published bed numbers.

SOLENT GROUP

Headon Hill Formation (Upper Eocene / Priabonian)

Osborne Member

Bed PC1 — Upper Osborne Member Context Beds

Intermittent exposures south-west and west of Burnt Wood include green, olive, grey and red calcareous clays, lime-rich muds and discontinuous concretionary limestones belonging to the upper Osborne Member. These soft beds are commonly obscured by vegetation, beach material and slipped sediment, but they provide the underlying context for the more resistant Bembridge Limestone ledges. The changing colours reflect alternating waterlogging, oxidation and soil development in a low-lying landscape of shallow lakes, lagoons, marshes and periodically exposed mudflats.

Typical Fossils

Fossils are generally less conspicuous than in the overlying units, but freshwater and low-salinity molluscs, charophytes, ostracods and plant material may occur in the calcareous muds and concretionary limestones. The assemblage is consistent with shallow lacustrine to marginal-lagoonal conditions rather than a fully open marine environment.

Bembridge Limestone Formation (Upper Eocene / Priabonian)

The full 6.7 m Bembridge Limestone reference succession measured at Gurnard Ledge must not be transferred directly to Porchfield. Around western Saltmead Ledge and west of Burnt Wood, only parts of the formation are intermittently available, and its upper beds change laterally because of erosion and contemporaneous structural movement. The PC divisions below therefore describe the locally relevant upper part of the formation rather than a complete measured log.

Bed PC2 — Upper Bembridge Limestone At Western Saltmead Ledge

Pale buff, cream and grey micritic limestone, clayey limestone, marl and lime-rich mud form low ledges and scattered foreshore blocks beneath the Bouldnor Formation. The limestones may be dense and finely crystalline or more porous where molluscan shells have dissolved, and they commonly weather much paler than the overlying grey and green marls. Freshwater gastropods, planorbids, charophytes, ostracods and aquatic plant remains characterize the formation, although fossil abundance and preservation vary considerably from bed to bed.

Depositional Environment

The upper Bembridge Limestone accumulated in shallow hard-water lakes, ponds and palustrine areas on a low-relief coastal plain. Fine carbonate mud formed in relatively clear alkaline water, while clay-rich intervals record greater influx of suspended sediment. Burrowing, root structures, calcareous soil development and local exposure surfaces show that the margins of these water bodies periodically dried or became shallow enough to support emergent vegetation.

Bed PC3 — Eroded Top Of The Bembridge Limestone And Porchfield Omission Surface

The top of the Bembridge Limestone beneath the lower Bouldnor Formation is irregular, locally burrowed, weathered and eroded rather than representing a simple uninterrupted bedding plane. The upper limestone unit changes laterally towards Burnt Wood and Saltmead Ledge, and parts of it were removed before deposition of the overlying marls. Reworked carbonate fragments may occur immediately above the surface where erosion was strongest.

This junction is important evidence for intra-Palaeogene movement along the Porchfield Anticline. Gentle uplift and warping altered the local topography while the Solent Group was still being deposited, allowing the upper Bembridge Limestone to be differentially eroded before renewed flooding established the muddy environments of the Bouldnor Formation.

Bouldnor Formation (Uppermost Eocene At This Locality)

Bembridge Marls Member / Gurnard Member

The British Geological Survey currently retains Bembridge Marls Member as its preferred formal name and lists Gurnard Member as an alternative. Recent stratigraphical work has increasingly used Gurnard Member for the same interval and has referred to Bed BW IV as the Insect Limestone Unit. Both sets of terminology describe the basal division of the Bouldnor Formation exposed here. The succession consists predominantly of laminated clay, silt, marl and lime-mudstone with thin shell beds and discontinuous limestone courses.

Only the basal part of the member is consistently relevant to Porchfield. The published 21.5 m complete reference thickness at Gurnard Ledge includes many higher horizons that cannot be demonstrated as a continuous section along this remote western shore and are therefore not reproduced here.

Beds BW I–II — Basal Algal, Shelly And Laminated Marl Interval

The basal Bembridge Marls comprise black, grey, green and blue-grey marls, lime-rich muds and finely laminated clay resting on the eroded Bembridge Limestone surface. Oyster-bearing shell material occurs near the base, but the thick sandy Bembridge Oyster Bed developed at Whitecliff Bay is absent from this north-western section. Instead, the basal flooding is represented by thinner shell-rich seams within predominantly argillaceous sediment.

Bed BW I near Burnt Wood contains small stromatolitic and oncolitic algal structures. These appear to be the only well-documented examples of this kind from the Palaeogene succession of southern England and indicate shallow water in which microbial coatings developed around grains, shells or small fragments. The associated oysters and low-diversity shells show that the first waters spreading across the eroded limestone surface were brackish and intermittently connected to more saline environments.

Bed BW II continues upward as laminated marly clay and lime-rich mud beneath the more conspicuous BW III shell band. Fine lamination, limited burrowing and the absence of coarse current structures indicate low-energy deposition in a quiet lagoon, lake-margin pool or sheltered estuarine embayment.

Bed BW III — Lower Shell Band Beneath The Bembridge Insect Bed

Bed BW III is a concentrated shell-bearing marker lying immediately beneath the insect-producing BW IV interval. Published logs of the Saltmead section show corbiculid bivalves together with small Tarebia-type gastropods and larger potamidid gastropods. The shells may be complete, fragmented, etched or concentrated into a thin lag, reflecting a short episode of brackish-water colonization and minor reworking before exceptionally quiet Insect Bed sedimentation began.

The low diversity but locally high abundance of the fauna indicates environmentally stressed water, probably subject to changing salinity. BW III is therefore an important field marker and not merely a fossiliferous part of BW IV: it defines the shell-rich lower boundary of the main insect-bearing unit.

Bed BW IV — Bembridge Insect Bed / Insect Limestone Unit (c. 0.65–0.85 m)

Bed BW IV is the principal fossil horizon at Porchfield and one of the most important Palaeogene insect deposits in Britain. Jarzembowski traced it for approximately 500 m through the lower cliff from near SZ 433 927 to around SZ 438 928, with fossiliferous blocks also occurring on the shore. The unit is approximately 0.65–0.85 m thick, although individual limestone and hard-marl courses vary laterally and may pinch out over short distances.

The traditional name “Insect Limestone” can be misleading because BW IV is not one continuous bed of solid limestone. It is a predominantly argillaceous interval of laminated marly clay containing four recognisable calcareous courses. These range from thin hard-marl lenses and tabular fine-grained limestone to isolated micritic concretions. Fresh material is generally blue-grey, while weathered surfaces develop a buff, ochre or brownish-yellow rind. The rock is exceptionally fine grained and commonly breaks with a curved or conchoidal fracture.

Thin silty and carbonaceous laminae contain ostracods, minute shell debris, reed and other plant fragments, seeds and insect remains. Small-scale folding, loading and irregular deformation developed before complete lithification, while early carbonate cementation protected delicate fossils from later flattening. Salt or gypsum pseudomorphs, shrinkage structures and the mixture of freshwater and salt-tolerant organisms show that water chemistry fluctuated and could become strongly evaporative at times.

Published Calcareous-Course Structure Of BW IV

The four BW IV sublayers described below are practical website divisions of the four calcareous courses figured in the published Saltmead Ledge section. The suffixes BW IVa–d are not formal bed numbers and should not be presented as replacements for the published collective designation BW IV. No unique fossil assemblage has been proved for each individual course, so the fauna and flora should normally be recorded as coming from BW IV unless the exact position is known.

Bed BW IVa — Lowest Calcareous Course And Basal Laminated Marl (site-use sublayer)

The lowest part of the Insect Bed begins above the BW III shell concentration with finely laminated marly clay containing an irregular course of hard marl and small micritic limestone bodies. The calcareous material is discontinuous and locally passes laterally into less strongly cemented marl. Fine plant detritus, ostracods and isolated arthropod fragments may occur within both the marl and the limestone, but rich concentrations are localized rather than continuous.

This lower course records the first phase of early carbonate cementation within the exceptionally quiet BW IV depocentre. The abrupt reduction in abundant brackish shells above BW III suggests that the water became less agitated and probably fresher or more strongly stratified.

Bed BW IVb — Lower-Middle Concretionary Course (site-use sublayer)

The second calcareous course comprises irregular hard-marl lenses and blue-grey micritic concretions enclosed by laminated clay. Published sections show a mixture of partly cemented marl and more completely lithified limestone bodies rather than a uniform tabular bed. The boundaries are gradational or irregular because carbonate cement developed around localized nuclei within water-rich sediment.

Early cementation could preserve insect bodies, wings, spiders and small crustaceans with substantial relief, while specimens enclosed only in the more argillaceous laminae tend to show greater compression. This contrast in preservation is one reason apparently similar BW IV blocks can vary greatly in fossil quality.

Bed BW IVc — Thin Upper-Middle Calcareous Course (site-use sublayer)

The third course is generally thinner and more laterally variable, occurring as a narrow hard-marl or micritic limestone lens within laminated marly clay. It may be locally reduced where large concretions developed in the overlying course. The original fine lamination can continue around the cemented body, showing that concretion growth took place shortly after deposition rather than representing a later limestone bed inserted into the succession.

Where fossiliferous, this course can preserve carbonaceous plant fragments and delicate arthropod remains, but there is no evidence that it consistently contains a different fauna from the other BW IV courses. Rich pockets may terminate abruptly within the same apparent lithology.

Bed BW IVd — Upper Large-Concretion Course (site-use sublayer)

The uppermost BW IV course contains the largest and most conspicuous micritic limestone concretions, commonly surrounded by less completely cemented hard marl. Some concretions are broad and lenticular and can occupy much of the thickness available, locally accounting for the thinning of the course beneath them. This is the most massive calcareous development in the Saltmead section but remains discontinuous along the outcrop.

The fine blue-grey micrite may preserve insects and spiders with wing venation, body outlines, setae and internal tissues visible at microscopic scale. Bird-feather fragments, tiny vertebrate remains and articulated aquatic crustaceans are exceptionally rare but demonstrate the preservational potential of the bed. BW IVd passes upward into laminated marly clay beneath the BW V shell band.

Bed BW V — Upper Shell Band Above The Bembridge Insect Bed

Bed BW V is the shell-bearing marker immediately above BW IV. Published Saltmead logs show a concentration of corbiculid bivalves with small Tarebia-type gastropods and larger potamidid gastropods at this level. The shells occur in laminated marly clay and may be disarticulated or concentrated along a bedding surface.

The return of abundant brackish-water molluscs marks renewed salinity change and greater water movement after deposition of the Insect Bed. BW III and BW V therefore bracket BW IV between two stressed brackish shell assemblages, making the three horizons especially useful for identifying correctly positioned blocks and temporary cliff exposures.

Bed PC4 — Lower Gurnard Member Marls Above BW V (partial interval)

Above BW V, intermittent exposures continue into grey, blue-grey, green and locally black laminated clay, silt and marl with occasional lime-mudstone, shell seams and small concretions. Some beds preserve quiet-water lamination, while others contain shell concentrations, burrows, shrinkage surfaces or evidence of temporary exposure. These features record continuing oscillation between brackish lagoonal water, freshwater pools, marshes and very shallow marginal environments.

The higher Bembridge Marls marker beds documented at Gurnard Ledge, including the main Corbicula and Nystia horizons, should not automatically be assigned to the narrowly defined Porchfield section. Exposure west of Burnt Wood is too intermittent to demonstrate a complete upward succession, and no artificial continuity should be created between isolated cliff and foreshore patches.

Typical Insects Of Bed BW IV

The Bembridge Insect Bed has produced approximately 220 described insect species and representatives of about fifteen orders. Particularly important groups include beetles, true flies, ants, bees and wasps, true bugs, termites, dragonflies and damselflies, lacewings, caddisflies, bark lice, thrips, stoneflies, scorpionflies, orthopterans and rare butterflies or moths. The fauna is dominated numerically by insects living in leaf litter, low vegetation and wetland habitats, although forms from trees, open water and the air above the lagoon are also present.

Named insects from the wider Bembridge Insect Bed assemblage include the butterfly Lithopsyche antiqua, the termite Mastotermes anglicus, the dragonflies Aeschnophlebia andreasi and Anglogomphaeschna eocenica, and numerous ants, wasps, flies and aquatic insect larvae. Recent work on wings collected at Saltmead and Sticelett ledges has continued to add new information, including the first definite libellulid dragonfly from the assemblage.

Spiders, Crustaceans And Ostracods

Spiders are represented by several dozen specimens, including the named forms Eoatypus woodwardii and Vectaraneus yulei. Aquatic arthropods include the anostracan or fairy shrimp Branchipodites vectensis, the isopod Eosphaeroma margarum, the ostracod Potamocypris brodiei and the clam shrimp Paraleptestheria mitchelli. Complete articulated examples of Branchipodites may occur together with insect remains, although exceptionally rich concentrations are highly localized.

Molluscs

Molluscs are much scarcer within BW IV than in the shell beds immediately above and below. The commonest Insect Bed species is the freshwater pulmonate Galba longiscata, traditionally recorded as Lymnaea (Galba) longiscata. Other recorded freshwater gastropods include Planorbarius discus, Gyraulus similis, Hippeutis headonensis, Viviparus lentus and the rare Viviparus angulosus. The more salt-tolerant Tarebia acuta is uncommon within the limestone itself, and Polymesoda (Pseudocyrena) obovata is the only confirmed bivalve from the Insect Limestone.

The mixture of freshwater hard-water gastropods, occasional brackish forms, salt pseudomorphs and salt-tolerant crustaceans suggests changing water chemistry. Many molluscs may have been washed into the quiet depositional centre from vegetated pond margins, freshwater channels or adjacent brackish shallows rather than living permanently in the deepest part of the water body.

Plants And The Bembridge Flora

BW IV is the principal source of the historic Bembridge Flora, although plant fossils occur in localized concentrations and are much less uniformly distributed than older collection labels sometimes imply. Reed-like leaves and fragments of Typha are among the most characteristic remains, accompanied by Azolla, Potamogeton, seeds, fruits, twigs, leaf fragments and minute carbonaceous plant debris. Winged and plumed seeds show that wind transport also contributed material to the lagoon.

Older collections combined material from several nearby horizons and localities, so not every plant historically labelled “Bembridge Insect Limestone” came from BW IV. Modern revision has excluded some supposed elements, including the palms, from the confirmed Insect Limestone flora. The most secure interpretation is of a wetland landscape with aquatic and marginal vegetation, reeds, herbs, shrubs and nearby trees rather than a single uniform tropical forest.

Rare Vertebrates

Vertebrate fossils are exceptionally uncommon but scientifically important. Recorded material includes unidentified teleost fish, lizards including a scincoid, unidentified birds, bird feathers and the theridomyid rodent Isoptychus. Fragmentary lizard skin and microscopic skeletal elements demonstrate that the same early cementation responsible for insect preservation could also retain extremely delicate vertebrate tissues. These finds should be regarded as exceptional rather than typical collecting material.

Fossil Preservation And Field Recognition

Fresh Insect Limestone is a very fine blue-grey micrite or argillaceous micrite, commonly surrounded by hard marl and weathering to a buff or brownish-yellow rind. The limestone tends to split along curved surfaces rather than perfectly flat bedding planes, and a fossil may occur partly on each half of a broken block. Insects can be preserved as dark or brown carbonaceous films, pale external moulds or partly three-dimensional bodies with calcite-replaced internal tissues.

The exceptionally fine matrix can retain minute wing veins, body segmentation, hairs and soft anatomical detail. This unusual preservation has led the rock to be compared with opaque amber, although it is a micritic limestone rather than fossil resin. Insect bodies in the more clay-rich laminae are generally more compressed, while those enclosed by early-cemented limestone may retain greater relief.

Fossiliferous debris can be more productive where marine erosion has released BW IV blocks from the lower cliff, but the lithology must be identified carefully. Many apparently suitable blue-grey limestone fragments contain only ostracods, shell debris or plant specks, and some contain no visible fossils. Rich insect-bearing pockets are localized, so the historical abundance of museum specimens should not be taken to mean that every BW IV block is fossiliferous.

Age And Correlation

Bed BW IV is late Priabonian in age and therefore belongs to the latest Eocene, approximately 34 million years ago. Older publications often placed the Bembridge Marls and their insect fauna in the earliest Oligocene because the position of the Eocene–Oligocene boundary was then uncertain. Modern stratigraphical correlation places the Insect Bed below that boundary.

The Burnt Wood horizon BW IV correlates with Bed GUR IV at Gurnard Ledge and with equivalent developments of the Bembridge Insect Bed elsewhere around the northern Isle of Wight. Its local four-course architecture at Saltmead Ledge differs from the two principal calcareous courses seen near Gurnard and the single concretionary development present at some eastern localities. These variations demonstrate that carbonate cementation was controlled locally within one laterally extensive muddy horizon.

Depositional Environment

The Porchfield succession records the replacement of shallow Bembridge Limestone lakes and palustrine areas by a broader muddy wetland system. Basal shell bands in Beds BW I–III record brackish flooding across the eroded limestone surface. BW IV then accumulated in an exceptionally quiet shallow lake, lagoon or sheltered estuarine pool, probably with freshwater inflow but subject to changing salinity, evaporation and occasional hypersaline conditions. BW V records the return of a more obviously brackish molluscan community.

Fine suspended sediment settled with very little disturbance in the BW IV depocentre. Terrestrial insects, seeds and plant fragments were blown or washed from surrounding vegetation, while aquatic crustaceans, ostracods and some molluscs lived in the water body or its margins. Rapid burial, early micritic cementation and limited scavenging protected fragile remains before decay and compaction could destroy them.

The wider environment was probably subtropical to warm temperate and seasonally wet, with extensive marsh, reed beds, freshwater pools, sluggish channels and locally wooded ground. Salt pseudomorphs and periodic shrinkage show that dry or strongly evaporative phases interrupted the wetter conditions, so the setting should not be reconstructed as continuously humid.

Structural Style And Exposure

The strata at Porchfield lie on the eastern limb of the Porchfield Anticline and descend gently eastwards towards the Thorness Bay Syncline. This structure was already beginning to influence sedimentation during the Palaeogene, as shown by the local erosion and overstep at the Bembridge Limestone–Bouldnor Formation junction. The present-day coast therefore exposes beds that were deposited across a subtly uneven and tectonically active surface.

The modern exposure is a low, discontinuous cliff-and-foreshore section rather than a single permanent rock face. Vegetation, slumping, mud, storm-beach material and tidal deposits repeatedly conceal parts of the succession. The Insect Bed was historically traceable for about 500 m, but its present visibility varies greatly, and fossiliferous pebbles diminish a short distance east of Saltmead Ledge. Isolated blocks cannot always be placed precisely without reference to the BW III and BW V shell bands.

Stratigraphic And Palaeontological Significance

Porchfield is one of the most important specialist fossil localities on the Isle of Wight because it includes the four-course Saltmead development of the Bembridge Insect Bed. For a unit less than one metre thick, BW IV has produced an extraordinary record of insects, spiders, crustaceans, molluscs, plants, feathers and rare vertebrates and is one of Britain’s finest examples of a Konservat-Lagerstätte.

The section is also geologically significant beyond its fossils. The eroded Bembridge Limestone surface, lateral changes in limestone development and relationship to the lower Bouldnor Formation provide important evidence for intra-Palaeogene movement along the Porchfield Anticline. It therefore records both an exceptional latest Eocene ecosystem and the local structural processes controlling deposition and preservation.

Total Thickness Covered Here

No defensible single total thickness can be assigned to the locally named Porchfield section because the Osborne Member, Bembridge Limestone and lower Gurnard Member are exposed intermittently in separate cliff and foreshore patches. The best-constrained unit is Bed BW IV itself, which is approximately 0.65–0.85 m thick and contains four calcareous courses. The page otherwise covers only the partial upper Bembridge Limestone and the basal few metres of the overlying Gurnard or Bembridge Marls Member; the complete 6.7 m Bembridge Limestone and 21.5 m Bembridge Marls measurements from Gurnard Ledge should not be transferred to this narrower locality.

References

Bristow, H.W., Reid, C. & Strahan, A. (1889). The Geology of the Isle of Wight.
White, H.J.O. (1921). A Short Account of the Geology of the Isle of Wight.
Reid, E.M. & Chandler, M.E.J. (1926). The Bembridge Flora.
Daley, B. (1969, 1972, 1973, 1974, 1999) on the palaeoenvironment, deformation structures, bed numbering, macroinvertebrates and stratigraphy of the Bembridge Marls.
Daley, B. & Edwards, N. (1971). Intra-Palaeogene earth movements in the Isle of Wight; and Daley, B. & Edwards, N. (1990) on the Bembridge Limestone succession.
Insole, A. & Daley, B. (1985). Revision of the late Eocene and early Oligocene lithostratigraphy of the Hampshire Basin.
Jarzembowski, E.A. (1980). Fossil insects from the Bembridge Marls, Palaeogene of the Isle of Wight.
Armenteros, I., Daley, B. & García, E. (1997). Lacustrine and palustrine facies in the Bembridge Limestone of the Isle of Wight.
McCobb, L.M.E., Duncan, I.J., Jarzembowski, E.A., Stankiewicz, B.A., Wills, M.A. & Briggs, D.E.G. (1998). Taphonomy of the insects from the Insect Bed, Bembridge Marls.
Ross, A.J. & Self, A. (2014). The fauna and flora of the Insect Limestone: introduction, history and geology.
Hayes, P.A. & Collinson, M.E. (2014). The flora of the Insect Limestone from the Isle of Wight.
Munt, M.C. (2014). Mollusca from the Insect Limestone of the Bembridge Marls Member.
Gallego, O.F. and co-authors (2019). The Crustacea of the Insect Bed, including the first British Cenozoic spinicaudatan.
Hooker, J.J., Evans, S.E. & Davis, M. (2019). Vertebrate remains from the Insect Limestone, latest Eocene, Isle of Wight.
Nel, A. & Ross, A.J. (2024). New dragonflies from the Upper Eocene of the Isle of Wight.
Stratigraphy of the Bouldnor Formation (Solent Group, Eocene–Oligocene) of the Isle of Wight, UK (2026). Proceedings of the Geologists’ Association, article 101209.
British Geological Survey Lexicon entries for Osborne Member, Headon Hill Formation, Bembridge Limestone Formation, Bembridge Marls Member and Bouldnor Formation.
Geological Conservation Review / JNCC accounts for Thorness Bay, Gurnard and the Bembridge Insect Bed.