Geological History of Pembrokeshire

The geology of Pembrokeshire is special for several reasons including the diversity of rock types to be found in a relatively small area. We are indebted to Sid Howells for compiling this section.

Cliff sections provide very good exposures

What's all the fuss about rocks?

The Pembrokeshire Coast National Park displays a greater variety of geological features and associated landforms than any equivalent area of the same size in the British Isles. The coastal scenery was the prime reason for the National Park's designation.

Pembrokeshire has 50 Geological Conservation Review (GCR) sites, of a total of only c.400 in the whole of Wales. These sites are, at least, of national significance and some are of international importance. The importance of the county's rocks and scenery is emphasised even more by comparisons of the area covered by it's GCR sites in relation to those in the remainder of Wales.

The text below provides a brief introduction to the geological history of the county. If you want to find out more you could join guided walks organised by the South Wales Group of the Geologist's Association or Pembrokeshire Coast National Park Authority. The booklets Geology of Pembrokeshire by Brian John, and Rocks and Scenery by Dyfed Elis-Gruffydd can also help to get you started but beware, there's a high probability you'll get hooked by this ever present, even at home or in town, interest.

And remember, geologists help find many of the raw materials we need for everyday life, including crude oil, metallic minerals and building materials).

As a general guide you should not damage biological or geological features, or cause unacceptable visual impact (eg. by hammering in the wrong places - better just to take photos) and you need to obtain permission to visit sites on privately-owned land. Contact the Countryside Council for Wales, the statutory organisation for nature conservation in Wales for more details (the local office is at Winchway House, Winch Lane. Haverfordwest SA61 1RP).

Geological time and 'continental drift'

- a few facts and figures.

To appreciate the vastness of geological time in relation to the geology and landscape of Pembrokeshire, consider the time since the Earth formed (c.4,700 million years ago) to be equivalent to one year in your life.

The oldest rocks on Earth would have formed some time in the spring of the imaginary year, but the oldest rocks in Pembrokeshire were not formed until volcanic eruptions occurred on November 15th (c.600 million years ago, towards the end of the Precambrian).

Land plants did not evolve until the last few days of November (the oldest fossil plants are of Silurian age - c.420 million years old).

All the solid rocks in now found in Pembrokeshire formed due to sedimentation or igneous activity which occurred before 9th December (ie. by the end of the Carboniferous Period, c.290 million years ago) at which time these rocks finally made it into the northern hemisphere, as a result of movement of the plates forming the Earth's crust. The older rocks had started just west of where Peru is today but had wandered down towards the South Pole before deciding to turn north (if they hadn't, the outdoor gear needed for a trip to Pembrokeshire would be very expensive!).

Any rocks less than 290 million years old that existed have been lost, principally due to weathering and erosion, particularly marine erosion just before Christmas (towards the end of the Cretaceous Period, c. 80 million years ago) when sea level was relatively high (>6OmOD).

Britain and North America rifted apart on the 26th of December (c.60 million years ago) and the northern part of the Atlantic Ocean (now c. 3,500km wide) began to open. On average it has widened by c.55mm/year but seafloor-spreading at the mid Atlantic Ridge is currently only around 5mm/year (about the same rate that your fingernails are growing).

At the same time the rocks of Pembrokeshire were lifted out of the sea and a drainage system was established on a relatively even cover of marine sediments that sloped gently southwards. The streams and rivers and rapidly cut through this material and began to dissect the solid rocks underneath.

The surface of Pembrokeshire was scraped clean of vegetation, soil and any remaining marine sediment by advances of the Irish Sea Ice Sheet that occurred on several occasions during the 'Ice Age', pushed southwards by glaciers moving out of the mountains of Scotland, the Lake District, the east coast of Ireland, and North Wales. At times the ice cover wasted away and the old river channels were deepened by the flow of meltwater.

The last glacial maxima occurred at around 11. 57pm on 31 December (c. 18,500 years ago) when ice covered north Pembrokeshire but you could walk in a direct line from Tenby to Barnstaple, across what is now the Bristol Channel (sea level was around 40m lower then because of the store of water held in continental ice sheets). On a previous advance the Irish Sea Ice Sheet had reached the north coast of Devon and Cornwall and carried the 'Bluestones' (some of which were later used at Stonehenge) from the Preseli Hills to Somerset.

Geological History of Pembrokeshire (greatly simplified !)     Geological map

late Precambrian (650-545 million years ago)

Volcanic eruptions followed by intrusion of molten magma into the accumulation of lavas and ashes. The oldest rocks in Pembrokeshire are the Treginnis Peninsula (SW of St David's) An associated intrusion (the St David's Granophyre) can be seen on the headland adjacent to Carreg Fran (west side of Porthlyski). Very rare 'jellyfish' fossils similar to those of the Ediacara fauna of Australia have been found in sedimentary rocks of this age in Carmarthenshire.

Towards the end of the Precambrian there was a period of uplift and erosion. The volcanic rocks were folded and the microgranite intrusion was exposed at the surface.

Cambrian (545-495 million years ago)

The volcanic landscape was flooded by the sea, leading to accumulation of a fining-upwards sequence of sediments (conglomerate, sandstone, mudstone) as the water depth increased. Shallow (coastal shelf) seas persisted through much of the Cambrian Period, except for deeper water conditions for part of the middle Cambrian. Fossils are fairly common in rocks formed after the early part of the Cambrian Period, and include 'giant' trilobites of the Solva area.

Ordovician (495-443 million years ago)

Another episode of uplift and erosion was again followed by marine transgression and eventual establishment of deeper water conditions (the 'Welsh Basin') characterised mainly by accumulation of muds commonly with fossils graptolites (extinct planktonic organisms. There was widespread underwater volcanic activity with some emergent volcanic islands. The basalt pillow lavas of Strumble Head and spectacular rhyolitic rocks of Ramsey Island provide good examples of the products of underwater eruptions. Associated intrusions (mostly sills) are marked by prominent tors visible along the north Pembrokeshire coast (eg. Carn Llidi, Penbiri, Garn Fawr) and in the Preseli Hills (eg. Carn Meini, source of the bluestones of Stonehenge). Turbidity currents (underwater landslides initiated on the margins of the adjacent coastal shelf ) also contributed to sedimentation in the Welsh Basin (good examples can be seen at Poppit Sands).

Silurian (443-417 million years ago)

Warm tropical seas - shallow shelf area with volcanic islands & lagoons (Skomer Volcanic Group). Brachiopods and corals especially abundant (eg. Coralliferous Group rocks at Marloes Sands).

Late Silurian and Devonian (420-354 million years ago)                    Geological map

Caledonian Orogeny (mountain building event caused by closure of Iapetus Ocean) creates folds and faults seen in the rocks of north Pembrokeshire (eg. at Ceibwr Bay). The Old Red Sandstone rocks of south Pembrokeshire represent sediments deposited on coastal plains with braided river channels and fringing 'saltmarsh' (early land plants were evolving at this time - examples from Freshwater East). Primitive armoured fish in rivers and amphibians burrowing on floodplains.

Early Carboniferous (354-323 million years ago)

Warm equatorial seas occupying broad gulf with abundant corals, crinoids (sea lily) and orthocones (squid-like creature with elongated conical shell). Transition from deeper water sediments (eg. muddy limestones of Castlemartin area) to fringing shoals (eg. oolitic limestones of Lydstep and Tenby) backed by lagoons.

Middle & Late Carboniferous (323-290 million years ago)

Delta swamps - accumulation of peat beneath 'giant' clubmosses, seedferns and horsetails provided source material for Pembrokeshire's anthracite coalfield. Periodic flooding by sea evidenced by horizons with marine fossils - goniatites (ammonoids) and pectenoid (scallop-like) bivalves.

At the end of the Carboniferous Period collision of continents causes the folds and faults of South Pembrokeshire eg. Middle Cove at Stackpole Quay, Cobbler's Hole (St Ann's Head), Ladies Cave Anticline (Saundersfoot).

Permian, Triassic and Jurassic (290-142 million years ago)

Older rocks weathered and eroded - no marine influence evident from Pembrokeshire but marine fossils (including ammonites, reptiles and sea mammals) of the adjacent seas found in Jurassic rocks of S. Wales (eg. Southern Down)

Cretaceous (142-65 million years ago)

Globally high sea levels of the Late Cretaceous (early example of 'Greenhouse Effect'). Deposition of the Chalk of south-east England. The plateau surfaces of Pembrokeshire (well developed around St David's and best viewed from Carn Llidi (Whitesand's Bay) or Carn Llundain (Ramsey Island) contour map and view from Carn Llundain, Ramsey Island) were cut by marine erosion (cf. modern wave-cut platform at Abereiddi, Freshwater West and Manorbier).

Tertiary (65-2 million years ago)                     Geological map

Block uplift associated with widening of northern Atlantic Ocean by mid-ocean spreading. Creation of new landmass c. 50 million years ago heralds development of the scenery which we see today. The main processes that have shaped this landscape are - marine erosion leading to development of spectacular coastline of the national park (eg. limestone cliffs of Stack Rocks area); weathering and erosion by streams and rivers - development of drainage pattern dominated by Eastern and Western Cleddau and glaciation (see below).

Quaternary (2 million - 10 thousand years ago)

Very cold conditions (at certain times) lead to glaciation of much of Britain. Ice from glaciers in the mountains of the western coast of Britain joins sea ice to form the Irish Sea Ice Sheet which advances across Cardigan Bay. During an early glaciation (c. 450,000 years ago) ice crosses Pembrokeshire and reaches north Devon Coast (most geologists think that the Preseli 'Bluestones' were transported most of the way to Stonehenge by this process). During the most recent glaciation (maximum extent c. 18,500 years ago) only north Pembrokeshire and St Brides Bay were covered by ice - south Pembrokeshire experienced tundra conditions.

Near glacial maxima sea-levels may have been up to 50m lower (explain reasons) ie you could have walked from north Pembrokeshire across Cardigan Bay to the Lleyn Peninsula. Conversely, during interglacials, when conditions were warmer, sea-levels were up to 5m higher than today (links to present melting of Antarctic ice sheets and predicted sea-level rise associated with global warming). Vast volumes of meltwater created Lake Teifi (vast but short-lived lake) and meltwater channels such as the Gwaun Valley and Cwm Dewi (near Dinas Head). Postglacial sea-level rise associated with migration of shingle ridges (eg. Newgale) and drowning of wooded coastal plains ('Cantref yr Gwaelod' )

Irish Sea Till (marine clays bulldozed by ice) at Abermawr and Druidston, and the 'raised beaches' at Poppit Sands and Broad Haven (South) provide evidence of these changes.

Present

Most obvious are modern coastal processes, including storm damage at Amroth.

Immediate future (prediction)                     Geological map

Possible rise in sea-level (perhaps relatively rapid) in response to global warming. Increased marine erosion. Stormier, milder and wetter conditions.

 

If you would like to be part of Pembrokeshire Online or if you have any comments about this site please contact us