Unesco Assisted Geopark

Edwen Quarry SH 51106850

In contrast to the Dinantian succession in South Wales, oolitic limestones are sparsely developed in the contemporary sequence in North Wales. The Edwen Quarry RIGS provides a section through one of the very few examples of such limestones in North Wales – the Edwen Oolite. This unit, of Brigantian age, forms part of the Dinantian sequence which crops out on both shores of the Menai Strait (Greenly, 1919, 1928). This sequence, as elsewhere (see Moelfre to Traeth Bychan RIGS), comprises a series of shoaling-upwards limestone cycles. The Anglesey succession as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. Edwen Quarry and the adjacent sea cliffs provide a section through the oolite-bearing Brigantian cycle. The cliffs east of the quarry expose the underlying, highly fossiliferous, dark grey limestones (wackestones and packestones) with thin mudstone partings. These strata contain abundant colonial corals, both massive and branching forms, as well gigantoproductid brachiopods; bryozoa and Chaetetes are also common. The fauna, listed by Greenly (1919), includes the typically Brigantian corals Actinocyathus floriformis, Lonsdaleia duplicata and Diphyphyllum latespetatum, as well as species of Syphonodendron and Syringopora. He also reports a spine of the echinoid Archaeocidaris. Most of the corals appear to be in life position, but rare inverted colonies are also present. The 8 m-thick, locally crossbedded Edwen Oolite, which abruptly succeeds these coral-bearing rocks, comprises a well-sorted ooid grainstone with sparce quartz and lithic pebbles. Seeded on a central nucleus, typically a tiny shell fragment, each of the well-rounded ooids (or ooliths) in this rock comprises layers of pink- or purple-stained radial-fibrous calcite. These grains, each less than a millimeter in diameter, are bound within a sparry calcite cement. The undulating top of the uppermost bed in the quarry is taken to mark the top of the oolite, and of the Brigantian cycle it forms a part, but the contact with the succeeding beds is not exposed.

The sequence of carbonate facies exposed in and adjacent to Edwen Quarry can be compared with other Brigantian cycles (see Moelfre to Traeth Bychan RIGS) seen on Anglesey, and is consistent with an upwards shoaling of the depositional environment. The richly fossiliferous wackestones and packstones which form the lower parts of the cycle record deposition in a setting where there was only limited winnowing of fines, but good circulation of oxygen and nutrient-rich waters. This lay below the damaging effects of frequent storm waves and currents, though inverted coral colonies suggest it did not escape the ravages of some more severe events. In contrast, the cross-bedded Edwen Oolite at the top of the cycle records deposition above fair weather wave-base since ooids can only form under conditions of near-constant agitation. The top of each Brigantian cycle, both on Anglesey and throughout North Wales, typically displays evidence of dissolution and calcrete formation; proof of a fall in sea level which led to the subaerial exposure of the platform surface (Davies, 1983, 1991). The undulating top of the Edwen Oolite may mark such an event. Isolated ooids are not uncommon in grainstone facies present at the tops of both late Asbian and Brigantian cycles on Anglesey, and elsewhere in North Wales, but facies dominated by ooids are extremely rare. The rarity of Dinantian oolitic limestones in North Wales compared with coeval sequences to the south is likely to reflect their different locations relative to local land masses, and to oceanic and atmospheric circulation patterns. The North Wales platform developed to the north of the Wales-Brabant High (St George’s Land of earlier literature). It is likely that this land mass served to reduce the impact of westerly and southerly winds and currents which evidently promoted extensive oolite shoal development to the south. The regional transgression, ongoing throughout the Dinantian, saw the progressive reduction of the Wales-Brabant massif, and of its sheltering influence, and, during Brigantian times, it was perhaps changes in the prevailing wind direction related to this, combined with unknown local factors, which are recorded by the Edwen Oolite (Davies et al., 2004; Waters et al., in press).

Fedw Fawr SH 60108220 to 60658190
Within the Anglesey Dinantian succession, the Fedw Fawr RIGS provides an important section through the lower portion of the cyclical late Asbian sequence of the Penmon area, permitting detailed comparison with Pedolau to Eglwys RIGS. Each cycle was initiated by a marine transgression, records a progressive shoalingupwards, and is capped by a palaeokarstic surface; the latter testifies to a period of dissolution in response to subaerial exposure. The limestones immediately underlying such surfaces display a complex array of textures and effects associated with plant colonisation and soil formation – features which fall under the general term of calcrete. The section at Fedw Fawr was critical in the development of a processdriven model for the sequential and spatial evolution of these features which has applications not only to the Dinantian succession in North Wales, but to a broader understanding of calcrete formation (Davies, 1991). This RIGS provides a section through the Tollhouse House Mudstone marker bed and is additionally important in exposing the margin of a fluvially incised channel and its late Asbian clastic fill – the Fedw Fawr Sandstone. The Anglesey succession as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. A wave-cut platform at the western end of the section [6000 8220] exposes early Asbian porcellaneous limestones at the top of the Leete Limestone Formation as seen at Tandinas Quarry & Cliffs RIGS. The cliffs and quarry benches above this level allow for careful analysis of the succeeding late Asbian limestone cycles. Five cycles are exposed, but the lower three provide the greatest insight. Grainstone textures predominate in these cycles, but the upper levels of each cycle comprise coarser, cross-bedded facies containing imbricate shell coquinas and rolled corals. The top of each cycle is defined by a well-developed palaeokarstic surface.

The lowest of these surfaces, stripped bare of the overlying strata, forms the prominent rock bench located 3 m above the base of the section. It displays superbly the typically pitted and hummocky form of such surfaces with a vertical relief of 0.6 m. Some of the hummocks have a veneer of finely laminated dark grey calcrete, and the grainstones below contain abundant dark brown, branching tubular structures formed by the calcification of fossil roots and termed rhizocretions (Davies, 1991). At the base of the cliffs above this surface is a thin, pale grey, clay palaeosol (a fossil soil). About 2m above this is a second palaeokarstic level, but one which displays marked lateral variation within the exposed cliff section. At the eastern end of the exposure, welded on to the cross-stratified grainstones at the top of the second cycle are grainstones at the base of the third cycle – the cycle boundary is scarcely discernible as a bedding feature. However, rhizocretions visible within the uppermost part of the second cycle terminate at the base of the overlying unit and provide unequivocal proof of subaerial exposure and plant colonisation at this level. As this horizon is traced westwards, a laminated calcrete crust appears at the contact between the two cycles; further west an overlying clay palaeosol appears infilling shallow pits up to 0.5 m deep. In association with these lateral changes the underlying limestone bed converts into a rubble of irregular limestone blocks into which the overlying clay has infiltrated. The lateral relationships observed at this site influenced Davies (1991) in his development of a model for the formation of palaeokarstic surfaces and calcretes within the Anglesey Dinantian succession. During the periods of lowered sea level and emergence of the platform recorded by such features, the unlithified carbonate sediment of the previously deposited cycle underwent dissolution and digenesis promoted by semiarid climatic conditions. This saw periods of intense evaporation alternate with rainy seasons and the downward migration of vadose groundwaters. Initially, following emergence, the roots of colonising land plants penetrated the loose sediment. On their death, dark carbonate cements were deposited as a sheath around the decaying structures to form rhizocretions.

The subsequent lithification of the sediment prevented root colonisation, but favoured the growth of laminated calcrete crusts on the now solid limestone surface. Wind-blown dust, much of it volcanic in origin, accumulated on this surface to form simple soils. Rainwaters percolating through this clay cover dissolved the limestones beneath to create the characteristic hummocky and pitted palaeokarstic relief. Locally, the newly formed limestones suffered fracturing and brecciation. This allowed rainwaters to penetrate to deeper levels, promoting secondary phases of cementation and calcrete crust formation, but also the downward infiltration of clay palaeosol material. Davies (1991) recognised that these processes could operate sequentially at a single site such that simple profiles marked only by rhizoliths could be viewed as immature, in contrast to more mature and complex profiles characterised by deep rubbly zones and secondary phases of calcretisation. However, the Fedw Fawr RIGS exposures were important in clearly showing that such contrasting profiles could also exist in close proximity to one another along the same palaeokarstic level. This prompted Davies (1991) to suggest that shallow depressions, into which surface drainage was directed and clay palaeosol material was washed, were likely to become sites of more complex profile development, whereas simple profiles would typify the adjacent highs. Cliffs in the centre of Fedw Fawr cove [6053 8191] expose fluvial conglomerates overlain by an estuarine sequence of interbedded sandstones and mudstones, all facies of the Fedw Fawr Sandstone (Greenly, 1919; Davies, 1983, 1994). This clastic sequence infills a deep channel which was cut through the cyclical late Asbian sequence seen to the west. Cliffs forming the western side of the cove reveal the steeply inclined margin of this feature, excavated by resurgent freshwater streams flowing across the Dinantian platform surface during one of the cycle-defining periods of subaerial exposure. Regional correlations suggest that this period of late Asbian incision and fill equates with that recorded by the Helaeth Sandstone of Pedolau to Eglwys RIGS (Davies,1983).

Flagstaff Quarry SH 635808
Comparison of the Flagstaff Quarry RIGS with the Tandinas Quarry RIGS reveals important lateral facies changes within the early Asbian sequence of the Penmon peninsula of Anglesey. Viewed as a whole, the Dinantian succession in North Wales records the establishment and growth of a carbonate platform during a protracted, but pulsed marine transgression. As part of this event, the early Asbian limestones of the Penmon area record the initial inundation of the older Ordovician and Precambrian rocks which form the basement to the Anglesey Dinantian sequence at a time when there were marked differences in facies between the sheltered platform interior and its exposed outer margin. The Flagstaff Quarry RIGS statement emphasises features not fully assessed in the GCR description provided by Adams and Cossey (2004). Exposed in the upper part of Flagstaff Quarry is the Tollhouse Mudstone Bed. This important late Asbian marker bed lies within the cyclical Loggerheads Limestone Formation (see Tandinas Quarry & Cliffs RIGS for lithostratigraphical usage and sources) and permits precise correlation with the Tandinas Quarry section and Great Orme’s Head (Warren et al., 1984). Such correlations reveal marked lateral facies changes within the underlying early Asbian sequences present at these localities. Distinctive, white-weathering, porcellaceous limestones are a feature throughout the 47 m-thick Leete Limestone Formation sequences exposed at Tandinas, and dominate the Flagstaff Member near the top of the formation (see Tandinas Quarry & Cliffs RIGS).

The latter has its type section at Flagstaff Quarry, but is the only portion of the section in which porcellaneous limestones are abundant. Here, this 8.5 m-thick unit is both underlain and overlain by thick sequences of variably dolomitised, cross-bedded, oolitic and skeletal grainstone, rich in abraded algal plates and with rolled brachiopods and corals. Thick, disarticulated valves of the chonetid brachiopod Daviesiella llangollensis present in the upper of these units confirm the early Asbian age of strata 12.5 m below the base of the Tollhouse Mudstone. These facies resemble the dolomitised grainstones which make up the Llandudno Pier Dolomite seen on the Great Orme, and which is now viewed as part of the regional Llanarmon Limestone Formation (Davies et al., 2004; Waters et al., in press). Contrary to Power (1977) and Davies (1983), therefore, it is appropriate to include the Flagstaff Quarry grainstones also in this division, and to exclude all but the Flagstaff Member from the Leete Limestone Formation. At Tandinas Quarry, the latter formation was interpreted as a series of progradation peritidal rhythms. In contrast, the sedimentary structures, grain types and textures of the Llanarmon Limestone of the Great Orme are consistent with deposition in offshore barrier and shoal settings (Davies et al., 2004). The Flagstaff Quarry RIGS provides clear evidence of an interleaving of high-energy barrier and low-energy back-barrier facies consistent with its intervening location.

Lligwy Bay SH 49858715 to 50008712
The site comprises exposures in the lower part of the local Dinantian succession including the Lligwy Bay Sandstone and the Lligwy Bay Disturbance. Complex relationships between siliciclastic and carbonate units reflect deposition within deeply incised channels and the subsequent collapse of the karstified channel margin. This unique site is critical in providing evidence of the scale of, and the processes associated with, penecontemporaneous fluvial incision and karstification of the Anglesey Dinantian sequence. This sequence as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. The Lligwy Bay RIGS significantly modifies the GCR description and interpretation proffered by Adams and Cossey (2004). Exposures at the western end of the site at Carreg Ddafad [4974 8710] reveal a poorly sorted boulder conglomerate resting on an irregular limestone surface (Greenly, 1919; Cope 1975). The conglomerate contains abundant angular blocks of dolomitised limestone as well as rounded clasts of local Precambrian and Lower Palaeozoic lithologies. Greenly (1919) recognized rock types similar to ones only visible today in the vicinity of Parys Mountain near Amlwch. To the south, the conglomerate passes up into pebbly sandstones and limestone conglomerates, the latter, well seen to the south of a narrow gulley, principally composed of rounded and tightly packed limestone clasts. At the base of cliffs which form the south-eastern side of Lligwy Bay, dips in the conglomerate and sandstone unit steepen sharply below the Lligwy Bay Disturbance (Greenly, 1919; Bates and Davies, 1981; Davies 1983). The latter comprises a bedded limestone sequence with intercalated mudstone and sandstone units but which is tipped on its side and dips steeply to the east. At its north-eastern end, bedding within the disturbed sequence becomes distorted and it gives way to a chaotic assemblage of limestone blocks set in a red silty mudstone with yellow sandstone beds. Davies (in Bates and Davies, 1981) was first to recognize that the bedded sequence within the Lligwy Bay Disturbance can be matched to an intact sequence at a higher level in the local stratigraphy, exposed in cliffs to the south; the uppermost beds within the disturbed zone equating with beds 50 m higher in the in situ stratigraphy.

He concluded that the Lligwy Bay Disturbance recorded the foundering of this higher stratigraphy and, self evidently, that this event post-dated the deposition of these rocks. Brachiopod and corals confirm that the succession at Lligwy Bay, including the strata involved in the Lligwy Bay Disturbance, can be assigned to the D1 Biozone, equivalent to the Asbian Stage of the Dinantian (Greenly, 1919; George et al., 1978). However, Davies (1983), based on thickness comparisons alone, suggested that the shelly limestones below the Lligwy Bay Conglomerate could be of early Asbian age and contrast with the late Asbian age of the succeeding units. Cope (1975) interpreted the limestone surface at Carreg Ddafad as a wave-cut platform and the upstanding masses of the limestone as the bases of fossil sea stacks. By inference the overlying conglomerate was interpreted as a boulder-beach deposit. However, the ill-sorted nature of the deposit indicated a fluvial origin to Davies (1983) (see also Bates and Davies, 1981) who interpreted the underlying limestone surface as the irregular floor of an incised river channel; one of many now recognized within the Anglesey Dinantian succession (Walkden and Davies, 1983; Davies, 1994). For these reasons, Davies (1983) included the conglomerate and sandstone sequence overlying the limestone in his ligwy Bay Sandstone, and viewed it as quite separate from Greenly’s (1919) basal Lligwy Sandstone. The relationship between the Lligwy Bay Sandstone and the Lligwy Bay Disturbance are poorly observed and remain enigmatic. The foundered strata of the disturbance may record the collapse of a karstic cavern either during the Dinantian, or much later, perhaps during Triassic or Tertiary periods of dissolution. However, palaeokarstic surfaces are a feature of the local Dinantian succession and record frequent periods of sustained subaerial exposure during which the carbonate platform was subjected to karstification, soil development and fluvial incision (Davies, 1991). The highest beds within the foundered sequence equate with one suchlevel (see Pedolau to Eglwys RIGS) and one, moreover, which displays evidence of such incision. It is feasible, therefore, to interpret the Lligwy Bay Sandstone and the Lligwy Bay Disturbance as features linked to, and contemporaneous with, this higher palaeokarstic event; to view the former as fluvial deposits preserved at the base of a 50 m-deep channel cut down from this higher surface; and the ‘disturbance’ as recording the local collapse of the margin of this same channel. This interpretation carries with it another radical possibility, namely that much of the nearby crop of Greenly’s (1919) Lligwy Sandstone, far from being a basal division of the local Dinantian sequence, may also form part of this younger channel fill.

Moelfre to Traeth Bychan SH 51308625 to 51458509
Within the Anglesey Dinantian succession, the Moelfre to Traeth Bychan RIGS exposes the local Asbian - Brigantian boundary as well as providing excellent sections in typical Brigantian transgressive-regressive carbonate cycles including evidence, at the tops of each cycle, of penecontemporaneous subaerial exposure. Exposures within this section were used to establish the general model for cyclic sedimentation for rocks of this age in North Wales (Davies, 1983, 1984). The section also displays some of the most fossiliferous strata within the local sequence, as well as several key marker beds and one of the best of the several sandstone pipe horizons which are unique to this succession. The Anglesey succession as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. Thickbedded packstones and grainstones with late Asbian foraminifera form the cliffs on either side of Moelfre harbour [513 863], and comprise the upper cycles of the local Loggerheads Limestone Formation sequence (= Moelfre Limestone of Davies, 1983, and Walkden & Davies, 1984). A prominent palaeokarstic surface marks the contact with the succeeding Cefn Mawr Limestone Formation (= Traeth Bychan Limestone of Davies, 1983, and Walkden & Davies, 1984). The latter contains typical Brigantian foraminifera and this basal surface is therefore taken to mark the local Asbian-Brigantian boundary. Low in the first Brigantian cycle are levels rich in bioclasts with thick laminated algal coatings (pisoliths). At a higher level is a thin but laterally extensive coral bed composed exclusively of Syphonodendron junceum colonies. This horizon, well seen in the quarried cliffs south of Moelfre [5126 8611], is widely developed within the Moelfre area and is an important marker bed used first in establishing the local cyclic succession (Davies, 1983). The palaeokarstic surface at the top of this lowest Brigantian cycle forms the extensive rock platform seen on the north side of Porth-yr-Aber [5133 8589] and displays well the dissolution pits and rhizocretions typical of such horizons. The foreshore and southern cliffs of Porth-yr-Aber expose the second Brigantian cycle, one more typical of the Cefn Mawr Limestone. Thinly interbedded dark grey, well-burrowed, wackestones and mudstone form the lower two thirds of the cycle and include levels rich in tests of the benthic formaminifera Saccaminopsis and, at a higher level, highly fossiliferous beds with abundant productid, including gigantoproductid brachiopods, many in life position, as well as solitary and colonial corals.

Thicker bedded and paler packstones underlie well-sorted oolitic limestones at the top of the cycle. The Saccaminopsis-rich facies within the Porth-yr-Aber cycle allow regional comparison with similar early Brigantian facies elsewhere in North Wales and northern England. Capping the Porthyr-Aber cycle, the basal bed of the Aber Sandstone occupies a series of deep cylindrical pipes penetrating the underlying limestone sequence. From the attitude of bedding surfaces within the sandstone fill of these features it is clear that they formed by dissolution beneath an already in place but unlithified sand cover. Therefore, they provide important confirmation of the model for sandstone pipe formation developed at Trwyn Dwlban RIGS; and by inference they too formed along the margins of a largely concealed fluvially incised channel. The calcareous sandstones which form the upper part of the Aber Sandstone sequence, display cut-and-fill structures, planar- and cross-stratification, abundant skolithus vertical escape burrows, shelly and pebbly lenses, as well as giant rhizocretions and a further set of shallow sandstone-filled pipes. These upper facies record beach and near-shore bar deposition within the confines of the Aber Sandstone channel during marine transgression of the underlying pipe-filling facies. The second set of pipes and associated rhizocretions provided clear evidence for a further period of subaerial exposure and testify to the complexity of such channel-filling siliciclastic sequences. A short distance to the south, at Morcyn [5170 8553], a higher cycle within the local Cefn Mawr Limestone sequence includes near its base a metre-thick unit composed of interlocking fossil corals – the Morcyn Coral Bed. The local foreshore exposes huge radiating colonies of the branching genera Siphonodendron, Diphiphyllum and Syringopora as well as massive forms of Actinocyathus and Palaeosmilia. Solitary corals and brachiopods are also present within this remarkable deposit and local marker bed. Many of the coral species are exclusively Brigantian. Seen in the cliffs to the south, as far as Traeth Bychan harbour [5145 8509], are orange-weathering colonies of Actinocyathus floriformis preserved in ferroan dolomite. A palaeokarstic surface within this sequence of rocks is overlain by the thin Traeth Bychan Sandstone Bed, a rare sheet sandstone within the Anglesey succession and which displays the distinctive spiral trace fossil Zoophycus.

Morfa Mawr - SH 453 717
Morfa-mawr RIGS consists of small grass-covered mounds, formerly spoil heaps, a the site of the abandoned Berw coal mine. The fossils in the mine waste provide unique evidence for Upper Carboniferous (Westphalian) conditions in the westernmost area of Britain. The deposits consist of coal and shale fragments, rich in plant fossils, particularly Lepidodendron (a club moss). In addition, fragments of fish (Coelocanth fish scales) and a bivalve, Carbonicola, have been identified as well as possible insect remains. These finds are good indicators of the environmental conditions in which the deposits accumulated, namely, freshwater and brackish streams and deltas with interdigitating wet forest areas, somewhat similar to the Amazonian and other equatorial forests today. Although the heyday of mining in this area was over 100 years ago, the spoil tips have remained surprisingly intact, possibly as a result of their marshy surroundings. The site is regionally important for reconstructing Westphalian conditions in Britain as well as being of historical interest as a representative of a once-extensive coal mining industry in the Malltraeth Marsh area of Anglesey. Evidence from the site complements that from more extensive Westphalian deposits elsewhere in Britain.

Mynydd Bodafon - SH 47408532
The site, on the south-eastern flank of Mynydd Bodafon, provides an exposure in the basal conglomerate and breccia of the Bodafon Formation in its type area. These strata form the unconformable lower division of the folded and cleaved Anglesey Old Red Sandstone succession which, sited over 100 km north-west of the nearest comparable outcrop in the Welsh Borderland, provides an important constraint on late Silurian to early Devonian palaeogeographical reconstruction, and on the timing and nature of late Caledonian orogenic events. The Bodafon Formation forms part of the sequence described and interpreted by J. R. L. Allen (1965) in his seminal paper on Old Red Sandstone sedimentology The ill-sorted conglomerate and breccia visible at the site are composed of rounded to angular pebbles and blocks derived from the adjacent mass of Precambrian quartzite that forms Mynydd Bodafon. From the site, it is clear that the nearby edifice forms part of an exhumed Old Red Sandstone landscape. Allen (1965) viewed the Bodafon Formation as a highly diachronous sequence of alluvial fanglomerates and breccias deposited at the margin of the Anglesey Old Red Sandstone basin and which interfingered with both the Traeth Bach and Porth y Mor formations (see Traeth Bach and Porth y Mor RIGS).

Pedolau to Eglwys Siglen - SH 50758717 to 51708695
Within the Anglesey Dinantian succession, the Pedolau to Eglwys Siglen RIGS provides excellent exposures of typical late Asbian transgressive-regressive carbonate cycles including evidence, at the tops of each cycle, of penecontemporaneous subaerial exposure, karstic dissolution and soil formation. It also includes a unique section in the Helaeth Sandstone, a sequence of fluvial conglomerates, estuarine mudstone and barrier-beach sandstones. The Anglesey succession as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. The late Asbian sequence on Anglesey, as elsewhere in North Wales and beyond, comprises a series of shoaling-upwards carbonate cycles each capped by a palaeokarstic surface. The latter record repeated falls in sea level (forced regressions) and the resulting subaerial exposure of the platform. The intervening rises in sea level (marine transgressions) flooded these surfaces, re-submerged the platform and initiated the next cycle of carbonate sedimentation. During the emergent phases, the sediment exposed at the platform surface underwent complex diagenetic changes associated with carbonate cementation, but also with soil formation (pedogenesis) and plant colonization. The resulting structures and fabrics are typical of modern-day calcretes formed under a semi-arid tropical climate. Wind-blown volcanic ash was also trapped and accumulated on the platform surface and, beneath this derived soil cover, the newly lithified limestones were prone to dissolution by downward percolating rain waters. The resulting complex array of dissolution, diagenetic and pedogenetic features displayed by the Anglesey palaeokarstic surfaces and their immediately underlying strata, have been described, illustrated and interpreted in detail by Davies (1991). Many of these phenomena are displayed by the Pedolau to Eglwys Siglen RIGS. Key amongst these are hummocky surfaces formed by dissolution below a still-preserved volcanic (bentonitic) clay fossil soil (or palaeosol); calcified root structures termed rhizocretions; laminated calcrete crusts and ooids; limestones altered to dark grey micrite; and growths of replacive microspar. In thin-sections, additional microscopic features such as early vadose cement fabrics and a range of brecciation and neomorphic replacement effects are also evident (Davies, 1991) In addition, the Anglesey Dinantian succession provides exclusive evidence that, during periods of subaerial exposure, freshwater streams flowed onto the Dinantian platform surface and incised channels into the underlying limestones. Sequences of fluvial, estuarine and marine siliciclastic facies were deposited, and are now preserved, within the confines of these features.

The Helaeth Sandstone, which has its type section within the Pedolau to Eglwys Siglen RIGS, provides one of the best and most easily accessible sections through such a sequence and was important in the development of a depositional model for these channel-fills (Davies, 1983; 1994). Strata exposed at, and to the east of, Pedolau [5082 8708] comprise a sequence of thin-bedded limestones with a distinctive ‘clotted’ texture and a product of their extensive replacement by microspar. These rocks are capped by a complex and mature palaeokarstic surface which displays evidence of fluvial incision and of more than one phase of subaerial exposure. A dark grey estuarine mudstone occupies an asymmetric channel incised through this surface, the mudstone pinching out to the south-east against the rising channel floor. Here [5100 8700] calcretised conglomerates rest on highly altered, brecciated and rubbly limestones with associated laminated calcrete crust and ooids. These features are preserved beneath a grey-clay palaeosol with fossil rootlets and a thin coal. This palaeokarstic levelbroadly equates with the youngest strata observed within the Lligwy Bay Disturbance and the latter may have formed in response to dissolution and fluvial incision during the same period of subaerial exposure (see Lligwy Bay RIGS). To the east, at the western side of Porth Helaeth [5102 8695], are exposures in pebble conglomerates with low-angle lateral accretion surfaces, resting on a bevelled limestone surface and locally infilling shallow potholes. An overlying, thick-bedded conglomerate unit displays trough cross-bedding. These rocks from part of the Helaeth Sandstone
(Greenly, 1919; Davies, 1983) and provide evidence of quarrying for millstones. At the base of cliffs on the eastern side of Porth Helaeth [5108 8683 to 5136 8682] are further exposures in this sandstone division. Here, at the base, a distinctive sequence of highly lenticular, hummocky cross-stratified, calcareous sandstone beds dips steeply to the south beneath a flat-lying truncation surface. Above this a thin sequence of limonitic mudstones with thin, tabular sandstone beds, underlies a capping unit of
planar- and cross-stratified calcareous sandstones with cut-and-fill structures, skolithus and diplocriterion escape burrows, shell coquinas, and granule and pebble lags. The Helaeth Sandstone exposures display the full range of facies associated with the fluvially incised channel features developed within the Anglesey Dinantian sequence. The conglomerates of the western exposures are of fluvial origin and record deposition by braided rivers, but with meandering reaches (Davies, 1983).

The eastern cliff section comprises facies deposited during the drowning of the channel as protective barrier spits were driven landwards across estuarine mud flats. The uppermost facies display sedimentary features and trace fossils typical of beaches and near-shore bars. Typical late Asbian limestone cycles, occurring above the Helaeth Sandstone, are well exposed in cliffs to the east [5136 8682 to 5170 8695] in the vicinity of Eglwys Siglen. In contrast to younger Brigantian cycles (see Moelfre to Traeth Bychan RIGS) these older examples are constructed principally from thickbedded, pseudobrecciated and mottled, skeletal and peloidal packstones. However, below capping palaeokarstic surfaces, thin units of well-rounded and sorted skeletal grainstones rich in dasycladacean algal plates are commonly preserved (Davies, 1983; 1984; 1991). This succession of facies within individual cycles reflects an upward decrease in the amount of interstitial carbonate mud preserved the larger skeletal and peloidal grains consistent with increasing energy conditions and decreasing depth of water. The lower packstone facies record deposition below fair-weather wave-base where mud was only partially winnowed and bioturbating benthos was able thoroughly to mix the sediment. In contast, the well-sorted grainstone units at the tops of the cycles record the effective winnowing and removal of mud-grade sediment which takes place in setting above fair-weather wave-base subject to constant agitation. This upwards shallowing of facies was a response to falling sea-level and culminated in subaerial exposure of the cycle top. Hummocky dissolution surfaces underlying bentonitic clay palaeosols define obvious and readily observed cycle boundaries. The immediately underlying limestones typically contain abundant rhizocretions and remnants of laminated calcrete crusts are locally preserved on some upstanding hummocks. Cracks and fissures in these crusts, some penetrating the underlying limestone, are locally filled with rounded calcrete-coated grains (calcreteooids). However, where clay palaeosols and hummocky surfaces are not developed, cycle boundaries are more difficult to recognize and the identification of some present in this section requires a diligent search for layers containing only sparse rhizocretions. Limestones throughout the Pedolau to Eglwys Siglen RIGS section contain fossil faunas typical of the late Asbian including the solitary corals Dibunophyllum bourtonense and Palaeosmilia murchisoni, and the colonial forms Siphonodendron affine and S. cf. martini, together with abundant species of Syringopora. In thin section the plates of the Asbian algae Koninkopora inflata can be observed.