Woodland Bay

Sandy limestones and shale, about 75 m in the Girvan foreshore section. Around the northern Ardwell–Whitehouse transition, detrital sandy limestones alternate with grey and green shale. The change from the underlying Ardwell rocks is sharp; this lower boundary lies at the southern geographical edge of the present account. Grading and Bouma divisions, the successive sedimentary structures within individual turbidites, indicate relatively proximal turbidity currents; some flows deposited composite, multiple units. Basal fragments include pieces eroded from older rocks, with fragments of Ardwell Flags recorded in the oldest bed.

The limestones contain broken shelf-derived fossils, including trilobites, brachiopods and corals, carried downslope with the sediment. The Tretaspis ceriodes species group is recorded, and conodont evidence includes the Amorphognathus superbus Zone. These transported remains record the source of the sediment as well as its final deep-water resting place.

Thin sandstone beds in grey-green shale, more than 45 m on the foreshore. The Threemile Formation, written “Three Mile” in older accounts, is less calcareous than Southshore. Its thin, very persistent sandstone beds and intervening shale are interpreted as more distal turbidites. Two or three thicker sandstone beds provide useful markers in the older foreshore description; these are unnamed beds, with no separately verified thicknesses, rather than formal members.

A missing shore interval and a nearby burn exposure. At Whitehouse, a reverse fault running approximately parallel to the strike cuts out the Penwhapple Formation, so it cannot be shown as a complete exposed interval between Threemile and Myoch on that shore. Dark shale with occasional sandstone is recorded in Myoch Burn a short distance upstream of the road, where graptolites indicate the linearis Biozone. The approximately 90 m section measured inland in Penwhapple Burn is a regional comparison, not a thickness for this coast. Historical references to the Byne-Hill Burn succession should be read with their original locality descriptions rather than assumed to identify every modern burn exposure.

Red and green silty mudstones dominate the Myoch Formation, with coarser sandstone and conglomerate near its base and locally within it. The BGS Lexicon gives a regional thickness of about 30 m and identifies Whitehouse foreshore as its type section. Faulting and lateral facies changes prevent that figure from being used as a measured thickness for each exposure. In the older British scheme the formation spans the Caradoc–Ashgill transition; modern discussion should retain the broader Late Ordovician context rather than treat the historical boundary as a universally exact fossil-zone boundary.

Basal conglomerate near Myoch Bridge. The fuller local succession begins with a coarse conglomerate containing a sandstone and mudstone matrix. It is distinct from the finer red and green mudstones higher in the formation. No secure local thickness is given for this basal unit.

Near-basal sandstone at Whitehouse. Pebbly, cross-bedded sandstone is exposed in a restricted low-water area southwest of the central shore. Its sedimentary structures indicate currents arriving from the north-west. The reverse fault removes other parts of the lower succession, leaving a limited view of this early coarse sedimentation.

Port Cardloch sandstone. Thick cross-bedded sandstone beds with shale clasts occur beside the road in a faulted patch at Port Cardloch. They show the near-basal coarse facies more clearly than the restricted Whitehouse exposure and are interpreted as channel-fill sands associated with early submarine-fan development. They are part of the Myoch succession, not a separate formation, and no numerical thickness is assigned to this patch.

Lower green silty mudstones. Above the coarse basal deposits, the lower fine-grained succession comprises green silty mudstone. This is the lowest of the three colour divisions described in the modern field account and should be distinguished from the overlying colour-banded transition and red upper mudstones.

Red and green transitional mudstones. Alternating red and green bands mark the passage from the lower green mudstones into the predominantly red upper interval. The banding is a useful field distinction, but the published coastal accounts do not establish a separate formal member or a reliable modern measured thickness for it.

Upper red silty mudstones and their deep-water fauna. Predominantly red silty mudstone contains an important in-situ deep-water association in its upper part. The cyclopygid–atheloptic association includes blind or small-eyed bottom-dwelling trilobites alongside large-eyed swimming forms, together with small brachiopods of the Foliomena community. Recorded trilobites include Dionide, Novaspis, Nankinolithus, Cyclopyge, Degamella, Microparia, Psilacella, Ellipsotaphrus, Telephina, Bohemilla and Raphiophorus; these names do not imply that every genus had the same mode of life.

The red mudstones are interpreted as fine overbank sediment deposited on a deep-water submarine fan. Their iron-oxide colour, considered alongside the benthic fauna, indicates an oxygenated sea-floor substrate rather than a terrestrial red-bed environment. The ecological interpretation is more secure than any attempt to turn it into an exact water-depth measurement.

Local sandstone–shale channel fill in the red mudstones. Near the road at Myoch Bay, a channel-fill package lies within the red beds. It occupies a plunging syncline in which red mudstones reappear in the core; a thrust truncates the exposure seawards, and an adjacent Myoch section lacks the channel fill. Its local thickness has not been established. Although the Balgaverie Member is a named upper-Myoch channel-fill unit in the inland Penwhapple Burn type context, the published evidence used here does not justify assigning that name or its approximately 8 m thickness to this coastal package.

Sandstone-filled fissures. Sandstone dykes cut the Myoch mudstones on the Whitehouse shore. These are sediment-filled fissures associated with disturbance of the sedimentary succession. They are neither additional layers in the normal depositional sequence nor the much younger igneous dolerite dykes also found along the coast.

The Mill Formation records renewed introduction of coarser and calcareous sediment, together with graptolitic shale. It is not represented by the same succession at every exposure: Myoch Bay preserves a fuller lower part, whereas erosion removed lower Mill beds before the reduced Whitehouse section accumulated. Its historical Ashgill assignment and complanatus-zone fossils are useful for correlation, but the lithological base and historical stage boundary should not be treated as automatically identical.

Myoch Bay: basal to near-basal graptolitic shale. Thin black shale occurs in tectonically interrupted exposures identified as localities 11A, 11B and 11G in Ingham’s foreshore account. It contains abundant Dicellograptus complanatus and D. gravis, with members of the Orthograptus calcaratus group. Williams’s revision places the assemblage low in the complanatus Biozone, close to the transition from the preceding linearis interval. The fossil assemblage is more informative than the apparent width of these faulted shale exposures.

Myoch Bay: calcareous siltstones above the basal shale. The succession coarsens upwards through calcareous siltstone towards the overlying conglomeratic channel fill. Lapworth’s historical Dictyonema interval probably belongs within this lower Mill package, but its precise equivalence to a particular modern bed or member is not independently established. His original description is retained separately below.

Myoch Bay: mudclast conglomerate and calcareous channel fill. This mass-flow deposit is less than 1 m thick where first encountered near the sea wall but thickens markedly seawards. A grey calcareous mudstone matrix surrounds deformed grey and green mudstone and siltstone clasts, with limestone fragments and rare igneous pebbles derived from a shallower platform. The unit is absent at Black Neuk, demonstrating that it is a restricted sedimentary body rather than a constant layer across the coast.

Its matrix carries transported shelf fossils, including brachiopods, trilobites and gastropods. Tretaspis cf. hadelandica convergens and Skenidioides greenoughi support the traditional early Ashgill correlation; Onniella and Orthambonites are also recorded. Cryptolithus lotus lotus is particularly associated with locality 11H. Its Laurentian affinity links the transported shelf fauna to the ancient North American side of the Iapetus Ocean. This fauna differs in origin from the largely in-situ deep-water association of the underlying Myoch red mudstones. The rare igneous-pebble occurrence at locality 11C is a scientifically important protected feature, not a collecting target.

Myoch Bay and Black Neuk: upper shale interval. About 16 m of grey-green silty shale overlies the conglomerate at Myoch Bay, with occasional graptolites of the complanatus Biozone. Much of this interval corresponds broadly to the thinner, shalier and sandier Mill succession at Whitehouse, but individual beds cannot be matched one for one. At Black Neuk, a thicker shale-dominated upper Mill succession lies beneath Shalloch without the terminal sandstone seen at Whitehouse.

Whitehouse: sharp base and disturbed mixed sediment. Here the Mill Formation rests abruptly on red Myoch mudstones, with the lower Mill succession seen at Myoch Bay missing. In the rock-pool exposure between the first and second small stacks, partly consolidated sediments are disturbed and mixed. This is interpreted as mass movement on an unstable submarine slope, explaining why a complete Myoch Bay log cannot simply be transferred to Whitehouse.

Whitehouse: lower shale member, an informal subdivision. Laminated grey and green silty shales occur on and around the three small stacks. Graptolites indicate the complanatus Biozone, while thin seams contain transported cyclopygid and other atheloptic trilobite remains. Dindymene and Aethidionide are specifically recorded in the older field guide. These beds supplied material associated with the historic Gray collection in the Natural History Museum; the published description does not give a separate numerical thickness for each fossil seam.

Whitehouse: upper sandy and silty member, an informal subdivision. Cross-bedded sandstone and siltier beds contain comminuted shell debris derived from the west and occasional more recognisable fossils. Slumping is especially clear on the seaward side of the middle stack. “Lower shale” and “upper sandy and silty” are useful descriptive divisions here, not formally named members deserving separate lithostratigraphic rank.

Whitehouse: terminal channel-fill sandstone. A distinctive thick sandstone bed occurs beyond the stacks towards low water, immediately below the basal lead-grey mudstone of the Shalloch Formation. No measured thickness is established in the cited account. Its absence at Black Neuk is an important lateral difference and it should not be assumed to mark the Mill top everywhere.

The Shalloch Formation is the youngest Ordovician unit exposed in this coastal account. Although older literature separated it as the “Barren Flagstones”, it is now placed within the Whitehouse Subgroup. More than 200 m is represented in the Girvan foreshore succession; the BGS estimate of approximately 300 m is a regional formation thickness, not a fully measured Woodland exposure. Its regional relationship to younger Ordovician rocks elsewhere cannot be substituted for the local erosional contact beneath the Silurian.

Basal leaden-grey mudstone, about 1 m. The basal marker is described as unfossiliferous. It is well displayed in a curved gully at Myoch Bay and, at Whitehouse, above the terminal Mill sandstone. At Black Neuk it instead overlies upper Mill shale. This thin marker must be distinguished from Lapworth’s much broader lower “Barren Flagstones” interval, which included fossil records from above the basal metre.

Main sandstone–shale alternation. Fine- to medium-grained turbidite sandstone alternates with siltstone and shale through the main Shalloch succession. Compared with the Myoch and Mill fossil horizons, these beds are generally fossil-poor. Near-vertical dips, fault repetition and sand-covered gaps make beach width an unreliable measure of thickness, even where the regular alternations appear to form a straightforward sequence.

Detrital carbonate horizon about 9 m above the base. On the Whitehouse shore, a thin carbonate-rich bed contains graptolites of the complanatus Biozone. It is an individual fossil-bearing horizon within the otherwise relatively unfossiliferous lower Shalloch succession, rather than evidence that every adjoining shale bed will contain the same fauna.

Conodont-bearing limestone 117 m above the base. A limestone horizon in the Girvan foreshore Shalloch succession yields a rich conodont assemblage of the Amorphognathus ordovicicus Zone. The measured position is reported by the GCR account, drawing on Bergström’s study. It identifies a stratigraphic horizon rather than a guaranteed present-day exposure beside a particular access point.

Higher graptolite and shelly horizons. Higher beds yield graptolites assigned to the anceps Biozone, and rare detrital carbonate beds contain lower to middle Ashgill shelly debris in the older regional terminology. The sources do not provide a single fixed thickness or a formal bed name for all these higher occurrences, so they should not be compressed into one precisely positioned marker.

Lapworth’s historical Myoch Bay and Barren Flagstones log

Lapworth’s 1882 account preserves bed-level observations that remain useful beside the modern formation names. The intervals below retain his descriptive divisions and approximate imperial measurements, with metric equivalents supplied for clarity. They are not newly named members, should not be added to modern formation totals, and cannot be imposed on the truncated Whitehouse section. Historical fossil names are retained as historical identifications.

Low coarse sandstone. Lapworth described a coarse, thick, gritty sandstone low in the succession and compared it with the faulted sandstone patch at Port Cardloch. No numerical thickness was given. It records the same broad coarse-to-fine transition recognised in the modern Myoch account without establishing an exact bed-to-bed correlation.

Green shale and harder gritty flags: at least 40 ft, about 12.2 m. This lower fine-grained interval was exposed south of Myoch Bay. The measurement belongs to Lapworth’s historical section and classification, not to an independently remeasured total for a modern formation.

Lower transitional band: about 15 ft, 4.6 m. Soft green mudstone and shale contain purple lines, marking the first historical colour transition above the lower green beds.

Purple mudstone band: estimated at no more than 30 ft, 9.1 m. Strong folding made the thickness uncertain. Its position between the lower and upper transitional bands is part of the historical log; the estimate should not be treated as an undeformed modern measured section.

Upper transitional band: estimated 20 ft, 6.1 m. Green mudstone with purple seams succeeds the more uniformly purple interval. This historical colour subdivision is more detailed than the broad modern description of red and green Myoch mudstones, but the two schemes do not have independently demonstrated identical boundaries.

“Complanatus zone” bed package: 5 ft, about 1.5 m. Carbonaceous black shale alternates with grey mudstone and seams of calcareous grit. Graptolites were abundant, including material reported under the historical name Diplograptus socialis. Lapworth’s thickness applies to this lithological package, not to the full thickness of the modern complanatus Biozone.

“Dictyonema zone”: 9 ft, about 2.7 m. Hard, flaggy shales with grey calcareous ribs contained fossils described as Dictyonema and Ganocladium, together with lingulids and other remains. These historical names and the bed description are useful evidence, but no unsupported modern taxonomic replacement or exact member correlation is implied.

“Dionide beds”: about 6 ft, 1.8 m. The original 1882 description specifies very calcareous mudstones, locally passing into impure nodular limestone. Peach and Horne’s later quotation used “sandstones”; the original lithological wording is followed here. This higher historical fossil interval is distinct from the in-situ Dionide-bearing fauna of the Myoch red mudstones. The whole 6 ft cannot securely be equated with a modern conglomerate member.

Lower “Barren Flagstones”: first 50 ft, about 15 m. Lapworth described pale-green mudstone and very thin flags, recording the historical fossil name Nematolites Grayii. This is a broad lower part of the old division, not the same unit as the approximately 1 m unfossiliferous basal Shalloch marker.

Middle “Barren Flagstones”: changing sandstone-bed thickness. Flags initially about 1 ft, 0.3 m, thick occurred at intervals of around 3 ft, 0.9 m. Higher in this part of the log, individual flagstone beds increased to 3–4 ft, about 0.9–1.2 m, with shale partings only inches thick. Upwards again, the flags became thinner and shale more prominent. These changes retain useful sedimentological information without requiring an unsupported modern member name.

Historical Woodland thickness estimate: 500 ft, about 152 m. Lapworth estimated the “Barren Flagstones” thickness from the width of the Woodland outcrop and its near-vertical bedding. It is an important early estimate, but the modern foreshore figure of more than 200 m and regional Shalloch estimate of about 300 m are preferred in their respective geographical contexts.

The Ordovician–Silurian break

Truncated Shalloch and an angular unconformity. The local Ordovician succession ends below an erosional surface, with the upper Cautleyan through Hirnantian interval missing. Younger Ordovician rocks, including the Drummuck succession and Auldthorns Formation, are not exposed in the coastal succession described here. At Woodland Point the discordance between the Ordovician and Silurian beds is about 8°. The relationship is interpreted in terms of submarine erosion and channel development, rather than a simple unbroken passage between the two systems.

The basal Silurian conglomerate changes sharply in thickness along the coast. At Craigskelly it forms a substantial channel fill, whereas at Woodland Point only intermittent thin lenses may intervene between Shalloch and Woodland sandstones. Where a lens is absent, Woodland sandstone rests directly on Shalloch. An older statement that basal conglomerate is entirely absent from Woodland Point is therefore too categorical.

The Woodland Formation at Woodland Point is late Rhuddanian, within the early Silurian Llandovery. The measured succession is 21.7 m thick, divided descriptively into a 10.2 m lower interval, 5.5 m middle interval and 6 m upper interval. The BGS Lexicon rounds its thickness to 21 m. “Woodland Formation” is the current formal name used here; historical names include Woodland Limestone and Striped Shale. The three lower, middle and upper divisions are unnamed, so they are described below without inventing formal geographical member names.

Lower member: 10.2 m of massive flaggy sandstone and calcareous shell beds. The lower interval contains strongly calcareous, shell-rich beds and lenses within massive flaggy sandstone. More than 30 brachiopod species have been recorded from the lower formation, with Stricklandia lens lens prominent in the historical account. Most shells in this lower part were transported before burial, contrasting with some of the life-position brachiopods higher in the formation.

Conodont-bearing calcareous bed, 2 m above the formation base. This distinct microfossil horizon contains an early form of Distomodus, figured by Aldridge in 1985 as D. kentuckyensis. The qualified identification is retained because the historical species assignment should not be made more definite than the locality account allows.

Middle member: 5.5 m of less massive flags and diminishing shell beds. Thin shell beds become less frequent upwards. A Stricklandia community occurs in the lower part of the middle interval, followed by a predominantly Clorinda community. Some brachiopods remain in life position, providing direct evidence for the shelf communities that inhabited the sea floor rather than only for shells carried in from elsewhere.

Upper member: 6 m of thin-bedded siltstone and shale. The upper interval is locally slumped and is only intermittently visible through beach cover. At Cow Rock, the corresponding upper Woodland shales are strongly folded and disrupted beneath the overlying conglomerate. That deformation does not provide a separate reliable thickness measurement for the Cow Rock exposure.

Graptolitic band, 4 m below the Woodland top. A diverse graptolite fauna gives the principal late Rhuddanian age control for the succession. The GCR describes it in the upper cyphus Biozone; the later BGS regional account uses revolutus, formerly cyphus. This change in zonal terminology does not shift the Woodland Formation out of the Rhuddanian. The fossil occurrence belongs to this particular upper-member band, not to every shale exposed in the bay.

Fossils and type material. Woodland Point is the type locality for the trilobites Stenopareia acymata and Acernaspis xynon, described by Howells in 1982, and for several brachiopods listed in the GCR account. Its type-locality list includes Lingula tenax, Triplesia woodlandensis, Katastrophomena woodlandensis, Leptaena? reedi, Leptaena valentia, Leptostrophomena jamesoni and Fardenia (Saughina) pertinax. These are the combinations used in that account, rather than a claim that every name remains taxonomically current. Cocks’s 2009 revision placed tenax in Leontiella and identified the Woodland Point lectotype B 74637, selected in 1978, together with topotypes B 74578–79, BC 1800–08 and BC 2255. The BGS regional account also illustrates Eostropheodonta mullochensis, specimen GSE 875, from Woodland Point. These records add to the importance of the measured succession, but they do not establish that every named species occurs in a single shell bed.

Coarse sandstone succession: 45 m exposed at Woodland Point, with the top unseen. Massive greywacke and coarse sandstone show normal and inverse grading, with conglomeratic bases to some beds. The BGS interpretation is of turbidite deposition. The formal age range is Rhuddanian–Aeronian, so the entire exposed formation should not be narrowed to the late Rhuddanian age established for the underlying Woodland beds. Floyd and Williams reported no fossils from Scart Grits; it is the sedimentary structures and basal conglomerate relationships that are especially informative here.