Chapman’s Pool Fossil Hunting

1875–1947 – Early measured sections
Blake and later Arkell described the upper Kimmeridge succession using marker beds and seepage lines. Their differing thickness estimates are useful historical records but were superseded by better correlation of the disturbed exposures.

1978 – Ammonite succession documented
Cope’s detailed ammonite work provided the taxonomic framework used by later studies of the Rotunda and Fittoni intervals. The distinction between flattened shale specimens and nodule-preserved material remains important.

2001 – The youngest Kimmeridge Clay remeasured
Gallois and Etches published an exact Houns-tout/Chapman’s Pool composite, identifying KC55–KC63 marker beds and recognizing that older correlations had omitted as much as ten metres from the disturbed middle succession. Nearby boreholes corroborated the revised interpretation but are not substituted for the outcrop log.

2011 – A recorded eastern-bay rockfall
West’s dated field account records a substantial fall in the eastern bay with material partly from the Rotunda interval. It documents active instability, not a recommendation to inspect a fresh fall.

The primary 2001 composite resolves the upper Kimmeridge Clay down to individual marker beds and subintervals. Disturbed exposures require a composite, so the measured succession is not a route to follow along or up the cliff.

ANCHOLME GROUP

The fossiliferous mudstones and upper silt-rich succession belong to the Kimmeridge Clay Formation. The Hounstout Clay and old Rhynchonella Marls belong here, although older alternative schemes assigned them to Portland Sand.

Kimmeridge Clay Formation

The exact Houns-tout/Chapman’s Pool composite measured section is the key source. It combines discontinuous exposures affected by landslips and weathering; it is not one continuously accessible cliff. Its upper beds are Tithonian in modern international usage, although older papers use Upper Kimmeridgian or Bolonian.

Age: Late Jurassic, Tithonian

KC55 lower division

Thinly interbedded grey mudstones, 3.10 m in the composite section, overlie an erosion surface bearing the Chapman’s Pool Pebble Bed. Ammonites include crushed and partly phosphatized Pavlovia.

Chapman’s Pool Pebble Bed

This basal KC55 concentration includes phosphatized ammonite body-chamber pebbles, oysters, other bivalves and belemnites recorded as Pachyteuthis. The latter is a belemnite, not the cuttlefish label in the older web quotation.

KC55 upper division: Blake’s Bed 2

Two oil-shale seams separated by calcareous mudstone total 0.90 m in the revised section. The ledge is recognizable on the beach between the stream and boathouse, but is slippery and must not be hammered. Older accounts give 1.25 m, distinct from the revised measurement.

KC56

Calcareous grey mudstone, 3.20 m, begins above the oil shale. Large crushed pavlovid ammonites occur in its lower part, including Pavlovia concinna and P. rotunda.

KC57: Rotunda Nodules interval

The revised interval is 3.10–3.30 m of calcareous mudstone, bounded by lines of dense carbonate burrowfill nodules. This should not be confused with an older 1.80 m narrowly defined “Rotunda Nodule Bed”.

KC57 lower nodule line

Smaller nodules commonly preserve less-compressed Pavlovia concinna, P. rotunda and the historical subspecies P. rotunda gibbosa. Early cementation protected some shell chambers from compaction.

KC57 upper nodule line

Larger burrowfill nodules, up to about 0.1 by 0.2 m in the study, enclose bivalves and rarer Pavlovia. The top of this line defines the base of KC58.

KC58: upper calcareous mudstones

A 16.30 m interval becomes more silty upward. It includes very pale calcareous beds and at least two thin, 10–20 mm shell-rich dark mudstones. Bivalves include Oxytoma, Protocardia and Thracia; Pavlovia concinna is recorded in its lower 13 m.

Hounstout silt-rich succession

The primary paper proposes member terminology and divides this into lower silt, clay and upper silt. BGS’s modern memoir retains these lithological distinctions, but a current standalone BGS member entry has not been verified. They are therefore treated here as attributed informal or research intervals rather than confirmed formal members.

KC59 basal Cidarid Siltstone

A 0.30 m tough pale siltstone has a bioturbated base and abundant marine fauna, including oysters, Entolium, belemnites, distinctive flattened cidarid spines and rhynchonellid brachiopods. The reported historical species assignments have not been independently verified against modern taxonomy.

KC59 middle division

Approximately 7.60 m of alternating silty mudstone and siltstone is poorly exposed or hidden by debris. This exposure limitation must accompany the log.

KC59 upper division and lower bituminous bed

The upper 2.40 m contains repeated silt/mud rhythms, capped by a thin 0.05–0.15 m laminated bituminous bed with pyritized oysters and other bivalves. A siltstone rib marks the base of this subdivision.

KC60: upper Lower Hounstout Silt

A 12.80 m siltstone and silty-mudstone interval contains beds commonly 0.10–0.40 m thick. Seven conspicuous coarsening-upward rhythms occur in its upper part; they are not individually named or assigned invented thicknesses.

KC61: lower Hounstout Clay and upper bituminous beds

A 4.90 m interval contains two oil-shale/bituminous seams with pale calcareous mudstone beneath and between. These form resistant weathered ribs above the lower silt succession.

KC62: upper Hounstout Clay

Bioturbated silty mudstone and subordinate siltstone total 12.70 m. An organic-rich rib up to 0.40 m thick lies about 5 m above its base. Burrows can be particularly clear in the silt-rich beds.

KC63 lower division: Upper Hounstout Silt

The lower 9.60 m alternates silt-rich and mud-rich beds mostly 1.1–1.7 m thick. Harder beds make bluffs and several seepage lines; seepage alone is an unreliable correlation marker.

KC63 middle division

A 2.30 m calcareously cemented silt/mudstone division forms prominent pale-weathering bluffs.

KC63 upper division

The upper 6.00 m of intensely burrowed silt/mudstone contains Teichichnus, Rhizocorallium and Arenicolites. Poorly preserved bivalves and ammonites occur, especially in its lower two metres.

PORTLAND GROUP

The overlying shallower-water marine beds cap the high cliffs. The fuller regional succession at St Aldhelm’s Head is not all accessible within the cove.

Portland Sand Formation

The modern boundary is placed at the base of the conspicuous Massive Bed. Older schemes extending Portland Sand downward into Hounstout beds are not followed.

Massive Bed

Fine-grained calcareous sandstone, 2.00 m in the exact composite, provides the clearest local formation boundary. Progalbanites at this level is important in historical Albani zonal definition.

Emmit Hill Marls

A marl/siltstone interval above the Massive Bed belongs to the east-Purbeck Portland succession. Its local Houns-tout thickness remains unverified here; the Winspit/St Alban’s measurements describe other sections.

White Cementstone

A hard marker above the lower marls is documented in the local cliff context. Glaucolithites caementarius at Houns-tout was used to define the base of the Glaucolithus Zone.

St Alban’s Head Marls

The higher marl interval is part of the east-Purbeck sequence; local cove thickness and detailed beds need independent measurement before expansion.

Parallel Bands

These resistant upper beds are dolomitic, despite the older sandy terminology used for this part of the succession.

Black Sandstones

The historical name refers to hard dolomitic beds, not simply quartz sandstone. Poorly preserved ammonites are difficult to identify; do not encourage extraction.

Portland Stone Formation

Only its basal part is represented on the Houns-tout/Chapman’s Pool capping described by West. The full St Aldhelm’s limestone succession is excluded.

Portland Chert Member: basal capping only

Chert-bearing limestone fragments and cliff-capping strata belong to this member. No whole-member local thickness or Portland Freestone extension is inferred.

References

Bgs Dorset Memoir
Bgs Kimmeridge Lexicon
Bgs Portland Chert
Chapmans Gallois 2001
Chapmans Swcp
West Chapmans