Runswick Bay Fossil Hunting

Runswick Bay is not a single clean cliff log. The centre of the bay is largely masked by landslip and glacial drift, bedrock is best seen on the rocky scars and foreshore around the village, Hob Holes and the north side of the bay, and the Wrack Hills plant bed occurs in a slipped undercliff slice rather than as an upright continuous cliff section.

Formal published Howarth bed numbers are retained below for the Cleveland Ironstone and Whitby Mudstone. Above the Lower Jurassic, the Dogger and Saltwick beds are described by named horizons and practical site-use intervals only, because at Runswick they are thin, faulted or slipped and do not form a single formal local bed-by-bed section.

LIAS GROUP

Cleveland Ironstone Formation (Upper Pliensbachian)

Cleveland Ironstone Section Character At Runswick Bay

Only the upper seam-rich part of the Cleveland Ironstone is usually accessible in the main Runswick Bay scars. The lower part of the formation, together with the underlying Staithes Sandstone Formation, is much better developed north-westward toward Port Mulgrave and Staithes. At Runswick the succession is dominated by repeated shallow-marine mudstone, siltstone and ooidal ironstone cycles with economically important seams and abundant late Pliensbachian ammonites.

Bed 41 — Ferruginous Shale With Basal Shell Bed (0.25 m)

A thin ferruginous shale resting on a basal shell bed, immediately beneath the Pecten Seam. Although not thick, it is an important marker because it introduces the upper seam-bearing part of the Runswick ironstone section. The shell concentration and ferruginous matrix indicate a marine omission or condensation surface within the otherwise cyclic muddy-silty shelf succession.

Beds 42–50 — Pecten Seam (c. 1.3 m)

The Pecten Seam consists of ironstone developed in five bands separated by shale partings, with shell beds in the intervening mudstones. It is one of the classic Cleveland ironstone seams and records repeated very shallow marine ironstone precipitation at the top of short upward-coarsening cycles. Bivalves are common, and the seam is one of the most useful field markers on the Runswick foreshore because its ironstone ledges stand out clearly from the softer shales.

Bed 51 — Black Hard Shale (0.45 m)

A dark, relatively hard shale interval above the Pecten Seam. Its more compact, bituminous and ferruginous character makes it a useful separator below the Main Seam. This bed records a return to quieter muddy shelf sedimentation after the ironstone-forming episode represented by the Pecten Seam.

Beds 52–53 — Main Seam Bottom Block (c. 1.7 m)

The lower part of the Main Seam is a thick ironstone with a shale separator. Burrows and rolled fossils show that the sea floor was intermittently firm and reworked, and ammonites, though uncommon, include forms close to Pleuroceras paucicostatum. This is one of the most economically important seams of the Cleveland Ironstone Formation and a classic example of berthierine-ooidal ironstone developed near the top of a small-scale marine sedimentary cycle.

Bed 54 — Main Seam Top Block (0.75 m)

The upper block of the Main Seam is a conspicuous ironstone marked by branching burrows, rolled fossils and a rich ammonite fauna. Recorded species include Pleuroceras paucicostatum, P. elaboratum, P. apyrenum, Amauroceras ferrugineum and A. lenticulare, with a unique record of Canavaria aff. cultraroi. The combination of burrowing, reworking and concentrated fauna shows repeated winnowing and condensation on a shallow marine shelf during the uppermost Pliensbachian.

Bed 55 — Hawskerense Shale (1.23 m)

A shale bed with common Pleuroceras hawskerense above the Main Seam. It is one of the clearest ammonite-bearing mudstone intervals in the top of the ironstone succession and represents quieter offshore mud deposition between ironstone-forming episodes. This bed is important biostratigraphically because it ties the upper Cleveland Ironstone at Runswick into the late Pliensbachian Pleuroceras-bearing interval.

Bed 56 — Upper Unnamed Ironstone (0.22 m)

A thin ironstone with an irregular top, commonly yielding Pleuroceras hawskerense and rare Protogrammoceras turgidulum. The bed forms a thin but distinctive hard marker immediately below the topmost shale of the formation. Like the Main Seam below, it represents a condensed iron-rich marine horizon at the top of a short cycle.

Bed 57 — Sandy Micaceous Shale (0.45 m)

The top of the exposed Cleveland Ironstone at Runswick Bay is a sandy micaceous shale with Pleuroceras hawskerense. Its more silty and micaceous character signals the gradual change toward the overlying Whitby Mudstone boundary interval, while still retaining the ammonite fauna typical of the latest exposed ironstone beds. Taken together, the exposed Runswick Cleveland Ironstone represents the uppermost late Pliensbachian ironstone cycles, broadly within the upper Margaritatus–Spinatum interval.

Whitby Mudstone Formation (Lower Toarcian)

Grey Shale Member

Grey Shale Member Section Character At Runswick Bay

The Grey Shale Member is exceptionally important at Runswick Bay because it lies within the Yorkshire type area for the Tenuicostatum Zone and lower Toarcian ammonite biohorizons. The shore section is best followed as a composite across different scars: the basal beds occur near low-tide exposures beside the top of the ironstone series, the Six Red Nodule beds are conspicuous on the reef, and the upper dogger-bearing beds are seen farther south below Hob Holes.

Beds 1–3 — Sulphur Band And Basal Boundary Shales

The base of the Whitby Mudstone Formation is marked by the Sulphur Band, a thin laminated pyritic mudstone with small jet lenses, above which lie the lowest grey boundary shales of the Grey Shale Member. Bed 3 is a continuous sideritic calcined mudstone weathering red in patches, and elsewhere on the Yorkshire coast this level yields some of the earliest typical Toarcian ammonites, including early dactylioceratids and Protogrammoceras paltum. At Runswick these basal beds are thin but critical because they carry the section across the Pliensbachian–Toarcian transition.

Beds 4–17 — Six Red Nodule Beds (c. 2.75 m)

This interval is dominated by grey shales with six closely spaced bands of calcareous and sideritic concretions, many weathering conspicuously dark red. Belemnites and bivalves occur through the shales, and the nodule bands make this one of the easiest lower Whitby Mudstone intervals to trace on the Runswick scars. The lithology indicates relatively quiet marine mud deposition interrupted by repeated early diagenetic concretion growth, probably linked to subtle pauses in sedimentation and organic decay fronts within the mud.

Bed 18 — Crosbeyi Bed (0.38 m)

A grey shale with small calcareous concretions and frequent Dactylioceras crosbeyi. This bed is one of the classic lower Toarcian biohorizon markers and shows how finely resolved the Runswick Bay Grey Shale succession can be when ammonites are collected carefully from the nodules rather than from weathered shale alone.

Beds 19a–19c — Clevelandicum Interval (c. 1.27 m combined)

These three beds comprise laminated bituminous shale below, a very thin grey shale with red-weathering lenticles and common Dactylioceras clevelandicum in the middle, and a thicker grey shale above. The interval captures a subtle but important environmental and faunal shift within the lower Grey Shale Member, with more organic-rich lamination at the base passing up into more typical grey shales with localized concretionary concentrations.

Beds 20–27 — Tenuicostatum Nodular Shales (c. 2.75 m)

Grey shales with several bands of small calcareous nodules and a basal double band of large calcified lenticular masses weathering red. Dactylioceras tenuicostatum is common in the smaller nodules, and the neotype of the species came from Bed 22. This is one of the most important ammonite intervals in the entire Yorkshire Lower Jurassic, and at Runswick it shows the full value of the Grey Shale Member for high-resolution biostratigraphy.

Beds 28–31 — Semicelatum Dogger-Bearing Shales (c. 3.55 m)

Grey shales with three bands of large calcareous concretions, including a conspicuous double row at the base that is often pyritic. Well-preserved Dactylioceras semicelatum, large belemnites and Cenoceras astacoides are recorded from these beds. The prominent doggers of Beds 28 and 30 are among the easiest upper Grey Shale markers to recognize on the shore and show repeated early cementation within otherwise soft offshore mud.

Bed 32 — Antiquum Shell-Bed Interval (1.85 m)

A grey shale with occasional flat calcareous nodules and widespread shell beds, especially near the base, where crushed Tiltoniceras antiquum may be abundant. Dactylioceras semicelatum and the thin-shelled bivalve Bositra also occur. Bed 32 forms the top of the Grey Shale Member and records a fossil-rich offshore mudstone phase immediately below the giant concretionary beds that mark the base of the Jet Rock.

Mulgrave Shale Member

Mulgrave Shale Member / Jet Rock Section Character At Hob Holes

In the middle of Runswick Bay, around Hob Holes, the Mulgrave Shale Member is classically exposed from just below the top of Bed 32 up to the Whalestones, with higher beds rising behind in the cliff and old jet workings. The lower part is the famous Jet Rock, a dark bituminous, pyritic mudstone succession with giant concretions, abundant ammonites and belemnites, and the jet horizons for which the Yorkshire coast became famous.

Bed 33 — Cannon Ball Doggers (0.15 m)

A narrow but very distinctive row of spherical concretions, commonly up to about 0.18 m across, at the base of the Mulgrave Shale Member. Elegantuliceras elegantulum is particularly characteristic, including both macroconchs and microconchs. This bed marks the abrupt change from the grey nodular shales below into the darker, more bituminous Jet Rock facies above.

Bed 34 — Elegantulum Shales (2.7 m)

Grey bituminous shale with frequent calcareous concretions and common Elegantuliceras elegantulum. The shale is darker, more pyritic and more organic-rich than the Grey Shale Member below, showing the shift into lower oxygen bottom-water conditions that characterizes the Jet Rock. Fossils are commonly preserved within concretions rather than in the soft shale matrix.

Bed 35 — Whalestones (0.9 m)

Large ovoid concretions, some up to several metres long, together with many smaller calcareous concretions in grey bituminous shale. Cleviceras exaratum is common, with less frequent Harpoceras serpentinum and Phylloceras heterophyllum. The enormous concretions make this one of the most striking beds in the Runswick Bay section and show very early cement growth within highly organic muddy sediment.

Bed 36 — Exaratum Shales (1.08 m)

A grey bituminous shale above the Whalestones in which Cleviceras exaratum is characteristic. The bed continues the low-oxygen offshore mudstone facies of the Jet Rock and forms part of the ammonite-rich exaratum interval that makes the lower Mulgrave so important for lower Toarcian correlation.

Bed 37 — Curling Stones (0.3 m)

A row of spheroidal calcareous concretions with pyritic skins, usually up to about 0.45 m across. Fossils include Cleviceras elegans, Harpoceras serpentinum, Dactylioceras semiannulatum, D. crassoides and Phylloceras heterophyllum. This is another classic jet-rock dogger line and a very useful field marker on the scar.

Bed 38 — Upper Pseudovertebrae Shale (1.54 m)

Grey bituminous shale with occasional concretions and the Upper Pseudovertebrae horizon about 0.3 m above the base. Cleviceras elegans, Harpoceras serpentinum and occasional Phylloceras heterophyllum occur. Like the other named jet-rock horizons, the bed represents dark organic-rich offshore sedimentation under restricted bottom-water oxygenation, with early diagenetic concretion development around fossil concentrations.

Bed 39 — Top Jet Dogger (0.23 m)

A continuous argillaceous limestone bed marking the top of the main Jet Rock concretionary interval. Cleviceras elegans and dactylioceratids are recorded. This hard, laterally persistent band is one of the clearest upper Jet Rock field datums and forms a natural marker for the transition toward the higher bituminous shales.

Bed 40 — Millstones (0.3 m)

Giant lenticular calcareous concretions, sometimes several metres across, set in grey bituminous shale. Fossils include Cleviceras elegans, Hildaites murleyi and dactylioceratids. The sheer size of the concretions is remarkable and reflects intense localized early cementation within organic-rich muds on the Toarcian shelf.

Beds 41–47 — Bituminous Shales

Above the Millstones the Mulgrave passes into a thicker interval of bituminous shales with fewer large calcareous concretions, but with widespread pyrite, local sideritic bands, fossil wood including large logs, and abundant crushed or pyritized ammonites. Bed 41, the lowest part of this interval, is exposed on the foreshore below the village, while higher beds rise into the cliff behind the old jet workings. Harpoceras falciferum, dactylioceratids, Phylloceras heterophyllum and related forms are characteristic. Hard jet occurs in the upper part of the Jet Rock, and softer jet continues into the higher bituminous shales, showing prolonged organic-rich accumulation under dysoxic to anoxic bottom conditions.

Bed 48 — Ovatum Band (c. 0.25 m)

A double row of large doggers, commonly sideritic and weathering dark red, with pyrite masses and local concentrations of belemnites. Ovaticeras ovatum, dactylioceratids and Phylloceras heterophyllum have been recorded. This is the classic topmost marker of the Mulgrave Shale Member and forms the boundary horizon below the non-bituminous Alum Shale Member.

Alum Shale Member

Alum Shale Section Character At Runswick Bay

The Alum Shale Member is less continuously exposed in the main bay than the Grey Shale and Jet Rock, but lower parts were historically worked in the Runswick alum quarry north of the village and can be followed northward toward Kettleness. The member consists of grey non-bituminous mudstones with abundant nodule lines and sideritic horizons, and it is sharply truncated upward by the Aalenian Dogger unconformity.

Beds 49–50 — Hard Shales (c. 6.35 m combined)

The base of the Alum Shale Member comprises harder, darker grey non-bituminous shales with sporadic calcareous doggers, pyrite masses and a continuous sideritic mudstone band weathering red. Crushed Dactylioceras commune is typical, and nodules higher in this interval have also yielded D. temperatum. These beds record the end of the strongly bituminous Jet Rock regime and the return to better oxygenated, though still quiet, offshore mud deposition.

Beds 51–54 — Lower Main Alum Shales (c. 7.1 m combined)

Grey shales with scattered doggers, large sideritic lenses weathering red, pyrite masses and fossiliferous nodule bands. Dactylioceras commune is abundant, with D. praepositum, D. temperatum, Hildoceras sublevisoni, Frechiella subcarinata and Phylloceras heterophyllum. In the Runswick alum quarry, nodules 22–24 feet above the Ovatum Band yielded many D. commune, and a fallen nodule from about the middle of Bed 51 produced Hildoceras sublevisoni. These beds are the key lower Alum Shale ammonite interval at Runswick.

Higher Alum Shale And Cement Shales

Above Bed 54 the Whitby Mudstone Formation continues upward through softer Main Alum Shales into upper Cement Shales with cementstone nodules, but this higher part is not shown as a clean continuous Runswick Bay cliff section. Regionally the Alum Shale Member is around 27 m thick at Whitby and up to about 37 m at Blea Wyke, but farther north-west many higher Toarcian zones are removed beneath the Dogger unconformity. At Runswick the important point is that the Aalenian transgression cuts down sharply onto this member.

RAVENSCAR GROUP

Dogger Formation (Aalenian)

Bed S1 — Dogger Erosion Surface And Ferruginous Sandstone

At Runswick Bay the Dogger is not exposed as a long, simple tabular bed but as thin, faulted or slipped remnants around the south-side undercliff and Caldron Cliff. Lithologically it is the usual Yorkshire coastal ferruginous sandstone to ironstone facies, grey-weathering to yellow-brown, locally with phosphatic pebbles and burrows descending into the underlying Alum Shale. Regionally the Dogger on the Yorkshire coast is generally only about 0.75–2 m thick. It records a marked Aalenian marine transgression and a pronounced disconformity above the Lower Jurassic mudstones, with substantial omission of upper Toarcian strata.

Saltwick Formation (Aalenian)

Saltwick Section Character At Wrack Hills

The Saltwick Formation at Runswick Bay is represented not by a straightforward foreshore section but by a slipped undercliff slice at Wrack Hills. Excavation and mapping showed that the fossil-bearing undercliff had moved downslope from the main cliff some distance behind, so the plant bed must be treated as a landslipped but still geologically meaningful fragment of the lower Ravenscar Group.

Bed S2 — Basal Saltwick Mudstone–Siltstone Interval

Below and around the plant-bearing horizon, the Saltwick Formation consists of grey mudstones, yellow-grey siltstones and fine- to medium-grained sandstones of non-marine to paralic character. Rootlets, plant debris, sideritic nodules and small channelized sand bodies are typical of the formation more generally, and at Runswick the same facies are inferred for the slipped undercliff slice that contains the flora. The interval records floodplain, crevasse-splay and abandoned-channel sedimentation on a low-relief delta plain.

Bed S3 — Wrack Hills Plant Bed

The Wrack Hills plant bed is a siderite-rich ironstone horizon within the Saltwick Formation and is the most important Middle Jurassic plant locality in Britain. The flora is exceptionally well preserved in three dimensions as a result of very early diagenetic lithification, and more than 30 species have been recorded. Characteristic plants include Williamsonia gigas, cones of Androstrobus, and the male cone of Elatides thomasii; freshwater mussels traditionally referred to Unio occur with the flora and confirm the non-marine character of the deposit. The bed represents quiet-water accumulation in a floodplain pool, abandoned channel or very low-energy channel-margin setting where plant debris was rapidly entombed and sideritized before collapse and decay.

Bed S4 — Overlying Channel Sandstones And Rooted Floodplain Beds

Above the plant bed, the Saltwick succession would originally have continued with erosive channel sandstones, rooted mudstones, thin coaly horizons and additional plant-bearing beds typical of the lower Ravenscar Group, though only a slipped remnant is preserved at Runswick. These higher beds record repeated switching between active river channels, abandoned channels, floodplain lakes and vegetated muddy surfaces. The Wrack Hills flora is therefore part of a broader fluvial to delta-plain succession rather than an isolated botanical anomaly.

Depositional Environment

Runswick Bay records a major environmental change from late Pliensbachian shallow-marine ironstone cycles in the Cleveland Ironstone Formation, into lower Toarcian offshore mud sedimentation of the Whitby Mudstone. The Grey Shale Member represents relatively open muddy shelf conditions with abundant ammonites and repeated concretion growth; the Mulgrave Shale Member records dark bituminous, pyritic, commonly dysoxic to anoxic offshore conditions that produced the Jet Rock and its jet horizons; the Alum Shale Member shows recovery to more oxygenated grey shelf mudstones before the Aalenian Dogger transgression cut down across the upper Toarcian. Above the unconformity, the Saltwick Formation records fluvial, floodplain and abandoned-channel environments with lush vegetation, freshwater mussels and localized sideritic plant preservation.

Total Thickness And Nature Of The Section

Runswick Bay is a composite locality rather than a single measurable vertical section. The main shore section shows the uppermost exposed Cleveland Ironstone and a substantial lower Toarcian Whitby Mudstone succession from the Sulphur Band through the Grey Shale and much of the Jet Rock, with lower Alum Shale worked and exposed north of the village; above that, a thin Dogger and a slipped slice of lower Saltwick occur on the south-side undercliff. The locality is therefore unusually valuable because it combines classic ammonite-controlled Lower Jurassic marine beds with a famous Aalenian plant horizon in one small bay.

References

Howarth, M.K. (1955, 1962, 1973, 1992) on the Cleveland Ironstone, Grey Shale, Jet Rock and Alum Shale ammonite stratigraphy of the Yorkshire coast.
British Geological Survey Lexicon entries: Cleveland Ironstone Formation, Staithes Sandstone Formation, Whitby Mudstone Formation, Grey Shale Member, Mulgrave Shale Member, Alum Shale Member, Dogger Formation and Saltwick Formation.
Geological Conservation Review and Geologists’ Association material on Staithes to Port Mulgrave and the Runswick Bay / Wrack Hills Middle Jurassic plant site.
Rawson, P.F. & Wright, J.K. on the Yorkshire Jurassic coast and biostratigraphic correlation.
Van Konijnenburg-van Cittert, J.H.A. & Morgans, H.S. on the Wrack Hills plant bed and its three-dimensionally preserved Saltwick flora.
Yorkshire coast field guides and memoirs dealing with the Lower Jurassic of Runswick Bay, Kettleness and the Whitby district.