Gristhorpe

At exceptionally low water, its sandstone can be seen beyond and below the Millepore ledge at Yons Nab. Channel sandstones cut older rooted deposits. Its exposure is partial, so a full regional member thickness would misrepresent the foreshore.

Plant horizon below the Millepore Bed

Harris’s recorded collecting horizons include a plant-bearing level about 5 m below the Millepore Bed. That historical position identifies a collecting horizon within the Sycarham Member; it is not an additional formal member or a guarantee that the layer is presently exposed.

The current BGS scheme includes both the Millepore Bed and Yons Nab Beds here. Some published sections place the Yons Nab Beds in the base of the Gristhorpe Member; those alternatives concern the boundary’s position, not two separate copies of the beds.

Millepore Bed — lower calcareous sandstone

Cross-bedded calcareous sandstone forms the lower part of the marine ledge. The GCR foreshore account describes about 7 m beneath its limestone cap, exposed only at very low tide. This exposure-based figure differs from the shorter stratotype interval quoted in the BGS Lexicon and should not be added to that separate measurement.

Millepore Bed — upper ooidal and bioclastic limestone

About 2 m of sandy ooidal limestone caps the foreshore ledge in the GCR account. Cross-bedding shows movement of sediment in shallow marine water. Fragments of the bryozoan historically called Millepora straminea, now commonly referred to as Haploecia straminea, explain the traditional name, although fragments are not abundant at every Yons Nab exposure.

Sideritic cap to the Millepore interval

The BGS type-section description records a sideritic nodule band marking the top of the informal Millepore Bed. It must be kept distinct from the younger ironstone-bearing layers within the Yons Nab Beds.

Yons Nab Beds — lower muddy interval

Shales with occasional ironstone bands and nodules overlie the Millepore ledge. A thin ripple-marked, burrowed sandstone interrupts the muddier lower beds. The transition records quieter conditions than the underlying mobile carbonate sand; fossils are concentrated unevenly rather than occurring throughout every shale layer.

Yons Nab Beds — upper flaggy interval and shell ledge

Alternating sandstone and shale pass upwards to a prominent ledge. Its iron-stained upper surface carries internal bivalve moulds, including Pholadomya, Trigonia and Pteroperna, with some shells in life position and large U-shaped burrows. The GCR gives approximately 5 m for the Yons Nab Beds, while the BGS stratotype account quotes 4 m; these are separately scoped descriptions.

Sandstones, siltstones and mudstones above the marine interval preserve roots, plant fragments and channel structures. The classic Gristhorpe Plant Bed lies low in this member. The non-marine beds are Bajocian and should not be assigned to the younger Scalby Formation merely because both contain plants.

Rooted and bioturbated basal sandstone

A resistant sandstone above the Yons Nab ledge contains iron-rich concretions and poorly preserved bivalves near its base, with substantial bioturbation higher up. The local field guide also records vertical roots. The published GCR account describes an approximately 4.5 m sandstone interval before the main plant bed; channel cutting makes simple layer-by-layer correlation unreliable around the point.

Channel sandstone beneath the plant bed

Near Yons Nab a north-east–south-west-trending channel cuts down into the Yons Nab Beds. Sandstone fill displays lateral-accretion and other channel structures. East of the point, ironstone-bearing sandstone with plant debris has yielded poorly preserved freshwater bivalves recorded as Unio. The fill is a lateral feature, not an extra bed to stack across the whole bay.

Gristhorpe Plant Bed — basal plant-debris claystone

The principal plant bed includes a lower claystone rich in plant fragments. It represents fine sediment accumulated as a channel was abandoned or in a lagoonal setting receiving nearby river-borne material. Interpretations vary in emphasis, but all require retention of the local sedimentary context rather than a generic swamp label.

Alternative three-part plant-bed description

The 2016 pollen study describes a plant bed up to 1 m thick between crevasse-splay sandstones: soft basal clay with the best-preserved specimens, an overlying clay-rich siltstone, then an iron-rich nodular interval with fewer plants. This is an alternative description of the same local package, not three extra beds to stack on the GCR’s differently worded subdivisions.

Gristhorpe Plant Bed — pale, soft claystone

The soft grey-white middle claystone preserves particularly fine dark leaves and reproductive structures. Delicate fossils include Sagenopteris, fern foliage and conifer shoots; Caytonia seed organs are central to the site’s scientific history. Preservation and floral composition change laterally within only a few metres. The classic bed is described as about 0.5 m in the palaeobotanical account; other accounts report around 0.6–1 m, and a larger 2.5 m estimate probably incorporates adjacent strata.

Gristhorpe Plant Bed — hard micaceous silty claystone

The upper part is firmer and more micaceous. U-shaped Diplocraterion burrows indicate renewed marine influence. Medium and darker grey silty rock generally carries more fragmented plant material than the pale claystone, so not all plant-bearing stones have equivalent preservation.

Black layers adjoining the Gristhorpe Bed

Historical collecting records distinguish a black layer below and another above the Gristhorpe Bed. They are named collecting horizons rather than a measured pair of formal beds, and must not be silently equated with the three lithological subdivisions of the main plant bed.

Bioturbated carbonaceous sandstone lens

A plant-rich, strongly burrowed sandstone lens occurs above the main plant-bearing interval. Its lens geometry and lateral variability are significant; the field guide illustrates this feature separately from the plant bed itself.

Rooted sandstone and upper sheet sandstones

Higher beds include sandstone with vertical horsetail roots and rooted sheet sandstones. Loaded bases and contorted lamination show deformation of wet sediment, while rooting records later colonisation. These upper horizons should not be used to claim that horsetail foliage is common in the classic Gristhorpe Plant Bed, whose flora is separately described.

In the GCR interpretation, about 1.3 m of finely laminated siltstone with ripple-drift and scour-and-fill structures lies at the base. A later field guide treats a roughly 1.4 m cross-bedded sandstone beneath its narrower Scarborough interval as upper Gristhorpe strata, while noting the alternative Helwath Beck assignment. These are alternative contact schemes and should not be duplicated as separate stacked units.

The GCR assigns the overlying approximately 2 m of soft muddy shelly limestone or shale to this member. Crushed Lopha, Meleagrinella and Trigonia occur with gastropods and belemnites; an ironstone-concretion layer lies near the top. The 2006 field guide describes a 2.8 m limy-shale section with two upper ironstone bands at Low Red Cliff. These are local section descriptions, not grounds for importing thicker northern members.

The basal channel sandstone at Yons Nab has been recognised as Moor Grit by some workers, whereas another sedimentological account regarded that member as absent. Preserve the documented uncertainty rather than forcing a regionally uniform basal unit.

Large basal channel and later nested channel

The channel east of Yons Nab erodes the top of the Scarborough Formation. A pyrite-nodule conglomerate marks its base; a smaller later channel cuts the fill and begins with a shale-clast conglomerate. These distinct erosional surfaces were described by Black in 1928 and are important field observations.

The upper Scalby succession comprises finer floodplain deposits and sandstone bodies. Plant-rich mudstone and siltstone contrast with coarser channel sand. A published loose block from Gristhorpe Bay preserves underprinted dinosaur tracks and disturbed bedding assigned to this member; its exact original bed cannot be reconstructed from the loose specimen alone.

Other historically named Scalby plant horizons

Harris’s locality register separately identifies Black’s Upper Estuarine Plant Bed, a grey-limestone occurrence, a bed above a channel, Hill’s named plant bed, Kendall’s cliff-foot bed and Mell–Casty Hill occurrences numbered 1, 3 and 4. These are historical collecting records across the Gristhorpe tract, not a continuous measured log or present-day access instructions. Their recorded names should be retained with locality provenance without inventing thicknesses or correlations.

Unit α1 — basal iron-rich limestone

A burrowed boundary rests on Scalby mudstone. The coastal composite range is 0.14–0.37 m for greenish-grey, reddish-weathering berthierine/sideritic limestone, becoming muddier towards Cunstone Nab. Bivalves dominate, including Entolium, oysters and burrowing forms; rare Macrocephalites provides age evidence. This range is for the described coast, not one universal Gristhorpe measurement.

Unit α2 — pale shelly limestone

About 0.15 m of pale-grey shell limestone is variably ooidal and locally micritic. Bivalves again dominate, but gastropods and Macrocephalites also occur. The named subdivision is part of the coastal comparative scheme; its presence must not be assumed at every separate exposure.

Unit α3 — erosive sandy limestone

This hard sandy limestone is developed in Gristhorpe Bay, where it reaches about 0.18 m, but is absent below Red Cliff. Its erosional base and bivalve-rich fabric record a separate phase of deposition; rare Macrocephalites kamptus was reported. The lateral absence matters as much as the thickness.

Unit α4 — shale transition

A thin silty or sandy shale grades towards the overlying Cayton Clay. It contains Entolium and Meleagrinella; a local Gristhorpe record includes the brachiopod Microthyridina. No single thickness is imposed on this variable transition. This is alpha 4 of the historical Cornbrash scheme. The modern BGS boundary at the change from limestone to shelly mudstone places this transitional shale with basal Cayton Clay; the historical numbering does not establish an unchanged modern formation assignment.

The well-developed type section is at Red Cliff, west of the plant-bearing point. Its numbered subdivisions can be compared into Gristhorpe Bay, but β2–β5 are successively truncated towards Cunstone Nab, where the member disappears. Composite maxima must not be drawn as a measured Gristhorpe stack.

Unit β1 — lower pale sandstone

Pale yellowish sandstone grades up from Cayton Clay and contains hard calcareous concretions. Most rock is sparsely fossiliferous, but some nodules contain small bivalves; Kepplerites is recorded near the top. The GCR coastal description gives a maximum 6.85 m, not an exposure-wide constant.

Unit β2 — iron-ooid sandstone

Rusty orange–brown sandstone contains berthierine ooids and a richer fauna than β1. Entolium, oysters and Modiolus occur with Kepplerites, Proplanulites and other ammonites. Its published maximum is 2.35 m in the comparative coastal succession.

Unit β3 — less ooidal fossiliferous sandstone

The next sandstone has few or no ooids, and a varied bivalve and ammonite assemblage with occasional belemnites. The coastal maximum quoted is 1.98 m; the bed is among those removed eastwards beneath the Langdale erosion surface.

Units β4 and β5 — impersistent upper Redcliff beds

At Red Cliff, β4 is an intermittently developed, pale-weathering ooidal sandstone up to 0.55 m, with shelly concretions and rare ammonites. A possible β5 soft sandstone, up to 0.65 m, is less securely recognised. These Red Cliff records are comparative markers, not confirmed complete beds everywhere in Gristhorpe Bay.

The sandstone and siltstone above the Redcliff beds rests on an erosional surface. It is absent at Red Cliff and becomes developed eastwards; at Cunstone Nab a 2.35 m fine–medium sandstone with ripple lamination and clay wisps rests on thinned Cayton Clay after the entire Redcliff Rock Member has been removed. That eastern measurement is retained as lateral context.

Cunstone Nab ammonite level within γ1

At Cunstone Nab, about 0.65 m above the Langdale base, poorly preserved Erymnoceras and Kosmoceras give Middle Callovian age evidence. This is an eastern reference horizon, not a find guarantee for the Gristhorpe plant-bed foreshore.

Hard greenish-grey, iron-ooid-rich rock is developed at Red Cliff. Eastwards in Gristhorpe Bay a tough ooidal limestone yields Quenstedtoceras; bivalves accompany comparatively uncommon age-diagnostic ammonites. The overlying contact varies locally and should not be represented as one universally complete boundary section.

Only the upper Oxford Clay succession is present in the Cleveland Basin. The boundary interval includes latest Callovian and earliest Oxfordian faunas; a lithostratigraphic formation boundary should not be assumed to coincide exactly with the stage boundary.

Superficial deposits, fallen blocks and access

Glacial deposits and landslips conceal parts of the cliff, while sand and shingle cover the foreshore intermittently. Fallen blocks may expose fossils clearly but lose their exact position in the sequence. None of the geological descriptions implies permission to excavate fixed beds or enter unstable cliff areas.

Plant organs and modern microscopic studies

Mesofossils collected in 2003 at TA 083842 were studied by Crane and Herendeen in 2009, revealing fine bennettitalean reproductive structures. Later work examined pollen from Androstrobus manis specimens 1370, 8509 and 7560 at Utrecht. Associations link Nilssonia compta, A. manis, Beania gracilis and Deltolepis crepidota, but their stem is unknown; Harris’s 1961 whole-plant reconstruction concerned a related plant, not this complete local organism.