Kimmeridge Bay’s dark Jurassic mudstones and resistant limestone ledges reveal a detailed marine succession. Ammonites and other shells can be seen in suitable exposures, and loose foreshore fossils may be collected under the local estate code, which prohibits hammering, digging and breaking rocks or nodules.
FIND FREQUENCY: ♦♦♦ – Ammonites and shells occur in the local succession; only naturally loose permitted material may be collected.
CHILDREN: ♦ – Supervise children on the accessible bay and avoid slippery ledges and cliffs.
ACCESS: ♦ – Signed toll-road and car-park approach; the natural shore remains tide- and weather-dependent.
TYPE: – Loose foreshore finds and observation of Kimmeridge Clay ledges; no hammering, digging or nodule breaking.
DIRECTIONS
♦ Follow the signed road from Kimmeridge village to the private bay car park and established access near the Wild Seas Centre. Check current toll-road conditions.
♦ Explore only the safely accessible bay with a clear return route and suitable tide, weather and swell. Chapman’s Pool is a separate remote locality, not an alternative entrance to this guide.
♦ Original geological locality reference: 50.60890°N, 2.12903°W. This is not a verified parking or descent point.
FOSSIL HUNTING
Ammonites are a good starting point at Kimmeridge Bay, but expect flattened shells, impressions and fragments more often than a complete three-dimensional specimen. Look at naturally exposed faces of loose foreshore pieces for a coiled outline or repeated ribs, rather than assuming every dark slab is fossiliferous. Many specimens lie along bedding surfaces and can be difficult to distinguish when weathered or coated. Fixed ledge fossils are useful to photograph and compare with museum examples even when there is nothing suitable to collect.
Aulacostephanus is a well-documented bay ammonite. On better-preserved material, ribs branch from small swellings near the inner edge of the whorl and stop short of a smooth strip along the outer rim. These features offer a better starting point than overall size or colour, though crushing can obscure them. Subdichotomoceras is another recorded genus; Gallois’s study describes both genera as abundant on some Washing Ledge interval bedding planes. That is a statement about particular fossil-bearing beds, not about the number of loose specimens a visitor will find.
The Washing Ledge succession also shows why preservation varies. Most mudstone ammonites are compressed, but concretions in W12–W14 have sometimes retained three-dimensional Gravesia and Subdichotomoceras. These are research observations to help interpret fossils, not an invitation to open nodules. Gravesia lafauriana is specifically recorded from W21 in Kimmeridge Bay. A loose piece without a secure source cannot be assigned to one of these numbered units simply because it was found near a ledge.
Bivalves add interest alongside the ammonites. The small oyster Nanogyra virgula, often called Exogyra virgula in older accounts, is recorded with Protocardia striatula in the Maple Ledge mudstones. Look for the outline of an individual valve and repeated shell growth lines, rather than mistaking any pale patch for a shell. Fish and marine-reptile remains are much less routine finds; celebrated museum specimens and old labels reading “Kimmeridge Bay” can represent a wider stretch of coast. They should not be used as a checklist for this beach.
The estate code permits loose foreshore fossils but prohibits hammering, digging and breaking rocks or nodules, including loose nodules. Leave attached fossils in place and follow any more restrictive on-site notice. Record the position and photograph an important-looking find before moving it; retain supporting matrix and ask the Etches Collection for advice, especially for bone or associated remains. Searching accessible loose material patiently is more useful than extending a visit into an unsafe tidal traverse.
A chronological record of geological research and discoveries associated with Kimmeridge Bay.
1875 – An early numbered succession
J. F. Blake numbered conspicuous units along the coast east of the bay. Later workers retained some marker names while substantially improving correlations; this was a regional framework rather than a single bay measured section.
1947 – Named ledges organize the local geology
Arkell’s district memoir used named stone bands and intervening shale intervals to describe the succession. Washing and Maple ledges remain recognizable field markers, although modern studies describe their intervening rocks as mudstones.
1979–2017 – A supposed large fault gap reassessed
A BGS borehole drilled in 1979 and later detailed correlation showed that the Maple Ledge Fault omits far less rock than older interpretations implied. Gallois’s 2017 account estimates less than half a metre.
2011 – Fifteen Maple rhythms mapped
Gallois correlated the exact cliff and foreshore successions using fifteen numbered units, aerial photographs and distinctive ribs. This improved the context of research specimens without making extraction lawful for visitors.
2017 – The lower bay succession revised
A detailed revision distinguished fourteen Flats and twenty-one Washing Ledge units, clarified ammonite marker beds and corrected several older correlations.
GEOLOGY
Kimmeridge Bay exposes part of the Kimmeridge Clay Formation within the Ancholme Group. Its dark marine mudstones alternate with more organic-rich laminated beds and harder carbonate horizons. Changes in the sediment and its organic content produced repeated rhythms, while later mineral growth strengthened particular layers into the ledges visible today. Waves remove softer material more readily, leaving the resistant bands as steps and reefs rather than one smooth shore platform.
The same contrast influences fossil preservation. Shells in mudstone were commonly compressed during burial; some carbonate concretions protected ammonites sufficiently to retain their three-dimensional form. A fossil’s flatness therefore reflects its burial and cementation history as well as the animal’s original shape. The Flats, Washing Ledge and Maple Ledge intervals divide the local succession into recognisable research units. Their numbered rhythms help scientists compare cliffs and foreshore exposures, but faults and bedding-plane movements can interrupt the apparent order.
The main bay succession is principally late Kimmeridgian in modern international terminology. Older “Bolonian” usage and the upper Maple interval’s historical stage-boundary interpretation need qualification, as explained below. The much longer coast towards Chapman’s Pool exposes additional, younger strata; its overall thickness and distant oil-shale markers are not measurements or collecting targets for this bay. The expanded account preserves the full local F, W and M subdivisions so that the visible ledges can be understood in their proper sequence.
The detailed geological succession and important exposure limits at Kimmeridge Bay.
A faulted marine mudstone succession produces the bay’s repeated ledges. The account separates modern formation rank from named research intervals and individual rhythms, and does not mistake the much larger regional type section for the thickness exposed in one bay.
ANCHOLME GROUP
The BGS places the Kimmeridge Clay Formation in the Ancholme Group. The bay exposes only part of this regional group.
Kimmeridge Clay Formation
Marine mudstones alternate with organic-rich laminated beds and harder carbonate-cemented ledges. The bay succession is Late Jurassic, principally late Kimmeridgian. The historical Kimmeridgian–Tithonian boundary interpretation in the upper Maple interval is discussed with its limitations below. The BGS lexicon uses a broad legacy Kimmeridgian age field; this is not a precise international boundary determination.
Age: Late Jurassic; boundary interpretation discussed below
Flats interval: F1–F14
Gallois’s numbered rhythms are useful exact-locality units, not verified BGS formal members. The complete interval occurs within Kimmeridge Bay. Organic-rich lower divisions are labelled a and the overlying less organic mudstone divisions b. The thinner Brandy Bay measurements describe a separate section.
F1
The Flats Stone Band forms the basal hard marker; the occurrence of Aulacostephanus autissiodorensis places the base of the Autissiodorensis interval within this marker rather than arbitrarily at its top.
F2
A thin oil-shale base passes into mudstone.
F3
A conspicuous oil-shale base is overlain by mudstone.
F4
A thin bituminous lower division is followed by mudstone.
F5
Bituminous mudstone below, less organic mudstone above.
F6
Bituminous lower beds pass into mudstone with a pale calcareous upper part and a micritic concretion horizon.
F7
The lower division combines bituminous mudstone and oil shale, followed by mudstone.
F8
A bituminous lower division and mudstone upper division form another recognizable rhythm; local bedding-plane shears complicate interpretation.
F9
This rhythm contains the Rebholzi Marker Bed with Sutneria rebholzi within F9.
F10
Oil shale at the base, mudstone above; part of the F10–F14 interval rich in small Nannocardioceras.
F11
An oil-shale base underlies mudstone; Nannocardioceras is recorded through this group of upper Flats rhythms.
F12
An oil-shale lower division is followed by mudstone.
F13
Bituminous mudstone and oil shale form the lower division, beneath mudstone.
F14
A thin organic-rich base and several incomplete rhythms precede the Washing Ledge marker. Individual laminae do not have independently established bed numbers.
Washing Ledge interval: W1–W21
All numbered units are exposed in Kimmeridge Bay. The section includes numerous organic-rich/calcareous rhythms and localized concretions. Most ammonites are flattened; occasional concretions preserve them in three dimensions. These are research subdivisions, not independently verified BGS members.
W1
The Washing Ledge Stone Band comprises two hard carbonate beds separated by about 0.1 m of oil shale.
W2
The lower part of the Volgae Marker interval contains Amoeboceras (Nannocardioceras) volgae; A. (N.) krausei and the last recorded Aspidoceras occur in W2.
W3
The lower part continues the Volgae Marker interval. An organic-rich base passes into mudstone and a pale calcareous top.
W4
A bituminous basal division is followed by mudstone with a calcareous upper part.
W5
Bituminous mudstone below and paler mudstone above repeat the local rhythmic pattern.
W6
An oil-shale base is followed by less organic mudstone.
W7
An oil-shale base passes into mudstone and a pale calcareous top.
W8
Oil shale below, mudstone above, becoming more calcareous toward the top.
W9
An organic-rich basal division passes into mudstone with a pale upper part; the graphic log distinguishes this from adjacent black oil-shale bands.
W10
The basal oil shale is followed by mudstone with an additional thin organic-rich layer and a paler upper part.
W11
A thin organic-rich basal bed is overlain by calcareous mudstone.
W12
Oil shale below pale calcareous mudstone with a concretion horizon.
W13
Bituminous mudstone underlies calcareous mudstone and concretions. Rare Pseudogravesia is recorded in W13–W14.
W14
A concretion-bearing organic-rich lower division passes into mudstone. This completes the W12–W14 concretion-bearing interval that can preserve less compressed ammonites.
W15
Oil shale at the base passes into mudstone with a calcareous upper part.
W16
Bituminous mudstone below, mudstone and a calcareous top above.
W17
A further bituminous-to-calcareous rhythm.
W18
An oil-shale base precedes the mudstone and calcareous upper division.
W19
Bituminous mudstone beneath pale calcareous mudstone.
W20
Bituminous mudstone beneath calcareous mudstone immediately below the final Washing Ledge rhythm.
W21
An oil-shale base passes into calcareous mudstone below Maple Ledge. Gravesia lafauriana is recorded here. The Maple Ledge Fault removes less than 0.5 m in the modern reconstruction, rather than the much larger gap inferred in older accounts.
Maple Ledge interval: M1–M15
The complete interval is 24.0 m in Kimmeridge Bay, compared with 20.6 m at Brandy Bay. It is exposed in northeast-bay cliffs and the intertidal area; M9–M15 below Hen Cliff are commonly algae-covered. Gallois calls this a member, but a matching modern BGS lexicon rank has not been verified; it is therefore treated here as an explicitly attributed research subdivision.
M1
Maple Ledge Stone Band, the resistant carbonate marker at the base.
M2
Mudstone immediately above the Maple Ledge Stone Band.
M3
An oil-shale and bituminous basal division passes upward into mudstone.
M4
A bituminous lower division and several thinner organic-rich layers lie within mudstone. The base carries a mapped bedding-plane shear.
M5
The basal oil shale has alternating kerogen-rich brown and pale coccolith-rich laminae. Mudstone above contains subsidiary thin organic-rich bands.
M6
The basal Stink Corner Rib is strongly cemented and its upper surface carries numerous ammonites and pyritic concretions; mudstone overlies it.
M7
Bituminous mudstone at the base is overlain by a thicker less organic mudstone division.
M8
An organic-rich base passes into mudstone with a calcareous upper part.
M9
The oil-shale/bituminous base includes a mapped shear; less organic mudstone forms the upper division.
M10
A mudstone-dominated unit with thin organic-rich divisions. The 2011 study places the last recorded Aulacostephanus 0.3 m below M11 and discusses its historical use as a stage-boundary proxy; that proxy is not a ratified global boundary.
M11
Waterfall Rib forms the basal resistant oil-shale bed, the lip of the east-bay waterfall and a foreshore ledge beneath Hen Cliff. Mudstone overlies it.
M12
A thin oil-shale/bituminous base is followed by mudstone with a pale calcareous upper part.
M13
A thin organic-rich basal layer is overlain by mudstone and calcareous mudstone.
M14
An oil-shale/bituminous lower division precedes mudstone containing a substantial calcareous interval.
M15
A bituminous lower division passes into mudstone with extensive calcareous beds toward the top. Blake’s Bed 42 is the marker above this numbered interval.
Geographical and provenance limits
The Blackstone, White Band, Rope Lake Head and Freshwater Steps sections belong to the wider coast eastward, not this bay log. The Saccocoma Marker Bed known from the Warlingham borehole was not identified in the bay section. Historic museum labels reading “Kimmeridge Bay” can refer broadly to the coast toward Encombe; these labels alone cannot establish which numbered bay bed yielded a fish or reptile.
References
Bgs Kimmeridge Lexicon
Bgs Kimmeridge Visit
Dwt Kimmeridge
Etches Code
Gcr Kimmeridge Fish
Kimmeridge Gallois 2011
Kimmeridge Gallois 2017
SAFETY
Keep well clear of cliff bases, overhangs and recent falls. Algae-covered ledges are exceptionally slippery. Tide times do not guarantee that exposed ledges or headlands are passable: swell and weather can block them. Do not wade around headlands, climb cliffs or undertake the long foreshore traverse described in the old guide. A helmet does not make a rockfall zone safe. Supervise children on the accessible bay and stay on established routes.
EQUIPMENT
Take a camera, notebook, tide information and wrapping for lawful loose finds. Wear footwear with grip on slippery ledges. Leave hammers and chisels at home: the local code also prohibits breaking loose rocks and nodules.
CLEANING AND TREATING
Photograph lawful loose finds before lifting them. Wrap fragile shale with supporting matrix and keep matching pieces together. Seek museum advice for vertebrate or pyrite-rich material before cleaning or consolidation. Do not apply acid or split nodules on site.
IDENTIFY YOUR FINDS
Need help identifying a fossil? Share clear photos, where you found it and its size with the community.
FURTHER READING
Etches Collection fossil collecting code
Gallois 2017: Flats and Washing Ledge succession
Gallois 2011: Maple Ledge succession
BGS Kimmeridge Clay Formation lexicon
BGS Kimmeridge Bay visitor geology
Dorset Wildlife Trust Wild Seas Centre
GCR fossil-fish site and provenance cautions
ACCESS RIGHTS
The current Etches Collection code states that Smedmore Estate allows collection of loose fossils from the foreshore. Hammering, digging and breaking rocks or nodules are prohibited. Specimens attached to ledges or cliffs must stay in place; important discoveries should be reported to the museum. This permission is specific to the stated estate arrangements, not the entire coast eastward. Follow any more restrictive on-site notice.
Protected-site information: SSSI Information – South Dorset.
It is important to follow our ‘Code of Conduct’ when collecting fossils or visiting any site. Please also read our ‘Terms and Conditions’.
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