Colwell Bay Fossil Hunting

1881 – Keeping and Tawney documented the fossils of the Venus Bed at Colwell Bay
One of the earliest major milestones for Colwell Bay was the publication of an old faunal list for the Venus Bed by Keeping and Tawney. This helped establish the bay as an important late Eocene fossil locality, especially for its rich molluscan assemblages.

1889 – Bristow, Reid and Strahan included Colwell Bay in the classic early accounts of the Isle of Wight Tertiary succession
Late nineteenth-century Geological Survey work helped place Colwell Bay firmly into the published geological framework of the Isle of Wight. This was part of the early scientific foundation for the bay’s later importance.

1921 – White described the section in one of the classic Isle of Wight accounts
White’s work became one of the standard early twentieth-century references for Colwell Bay, helping to document the Headon Hill Formation succession and its fossils in more detail.

1963 – Marjorie Chandler described plant macrofossils from the Colwell Bay succession
Chandler’s work showed that Colwell Bay was important not only for shells and stratigraphy, but also for fossil plants, especially water plants. This added an important palaeobotanical dimension to the site.

1967 – Edwards re-described the section in detail
Edwards’ work was one of the important modern re-descriptions of the Colwell Bay succession and helped refine understanding of the beds exposed in the bay. It became part of the key body of literature used in later reviews.

1971–1974 – major work clarified the fossils and environments of the Colwell Bay Member
Studies by Stinton, Insole, Curry and colleagues, Daley and Edwards, and Murray and Wright during the early 1970s greatly improved understanding of the bay. These works documented the rich molluscan faunas, foraminifera and ostracods, and showed that much of the Colwell Bay Member was deposited under hyposaline lagoonal or estuarine conditions rather than fully marine ones.

1978 – the Colwell Bay beds were generally accepted as late Eocene in age
By the late 1970s the Colwell Bay succession was generally being treated as late Eocene rather than Oligocene. This was an important step in placing the bay accurately within the broader Hampshire Basin chronology.

1985 – Colwell Bay was designated the stratotype for two members of the Headon Hill Formation
Insole and Daley designated Colwell Bay as the type section for the Colwell Bay Member and the Linstone Chine Member. This was one of the most important scientific milestones for the locality and gave it a permanent place in the formal stratigraphy of the Isle of Wight Palaeogene.

1999 – Daley re-described the section in the Geological Conservation Review
Modern review work confirmed Colwell Bay as a major lithostratigraphic and palaeontological site. The bay was highlighted for its varied environments, from freshwater to almost wholly marine conditions, and for the richness of its molluscs, microfossils and plant fossils.

2010 – the Colwell Bay Member was analysed as a key late Eocene depositional sequence
Later work by Chris King showed that the Colwell Bay Member represents a single depositional sequence divided into several parasequences, with environmentally controlled mollusc assemblages recording changing marginal-marine conditions. This reinforced the wider scientific importance of the bay beyond the Isle of Wight itself.

Modern understanding – Colwell Bay remains one of the Isle of Wight’s key late Eocene fossil localities
Today Colwell Bay is recognised as a major late Eocene site within the Headon Hill Formation, especially for the richly fossiliferous Colwell Bay Member. The bay is best known for molluscs such as Theodoxus, Potamides and Corbicula from the Neritina Bed and Venus Bed, together with important ostracods, foraminifera and plant macrofossils.

SOLENT GROUP

Headon Hill Formation (Upper Eocene, Priabonian)

Section Character

Colwell Bay is a soft-cliff and foreshore section rather than a single permanently open vertical log. Exposure varies with beach sand, slipped debris and active cliff recession, and the succession is best followed as a composite from Warden Point and Colwell Chine northward past Brambles Chine and Linstone Chine to Cliff End. A small anticlinal flexure near Linstone Chine is important because it brings back a thin local limestone representative above the Linstone Chine sands toward the north end of the bay.

Type-Section Significance

Colwell Bay contains the type section of the Colwell Bay Member beside Brambles Chine, the type section of the Linstone Chine Member between Brambles and Linstone chines, and the type section of the Cliff End Member at the north end of the bay. No formal published bed numbering is in standard use for the Colwell Bay cliff itself, so the CB bed numbers below are site-use divisions based directly on named beds, member boundaries, erosion surfaces and distinctive lithological packages.

Totland Bay Member

Bed CB1 — Warden Ledge Limestone (local marker within the middle Totland Bay Member)

At the southern approach to Colwell Bay, between Totland Bay and Warden Point, the Warden Ledge Limestone forms a resistant local marker within the otherwise soft Totland Bay Member succession. It is a practical field datum rather than a thick independently mappable limestone body in the Colwell Bay cliffs, but it shows that the lower Headon succession here included repeated calcareous and shell-bearing phases within a dominantly muddy coastal-plain succession. The wider Totland Bay Member in west Wight is chiefly silty mud, calcareous mudstone and thin sand with freshwater limestones, laid down in freshwater to brackish marginal-marine and non-marine settings.

Bed CB2 — Upper Totland Bay Member And How Ledge Limestone (How Ledge c. 2 m)

The highest Totland Bay beds are exposed in the low cliff north of Colwell Chine and on the foreshore as How Ledge. The How Ledge Limestone is a prominent pale freshwater limestone rich in pulmonate gastropods such as Galba and planorbids, with Viviparus also present. It is the uppermost major freshwater limestone of the lower Headon succession here and should not be confused with the much thicker Hatherwood Limestone higher in the Headon Hill cliff farther south. Muds and marls around and above the How Ledge horizon show increasing brackish influence, with molluscs such as Potamomya, Melanopsis, Potamides, Tarebia and Theodoxus recorded in the upper Totland Bay interval. The top of the member is a sharp interburrowed omission surface penetrated by Thalassinoides, marking abrupt onset of the overlying Colwell Bay marine incursion.

Colwell Bay Member

Bed CB3 — Basal Omission Surface And Neritina Bed

The Colwell Bay Member begins abruptly above the bored omission surface at the top of the Totland Bay Member. At Colwell Bay, the fully marine Brockenhurst Bed developed farther east and on the mainland is not represented as such; instead the lowest Colwell Bay beds are the brackish marginal-marine Neritina Bed, a dark brown clayey sand to sandy silty clay. Fossils include Theodoxus (“Neritina”), Potamides, Corbicula, Batillaria and Melanopsis, together with other molluscs and some drifted freshwater shells. This is the local expression of the basal Colwell Bay transgression, recording estuarine to lagoonal conditions with strong marine influence but lower salinity than the classic fully marine Brockenhurst facies farther north-east in the basin.

Bed CB4 — Venus Bed And Colwell Oyster Bed

The middle and most famous part of the Colwell Bay Member consists of blue-green, grey and brown shelly silty clay and muddy sand known as the Venus Bed. Its characteristic and abundant bivalve is Sinodia suborbicularis, the shell formerly known as “Venus incrassata”, from which the bed takes its traditional name. At Colwell Bay, especially around Brambles Chine, this interval may be replaced laterally or interrupted locally by the Colwell Oyster Bed, a very shelly muddy sand crowded with oysters and developed as a lenticular channel-fill in an erosion hollow cut into the Venus Bed. The Oyster Bed is generally interpreted as a tidal-channel fill. These beds represent the most fossiliferous and most marine part of the bay section, deposited in shallow hyposaline lagoonal to inner-sublittoral conditions with repeated winnowing and local channel scour.

Bed CB5 — Upper Colwell Bay Member / Batillaria Bed

The uppermost Colwell Bay Member is made up of greenish interbedded fine-grained sand and silty clay with shell seams, traditionally known as the Batillaria Bed. The member as a whole can be divided into a lower greyer, more sandy part and an upper brighter green, muddier part, and the Batillaria Bed belongs to this upper shoaling phase. Shell seams include Batillaria and other low-salinity tolerant molluscs, showing a shift away from the more openly marine conditions of the Venus Bed toward more restricted marginal-marine deposition. Beds CB3–CB5 together make up the 9 m-thick type development of the Colwell Bay Member at Colwell Bay.

Linstone Chine Member

Bed CB6 — Linstone Chine Channel-Fill Sands (c. 4 m)

The Linstone Chine Member is a localized, heterolithic, fine- to medium-grained sand body with plant debris, best developed in the upper cliff between Brambles and Linstone chines. It has an erosive base cut down into the Colwell Bay Member and is interpreted as a marginal-marine channel fill rather than a regionally persistent sheet sand. Its lenticular geometry and restricted development are critical to understanding the Colwell Bay section, because over much of the bay it is this channel-filling sand body, not a continuous laterally uniform bed, that separates the marine Colwell Bay Member below from the more non-marine muds above.

Hatherwood Limestone Member

Bed CB7 — Thin Local Limestone At Cliff End, Probably A Hatherwood Representative (c. 0.5 m)

Along most of Colwell Bay the Hatherwood Limestone Member is absent. Toward Cliff End, however, a thin buff shelly limestone about 0.5 m thick reappears above the Linstone Chine sands. It is Galba-bearing and its top is truncated by an erosion surface. This bed is generally interpreted as a very attenuated local representative of part of the Hatherwood limestone succession seen much more fully on Headon Hill. It indicates brief renewal of freshwater limestone deposition before further erosion and replacement by the overlying clastic sequence. The bed owes its local reappearance to the structural complications around the Linstone Chine fold area and should not be mistaken for the full Headon Hill limestone development farther south.

Lacey’s Farm Limestone Member — Not Developed At Colwell Bay

The Lacey’s Farm Limestone Member, well developed around Headon Hill and inland, dies out northward and is completely absent at Colwell Bay. This absence is one of the key ways in which the Colwell Bay succession differs from the fuller south-west Headon Hill section.

Cliff End Member

Bed CB8 — Basal Cliff End Erosion Surface And Lower Limestone-Bearing Muds

Over much of the bay, the Cliff End Member rests directly on the eroded top of the Linstone Chine Member; at the extreme north end it overlies the thin local limestone described above. The basal Cliff End beds are grey-green to brown shaly muds and clays with thin shell layers, and a thin brown limestone occurs about 3.5 m above the erosional base at the type site. Fossils are dominated by Potamomya and Viviparus. These beds show that after the localized sand-filled channel and local freshwater limestone phase, sedimentation returned to quiet muddy conditions on a low-energy coastal plain with ponds, marshes and low-salinity water bodies.

Bed CB9 — Main Cliff End Potamomya–Viviparus Muds And Thin Sands (over 10 m at the type site)

The greater part of the Cliff End Member consists of grey-green and brown mud, shaly in part, with clay carrying thin shell layers and subordinate thin sands. The fauna remains low-diversity and low-salinity in character, with Potamomya and Viviparus especially typical. These beds are increasingly clay-rich northward toward Fort Albert, where modern landslip and slumping commonly obscure parts of the cliff. Overall, the member records non-marine coastal-plain deposition, with broad muddy flats, shallow standing water and only limited clastic channel input.

Fishbourne Member

Bed CB10 — Fishbourne Thin-Bedded Clays With Shell Seams (c. 3 m at Colwell Bay)

Above the Cliff End Member, the Fishbourne Member is represented in Colwell Bay by only about 3 m of brown and blue-grey thin-bedded clay with thin shell beds. Although much thinner here than in Whitecliff Bay, it is an important upper Headon unit because it records another shift toward marginal-marine to non-marine conditions with thin shelly horizons rather than freshwater limestone. In the Colwell Bay cliffs these beds are not as visually striking as the Venus Bed below, but they help show that the upper part of the local Headon succession remained environmentally variable rather than simply becoming monotonous mottled floodplain clay immediately above the Cliff End Member.

Osborne Member

Bed CB11 — Osborne Colour-Mottled Fluviatile Clays And Silts (c. 13.5 m at Colwell Bay)

The uppermost Headon Hill beds represented in Colwell Bay are about 13.5 m of red and green colour-mottled calcareous clay and silt belonging to the Osborne Member. These beds are largely unfossiliferous and represent a non-marine fluviatile to overbank floodplain environment with pedogenic mottling, oxidation and repeated subaerial exposure. In the modern bay they form much of the higher slipped and degraded upper cliff ground rather than clean fossiliferous ledges on the foreshore. Their presence is important because it shows that Colwell Bay does not end with the Cliff End Member alone, but continues upward into the drier, more obviously terrestrial upper Headon succession.

Depositional Environment

Colwell Bay records repeated environmental oscillation across the Priabonian Headon Hill Formation. The Totland Bay Member represents freshwater to brackish coastal-plain lakes, marshes and marginal-marine muds, with shell-rich freshwater limestones such as the How Ledge Limestone. The Colwell Bay Member records the major marine to marginal-marine incursion of the local succession, beginning above a bored omission surface and passing from brackish basal transgressive sands into the fossil-rich Venus Bed and local tidal-channel Oyster Bed before shoaling upward into the Batillaria Bed. The Linstone Chine Member is a localized sand-filled marginal-marine channel cut into those marine beds. Above it, the thin local limestone near Cliff End marks a brief return to freshwater carbonate deposition, and the Cliff End, Fishbourne and Osborne members together record the restoration of mainly non-marine coastal-plain, lagoonal and fluviatile conditions.

Total Thickness Covered Here

Colwell Bay represents more than 40 m of Priabonian Headon Hill Formation in composite cliff and foreshore exposure, from the upper part of the Totland Bay Member through the 9 m-thick Colwell Bay Member, the 4 m-thick Linstone Chine Member, a very thin local Hatherwood representative, the Cliff End Member, the 3 m Fishbourne Member and the 13.5 m Osborne Member. The full western Totland Bay Member is thicker than the part normally visible in Colwell Bay itself, so the locality should be treated as a composite west Wight Headon section rather than as a single continuously open measured cliff.

References

Daley, B. & Balson, P. (1999). British Tertiary Stratigraphy, Geological Conservation Review Series No. 15, especially the Colwell Bay chapter.
Insole, A. & Daley, B. (1985) on the revised formal member nomenclature of the Headon Hill Formation and Solent Group.
Insole, A., Daley, B. & Gale, A. (1998). The Isle of Wight, Geologists’ Association Guide No. 60.
British Geological Survey Lexicon entries for Headon Hill Formation, Totland Bay Member, Colwell Bay Member, Linstone Chine Member, Hatherwood Limestone Member, Cliff End Member, Fishbourne Member and Osborne Member.
British Geological Survey (2008). Geology of the Isle of Wight, part sheets 300, 331, 344 and 345: brief explanation.
White, H.J.O. (1921). A Short Account of the Geology of the Isle of Wight.
Keeping, W. & Tawney, E.B. (1881), Wright, T. (1851) and Reid, C. & Strahan, A. (1889) for the classic How Ledge, Neritina, Venus and Oyster bed nomenclature.
King, C. (2010). Stratigraphy, depositional environments and palaeogeography of the Colwell Bay Member (Headon Hill Formation, Solent Group, late Eocene, Hampshire Basin).