Durlston Bay Fossil Hunting

19th century – the Durlston Head side of the bay became recognised as one of the finest Purbeck sections in Britain
The stretch from the middle of Durlston Bay to Durlston Head became important early in the history of Purbeck geology because it exposes a superb succession of Middle and Lower Purbeck beds. This helped establish the southern half of the bay as a classic reference section for the transition from lagoonal limestones and shales down into the evaporitic and brecciated lower beds near Durlston Head.

1898 – Strahan sketched the succession in the southern part of Durlston Bay towards Durlston Head
One of the key early milestones for the southern half of the bay was Strahan’s published cliff profile of the section from the Zigzag path southwards. This became an important early guide to the stratigraphy of the middle bay to Durlston Head stretch.

1915–1919 – A. Smith Woodward showed that the Middle Purbeck beds of Durlston Bay were exceptionally important for fossil fishes
Woodward’s work demonstrated that some of the best fish specimens from Durlston Bay come from the Intermarine Member or Upper Building Stones of the Middle Purbeck succession. This helped establish the bay as one of the most important Late Jurassic–earliest Cretaceous fish localities in Britain.

1969 – Torrens brought renewed attention to the Durlston Bay bed succession
Twentieth-century work revived detailed interest in the numbered bed succession of Durlston Bay, helping tie the historic fossils to a more precise modern framework. This was an important step towards the later detailed DB bed-number system used through the bay.

1985 – West and El-Shahat documented dinosaur footprints from the Corbula Beds
The middle to southern part of Durlston Bay became especially important for trace fossils when dinosaur footprints were recorded from the Corbula Member. This confirmed that the bay preserves not only bones, teeth and shells, but also direct evidence of dinosaur activity on the Purbeck lagoon margins.

1987–1988 – Ensom described mammal, fish and microvertebrate material from the Cherty Freshwater Member
Important work in the late 1980s showed that Bed DB102 in the Cherty Freshwater Member had yielded a diverse microvertebrate assemblage, including fish, amphibians, reptiles and mammals. This added major significance to the middle part of the bay, showing that small vertebrate remains were concentrated at specific horizons and not just scattered randomly through the succession.

1991 – Nunn published a detailed geological map of the Purbeck beds in Durlston Bay
Nunn’s mapping was one of the most important modern milestones for the bay because it gave a detailed geological survey of the Purbeck Beds and helped clarify the bed relationships through the central part of the section. This became especially useful for understanding the middle bay succession southwards toward the major faults and Durlston Head.

1991 – Ensom and colleagues described diverse reptile faunas from the bay
Later work on the Durlston reptile fauna helped confirm the site as one of Britain’s richest earliest Cretaceous reptile localities. The middle and southern beds are especially important for turtles, crocodilians and small reptiles.

1993 – Clements published the bed-by-bed reappraisal of the Durlston Bay succession
Clements’ 1993 reappraisal of the stratigraphy provided the standard modern numbered DB bed framework for Durlston Bay. This was one of the most important scientific milestones for the site because it allowed fossils from the middle of the bay to Durlston Head to be placed much more accurately within the succession.

Modern era – Richard Edmonds discovered a fine crocodile skull at the base of the Shed Bed and the junction with the Shingle
One of the most notable later vertebrate finds from the middle bay section was a fine crocodile skull discovered by Richard Edmonds at the base of the Shed Bed and the junction with the Shingle. This is one of the clearer named vertebrate discoveries tied to the central Durlston Bay succession.

Modern understanding – the middle bay to Durlston Head stretch remains one of the key Purbeck reference sections in Britain
Today this part of Durlston Bay is recognised as especially important for the Purbeck bed succession, dinosaur footprints, fish-bearing limestones, turtle and crocodile horizons, and the structurally complex lower beds near Durlston Head. Its importance lies not only in individual fossils, but in the way the fossil horizons can be tied into a detailed bed-by-bed stratigraphic framework.

PURBECK GROUP

Only the middle of Durlston Bay southward to Durlston Head is treated here. That means the page focuses on the basal Durlston Formation in the central bay and then the much thicker Lulworth Formation reference section southward to the headland. The younger Peveril Point Member and the main northern Upper Purbeck section nearer Peveril Point are deliberately excluded.

Durlston Formation (Lower Cretaceous, Berriasian)

Within the middle-bay-to-head subsection, the Durlston Formation is represented only by the basal Stair Hole Member and by a small fault-repeated slice of the lower building-stone succession in the central spur. The full Stair Hole Member is much thicker farther north in Durlston Bay, but only its base and a structurally repeated part of its lower shell-beach facies belong to the area covered here.

Stair Hole Member

Bed DT1 — Cinder Bed (formal DB111; c. 2.95 m)

The Cinder Bed is the key marker at the top of the section covered here and forms the formal base of the Durlston Formation. It is a bluish-grey, massive, oyster-rich shelly limestone and calcareous mudstone, crowded with Praeexogyra distorta and accompanied by Serpula coacervata, the echinoid Hemicidaris purbeckensis and rarer larger bivalves such as “Protocardia” major. In the field it forms the obvious ledge at the centre of Durlston Bay and is repeated again south of the Zigzag Path by faulting. It records a short but important marine incursion across the otherwise mainly lagoonal and freshwater Purbeck succession and is one of the most laterally persistent marker horizons in the whole group.

Bed DT2 — Fault-Repeated Lower Stair Hole Building-Stone Slice (includes formal Bed DB118 locally)

Between the two central bay faults, a limestone spur preserves a local repeated slice of the lower Stair Hole Member above the Cinder Bed. These beds belong to the traditional lower Intermarine or Upper Building Stone facies: shell-rich biosparites, biomicrites and mudstones deposited in low-salinity lagoon-margin shell-beach and shallow-water settings rather than in fully marine open shelf conditions. The most important local horizon is DB118, which preserves an in situ dinosaur footprint in the central bay. This repeated spur is structurally important because it proves that the central bay is not a single uninterrupted downward section from the Cinder to Durlston Head, but a faulted composite in which part of the lower Stair Hole Member is reintroduced above the older lower Purbeck beds.

Lulworth Formation (Latest Jurassic To Earliest Cretaceous, Tithonian–Berriasian)

The middle of Durlston Bay to Durlston Head is one of the primary reference sections for the Lulworth Formation. Here the succession descends from the top of the Worbarrow Tout Member through the difficult-to-separate Ridgeway interval into the Mupe Member at the headland. The Jurassic–Cretaceous boundary lies low in this lower Purbeck section, around the Cypris Freestones–Soft Cockle part of the succession in different published schemes, so this subsection spans latest Tithonian into Berriasian time.

Worbarrow Tout Member

Bed DT3 — Upper Cherty Freshwater Beds (formal beds DB87–110; uppermost Worbarrow Tout Member)

The top of the Lulworth Formation at Durlston Bay consists of thicker-bedded, chert-rich freshwater micrites with interbedded mudstones and shales. Chert remains visible as low as DB87, and the member as a whole carries freshwater gastropods, ostracods and charophytes rather than the more saline faunas of the lower lower-Purbeck units. These beds represent shallow, carbonate-rich low-salinity lakes and ponded marginal-waterbody deposits developed immediately below the marine Cinder Bed transgression. At Durlston they form the cliff-foot limestones directly beneath the Cinder and are among the most important vertebrate-bearing horizons in the lower half of the bay.

Bed DT4 — Feather Bed And Associated Cherty Freshwater Vertebrate Horizons (formal beds DB102–108 in part)

The upper Cherty Freshwater succession contains several especially important vertebrate and footprint horizons. DB102 has yielded a diverse microvertebrate assemblage including fish, amphibians, reptiles and mammals. The Under Feather level (DB106) yielded material named as Iguanodon hoggi in the older literature, while the Feather Bed itself (DB108) produced postcranial remains referred to Nuthetes, granicones and numerous small crocodilian remains including forms historically called Nannosuchus and Theriosuchus. Dinosaur footprints have been recorded lower in the same part of the sequence, including DB94, DB102 and DB103. Lithologically these beds are a mix of cherty limestones, carbonaceous shales and freshwater micrites laid down in shallow lake and marginal-mudflat settings where episodic drying and flooding repeatedly created surfaces capable of preserving tracks and concentrating small vertebrate debris.

Bed DT5 — Marly Freshwater Beds And Mammal Bed (formal Bed DB83 in part)

Below the cherty unit lies the Marly Freshwater interval, an argillaceous freshwater-lake succession rich in Hydrobia, Viviparus, Planorbis, Physa and charophytes. Its most famous bed is the Mammal Bed, DB83, a dark grey, shelly, carbonaceous calcareous clay and shale resting on an irregular surface and in part grading into the bed below. Despite its old quarry name as a “Dirt Bed”, it is not a rooted palaeosol like the Great Dirt Bed of the basal Purbeck; instead it is a carbonaceous freshwater lake-margin or swamp deposit with abundant freshwater shells and accumulated vertebrate debris. This horizon yielded the classic Beckles mammal assemblage, historically recorded as around 14 genera and 18 species of early mammals, together with lizards, crocodilians, dinosaur remains including Echinodon and the type jaw of Nuthetes, fish, plants and invertebrates. It is one of the most important microvertebrate beds in the British Early Cretaceous.

Bed DT6 — Soft Cockle Beds (middle Worbarrow Tout Member)

The Soft Cockle Beds form a long evaporite-rich interval of softer marls, micrites and dolomitic beds with abundant evidence of primary and early diagenetic evaporites. The characteristic bivalve is Protocardia purbeckensis, and Serpula coacervata is common. Particularly important at Durlston are the secondary gypsum beds developed after burial anhydrite and ultimately after primary gypsum, commonly with nodular or chicken-wire textures. Cream-coloured dolomitic beds with halite pseudomorphs, some celestite and minor stromatolitic development also occur. These strata represent restricted saline to hypersaline lagoon and sabkha cycles under semi-arid seasonal conditions, and dissolution of the gypsum contributes significantly to the modern instability of the central and southern Durlston cliffs.

Bed DT7 — Hard Cockle Beds (basal Worbarrow Tout Member at Durlston)

In western Dorset the Hard Cockle Beds are sandy bivalve limestones, but at Durlston Bay they pass into a more basinal facies of marls and marlstones. Protocardia purbeckensis remains common, evaporite pseudomorphs are present, and insect horizons are known, including fossil wasps at DB36c. Freshwater fossils and normal fully marine faunas are absent, showing that these beds accumulated in strongly restricted hypersaline lagoonal conditions. The change from the more obvious bivalve-rich limestones farther west into marly basinal facies at Durlston is one of the important eastward facies changes within the Purbeck Group.

Ridgeway Member

Bed DT8 — Ridgeway Transition Interval (poorly distinguished at Durlston Bay)

The Ridgeway Member is difficult to define at Durlston because the mudstone-rich facies seen farther west pass eastward into monotonous micritic limestone little different from parts of the Mupe Member below and the Worbarrow Tout Member above. Where present it represents a quieter, darker, more carbonaceous muddy phase between the thick ostracod-rich limestones of the Mupe Member and the more quartz-detrital lower Worbarrow Tout beds. In the field around middle Durlston Bay this transition is largely masked by facies change, weathering and structural disruption, so it is best treated as an indistinct but real stratigraphical interval rather than forced into an artificial sharp boundary.

Mupe Member

Bed DT9 — Cypris Freestones And Archaeoniscus Bed (includes formal DB11; Archaeoniscus Bed c. 1.3 m)

The upper Mupe Member at Durlston Head consists mainly of ripple-laminated ostracodal and pelletoidal limestones traditionally known as the Cypris Freestones. They formed in shallow carbonate flats and lagoons with salinities commonly around twice that of normal seawater, and desiccation of the lagoon floor left halite crystals that were later dissolved to produce casts in the fine carbonate sediment. A conspicuous marlstone, the Archaeoniscus Bed (DB11), lies above the Broken Beds and is easily seen at Durlston Head; it contains the isopod Archaeoniscus brodiei. Dragonfly larvae are rare. The whole interval is a distinctive restricted-lagoon carbonate facies with seasonal drying and repeated reworking of ostracod-rich sediment.

Bed DT10 — Broken Beds (major evaporitic breccia interval)

The Broken Beds are among the most distinctive strata in the whole Durlston Bay subsection. At Durlston Head they comprise partly calcitized evaporites and evaporitic breccias, commonly very porous, with celestite residues and associated brittle-fracture structures. In the central bay their exposure has been partly obscured by sea-defence works, but at the headland they are well shown just north of the major fault against the Portland Stone. A major structural downbulge thickens the Broken Beds here and shows them to have been squeezed and displaced northward in association with extensional movement. This is not a normal undisturbed lagoonal bedset but an evaporite-rich interval strongly modified by very early tectonism, collapse and plastic flow of former anhydrite or gypsum.

Bed DT11 — Caps (basal Purbeck strata at Durlston Head)

The Caps form the basal Purbeck beds at Durlston Head. They are mostly evaporitic, but include stromatolitic limestone at the base and the Lower Celestite Bed. Unlike the classic basal Purbeck sections at some other Dorset localities, the Great Dirt Bed is absent here and no in situ tree stumps have been found. These beds therefore represent a very restricted lagoonal to sabkha setting developed immediately above the Portland limestones, but in a more basinal evaporitic facies than the tree-bearing basal Purbeck sequences better known elsewhere in Dorset.

PORTLAND GROUP

Portland Stone Formation (Upper Jurassic, Tithonian)

Bed DT12 — Faulted Portland Stone South Of Durlston Head (context below the Purbeck Group)

South of the major fault at Durlston Head, the Purbeck strata rest against the upper Portland limestones. The exposed Portland Stone includes sea-worn cherty limestones of Portland Chert Member type and higher ooidal freestones of Portland Freestone type. These rocks represent shallow fully marine carbonate-shelf deposition before the abrupt environmental change into the restricted lagoonal and evaporitic Purbeck world above. They are included here because the Purbeck–Portland relationship is one of the defining geological features of the Durlston Head end of the section.

Structural Style And Composite Nature Of The Section

The middle-to-southern Durlston Bay section is not a single uninterrupted coastwise log. In the middle of the bay two obvious normal faults downthrow southward and repeat part of the section, producing a limestone spur in which Cherty Freshwater, Cinder and a slice of Intermarine beds reappear. Around the old flats and sea-defence works, parts of the lower Purbeck are obscured. At Durlston Head, major extensional faulting, evaporite flow and the Durlston downbulge tectonically thicken and distort the Broken Beds close to the Portland contact. Any realistic website log must therefore be treated as a composite, faulted section rather than a simple continuous cliff profile.

Depositional Environment

This subsection records repeated oscillation between freshwater, brackish, hypersaline and briefly marine conditions on the latest Jurassic to earliest Cretaceous Purbeck coastal plain. The Cinder Bed marks a short normal-marine incursion with oysters, serpulids and echinoids. The fault-repeated lower Stair Hole slice records low-salinity shell-beach and shallow lagoon conditions that preserved dinosaur footprints and vertebrate debris. The Worbarrow Tout Member below passes from freshwater lake and swamp deposits of the Mammal and Cherty Freshwater beds, rich in molluscs, charophytes, mammals, reptiles and tracks, down into the more saline Soft and Hard Cockle lagoon–sabkha facies with evaporites, halite pseudomorphs, gypsum and dolomite. The Mupe Member continues the restricted lagoonal and evaporitic story in an even more basal form, while the Portland Stone beneath represents the preceding open-marine carbonate shelf.

Total Thickness And Stratigraphic Range Covered Here

The middle-bay-to-head subsection includes the base of the Berriasian Durlston Formation at the Cinder Bed, then descends through most of the upper and middle Lulworth Formation to the Purbeck–Portland junction. The Worbarrow Tout Member alone reaches about 39 m at Durlston, and the full Durlston Formation reaches about 57 m in the bay as a whole, though most of its upper part lies outside the present focus. Because the section is repeated and locally thickened by faults, especially in the central bay and at Durlston Head, it is more accurate to say that this page covers most of the lower Purbeck reference section from the Cinder Bed down to the Portland Stone rather than to assign a single simple measured thickness to the subsection.

References

Clements, R.G. (1993). Type section of the Purbeck Limestone Group, Durlston Bay, Swanage. Proceedings of the Dorset Natural History and Archaeological Society, 114, 181–206.
Westhead, R.K. & Mather, A.E. (1996). An updated lithostratigraphy of the Purbeck Limestone Group in the Dorset area. Proceedings of the Geologists’ Association, 107, 117–128.
Arkell, W.J. (1947). The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain.
Barton, C.M., Woods, M.A., Bristow, C.R. & Newell, A.J. (2011). South Dorset and south-east Devon and its World Heritage Coast. British Geological Survey Special Memoir.
British Geological Survey Lexicon entries: Durlston Formation, Stair Hole Member, Lulworth Formation, Worbarrow Tout Member, Ridgeway Member, Mupe Member, Portland Stone Formation and Portland Chert Member.
Benton, M.J. & Spencer, P.S. (1995). Fossil Reptiles of Great Britain, Geological Conservation Review Series, including the Durlston Bay account.
Dineley, D.L. & Metcalf, S.J. (1999). Fossil Fishes of Great Britain, Geological Conservation Review Series, including the Durlston Bay account.
Jarzembowski, E.A., Siveter, D.J., Palmer, D. & Selden, P.A. (2010). Fossil Arthropods of Great Britain, Geological Conservation Review Series, including the Durlston Bay account.
West, I.M. (1965, 1975) and Salter, M.G. & West, I.M. (1965) on Purbeck gypsum, celestite, Broken Beds and evaporite tectonics at Durlston Head.
Nunn, J.F. (1991). A geological map of Purbeck Beds in the northern part of Durlston Bay. Proceedings of the Dorset Natural History and Archaeological Society, 113, 145–148.
Allen, P. & Wimbledon, W.A. (1992) on integrated Jurassic–Cretaceous boundary correlation in the Purbeck succession; and Ensom, P.C. and co-authors on the Mammal Bed, Feather Bed, microvertebrate horizons and dinosaur footprints of Durlston Bay.