Osmington Mills Fossil Hunting

1877 – The oolite succession named
Blake and Hudleston introduced Osmington Oolite for a narrower set of beds than the formation recognized today. Later revisions broadened the unit, explaining differences between early descriptions and modern charts.

1936 – Bran Point reference sections
Arkell established the Nodular Rubble at Bran Point and selected a standard section for the shell-rich Clavellata beds. These local benchmarks remain important even where weathering reduces exposure.

1947 – The detailed Dorset memoir
Arkell’s south Dorset memoir documented the Osmington coast in detail, preserving bed descriptions used in later field guides. Historical measurements remain valuable when their exact locality and terminology are retained.

1986 – Modern members and a Bencliff reference section
Wright revised the stratigraphy and selected the exposed Bencliff Grit east of Osmington Mills as a standard section. The former type locality near Weymouth had been built over.

1989–1998 – The fine-grained “grit” investigated
Studies by Allen and Underhill, Sun, and Goldring and colleagues analysed the local Bencliff structures. They showed how changes in currents, storms and sediment supply could build the striking interbedded sand-and-mud succession.

The eastern route shows a changing, faulted Upper Jurassic succession. The units below are arranged oldest to youngest. Historical A-bed labels are retained for field usefulness, with unresolved thickness differences made explicit.

CORALLIAN GROUP

Oxfordian, Upper Jurassic. The succession records repeated changes between sandy sea floors, muddy shelf conditions and shallow-water carbonate deposition. Osmington Mills lies west of Ringstead Bay.

Age: Oxfordian

Redcliff Formation

Redcliff is the modern spelling. Its local exposed parts include Nothe Grit, upper Nothe Clay and Bencliff Grit, but the much wider GCR composite does not represent one complete cliff section at Osmington Mills.

Nothe Grit Member

A small exposure occurs immediately east of the slipway. Fine sandstone and calcareous sandy beds contain bivalves; extensive measured sections farther west are outside this guide’s core locality.

Preston Grit beds

The shelly beds at the top of the low cliff near the slipway are traditionally called Preston Grit. Current BGS defines them within the upper Nothe Grit, although the GCR used separate member rank. The waterfall marker described in the original guide remains subject to field confirmation; it does not establish a separate formal member.

Nothe Clay Member

The upper part and its transition into Bencliff Grit are exposed east of Osmington Mills. Muddy beds become sandier upward. The full 12 m GCR section is described at Redcliff and is not a measured local Mills thickness.

Bencliff Grit Member

The reference section east of Osmington Mills contains 6.7 m of fine sand and silt, despite the coarse-sounding name. Large carbonate-cemented concretions preserve striking cross-stratification. Most material is not richly fossiliferous.

Bencliff event-bed facies

Published studies distinguish an erosive sandy-silt base, laminated mud, mud-and-sand flaser bedding and ripple-laminated sand. Individual event sequences are incomplete in places, so show these as recurring facies rather than four continuous named beds. Burrows descend from the sandier tops.

Osmington Oolite Formation

The type coast extends from Frenchman’s Ledge to Bran Point. An abrupt burrowed surface above Bencliff Grit marks its base. The following A-numbers preserve the original guide’s useful Arkell-style field sequence, as reproduced by Ian West; they are informal bed labels, not modern formations. Thickness and facies vary rapidly along the coast. Throughout the A-bed descriptions below, thicknesses attributed to West’s historical field log are distinct from the GCR member totals. The differing geographical scopes and unresolved numerical discrepancies are retained where they apply.

Upton Member

Sandy limestone, marl and clay form the lower part. The GCR gives 6.7–8.8 m across the local outcrop. Bed-level values below belong to the historical field log, not a new measurement.

A1a — First Limestone

Sandy, muddy limestone with Nanogyra nana and conspicuous Thalassinoides. Its ledges are useful field markers; thickness increases westward. West’s historical A-log reports up to 0.60 m here.

A1b — sandy oolitic marl

Oyster-bearing marl grades down into A1a. Arkell combined the two as A1, with a reported total of 1.98 m; do not sum the variable A1a maximum as a more precise total. West’s historical A-log reports 1.37 m here.

A2 — historical Chlamys qualicosta Bed

Hard, dark, shell-rich oolitic limestone forms part of Bran Ledge. The top 0.15 m is a distinct course. Historical species naming is retained only as the bed label pending taxonomic review. West’s historical A-log reports 0.76 m here.

A3 — dark clay

Clay or local marl contains compressed, fragile bivalves. Such preservation does not support aggressive washing or hammering. West’s historical A-log reports 0.60 m here.

A4 — Pisolite

The conspicuous coarse oncoidal limestone contains shell nuclei and fragments, with recorded Perisphinctes and Cardioceras. The bed is repeated by faulting near Bran Point. West’s historical A-log reports 0.46 m here.

A5 — nodular clay

Clay with pale calcareous nodules has yielded Perisphinctes. “Littlemore Clay Beds facies” is an old descriptive comparison, not an additional formal formation. West’s historical A-log reports 3.81 m here.

Shortlake Member

The ooidal middle succession changes thickness and bedding character markedly. The GCR records 5.2 m at Bran Point compared with 10.4 m at the separate Shortlake–Black Head type section. The historical A6–A11 transcription should not be summed or presented as an independently reconciled Bran Point measured log.

A6 — Middle White Oolite

Cross-bedded ooidal limestone becomes more marly eastward, leaving only the uppermost approximately 0.6 m solid at Bran Point. Ian West’s transcription gives component values of 2.29 and 1.06 m but a stated total of 3.05 m; these disagree. The eastward lithological change is documented, but a corrected total has not been established.

A7 — oolitic marlstone

Soft, rubbly oolitic marlstone with burrows and small oysters interrupts the harder oolitic succession. The historical A-log explicitly records Thalassinoides and common Nanogyra nana; A7 remains unmeasured.

A8 — clay and white nodules

Clay has three nodular mudstone bands in its upper 1.22 m in the historical log. West’s historical A-log reports 2.51 m here.

A9 — lower Upper White Oolite

Cross-bedded ooidal limestone contains vertical burrows. West’s historical A-log reports 0.60 m here.

A10 — grey clay

The lower approximately 0.3 m contains ooidal calcareous nodules. West’s historical A-log reports 0.99 m here.

A11 — upper Upper White Oolite

Thin fissile ooidal limestone layers alternate with clay and abundant small oysters. West’s historical A-log reports 0.30 m here.

Nodular Rubble Member

The upper member is a conspicuous nodular limestone and marl unit. GCR reports 3.2 m at Bran Point; the historical A12 transcription gives 3.35 m. This modest difference should remain attributed, rather than averaged.

A12 — Nodular Rubble

Two poorly separated courses contain abundant microscopic sponge spicules attributed to Rhaxella. Shell and echinoderm debris accompany records of Nucleolites scutatus, Nanogyra nana and bivalve and gastropod moulds. The basal 0.30 m is more ooidal and muddy. West’s historical A-log reports 3.35 m here. Moulds identified historically as Pholadomya, Pseudomelania and Natica are reported; these are not newly verified taxonomic determinations.

Clavellata Formation

Shell-rich limestone lies above an eroded, bored top to the Osmington Oolite. The historical Bran Point standard section is important, but its weathered state led later authors to describe nearby Black Head instead. Those Black Head measurements describe a separate section. In the BGS 2011 mapped hierarchy, Clavellata is undivided, while Sandsfoot Clay belongs to Sandsfoot Formation. The GCR’s contrasting placement is historical scheme context, rather than a measured account of this section.

Sandy Block

The lowest informal division comprises muddy, shell-fragment-rich limestone above the bored basal surface. No exact Bran Point thickness has been confirmed in the sources read.

Chief Shell Beds

Concentrations of Myophorella clavellata distinguish these beds. At Bran Point the division is reported only about half as thick as at Black Head, a direct warning against importing the neighbouring measured log.

Clay Band

A shelly muddy interruption separates shell-rich limestone from the overlying red-weathering beds. This informal marker is recognised here, but the Black Head measurement belongs to a separate section.

Red Beds

Iron-rich limestone and more muddy ooidal layers weather reddish. Published Dorset-coast ammonite holotypes include specimens from several separate exposures; an Osmington species name is not proof that a specimen came from this guide’s exact beach.

Upper Corallian exposure limit

BGS 2011 maps Clavellata undivided and puts Sandsfoot Clay in Sandsfoot Formation, consistent with the SFC Lexicon parent. The GCR uses a contrasting scheme. Its Bran Point account reports no exposed Sandsfoot Clay or Sandsfoot Grit, so neither an added local exposure nor the complete Black Head section follows from this nomenclatural correction.

References

Bgs Bencliff Lexicon
Bgs Dorset Memoir
Bgs Nothe Lexicon
Bgs Osmington Lexicon
Bgs Redcliff Lexicon
Bgs Sandsfoot Clay Lexicon
Gcr Osmington
Smugglers Parking
West Osmington