St. Oswald’s Bay Fossil Hunting

1965–1996 – The deep solution pipes investigated
House’s studies, discussed in West’s field account, examined the unusually deep pipes and their sediment fills. A lower exposed pipe contained lignitic material but did not yield the expected Tertiary pollen, leaving the timing of filling uncertain.

29–30 April 2013 – A major Chalk collapse
A large rockfall occurred overnight in the bay. West photographed the deposit on 3 May and again during subsequent erosion, linking the failure to the steeply overturned, sheared Chalk. This directly contradicts the old description of the site as perfectly safe. Follow-up photographs in 2014 documented continued erosion and are part of this event record, not a separate discovery.

The bay cuts across a faulted, steeply tilted succession from Jurassic limestone to Cretaceous Chalk. Its original collecting observations are retained, while neighbouring logs and uncertain Chalk formation assignments remain explicitly bounded.

PORTLAND GROUP

Older marine limestone survives at Dungy Head and the offshore rocks along the bay’s southern edge. These remnants help explain the former barrier that once sheltered the eroding coves.

Age: Late Jurassic, Tithonian

Eastern Dungy Head Portland Sand: excluded adjacent context

West’s Portland Sand description lies round the eastern side of the headland. It is excluded from this northern-bay beach inventory; no traverse, exposure continuity or measured thickness is implied.

Portland Stone Formation

The offshore Man O’War and Norman rocks and Dungy Head exposures provide the resistant limestone framework. The full measured Portland logs from other Dorset localities do not establish the exact local succession.

Portland Chert Member

The Dungy Head part of the section includes sponge-spicule-rich limestone with chert. Older research accounts use Dungy Head/Dancing Ledge member names; the current BGS framework uses Portland Chert Member. West reports 11 m at Dungy Head, citing Arkell; this is an attributed local historical value, not a northern-bay beach measurement.

Puffin Ledge

A thin conspicuous burrowed marker within the Dungy Head cherty succession is recorded by West; Thalassinoides and large ammonites occur at this general level. No precise local thickness or specimen identity is asserted.

Portland Freestone Member

The upper Portland limestone forms part of the regional barrier succession at the Dungy Head margin. Exact local quarry-bed measurements have not been independently extracted for this guide.

PURBECK LIMESTONE GROUP

Tilted lagoonal limestone and mudstone occur at the bay margins. The source-supported context does not justify transplanting the entire Lulworth Cove or Durlston measured section.

Age: Latest Jurassic–Early Cretaceous

Lulworth Formation

At the specifically included Dungy Head margin, basal caps and soils are documented but tectonic displacement may omit strata. No complete northern-bay Purbeck log is claimed.

Mupe Member

Modern BGS places the basal caps, soils and Broken Beds within Mupe. The following observations are specifically Dungy Head, not imported Fossil Forest measurements.

Lower Dirt Bed

West records the soil beneath the well-developed Hard Cap; no local thickness is supplied.

Hard Cap

The lowermost part immediately above Lower Dirt Bed is thrombolitic. The source describes the bed at the cliff and cliff top but does not justify a visitor scramble.

Great Dirt Bed

A well-developed pebbly soil separates Hard and Soft caps at Dungy Head. Its local thickness has not been established.

Soft Cap

The Dungy Head bed is thin and laminated, without the obvious thrombolites described at some other localities. Its facies therefore differs from the better-known Fossil Forest examples.

Broken Beds: deformed remnant

The poorly developed thin breccia contains some chert and has been strongly affected by deformation. Its original thickness is uncertain; Transition Bed is not seen and its absence may have more than one explanation.

Durlston Formation

The thin local Cinder Bed marks the base. Higher Dungy Head details are poorly exposed and structurally disturbed.

Stair Hole Member

The Cinder and Chief Beef observations belong within this modern member; the intervening complete member log is not established.

Cinder Bed

A particularly thin, poorly developed Cinder horizon is explicitly described at Dungy Head. No exact thickness is supplied.

Chief Beef Beds

Distorted fibrous calcite is documented in the local interval. Its deformation is an observation; the source’s proposed crystallization mechanism remains interpretation rather than established fact.

WEALDEN GROUP

Non-marine mudstone and sandstone are faulted and strongly reduced in apparent thickness near Dungy Head. Regional Swanage totals are not bay measurements.

Age: Early Cretaceous

Wessex Formation

The local Wealden beds contain plant debris and pyrite-bearing sandstone. West reports an approximately 48 m remnant, affected by stratigraphical thinning and faulting, not a complete regional formation thickness. Iron-rich seepage and yellow jarosite are modern alteration products, not fossil sulphur.

SELBORNE GROUP

This is the modern parent group for the Gault and Upper Greensand. It records the return of marine conditions over the older non-marine deposits.

Gault Formation

Marine grey mudstone is recognized at the Dungy Head margin and becomes sandier toward the Upper Greensand. Weathering and movement commonly obscure its fossils.

Age: Albian

Basal Gault pebble bed

Rounded quartz and black chert pebbles mark the marine transgression above the older beds. The BGS excursion illustrates this exact eastern Durdle/Man O’War context; no White Nothe bed thickness is imported.

Upper Greensand Formation

Glauconitic sand, silt and cemented beds yield marine fossils in the original collecting area. Upper parts include chert and condensed phosphatic horizons. The available detailed Durdle Cove log lies west of the promontory and is not copied here as a St Oswald measured section.

Age: Predominantly Albian in this local account

Upper Greensand–Chalk contact and basal phosphatic horizon

The old field guide records a glauconitic limestone with phosphatic pellets and reworked fossils. Rolled or phosphatized ammonites can be older than their enclosing deposit, so matrix and preservation must be recorded rather than assigning every loose fossil to one age.

CHALK GROUP

Steep and locally overturned chalk forms the high northern cliffs. Faulting truncates and repeats the section, so cliff height is not stratigraphic thickness.

Age: Late Cretaceous

GREY CHALK SUBGROUP

Cyclic marly and purer chalk is documented on the eastern side of Man O’War Head. Flint is generally absent from this lower part. A local formation-by-formation measured log has not been verified, and the Durdle Cove counterpart must not be assumed identical.

Age: Cenomanian

WHITE CHALK SUBGROUP

The main high cliffs include flinty chalk affected by overturning and shear. The old “White Group Subgroup” wording is corrected to White Chalk Subgroup.

Lewes–Seaford attribution: qualified local horizon

West places the 2013 rockfall in old “Micraster Chalk”, probably Coniacian, and cautiously suggests part of the Lewes Nodular Chalk to Seaford Chalk interval. The exact formation and bed were not established; a detailed succession cannot be established from this record.

Red Hole and other solution pipes

Deep dissolution features contain rusty sand, clay and flint debris. Their age and relationship to folding have long been debated; neither an Eocene nor Pliocene fill age should be presented as definitively established for every pipe.

References

Bgs Mupe
Bgs Oswald Guide
Bgs Portland Chert
Bgs Portland Freestone
Bgs Selborne
Bgs Stairhole
Bgs Wessex Lexicon
Holiday Oswald
Lulworth Collecting
Lulworth Durdle
West Dungy
West Lulworth Strata