The Bedrock Formations of West Cork

For an interactive map of the environments represented by these formations, and how they developed through time, click here (opens in a new tab).

The principal geological units of West Cork – including relevant neighbouring areas – are listed below in age order, with the oldest at the top. They are not, however, strictly contiguous; some formations were deposited in one location at the same time as one or more others elsewhere. Older deposits exist, at depth, but they are not exposed so either have not been studied, or have no direct relation to the rocks underlying West Cork, except maybe by inference. They are therefore not listed. Thicknesses given are of the rock formations at the present time; the sediment when deposited would have been of greater thickness containing a higher percentage of water. As the sediments were buried they were compressed and, under higher pressures and temperatures, the water was squeezed out, some crystalline structures changed, and the deposits developed into rock.

There were two factors that enabled such immense depths of sediment - 7 km at the centre around the Kenmare River area - to be deposited on the land surface;

  • 1. the land surface was subsiding, initially as vertical movement at the fault bordering the north of what became the Munster Basin caused the land south of the fault to drop; and
  • 2. an ample and continuing supply of sediment, provided by ongoing erosion of the high ground north of the fault, was washed south onto the subsiding land surface.
  • The important points to note are the changing environments as the various sediments were deposited in different places across the region. It is clear that over the period of deposition, at least 35 million years of terrestrial deposition and a further 35 million years of marine deposition, the environments were gradually changing at different rates in different places.

    The terrestrial deposits took place in a semi-arid plain with mountains and high ground to the north and it seems also to the east and west, and probably some high ground initially to the south of the basin. These high areas were eroding as a result of weathering on a land surface that was dry, largely unconsolidated, and not held in place by plant roots – plants were at an early stage of development and those that existed did not have extensive root systems– and this was a semi-arid area so maybe not many plants grew in this region.

    The marine deposits were laid down on the bed of a shallow warm shelf sea, similar to maybe the Arabian Gulf, or areas of the Caribbean today, with more life forms than were found on land.

    We start off at about 390 million years ago in the later stages of the middle Devonian. The Valentia Slate formation is the oldest rock formation in the Munster Basin that comes to the surface and is exposed, and can therefore be studied. There are older formations beneath the surface, and the Munster Basin is thought to have started developing in the Early Devonian, maybe 10 or 20 million years earlier. But the first good evidence of the environment in the Munster Basin dates from the Valentia Slate, and that's where we start.

    Middle Devonian – 393 to 385 million years ago

    During this period of about 8 million years, the forces of erosion - wind, rainfall and gravity - caused sediment to be washed from high land in the north down into the lowland areas of the developing Munster Basin. Streams, rivers and wadis drained into the area, leaving coarser sediments nearer the source and taking finer sediments further south. The climate was hot and semi-arid, few plants covered the land surface and animal life was concentrated in wet regions. The sediments were terrestrial sands, silts and muds, with a high proportion of silica and quartz (siliceous) minerals.

    Valentia Slate FormationVS

    On the Iveragh peninsula this formation is between 1450 and 3200 m thick, comprised of silt and sand with some fine grained sand. At the time of deposition the coastline lay some 200 km to the south. Drainage from the highlands in the north, with sheet floods and some crevasse splays (these are areas where flood waters overtop and break the banks of the channel, spreading out, and taking sediment as it goes) spreading across a semi arid plain, leaving ripple marks in some places. Further north, nearer the source (proximal) some beds contain pebbles (conglomerate) suggestive of movement of the larger grain-sized deposits from foothill sediment fans, but further south, further from the source (distal), the sediment is finer - fine sand, silt, and mud. Drying out of these flood deposits caused mudcracks to form and allowed oxidation of the surface sediments, resulting in red iron-oxide rich beds. Some lakes and persistent river channels were inhabited by fish, with mudflats showing burrows (possibly of lungfish), and crossed by four legged (tetrapod) amphibians. This is when the tetrapod tracks on the north side of Valentia Island were formed. Well worth a visit.

    Bird Hill FormationBH

    At about the same time as the Valentia Slate was being deposited, layers of fine grained sands and silts were laid down on the land that now lies to the south and east of Iveragh, giving a depth of over 900 m. These were persistent low energy drainage channels and floods depositing sand and silt across the plains and lake beds to the east, sometimes interrupted by high energy floods across plains resulting in flood deposits and ephemeral lakes. The drying out of these lakes left dessication cracks which were preserved in the rock. New sediment flows often eroded the surfaces they flowed over, before depositing the sediment load. Some burowing aquatic animals, possibly lungfish, left traces which became fossilised burrows. The significance of the energy of the drainage is that the higher the energy - i.e. more rapid flow of greater volumes – the larger the size of sediment grains that the water can move, so the size of the sediment grains can serve as a proxy for the energy of the drainage system at that point. The energy generally lessens with increasing distance from source.

    Glenflesk Chloritic SandstoneGC

    This formation comprises the close-to-source (proximal) deposits of a large drainage dispersal system extending southwards from the northern edge of the basin. Lands to the east of Iveragh on the northern edge of the basin received sediment in high volume and fairly direct drainage flows that resulted in this formation reaching over 3700 m in depth. This was similar to the Valentia Slate formation, being medium grained sands, conglomerate and pebbly sands, and silt. Rare mud cracks and deeper dessication cracks are the result of some floods and drainage channels drying out. Some plant fragments occasionally caught up in the drainage flows have been preserved as plant fossils. The presence of the mineral chlorite in these deposits is suggestive of more magnesium present at the time of deposition - derived from the parent rocks - and is also an indication of the age of the rock. This formation, of which the Slaheny and Gortanimill formations represent more distant deposits, was a result of a renewed drainage system 'overprinting' that of the Valentia Slate and Bird Hill formation systems.

    Lough Guitane RhyolitesGCr

    During the deposition fo the sediments that were to become the Glenflesk Chloritic Sandstone, volcanic eruptions in the area that is now east of the Iveragh, at Loch Guitane, gave rise to outpourings of acidic rhyolite lava. Being acidic, the volcanoes were more explosive than flowing.

    Slaheny FormationSL

    These were braided rivers flowing south west across flood plains, their channels diverging and then meeting again in a random network, finally discharging into ephemeral basins. Occurring further south than the Gleflesk beds, in the areas which are now the eastern Beara peninsula, these are finer grained deposits of silt and medium to coarse grained sands, with some conglomerates, and represent more distant (distal) areas of the drainage system. Some sand contained micas (a silicate mineral that may contain aluminium, iron and magnesium), possibly derived from the erosion of igneous rocks. In the Beara peninsula this formation is about 300m thick, thickening eastwards to 700m.

    Gortanimill FormationGM

    Comprised of finer grained sands, showing some cross stratification indicating changes in flow direction as diverging drainage channels flowed across the course of older channels, and also some medium grain sands, and silt, this environment was of a more centrally positioned distal area of the same drainage system that resulted in the Glenflesk Sandstone. Sediment was finer grained because of the greater distance from source, with the drainage flowing south to south east. These deposits were laid down in an area further east from the Slaheny formation, and south of the Glenflesk, resulting in a thickness of between 250m and 650 m. They were succeeded by the Caha Mountain Formation, and further east by the Ballytrasna Formation.

    Upper Devonian – 385 to 359 million years ago

    The more extensive deposits up until this point were largely from the same period (contemporaneous) and represent environments at different distances from the source in the high lands to the north. With the northern boundary of the evolving Munster Basin situated (probably) along the Dingle Bay – Galtee Mountains Fault zone, the earliest subsidence of the basin was in these northern areas that were directly receiving sediments washed out from the mountains. But from this time onwards we see increasingly more deposition further south, coming from highlands more to the east and to the west as well, and causing greater subsidence increasingly further south.

    St. Finan's Sandstone FormationSF

    In the Iveragh peninsula reaching a depth of about 1450 m, these fine grained sands and silts contain ripple marks, some lenses of pebbly sands, and some fish fossils. They were laid down in an environment of alluvial flood plains in which lakes formed. These sediments succeeded the Valentia Slate deposits in the northern part of the Iveragh and further east contemporaneous beds have been identified as the Glenflesk Chloritic Sandstone and Slaheny sandstone. Within this environment there was emplacement of sills of igneous rock at shallow depth, which broke the surface and erupted locally, and a probably separate volcanic eruption causing significant ash to fall - as much as 10 m deep in places. These volcanic eruptions are associated with continued extension faulting caused by extension stresses within the crust, causing continued subsidence as the basin developed.

    Ballinskelligs Sandstone FormationBJ

    The environment of deposition of this formation is of fluvial channels draining across an extensive flood plain depositing medium grained sands with some silt. The deposits were contemporaneous with, and transition into, the Gun Point Formation deposits to east and south. A deposit of the western Iveragh (with depths over 750 m, thickening to 1500 m in the east).

    Caha Mountain FormationCH

    This is a distal deposit receiving silt and fine grained sand from drainage from the high ground in the north and north east. By the time the rivers reahced this area they had developed,slowed and spread into braided rivers with wide spreading courses, which at times of high energy expanded into sheet floods across the plains. Some current and wave ripple marks can be seen in the rocks of this formation. This widespread and deep deposit covered the land in the areas of the Sheep's Head (depth over 350 m), Beara (1500 to 2000 m), and the Shehy Mountains (over 1500 m).The major part of the Beara peninsula is composed of the Caha Mountain formation, though both to north and south lie outcrops of the Gun Point, Toe Head and Old Head Formations.

    Sherkin FormationSK

    This deposit was laid down by braided rivers flowing across coastal flood plains. At the base of this formation, and thus occurring in the early days of this period, is the Foilcoagh Bay Member (see below), representing a small and temporary, but siginifcant, change in environment. This is a relatively localised deposit in the area of the South Coast, from Clear Island to Rosscarbery, but reaching a depth of over 1000 m. The alternating, and mixed, medium and fine grained sands, with lesser muds, contain sand layers up to 1.5 m thick wih cross bedding, and some erosional bases. Lenses of mud, some with ripple bedding, mud cracks or burrows, are also evident. The vertical bedded rocks to the west of the daymark at Baltimore are the Sherkin formation (photo); in fact the daymark lies pretty much on the junction of the Sherkin Formation and the Castlehaven Formation. These latter form the coastline south of Baltimore and running east to, and beyond, Toe Head.

    Foilcoagh Bay MemberSKfb

    At 60 m depth, this black mud shows signs of slumping, and contains a thin layer of volcanic ash (tuff). This member signifies a period of marine lagoon or incursion, probably from the south east, due to a high sea level event. Marine phytoplankton, algae, marine worms, wood fragments, and plant spores have all been found in this member. This marine transgression may have been due to one or more of (1)local basin subsidence southwards, (2)changes in sea level on a global scale, which appears to have been characteristic of the Devonian period, or (3)fault movement of the northern bounding fault of the developing Munster basin. The combination of volcanics with transgression of the sea across the land to such a distance may well be be suggestive of the cause.

    Gun Point FormationGP

    Braided channels flowing southwards across a wide and extensive floodplain, with flooding over the banks, deposited sands with some interbedded breccias, with grain size decreasing as time went on. These deposits succeeded both the Ballinskelligs Formation in southern Iveragh, and the Caha Mountain Formation across the most of West Cork. Another very extensive formation with deposits across the Iveragh (200 to 300 m), Beara (300 to 2500 m), the Sheep's Head (400 m), and further eastwards (1200 to 2500 m).

    Ballytrasna FormationBT

    In north Cork, succeeding the Gortanimill Formation - (360 to 1500 m). 90% mud, with fine and medium grain sand. This environment occurred at much the same time as that of the Ballinskelligs Formation described above, and then carried on, continuing while the Caha Mountain, Gun Point and Castlehaven Formation deposits were being laid down further west.

    Castlehaven FormationCE

    This environment of braided rivers draining across and into a distant flood plain resulted in deposits of mud and silt with some thin units of fine grained sand. Some muds show burrows and mudcracks. There was some redistribution of existing deposits as new sediment from the source areas of erosion reduce, and thinner layers of sediment were deposited in shallow floods as the source material becomes scarcer - the source of sedimentary material is gradually 'drying up'. The Castlehaven Formation commenced in the Mizen area, and inland, at much the same time as the Sherkin formation to the south, and the Caha Mountain Formation further north and east, continuing and building up a great depth while the Gun Point and Castlehaven Formations were deposited to the north, north west, and east, and finally succeeding all of these across most of West Cork. Iveragh (200m), Beara (250m), the Sheep's Head (200 to 250 m), Mizen (1500m), the South Coast (900 m), and Inland (280 m). This formation can be seen in the southern coastline from Baltimore to Toe Head and beyond; the dramatic and beautiful caves on both sides of Barloge Creek at the entrance to Lough Hyne are within the Castlehaven formation (photo). Most of the islands in Roaringwater Bay are remnants of this formation.

    Toe Head FormationTH

    Across the Iveragh, Beara, Sheep's Head, and Mizen peninsulas, the South Coast, and further inland. Fluvial channels in a coastal flood plain deposited sand and silt, with some cross stratification as flow directions changed, and mud. These layers contain some plant fossils and mudcracks. It is clear that continued erosion of the high ground has moved much of that high ground down into the Munster Basin, bit by bit, and the source area is by now further away and producing less material. This is the last terrestrial deposit in the Munster Basin. Iveragh 200 m, Beara 100 to 500 m, Sheep's Head 200 m, Mizen over 1000 m, South Coast 200 to 300 m, inland 150 to 200 m.

    Uppermost Devonian (Strunian) c 359 million years ago

    At this time the coastline, which has been steadily moving north from the area later occupied by South Devon, across the Cornubian Peninsula of Somerset, Dorset, Devon and Cornwall (which, as a small crustal plate, was being pushed steadily westwards into place along a fault, similar to the San Andreas fault today, named the Bristol Channel - Bray Fault), and then across the Celtic Sea, finally reaches Cork. As a consequence the environment has gradually become more coastal, and although sediments were still terrestrial (aeolian, that is, deposited in the presence of air and thus subject to movement by air (wind)), they now finally became marine. In these early stages of marine deposition the shallow tidal flats show signs of having periodically dried out and at those times to have supported some plant communities. The drying was probably due to continuing fluctuations in sea level for which the Devonian period is well known.

    Old Head FormationOH

    This deposit represents the start of marine conditions, in a shallow marine shelf environment. The base of this formation is thought to have been deposited in a tidal mudflat environment, grading upwards into shallow tidal sea. Thus the basal beds are principally mudstone with disturbance by burrowing creatres - bioturbation - whilst higher up there is more fine grained sand with some interbedded silts and other small rock debris. The features within the bedding, such as cross stratification with troughs and coarsening upward within beds, suggests regular small probably tidally influenced intakes of sediment. The sediment is more silt towards the top with some fossils. In some localised areas it would seem that deposition took place in isolated shallow lagoons where no tidal influence was felt. The development of a separate subsidence system as the South Munster Basin south of Sheep's Head high, and also a less subsiding area, the Glandore High, across the South Coast, all indicating differential movement developing across the basin. Iveragh 525m, Beara 1100m, Sheep's Head, Mizen and South Coast 900 to 250m eastwards, Inland up to 1000m.

    Bere Island MemberOHbi

    Still in a shallow shelf sea, this is a localised deposit within the Old Head formation, on the western end of Bere Island. These beds of mud are rich in crinoid and brachiopod fossils, with some inclusions of carbonaceous and calcareous material. It is interesting to note the appearance of carbonates in the marine record; the terrestrial deposits up to this time have been very quartzitic with little if any carbonate i.e. lime (Ca).

    Lower Carboniferous (Dinantian – Courceyan) – 359 to 326 million years ago

    The formations and members below, as far as and including the Lispatrick formation, date from the Courceyan sub stage of the Carboniferous, from 359 to 326 million years ago.

    Kinsale FormationKN

    The following six formations all comprise separate members of the Kinsale Formation. Where these can not be individually distinguished the generic Kinsale Formation name is used.

    Castle Slate MemberKNcs

    A deepening marine shelf receiving shallow offshore marine muds in a relatively quiet sea. This is a dark grey mud with occasional pyrites, with possiby anoxic conditons on the seabed, and crinoid fossils. This mud is about 10 m thick, thickening towards the east and the Kinsale area. This thin and extensive bed largely defines the start of muddy shelf sea deposits across the whole of West Cork south of Beara and in fact is so widely found that it is used as a marker horizon for the transition from the Devonian to the Carboniferous, of which the Castle Slate is the basal member. An outcrop of the Castle Slate member occurs at the head of Dunmanus Bay, just west of Durrus, off the road to Ahakista and Kilcrohane (photo).

    Narrow Cove MemberKNnc

    A tidal shelf sea close by the coastline, with deposits of principally sand, with some tidal and wave ripple bedding, and some mud. Both the Narrow Cove and Pigs Cove members were deposited across the southern part of West Cork. Some vertically bedded exposures, showing ripples very clearly, are to be seen on the northern side of the R586 just east of Clashnacrona gorge, between Dunmanway and Drimoleague (photo).

    Pigs Cove MemberKNpc

    As the shelf sea deepened and the coastline moved north, more mud and silty mud were deposited, with some thin layers of sand. Narrow Cove and Pigs Cove members together reach 1000m thickness. The ridge running alongside and to the north of the coastal road from Leap heading east follows the line of the meeting between the younger, mudstone Pigs Cove Member and the significantly higher ridge of Narrow Cove sandstone (photo). The high ground to the south of the R586 running east from Drimoleague all lie within the Pigs Cove member - Lahanaght Hill, Killaveenoge, and Maulanamirish (photo).

    Ardaturrish MemberKNat

    Interbedded sand and mud (heterolithic) in a shallow tidal environment, some sand , silt and mud layers. Some bioturbation, rare dessication cracks. Burrowing marine creatures, but in a shallow sea that occasionally dries out, possibly intertidal. In the Bantry Bay and Southern Beara, this is a deposit being laid down at the same time as the Pigs Cove and Narrow Cove members further south, and reaches a depth of 100 to 180 m.

    Reenagough MemberKNrg

    Continuing the shallow tidal shelf sea close to land, following on from the Ardaturrish member in the Bantry Bay area. Mud sand and silt at the base giving way to later sand with ripple bedding from tidal movement. Some plant fossils occur. Depths between 350 and 600 m.

    Ardnamannagh MemberKNam

    Still in the Bantry Bay area, this deposit possibly represents the outer edge of a delta outlet. They continue these later sediments of the Bantry Bay area with further heterolithic deposits of mud, sand, and silt, the overal particle size getting smaller towards the top indicating a lower energy environment. Some crinoid fossils, and some bioturbation from burrowing marine animals, and some plant fossils. Depths of 100 to 250 m.

    Reenydonagan FormationRD

    Still principally in the Bantry Bay area, the area had developed into a warm shallow sea with abundant marine life. Mud, with some silt, lime silt and crinoid fossils, some silica sand but increasingly lime rich (calcareous). Outcrops of this rock, containing fossilised crinoid stem rings (columnals), can be seen on Black Ball Head on the southern side of the Beara peninsula (photo). In total this formation reaches no more than 125 m in depth.

    Courtmacsherry FormationCY

    Across southern West Cork, the increasing amount of lime – up to now most sediments have been silica based (siliceous) and derived from terrestrial erosion and weathering – is probably a result of marine life in the warm shallow seas. The remains of these organisms, as lime rich (calcareous) skeletons and shells, fall to the bed of the sea, rather like in the Caribbean today. This bed of calcareous and non calcareous muds, non-calcareous silt and sand, and increasing lime mud towards the west, reaches 342m depth at Kinsale, thinning west.

    Lispatrick FormationLF

    During deposition of the Pigs Cove muds, this localised layer of a black mud containing goniatite fossils. Goniatites are cephalopods, like ammonites and nautiloids, with shells of lime (calcium carbonate). This is a small outcrop across the head of the Glandore inlet.

    Dinantian LimestonesDIN

    Lime mud deposited in a warm, shallow, low energy, carbonate, shelf sea, with marine fossils. In fact the Dinantian limestone is the only true limestone to outcrop in West Cork on land, and is found in only one small location. Large cobbles and pebbles can be found on Beach beach on the southern shore of Bantry Bay, opposite Whiddy Island (photo). It is thought that further outcrops may exist on the bed of Bantry Bay.

    Middle Carboniferous (Namurian) – 326 to 313 million years ago

    These last three formations – below - are found solely on Whiddy Island. They represent the latest sediment deposits made into the Munster Basin that can be seen and identifed in outcrop. Dating from the Namurian (326 to 313 million years ago) these outcrops represent the remains of strata that overlay the Reenydonegan formation, formed the core of the downfolded syncline in Bantry Bay and, as softer, partially calcareous rocks, have been almost entirely eroded away. There are no extant deposits that have been found for the period of time between the Dinantian and the Whiddy Island group (below). However, the environment of deposition fits and it is doubtful if anything more than a shallowing of the shelf sea occurred. This may have caused some of the latest Dinantian deposits to become exposed and eroded. During the Whiddy Island deposition the basin appears to have shallowed further due to filling by these sediments - this is the probable reason for the gradual upward transition from mud to silt and then to fine sand.

    East Point FormationEP

    Deposits of black mud in a warm deep sea, a low energy environment with poor bottom circulation, resulting in anoxia in the mud. Black mud transitioning between siliceous and calcareous, some pyrites. Marine fossils. The lime probably derived from erosion of Dinantian limestone deposits which may have been exposed and eroded after the Reenydonegan formation deposition.. Depth - over 170 m.

    Middle Battery FormationMB

    In a similar environment, and directly succeeding the East Point deposits, was this generally black mud, with an increasing proportion of sand upwards. The basin has stopped subsiding and is now filling with sediment. The shallowing sea means the seabed is coming into more energetic environments where wave, current and tidal action are felt. 230 m.

    Kilmore FormationKL

    Further up the succession, this deposit has transitioned into fine grained sands, with some small amounts of silts and mud at lower levels. Over 110 m in depth.

    Futher Developments after deposition

    Looking at a geological map of West Cork, several repeating patterns of these formations are clear. The deposits described above were all laid down either by flood or drainage channels on land, or on the bed of a shallow sea. They were deposited in a horizontal position, and yet both the patterns on the map, and the sight of vertically bedded rocks in the countryside and on the shorelines, tell a story of momentous upheavals and tremedous forces. These forces squeezed the land in a reversal of the pulling apart that caused the basin to form initally. This compression resulted in a shortening of the land surface, north to south, by as much as 50%, and are described in more detail on the page on the Palaeogeography of West Cork.