Black Head Fossil Hunting

1875 – Blake discussed the boundary beds of the Weymouth district, including the Black Head and Osmington Mills coast
J. F. Blake’s work on the Kimmeridge Clay and associated boundary beds helped establish the Osmington Mills to Black Head coast as an important Upper Jurassic section. This was one of the early scientific milestones in recognising the significance of the beds exposed here.

1877 – Blake and Hudleston further documented the Corallian and Kimmeridge succession of this coast
Later nineteenth-century work refined understanding of the uppermost Corallian beds and the passage into the Kimmeridge Clay along the Weymouth district coast. Black Head and Osmington Mills became part of this classic stretch for studying the Oxfordian–Kimmeridgian transition.

1933–1947 – Arkell’s classic work fixed Black Head and Osmington Mills within the standard Upper Jurassic framework of Dorset
W. J. Arkell’s major studies and Geological Survey memoirs treated the Black Head and Osmington Mills section as part of the classic Corallian and Kimmeridge Clay coast of south Dorset. His work helped cement the locality’s reputation for fossiliferous Upper Jurassic beds and their structural complexity.

1978 – Birkelund and colleagues used the nearby Weymouth district succession in important work on Rasenia faunas
Work on Lower Kimmeridgian ammonites, especially Rasenia, drew on the wider Weymouth district succession that includes Black Head and Osmington Mills. This helped reinforce the importance of the locality in studies of the Cymodoce Zone and the basal Kimmeridge Clay faunas.

1981 – Cox and Gallois refined the basal Kimmeridge Clay succession of the Weymouth district
Detailed later twentieth-century work clarified the lithostratigraphy and faunas of the beds above the Corallian in the Weymouth area, helping place the Black Head and Osmington Mills succession into a more precise modern framework.

2001 – the Geological Conservation Review published its Black Head account
The review documented the locality’s lower Kimmeridge Clay section, marker beds and relationship to the underlying Corallian. Its importance includes the lower intervals absent from Kimmeridge Bay and the history of research on the Oxfordian–Kimmeridgian boundary. This is the publication date of the account, not a newly assigned legal designation date.

2009–2013 – the Weymouth Bay pliosaur entered the museum and was named Pliosaurus kevani
The original scientific paper records recovery over eight years, mostly by Kevan Sheehan, with contributions from other collectors. Dorset Museum’s acquisition was announced in 2009, the prepared skull went on display in 2011, and Benson and colleagues named the species in 2013. Its source was the Wyke Siltstone in the lower Kimmeridge Clay at Osmington Bay.

Modern understanding – Black Head and Osmington Mills remain important for Corallian and Kimmeridge Clay fossils
Today the locality is best known for its fossiliferous Upper Jurassic succession rather than for a long list of individual finds. The coast can yield oysters, bivalves, ammonites and other marine fossils from the Corallian and Kimmeridge Clay, together with marine reptile remains, while also remaining one of the best places to see the structurally disturbed Jurassic rocks of the Weymouth Anticline.

BH labels are editorial cross-references, not a new measured bed scheme. The detailed lower Kimmeridge framework follows the separate Black Head columns of GCR Figure 2.13a; its Osmington Mills column is a neighbouring comparison, not silently merged. The higher historical “Upper Kimmeridge” markers are lithostratigraphical context and the old terminology is not used as an assertion that every level falls within the modern Kimmeridgian Stage.

CORALLIAN GROUP

Osmington Oolite Formation (Upper Jurassic — Oxfordian)

Upper Oolitic Reef And Foreshore Limestones (Informal Description)

Upton Member — Truncated Local Rock-Platform Section

Black Head Upton Succession

The GCR records the Upton succession in the lower Black Head platform [SY 725 819], where erosion beneath Shortlake truncates the fuller Bran Point sequence. Sandy limestone, calcareous mudstone, nodular clay and sandy bioturbated marl provide the coastal reference framework. The Bran Point 6.7–8.8 m member range and individual reference-bed dimensions are not assigned to this truncated Black Head exposure.

Shortlake Member — Middle White Oolite Context

Bed BH1 — Middle White Oolite And Associated Reef Limestones

Hard oolitic limestones and sandy limestones forming reefs on the foreshore at Black Head and west of Osmington Mills. These resistant Corallian rocks make the wave-cut platform at low tide and underlie the dark Kimmeridge cliff behind. The limestones are shallow-marine carbonate deposits laid down in a warm Oxfordian sea, and they commonly show rhythmic bedding, shell debris and local cementation. Much of the lower Corallian is not easily accessible in the cliffs, but the foreshore reefs provide a good view of the general lithology.

Typical Fossils

Oolitic shelly limestones with bivalves, brachiopods and other shallow-marine Corallian fossils, together with ex situ ammonites and reef-derived material from nearby higher beds.

Shortlake Type Section: Shortlake Steps To Black Head (10.4 m)

The local type section extends from [SY 723 819] to [SY 725 818]. Cross-bedded ooid limestones, with westward-dipping foresets reaching 1 m in height, alternate with burrowed oolites and clays. Individual beds correlate only over short distances; the 1 m figure describes foreset height, not a whole-member thickness.

Nodular Rubble Member

Black Head Nodular Rubble And Burrow Networks

The GCR explicitly records conspicuous Thalassinoides networks in the Black Head low cliffs, explaining many limestone nodules as cemented burrow fills. Its 3.2 m reference thickness and 0.5 m clay bands describe the complete Bran Point section; they are not asserted as measured Black Head values.

Clavellata Formation (Upper Jurassic — Oxfordian)

Clavellata Member — GCR 2001 Scheme

Upper Corallian Clays, Ironstones And Trigonia Beds (Informal Description)

Bed BH2 — Upper Clavellata Beds And Red Beds

Ferruginous and locally reddish sandy limestones, oolitic ironstones and clayey beds representing the upper part of the Corallian succession at Black Head. These beds are much more mixed in lithology than the uniform mudstones of the Kimmeridge Clay above and preserve the last clearly Corallian shallow-marine fauna beneath the Kimmeridgian transgression. They are best seen in the low cliff and loose foreshore blocks at the foot of the main headland.

Typical Fossils

Myophorella clavellata and allied trigoniid bivalves are characteristic, together with other shelly Corallian faunas. The topmost Corallian also includes the ironshot muddy limestone facies traditionally known as the Osmington Mills Ironstone.

Sandy Block — Black Head Section (2.42 m)

At [SY 726 819], the GCR description records four or five beds of grey muddy limestone, immature ooids and fine shell debris, with only about 5% quartz sand. Scattered Myophorella and Perisphinctes occur. The underlying Osmington Oolite surface is bored and eroded.

Chief Shell Beds — Black Head Section (2.07 m)

Five distinct Myophorella clavellata-rich shell layers occur in impure ooid limestone, largely as separated valves. Disseminated siderite gives pale-red weathering. The cited Black Head account also records Amoeboceras glosense, Decipia lintonensis and Perisphinctes (Pseudarisphinctes). No unreported individual thicknesses are assigned to the five shell layers.

Clay Band — Black Head Section (0.6 m)

A separate silty, shelly, iron-rich clay interrupts the limestone succession.

Red Beds — Black Head Section (2.02 m)

Hard grey sideritic limestones weather bright red and alternate with softer muddy oolite. The larger catalogue of ammonite holotypes in the GCR is explicitly from Dorset coast Red Beds, not exclusively this measured Black Head section.

Sandsfoot Clay Member — Coastal Reference In The GCR Clavellata Scheme

GCR 2001 includes Sandsfoot Clay in its expanded Clavellata Formation. BGS 2011 also explains the older mapped usage placing it in Sandsfoot Formation. This reference passage follows the stated GCR hierarchy; the following three measured subdivisions are coastal reference values, not newly verified measurements at the Black Head outcrop.

Sandsfoot Clay Reference Division I (0.85 m)

Sandy mudstone is richly fossiliferous in the cited coastal description, with Myophorella clavellata, Gervillella aviculoides, Perisphinctes and Decipia decipiens.

Sandsfoot Clay Reference Division II (1.45 m)

A separate burrowed sandy clay follows the lower mudstone.

Sandsfoot Clay Reference Division III (1.6 m)

Fine silty clay completes the 3.9 m reference succession; the higher divisions yield bivalves including Goniomya literata and Pleuromya uniformis, without an unsupported assignment of each species to one subbed.

Sandsfoot Formation (Upper Jurassic — Oxfordian)

Sandsfoot Grit Member

Sandsfoot Grit Unit I — Black Head (0.95 m)

Poorly cemented sand contains occasional limonite ooids in the local section cited by the GCR.

Sandsfoot Grit Unit II — Black Head (3.9 m)

Soft, uncemented clay-bearing sand forms the middle division.

Sandsfoot Grit Unit III — Black Head (2.5 m)

Fine- to medium-grained ferruginous sandstone becomes strongly burrowed upward, with bivalves, limonite ooids and quartz pebbles. The cited matrix yielded Amoeboceras rosenkrantzi and Microbiplices anglicus. The three units total the published local 7.35 m; erosion below Ringstead Clay varies across the coast.

Ringstead Clay Member

Ringstead Clay Between Black Head And Osmington Mills (3.5 m In The GCR Account)

The cited intervening cliff exposures show fossiliferous mudstone with abundant orange-weathering ferruginous concretions in the upper part. The lower pale clay and upper sideritic silty-clay contrast is recognised regionally, but the 1 m and 0.5 m partial visible values at Ringstead Bay are not transferred to Black Head.

Osmington Mills Ironstone Member

Osmington Mills Type-Section Lower Dark Clay (0.28 m)

The nearby type section [SY 733 818] contains dark silty clay at the bottom of the ironstone member; this is a separate reference measurement.

Osmington Mills Type-Section Ferruginous Oolite (0.20 m)

Limonite-ooid calcareous mudstone overlies the clay and yields Ringsteadia evoluta. Together these make the published 0.48 m type-section package, a source-scoped comparison for the Black Head ironstone rather than a uniform thickness throughout the coast.

Bed BH3 — Osmington Mills Ironstone

A shelly oolitic ironstone at the top of the Corallian, replacing the Ringstead Coral Bed in the Black Head–Osmington Mills area. This bed lies immediately beneath the base of the Kimmeridge Clay and marks the final mixed-lithology Oxfordian shelf deposition before the abrupt switch into dark grey Kimmeridge mudstones. The contact above is an important minor erosion surface and faunal break.

ANCHOLME GROUP

Kimmeridge Clay Formation (Upper Jurassic)

The Black Head section is one of the key Dorset localities for the lower part of the Kimmeridge Clay. In contrast to Kimmeridge Bay, where the section begins much higher in the Lower Kimmeridge, Black Head exposes the basal Kimmeridgian beds and much of the lower zonal succession. Dips are steep, commonly around 80° near the base and 50–60° higher up, and landslip plus camber mean that the published succession must be reconstructed from discontinuous exposures and detached material rather than read as one simple continuous cliff face. Do not enter or clear slip faces to examine it.

Basal Lower Kimmeridge Clay (Baylei Zone) (Informal Description)

Bed BH4 — Inconstans Bed (Published Bed 1)

The basal bed of the Kimmeridge Clay and the traditional Dorset Oxfordian–Kimmeridgian boundary in the cited Black Head framework. It is a dark grey, intensely bioturbated clay, up to about 0.4 m thick, with wisps and burrow fills of silt and fine-grained sand and scattered limonite ooids derived from the underlying Osmington Mills Ironstone. Phosphatic pebble beds occur at the base and about 0.3 m above the base. This is the critical boundary bed at Black Head and marks the abrupt change from mixed Corallian lithologies into relatively uniform Kimmeridge mudstone sedimentation.

Fossil Content

Torquirhynchia inconstans, Pictonia, Goniomya, Pholadomya, Pleuromya, phosphatised serpulids and the gastropod Pleurotomaria. These fossils occur both in the pebble-rich lag levels and through the intervening clay.

Inconstans Basal Phosphatic Lag

The GCR records phosphatic pebbles at the bed’s base, with reworked limonite ooids concentrated low in the clay. Fossils include phosphatised serpulids, molluscs, Torquirhynchia and Pictonia.

Inconstans Upper Pebble Level — About 0.3 m Above The Base

A second phosphatic concentration is explicitly recorded at this local height. Fossils also occur between the two lags, including crushed shells, Chlamys clay casts, Lopha clusters and trigoniids; the bed is not one undifferentiated pebble seam.

Bed BH5 — Nana Bed

A thin but important shelly bed just above the Inconstans Bed, rich in the small oyster Nanogyra nana. It provides an immediate marker above the Kimmeridgian boundary and is useful in tracing the lowest Kimmeridge Clay across Black Head, Osmington Mills and Ringstead Bay.

Bed BH6 — Dark Grey Mudstones With Abundant Deltoideum Delta

Dark grey mudstones overlying the Nana Bed, characterised in the Black Head graphic section by abundant Deltoideum delta. These beds represent early fully marine Kimmeridge shelf mud deposition and form the first substantial mudstone interval above the boundary beds.

Bed BH7 — Pale Grey Mudstones With Thin Red-Weathering Tabular Clay Ironstones

A paler calcareous mudstone unit with thin red-weathering tabular clay-ironstone bands. This interval is typical of the rhythmic sedimentation in the lower part of the Lower Kimmeridge Clay, where siltstone or calcareous mudstone may alternate with darker mudstones in repeated packages.

Zone Significance

These basal beds belong to the Pictonia baylei Zone and make Black Head one of the most important Dorset sections for the lowest Kimmeridgian, because this interval is not seen at Kimmeridge Bay.

Lower Kimmeridge Siltstones And Lower Cymodoce Beds (Informal Description)

The GCR graphic log distinguishes two or three thin tabular siderite-mudstone bands in the pale calcareous mudstone above the Deltoideum-rich interval. These are local observed subdivisions, without thicknesses inferred from the drawing.

Bed BH8 — Wyke Siltstone (Published Bed 5)

A prominent hard band and line of seepage in the lower part of the Kimmeridge Clay. The Wyke Siltstone consists of intensely burrowed, finely cross-laminated, muddy quartz silt, crowded in its lower part with bivalves, particularly myids in growth position. Common phosphatic pebbles occur in the lowest part and the bed rests on an irregular interburrowed contact. It is one of the most useful lower Kimmeridge correlation beds in Dorset.

Bed BH9 — Black Head Siltstone (Published Bed 8)

A second prominent siltstone bed above the Wyke Siltstone, named from this locality. It is shelly throughout, with small oysters and Thracia, and contains abundant Amoeboceras (Amoebites) preserved in calcite ghost preservation. Like the Wyke Siltstone, it contains phosphatic pebbles in its lowest part and rests with an irregular, interburrowed base on the mudstones beneath. This bed is especially notable at Black Head because dinosaur bone has been found part-way up the cliff in the Black Head Siltstone.

Bed BH10 — Lower Cymodoce Mudstones And Concretionary Shelly Beds

Dark grey mudstones with more shelly interbeds and common concretions above the Black Head Siltstone. The graphic section shows intervals rich in shelly material, septarian concretions and ammonites including Rasenia and Xenostephanus. These beds represent continued lower Kimmeridge mudstone deposition after the siltstone influxes had waned and more typical soft shale facies returned.

Bed BH11 — Xenostephanus Beds (Published Bed 14)

A named lower Kimmeridge horizon in the Black Head succession, lying above the Black Head Siltstone and within the Cymodoce succession. It marks one of the characteristic shelly and ammonite-rich levels in the lower Kimmeridge mudstones before the more uniformly dark mutabilis-zone shales above.

Ammonites

Rasenia cymodoce, Xenostephanus and associated Cymodoce faunas.

Middle Lower Kimmeridge Clay (Mutabilis And Eudoxus Zones) (Informal Description)

Pumpkin-Shaped Septarian Concretions Near The Cymodoce–Mutabilis Transition

The Black Head log depicts this concretionary level above the very shelly Rasenia–Xenostephanus package. The younger mudstones become moderately shelly, with Aulacostephanus including A. eulepidus and small oysters. No numerical thickness is inferred from the graphical scale.

Bed BH12 — Mutabilis-Zone Dark Mudstones And Calcareous Bands

Dark grey mudstones and bituminous shales with intermittent pale calcareous mudstones and thin cemented bands. This part of the succession is less obviously divided into single striking lithological beds when seen from a distance, but repeated calcareous and shelly horizons are recorded beneath the weathered clay crust. This geological observation is not advice to clear or dig the cliff face. These beds yield ammonites of the Aulacostephanus mutabilis group and show the characteristic Kimmeridge alternation of darker organic-rich mudstones and paler, more calcareous intervals.

Mutabilis Interval: Shell Plasters And Myid–Lopha Levels

The GCR Black Head column separates smooth mudstone bearing shell plasters and iridescent Aulacostephanus eulepidus from a more shelly level with myid bivalves and Lopha. These are described local lithological intervals, not newly numbered formal beds.

Mutabilis Interval: Calcareous Mudstone And Septarian Horizon

Above these beds, the published column shows widely spaced septarian concretions within a relatively uniform, blocky-weathering calcareous mudstone package with small oysters and echinoid debris. The drawn bedding-plane shear is a structural complication, not an invented depositional gap.

Bed BH13 — Supracorallina Bed

A pale calcareous mudstone horizon with abundant Neocrassina extensa (formerly referred to as Astarte supracorallina) and Aulacostephanus eulepidus. It is one of the recognised marker beds in the Black Head–Osmington Kimmeridge succession and helps tie the mutabilis-zone strata into the wider Dorset framework.

Upper Mutabilis Shelly Levels And The Transition Into Eudoxus

Above the Supracorallina marker the published Black Head log retains very shelly beds with Aspidoceras, Aulacostephanus, Lopha and rhynchonellids, followed by shelly and sparsely shelly mudstones with iridescent ammonites. Its Eudoxus part includes Amoebites, Aspidoceras with Laevaptychus, Aulacostephanus of the eudoxus group and Sutneria eumela.

Bed BH14 — Eudoxus-Zone Shelly Mudstones

Shelly dark mudstones and calcareous mudstones yielding Aulacostephanus eudoxus and allied faunas. These beds are particularly important because the Black Head succession continues well below the level reached at Kimmeridge Bay, where the cliffs descend no lower than about 10 m into the eudoxus zone. At Black Head, the lower zones beneath eudoxus are fully represented, making the locality one of the most valuable lower Kimmeridgian reference sections on the Dorset coast.

Fossil Content

Aulacostephanus eudoxus and associated shelly mudstone faunas occur in the Eudoxus interval. West also records local loose slabs with Lucina minuscula and small Protocardia, and vertebrate-bearing fallen nodules and blocks. Those loose finds do not establish an exact Eudoxus-bed origin for every specimen.

Unexposed Interval Below The Higher Eudoxus Packages — Approximately 7 m In The Published Log

The GCR explicitly marks this gap; soil fragments indicate shelly mudstone with thin oil-shale interbeds. It is kept as an exposure gap rather than reconstructed into a measured bed-by-bed section.

Higher Eudoxus Oil Shales And Shelly Mudstones

The exposed column above that gap includes a pyritised Saccocoma–Sutneria level and rhythmic shelly oil shale alternating with shelly or sparsely shelly mudstone. Its iridescent ammonites include Aspidoceras, Amoebites and Aulacostephanus of the nodosus group.

Bed BH15 — Virgula Limestone

A soft muddy limestone composed largely of the oyster Nanogyra virgula. This is one of the most conspicuous tabular marker bands in the higher part of the Black Head lower Kimmeridge sequence and divides the shale succession into major field packages. It records a brief, shell-rich calcareous phase within otherwise mud-dominated sedimentation.

Above The Virgula Limestone: Shelly Calcareous Beds

The local graphic section records Crussoliceras, Nanogyra virgula and serpulids above the oyster limestone. This named fossil association is retained separately from the limestone itself.

Unexposed Interval Below The Nannocardioceras Beds — Approximately 8 m In The Published Log

The primary figure records this exposure gap explicitly. Soil fragments suggest oil shale bearing Nannocardioceras with shelly and sparsely shelly mudstones; a continuous measured subdivision is not supplied.

Lower Nannocardioceras Concretionary And Oil-Shale Levels

Below the main cementstone, the GCR column shows concretions containing crushed bivalves and Nannocardioceras, then poorly exposed interbedded oil shale and mudstone with abundant examples of that ammonite. These are distinct from the higher cemented oil-shale horizon that preserves fossils in solid calcite.

Bed BH16 — Nannocardioceras Cementstone

A nodular cementstone horizon preserving Amoeboceras (Nannocardioceras) in uncrushed translucent calcite. This is one of the classic higher marker beds in the Black Head succession and demonstrates how early cementation locally protected ammonites from compaction within the Kimmeridge mudstones.

Regional Thickness Estimate: About 244 Metres For The Wider Black Head–Osmington–Ringstead Area (Cox And Gallois, 1981), Not A Measured Single Black Head Cliff

Higher Kimmeridge Marker Beds In The Black Head–Osmington Landslip Succession (Informal Description)

Bed BH17 — Possible Blake’s Bed 42

The GCR identifies a possible equivalent of Blake’s Bed 42 in the higher Black Head–Osmington succession; its question mark is retained here. It is not always easy to isolate in the field on the main ridge, but it is part of the established Dorset correlation framework and lies above the Nannocardioceras Cementstone in the higher Kimmeridge shales.

Bed BH18 — Grey Ledge Stone Band

A more resistant stone band higher in the succession, useful as a local datum where present in slipped or partially obscured cliff sections. It represents another calcareous or cemented phase interrupting the soft mudstone-dominated sequence.

Bed BH19 — Blackstone

A darker, more organic-rich stone band in the upper part of the Kimmeridge Clay succession. In the wider Dorset Kimmeridge it is associated with highly organic facies, and at Black Head it forms part of the upper marker-bed succession recognised in the landslipped cliff and debris. The GCR specifically records tiny pyritised Saccocoma plates in this marker.

Rope Lake Head Stone Band Equivalent — Intermittent Higher Exposure

The GCR figure explicitly includes this named equivalent between the Blackstone and White Stone bands in the intermittent higher Black Head succession. Poor exposure prevents a complete local measured log between these markers.

Bed BH20 — White Stone Band

A conspicuous white coccolith limestone in the higher Kimmeridge sequence. Field guides note that the Black Head cliffs extend upward nearly to the level of the White Stone Band, though the upper part of the succession is more obscured and landslipped than the lower reference section. It is therefore best treated as a higher marker horizon in the broader Black Head–Osmington landslip complex rather than as a continuously exposed bed in the main lower cliff section.

Higher Zonal Interpretation

The lower Kimmeridge succession at Black Head runs upward from the Pictonia baylei Zone through the Rasenia cymodoce, Aulacostephanus mutabilis and Aulacostephanus eudoxus zones, with higher beds in the landslip complex extending toward younger upper Kimmeridge levels. This is why Black Head is so important: it supplies the lower zonal succession that is absent from the better-known Kimmeridge Bay cliffs.

Reworked Cretaceous And Superficial Beach Debris — Outside The In-Situ Jurassic Succession

Upper Greensand Boulder Field And Mudslide Debris (Informal Description)

Bed BH21 — Reworked Upper Greensand Boulders And Flint Pebbles

East of Black Head, and locally along the route from Osmington Mills, the beach carries abundant large boulders derived from major mudslides involving Lower Cretaceous rocks. These include Upper Greensand “cowstones” with serpulid worm tubes such as Rotularia concava, echinoid fragments, burrows and occasional ammonites. These are not part of the in-situ Jurassic cliff section at Black Head, but they are an important part of the collector’s locality because they add a second, transported fossil assemblage to the beach material.

Depositional Environment

Black Head records the passage from warm shallow Oxfordian carbonate shelves of the Corallian Group into the more uniform dark mudstones of the Kimmeridge Clay, deposited in a deeper, quieter marine basin prone to low oxygen at the sea floor. In the lower part of the Kimmeridge Clay, rhythmic alternations of silty mudstone, dark mudstone and pale calcareous mudstone are common, and several marker siltstones, limestones and cementstones can be traced across the Dorset type area. Landslip and camber complicate the cliff profile, but marine erosion continually refreshes the lower section, which is why Black Head remains one of the best places to study the basal and lower Kimmeridge Clay.

Thickness Note: No Single Complete Local Thickness Is Assigned To This Black Head Composite; Regional Estimates For The Wider Black Head–Osmington–Ringstead Area Are Not Measurements Of One Exposed Cliff

References

Cox, B.M. & Gallois, R.W. (1981). The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences.
British Geological Survey Lexicon: Kimmeridge Clay Formation; Osmington Oolite Formation.
Geological Conservation Review: Black Head and Osmington site accounts.
Ian West, Wessex Coast Geology field guide for Osmington Mills–Black Head.
House, M.R. (1993) for the Xenostephanus bed framework in the Dorset Kimmeridge succession.

GCR 21, Black Head, printed pages 31–34 and actual Figure 2.13a: separate bed packages, explicit exposure gaps and higher marker uncertainties. West’s direct Black Head observations: Corallian reefs, local loose fossils and transported Cretaceous debris.