Barton on Sea Fossil Hunting

Early 19th century onwards – Barton Cliff became one of Britain’s classic Eocene fossil localities
Barton Cliff has been collected for more than two centuries and quickly became famous for its rich Eocene fossils. Its importance lies not only in the huge number of shells, but also in its vertebrates, including sharks, bony fish, turtles, birds and mammals.

1888 – Gardiner, Keeping and Monckton documented the Barton Beds succession and faunal changes
The classic 1888 work by Gardiner, Keeping and Monckton divided the Barton Beds into Lower, Middle and Upper parts using changes in the fossil fauna. This helped establish Barton as one of the key reference sections for British Eocene fossils.

1925–1926 – Marjorie Chandler described the famous Hordle Flora from the Barton Cliffs succession
Chandler’s work on the fossil leaves, fruits and seeds from the Barton Cliffs succession made the Hordle Flora one of the best-known British late Eocene plant assemblages. These fossils became very important for reconstructing ancient environments and climate.

1972 – first land mammals recorded from the marine Barton Beds by J. J. Hooker
One of the most important scientific breakthroughs at Barton came in 1972, when J. J. Hooker published the first land mammals from the marine Barton Beds of Hampshire. This was especially significant because mammals in clearly marine Eocene strata are unusual and helped improve correlation with continental faunas.

1979 – Notorhynchus kempi described from Barton Cliff material
Barton Cliff produced material later named Notorhynchus kempi by Ward in 1979. This added to the site’s importance for fossil sharks and showed how significant Barton is for Eocene vertebrate faunas as well as shells.

1986 – Hooker’s major mammal study established Barton as a key Eocene mammal site
Hooker’s 1986 monograph on mammals from the Bartonian of the Hampshire Basin made clear just how important Barton is for Eocene mammals. Barton Cliff became one of the key British reference sites for understanding mammal faunas of this age.

Late 20th century – Barton recognised as Britain’s most productive Mid Eocene reptile site
Later reviews showed that Barton Cliff is the most productive British Mid Eocene site for reptiles, with turtles, lizards and snakes all recorded. It also remained one of the richest British Eocene fish localities, with at least 30 chondrichthyan taxa recognised from the section.

Modern work – Barton continues to be a key reference site for Eocene marine fossils
Barton-on-Sea remains one of the most important British Eocene fossil sites, with ongoing study of museum collections and continued collecting from the cliffs and foreshore. Its importance now lies in the combination of abundant molluscs, major vertebrate faunas and the classic plant assemblages from the wider Barton Cliffs succession.

BARTON GROUP

Barton Clay Formation

The modern lithostratigraphy used here follows the British Geological Survey and Edwards & Freshney scheme, in which the classic marine Barton succession at Barton-on-Sea is divided into the Barton Clay Formation, the Chama Sand Formation and the Becton Sand Formation. For bed-scale detail, the published Burton bed lettering is retained because it remains the standard practical horizon scheme at the type locality: the Barton Clay extends up to and including Bed G, Bed H is the Chama Sand, and Beds I–K belong to the Becton Sand.

Bed A0 — Basal Pebble Bed / Transgressive Lag

At the accepted lithostratigraphic base of the Barton Clay, a rounded flint pebble lag rests sharply on the underlying Boscombe Sand. This bed is the natural transgressive break at the base of the marine Barton sequence and records erosion and ravinement at the start of a new Eocene marine incursion. In Hooker’s cyclic notation this basal lag is treated as A0, although Burton’s original lettered bed scheme begins with A1 above it.

Bed A1 — Nummulites prestwichianus Bed

This green glauconitic sandy clay is the classic Nummulites prestwichianus horizon near the base of the Barton Clay. Historically, the base of the Bartonian was long taken at the bottom of this bed, even though the modern lithostratigraphic base of the formation is placed lower, at the basal pebble bed. It belongs to the shell-rich lower Barton interval and marks the onset of fully marine Barton Clay deposition. Beds A1–A3 together have yielded a notable vertebrate fauna including sharks, rays, bony fishes and the rare frog record from the Barton succession, though A1 itself is now commonly poorly exposed because of sea-defence works and weathering.

Bed A2 — Lower Barton Sandy Clay Interval

Modern exposure of A2 is usually poor, but in Burton’s scheme it forms the intermediate sandy-clay part of the first coarsening-upward Barton cycle between the green glauconitic nummulite bed below and the more obviously reworked A3 sands above. It was part of the very fossil-rich lower Barton succession, probably as shelly loamy sand and sandy clay of open-marine aspect, but the bed is now too commonly hidden, decalcified or protected to justify over-precise modern field description. It is best treated conservatively as a sandy lower Barton shell-bed interval rather than as a continuously visible cliff unit.

Bed A3 — Upper Lower Barton Sands / Reworked Lower Barton Horizon

A3 forms the top of the first Barton cycle and is sandier, more reworked and more regressive in character than the beds below. Older descriptions place rusty sands with abundant Pholadomya margaritacea toward this level, and Hooker noted abraded fossils here, showing considerable reworking during shallowing. Volutospina ambigua is especially characteristic. The lower Barton interval as a whole can also yield small corals, echinoderm remains, crab material, fish teeth, turtle fragments, worn freshwater shells and driftwood, showing both rich marine life and the proximity of land.

Bed B — Basal Transgressive Bed Of The Second Barton Cycle

Bed B begins above a sharp burrowed junction and records renewed marine flooding after the regressive top of A3. Glauconitic silty clays and sandy muds are characteristic, and the bed marks the return to deeper, more argillaceous shelf conditions at the start of the second major Barton cycle. Rich molluscan faunas reappear, and Volutospina athleta is particularly characteristic of this horizon.

Bed C — Voluta suspensa Bed

Bed C is one of the most recognisable beds in the modern Barton cliff, especially between Naish Farm and the first Barton sea defences. It is a more truly argillaceous marine clay than the lower Barton sands and is easily identified because it lies between septarian nodule bands and includes a conspicuous pale, burrowed marl or “white band” in its central part. The bed is named from the classic large gastropod horizon traditionally called the Voluta suspensa Bed and forms one of the best field marker levels in the middle part of the Barton Clay.

Bed D — Middle Barton Clay And Nodule Interval

Bed D is generally seen now in slumped scarps and disturbed cliff rather than as a clean, continuous in situ face. It consists of shelly clay and sandy clay with nodule horizons and belongs to the same septarian-bearing middle Barton interval as Beds C and E. Sycostoma pyrus is especially characteristic. In the present cliff it is important not only stratigraphically but also geomorphologically, because this middle Barton clay package commonly coincides with zones of slipping and bench development.

Bed E — Earthy Bed

This earthy, shell-rich clay and silt horizon is one of the great fossil beds of Barton-on-Sea. A thin but persistent oyster seam at its base has yielded fish vertebrae and fragmentary chelonian remains, and the bed as a whole is famous for beautifully preserved aragonitic molluscs. Turritella imbricateria, Volutospina ambigua, the large Volutospina luctator, and Xenophora agglutinans are particularly characteristic. In collector terms it is one of the richest horizons in the entire Barton Clay succession.

Bed F — Upper Barton Clay Above The Earthy Bed

Above the Earthy Bed, Burton’s Bed F comprises upper Barton clays and sandy clays that are now commonly obscured by slumping, vegetation and coastal engineering works. The interval continues the upward-shallowing trend of the second Barton cycle and includes nodule-bearing weak horizons with increasing sandy content. Modern cliff and ground-model studies distinguish F1 and F2 levels within this upper Barton clay package, and the base of F2 in particular is an important shear surface that controls landslide benches and slope movement along much of the Barton-on-Sea frontage.

Bed G — Stone Band / Shell Band

Bed G is the classic top of the Barton Clay and one of the most distinctive marker beds in the whole section. It is a highly winnowed, brown to reddish, sideritic shell lag rich in Turritella and bivalves, commonly forming hard slabs on the beach. Sycostoma pyrus is also characteristic. Sedimentologically it represents the most strongly reworked and shell-concentrated horizon of the second cycle, probably a storm-accumulated and/or wave-winnowed shell concentration laid down in very shallow marine conditions immediately before the Chama Sand.

Total Thickness Of Barton Clay Formation In The Classic Barton Cliffs Type Section: Somewhat Over 30 Metres

Chama Sand Formation

Bed H — Chama Bed / Chama Sand Formation (c. 5.5 m)

Bed H, the old Chama Bed and the modern Chama Sand Formation, is about 5.5 m thick at Barton. The lower c. 3 m consists of bluish-grey sandy clay with numerous fossils; the upper c. 2.4 m is greenish-grey to bluish-grey clayey sand with fewer fossils, chiefly bivalves. The blue-green colour reflects abundant glauconite. The conspicuous white, thick-shelled bivalve Chama squamosa is the best field fossil, especially in the lower part with Lyria decora; the upper part is characterised by “Meretrix” incurvata and Volutilithes pertusus. Unlike many lower Barton shell seams, fossils are scattered through the bed rather than concentrated in thin shell layers. The formation marks a major shoaling event and heralds the more restricted conditions that follow upward.

Total Thickness Of Chama Sand Formation At Barton-on-Sea: Approximately 5.5 Metres

Becton Sand Formation

Bed I — Lower Becton Sand (c. 7.9 m)

Bed I forms the lower part of the Becton Sand Formation and consists of pale grey, yellow and white fine micaceous sands, clayey and silty at the base and only weakly coherent in many exposures. Fossils are generally sparse, though shell moulds occur in the upper part. The trace fossil Ophiomorpha is especially important here, indicating sandy middle- to upper-shoreface conditions without evidence of subaerial exposure. This bed represents the clean-sand, relatively high-energy phase above the Chama shoal.

Becton Bunny Member

Bed J — Becton Bunny Bed / Olivella branderi Bed (c. 7.9 m)

Bed J is the muddy middle division of the Becton Sand and corresponds to the Becton Bunny Member. It is about 7.9 m thick, with drab grey sandy clay in the lowest c. 1.8 m and about 6 m of greenish-grey sandy clay above, weathering pinkish-drab because of ferruginous matter; small spheroidal ferruginous concretions occur around 3 m above the base. A marked break at its base separates it from the cleaner sands below. Fossils are numerous but usually fragile and include abundant Olivella branderi, together with Nucula, Pitar, Potamides, Corbicula, Bayania, rare Nautilus, and callianassid crustacean remains. The mixed fauna shows a shift from marine into appreciably brackish conditions in a more sheltered sandy-muddy setting.

Bed K — Upper Becton Sand / Long Mead End Sands

Bed K forms the upper part of the Becton Sand above the Becton Bunny Member and includes the cleaner Long Mead End sands. It is mainly composed of white and yellow fine sands with some sandy loams, less muddy than Bed J and locally more beach- or barrier-like in aspect. Brackish shells occur in the loamier parts, including Cyrena, Dreissensia and Erodona, while the higher white and yellow sands have yielded Lucina gibbosula and Batillaria pleurotomoides. A thin greenish clay marks the approach to the overlying Headon Hill Formation farther east. This bed completes the marine Barton succession at Barton-on-Sea and records the final shoaling into restricted lagoonal and, beyond the section, freshwater conditions.

Total Thickness Of Becton Sand Formation In The Barton Cliffs Type Area: About 22 Metres

Stratigraphic Significance

The Barton coastal section between Friar’s Cliff and Beacon Cliff is internationally important because it is the type section for the Bartonian Stage and includes the type sections of the Barton Clay, Chama Sand and Becton Sand formations. Historically the Bartonian was recognised from the Nummulites prestwichianus horizon near the base of Bed A1, although modern lithostratigraphy places the base of the Barton Clay at the basal pebble bed below. The section is also a key calcareous nannoplankton reference, including the NP17 type area and the approximate NP16–NP17 boundary around Bed E.

Depositional Environment

The Barton-on-Sea succession records three linked marine regressive–transgressive cycles. The basal pebble bed and lower Barton beds mark marine transgression over older sandy strata; A3 shows shallowing and reworking at the top of the first cycle; Bed B deepens again into a more argillaceous shelf-mud phase; Beds G, H and I record progressive shoaling from shell lag to glauconitic sandy shoal and then shoreface sand; and Beds J and K show increasing restriction, brackish influence and barrier or nearshore sand accumulation before the succeeding Headon lagoons. The Barton Clay fauna indicates a low-energy warm marine shelf with nearby land, and estimated water depths in the broader Barton sequence were variable, from relatively shallow inner-shelf settings to rather deeper shelf waters at some horizons.

Structural Style And Exposure

Barton-on-Sea must be treated as a composite landslipped coastal section, not as one neat continuously exposed cliff face. Sea-defence works and cliff instability have hidden much of the lower Barton at the central frontage, so Beds A1 and A2 are now rarely seen well; A3 may appear near the west end of the Naish Farm section depending on erosion; Bed C is the clearest middle Barton field marker; Beds D–F are commonly visible only in limited slumped scarps; Bed G is now uncommon but may appear in landslide debris; the Chama Sand is often best seen at the eastern end of the sea defences; and Beds I–K of the Becton Sand are best examined eastward near Becton Bunny and toward Long Mead End. The geology and the modern geomorphology are closely linked, with the upper Barton clay horizons, especially within Bed F, forming important slip surfaces.

Total Thickness Covered Here

The classic marine Barton-on-Sea succession described here totals roughly 58 metres in the Barton Cliffs type area: somewhat over 30 m of Barton Clay Formation, about 5.5 m of Chama Sand Formation, and about 22 m of Becton Sand Formation, before passage upward and eastward into the lower Headon Hill Formation. Older publications and differing boundary concepts may give somewhat larger figures for the Barton Clay or the whole Barton Beds, but these are the most useful modern formation-scale thicknesses for the coastal type section.

References

Gardner, Keeping & Monckton (1888) on the classic Lower, Middle and Upper Barton divisions.
White (1917, 1921) on the Barton and allied Hampshire Basin shell beds.
Burton, E.St.J. (1929, 1933) on the faunal horizons and lettered bed scheme of the Barton succession.
Curry & Wisden (1958). Geologists’ Association guide to the Hampshire Basin.
Hooker, J.J. (1976, 1986) on cyclicity, mammalian biostratigraphy and the formal Barton Clay–Becton Sand lithostratigraphy.
Edwards, N. & Freshney, E.C. (1987a, 1987b) on the Barton Group stratigraphy and the Barton Clay, Chama Sand and Becton Sand formations.
Bristow, Freshney & Penn (1991). British Geological Survey memoir for the Bournemouth district.
British Geological Survey memoir for the Southampton district.
British Geological Survey Lexicon: Barton Group, Barton Clay Formation, Chama Sand Formation, Becton Sand Formation, Becton Bunny Member and Headon Hill Formation.
Geological Conservation Review / JNCC accounts for Barton Cliffs and Barton Cliff.
Aubry and co-authors on calcareous nannoplankton zonation of the Bartonian type section.