Tidmoor Point Fossil Hunting

1947 – W. J. Arkell described Tidmoor Point as one of the most celebrated fossil localities in England
Tidmoor Point was already recognised by Arkell as a classic Oxford Clay locality because of its prolific pyritic ammonite fauna. This is one of the clearest historic statements of the site’s importance.

1999 – Chapman reviewed the Oxfordian-age clays east of Tidmoor Point
Later geological work treated the Tidmoor Point area as an important Oxfordian coastal exposure within the wider Fleet Lagoon succession, helping place the fossil-bearing clays into a clearer stratigraphic framework.

2003 – Tidmoor Point was included in discussion of classic Oxford Clay sections for the Oxfordian Stage
Work on the search for a global standard for the Oxfordian Stage included the Fleet Lagoon Oxford Clay sections, specifically mentioning Tidmoor Point among the classic localities reviewed for their ammonite faunas and boundary importance.

Modern collecting – Tidmoor Point remains best known for pyritised ammonites such as Quenstedtoceras
Modern field guides continue to highlight Tidmoor Point for pyritised ammonites, especially Quenstedtoceras, together with small belemnites and crinoid ossicles weathering from the Oxford Clay.

Section Note

No formal published bed-by-bed Tidmoor Point log exists for the main ammonite-producing ground, because the exposures are low, slipped, intermittently visible and locally repeated or obscured. The numbered units used below are therefore a practical site-use scheme tied to real lithological and stratigraphic packages recognized in the literature: trace Kellaways exposures west of the point, patchy Peterborough Member mudstones in the bay west of the promontory and around Chickerell Hive Point, the slumped Stewartby Member banks of Tidmoor Point itself, and the younger Weymouth Member cropping out east of the point.

ANCHOLME GROUP

Kellaways Formation (Callovian)

Kellaways Clay Member

Bed TP1 — Trace Kellaways Mudstones West Of Tidmoor Point

Only thin and intermittent traces of the Kellaways Formation survive west of Tidmoor Point, around Butterstreet Cove and the East Fleet stream, but they are important because they underlie the Oxford Clay exposed at the point. Grey, locally silty to muddy sandstone and sandy mudstone contain shell fragments and bivalves including Goniomya literata, Nanogyra, Pholadomya and Pleuromya uniformis, together with Cadoceras and a nautiloid. Farther toward East Fleet, large septarian concretions and large body chambers of Proplanulites have been recorded. These exposures represent Kellaways Clay Member facies; a separate Kellaways Sand Member is either extremely thin or not demonstrable here. Ammonite evidence indicates Koenigi-zone levels in the East Fleet–Butterstreet sector, and loose pyritic septarian concretions near East Fleet suggest the Calloviense Zone, Enodatum Subzone. Depositional environment: quiet marine shelf mud with intermittent silt and sand influx, shell concentrations and early-cemented septaria.

Oxford Clay Formation (Callovian To Lower Oxfordian At This Locality)

Peterborough Member

Bed TP2 — Septarian Bituminous Mudstones Of The Lower–Middle Peterborough Member

Patchy outcrops on the north-east side of Chickerell Hive Point expose higher beds than the East Fleet Kellaways and belong to the Peterborough Member of the Oxford Clay. They are dark grey to brown bituminous shale and mudstone with layers of large septarian cementstone concretions, some up to about 0.6 m in diameter. Recorded ammonites include Kosmoceras castor and Kosmoceras grossouvrei, showing middle Callovian Peterborough Member strata below the classic Tidmoor Point Lamberti fauna. Typical fossils are crushed ammonites, belemnites and shell debris preserved in organic-rich shale and septarian concretions. Depositional environment: low-energy offshore mud accumulation under relatively dysoxic bottom-water conditions, with early diagenetic cementation forming the septaria.

Bed TP3 — Higher Peterborough Mudstones West Of Tidmoor Point

In the bay on the west side of Tidmoor Point, shales with Kosmoceras transitionis have been recorded from slightly higher Oxford Clay levels. These beds are still within Peterborough Member facies and represent the upward continuation from the darker bituminous septarian shales toward the upper Callovian mudstones below the slipped banks of the point itself. Lithologically they were probably still dark grey to brown organic-rich clay and shale, but the exposure is too poor and fragmentary for a formal local log. Their importance lies in showing that Peterborough Member strata continue almost to Tidmoor Point, even though the famous pyritic fauna of the point itself is mostly derived from higher beds.

Peterborough Member Note

At Tidmoor Point and its western approach, the Peterborough Member is exposed only in isolated patches, but the recorded faunas show that the locally represented interval spans at least Jason- to Athleta-age Oxford Clay. The continuous measured reference section for these beds in the district is nearby Crookhill Brickpit rather than Tidmoor Point itself.

Stewartby Member

Bed TP4 — Slipped Stewartby Mudstones Of Tidmoor Point

The low banks of Tidmoor Point itself are made of slipped pale- to medium-grey, smooth-textured, variably calcareous mudstones that correspond to Stewartby Member facies. Occasional pale phosphatic and calcareous nodules occur, many of them representing ammonite body chambers. Compared with the darker Peterborough shales below, these clays are less fissile, more blocky and generally less strongly bituminous, but slumping and weathering have blurred bedding and destroyed any continuous section. Ammonites and other macrofossils are usually preserved as pyritized internal moulds. Depositional environment: offshore marine mud deposition, still low-energy and below normal wave base, but in a paler and more calcareous facies than the Peterborough Member below.

Bed TP5 — Famous Pyritic Lamberti-Zone Foreshore Horizon

On the flat foreshore below the point, mud lumps and thin clay partings yield the classic pyritic and limonitic ammonite nuclei for which Tidmoor Point is famous. This is not a formal published bed, but a practical site-use horizon representing fossiliferous mud derived mainly from the upper Stewartby Member. The assemblage is stratigraphically mixed and largely ex situ, yet it is dominated by Upper Callovian faunas of the Quenstedtoceras lamberti Zone, with evidence of both the Henrici and Lamberti subzones; a late Athleta Zone, Spinosum-subzone component may also be represented.

Typical Fossils And Ammonites

The classic Tidmoor Point fauna includes genera such as Alligaticeras, Distichoceras, Euaspidoceras, Grossouvria, Hecticoceras, Kosmoceras, Pachyceras, Paralcidia, Peltoceras and Quenstedtoceras. More specifically recorded forms include Quenstedtoceras ex gr. lamberti, Q. leachii, ammonites of the Kosmoceras spinosum group, Euaspidoceras hirsutum, Distichoceras bicostatum and Pachyceras lalandeanum. Associated fossils include common belemnites, especially Hibolithes hastatus, nautiloids such as Paracenoceras, pyritized bivalves including Grammatodon and Nuculana, cerithiid gastropods and pentacrinoid stem fragments. The usual preservation is as small pyritic internal moulds or limonite after pyrite, whereas larger fossils commonly survive only as body-chamber nodules.

Historical And Stratigraphic Caution

Although Tidmoor Point supplied many classic Oxford Clay ammonites and almost certainly yielded some important type material, the exact source of some older museum specimens is uncertain. Some may have come from now-obliterated brickpits, Radipole Backwater or Weymouth Pottery rather than from the modern foreshore. This is one of the main reasons why the historic faunal lists cannot be converted into a spurious precise cliff log: the fauna is real and very important, but the modern exposure is mixed, slipped and only partly in situ.

Bed TP6 — Stewartby–Weymouth Boundary Interval

The Stewartby–Weymouth boundary is one of the key problems at Tidmoor Point. In other districts the top of the Stewartby Member is marked by the Lamberti Limestone or an equivalent shell bed or calcareous siltstone, and the passage can be tracked by faunal change from Callovian assemblages below into Lower Oxfordian faunas above, with supportive change from Gryphaea lituola below to Gryphaea dilatata above. No such clean marker can be demonstrated at Tidmoor Point itself. Slumping, ex situ fossils and the absence of a continuously exposed face mean that the Callovian–Oxfordian boundary cannot be fixed here with the precision possible in the better exposed Weymouth district sections farther east.

Weymouth Member

Bed TP7 — Lower Weymouth Member East Of Tidmoor Point

East of Tidmoor Point, beyond the classic ammonite-producing banks, younger Oxfordian beds of the Weymouth Member crop out on the foreshore but are commonly masked by seaweed and superficial mud. They are pale grey, smooth, blocky, calcareous mudstones, generally less organic-rich than the Peterborough Member and carrying the more typical Weymouth Member fauna of large oysters and pyritized cardioceratid ammonites. At Tidmoor Point itself these beds are not sufficiently exposed to justify a detailed numbered log, but they represent the incoming Lower Oxfordian mudstones above the famous Tidmoor Point Lamberti fauna and are probably equivalent to the basal Weymouth Member that is better exposed eastward in the Weymouth area.

Depositional Environment

The Tidmoor Point succession records offshore marine mud deposition on the south Dorset shelf during late Callovian to early Oxfordian time. The Kellaways traces represent a somewhat sandier muddy shelf with shell beds and septaria; Peterborough Member deposition took place under relatively deeper, quieter and often dysoxic bottom waters that favoured preservation of organic-rich mud and crushed ammonites; Stewartby Member sedimentation brought paler, more calcareous blocky mudstones still deposited offshore but yielding abundant pyritized cephalopods; and the basal Weymouth Member marks the incoming Lower Oxfordian pale calcareous mudstone facies with large oysters and cardioceratid ammonites. Throughout the site, slumping, low relief and foreshore reworking mean that fossils are commonly recovered from derived mud lumps and nodule debris rather than from a single clean in-situ bed.

Total Thickness Note

No reliable total thickness can be measured at Tidmoor Point itself. The locality exposes a discontinuous and partly slipped composite of the upper Kellaways Formation and Oxford Clay Formation, spanning Kellaways Clay Member traces, Peterborough Member mudstones from at least Jason to Athleta age, Stewartby Member clays yielding the classic mixed late Athleta–Lamberti ammonite fauna, and the base of the Lower Oxfordian Weymouth Member east of the point.

References

Arkell, W.J. (1947). Geology of the Country around Weymouth, Swanage, Corfe and Lulworth.
Cox, B.M. & Page, K.N. (2002). Shipmoor Point–Butterstreet Cove and Tidmoor Point–East Fleet Coast, Dorset, in British Middle Jurassic Stratigraphy, Geological Conservation Review Series 26.
Page, K.N. (2002). Crookhill Brickpit, Dorset, in British Middle Jurassic Stratigraphy, Geological Conservation Review Series 26.
Chapman, N.D. (1997). Ammonites from the Oxford Clay near Budmouth School and Tidmoor Point, Weymouth, and their bearing on the Callovian/Oxfordian boundary.
Callomon, J.H. & Cope, J.C.W. (1995) on the Callovian–Oxfordian boundary beds and Oxford Clay succession of the Weymouth district.
British Geological Survey Lexicon of Named Rock Units: Kellaways Formation, Kellaways Clay Member, Kellaways Sand Member, Oxford Clay Formation, Peterborough Member and Stewartby Member.
British Geological Survey Stratigraphical Framework Series: Oxford Clay Formation.
British Geological Survey Special Memoir: South Dorset and south-east Devon and its World Heritage Coast.