Rocks at Dunquin on the Dingle Peninsula

The Blasket Islands are deserted now but at one time the small pier at Dunquin was a lifeline for the small island community. It is hard to imagine the hardships of their existence and the way they would have navigated in all weathers across the short stretch of water to the mainland of the Dingle peninsula in tarred canvas-covered open boats called curraghs. For the islanders wanting to buy or sell goods, needing a doctor, having to attend church, confession, christenings, weddings, or funerals, or to visit mainland friends and family, Dunquin was an important landing place. The very basic, even primitive, life of the islanders is movingly and simply told in The Islandman by Tomás O’Crohan  who lived and died on Great Blasket Island (1856 to 1937). Nowadays, it seems to be mostly small boats that launch from the pier to ferry tourists to the uninhabited islands .

Dunquin harbour not only has this important historical association but it is also a noteworthy geological location. Walking down the steep, zig-zag path from the stone-walled green fields above to the beach and pier below, there are great views of the cliffs to the north and south of the harbour. It is a transition zone between two major geological periods – where a predominantly marine environment changed to a mainly terrestrial one due to vascillating sea level relative to the land. It is the location where yellow marine siltstones belonging to the Drom Point Formation of the Silurian Dunquin Group lie next to the reddish, purplish, and greenish sandstone strata of the Silurian/Devonian Dingle Group which are terrestrial in origin.

The cliff faces seem to be striped in contrasting subtle hues. The rock layers are steeply angled now following earth movements over the many millions of years since they were originally laid down in a horizontal position. Odd circular or spherical formations can be seen in some layers. The bedding plane of one outcrop next to the pier has a roughly polygonal pattern of drying mud cracks preserved in the stone. Curving veins of quartz cut across the strata to the south of the harbour.

Altogether a very good place for rock enthusiasts and well worth a return visit. Next time I would like to take a boat trip to the Blasket Islands where (on Inishvickillane at least) the rocks are mainly volcanic tuffs and lavas.

REFERENCES

O’Crohan, Tomás,  1937 The Islandman, Oxford University Press, ISBN 978-0-19-281233-9, re-issued 2000.

Horne, Ralph R., 1976, Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, Minister for Industry and Energy, Geological Survey Office. Reprinted 1999.

Rocks at Trabeg on the Dingle Peninsula

Close-up of the Devonian conglomerate at Trabeg on the Dingle Peninsula

The sand looks black from a distance as you descend to the shore at Trá Chathail near An Trá Bheag (Short Strand) – otherwise known as Trabeg. The path cuts down deep through the stratified red rocks to get to the beach which is strewn with pebbles, mostly shades of red, maroon, green, grey, and white.

Trabeg is on the south coast of the Dingle Peninsula in Ireland, and is the “type section” of the Trabeg Conglomerate Formation which is exposed in the cliffs on the beach. This is place where that particular rock type was first described. The rock layers constitute part of the Dingle Group and were formed in the Devonian period between 345 and 395 million years ago. The conglomerates are composed of fairly well rounded pebbles of red sandstones and mudstones, with white vein quartz and chert. A few pebbles of volcanic rock and of grey limestone are also present.

The way in which the conglomerate rock has formed from the mass movement and subsequent accumulation of debris from terrestrial locations during, for example, river flood events, means that the pebbles are derived from a wide area covering many different geological types. The pebble beds or conglomerates are inter-bedded with layers of red sandstones and mudstones, the finer sediments of which were deposited normally by rivers during non-storm/flood times. The alternating layers are now tilted from the original horizontal orientation in which they were first deposited, and are clear to see dipping south at about 70 degrees.

As the cliffs at Trá Chathail are worn away by the action of waves and weathering, the pebbles contained in the conglomerate matrix are freed up and remain the shore below – an instant pebble beach. Added to these are pieces of other rock or matrix that became rounded into pebbles after they arrived on the beach. Some pebbles and rocks may have been transported by wave action from further along the coast were the geology is quite different: from the Eask Formation, West Cork Sandstone, Bulls Head Formation, and the earlier Silurian rocks of the Dunquin Group.

REFERENCE

Horne, Ralph R. (1976) Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, reprinted 1999.

Seashore Rocks at Manorbier

The red-rock wave-cut platform on Manorbier beach in South Pembrokeshire is pretty spectacular. Row after row of vertically aligned strata stretch from the shore across the bay and up the cliffs in the distance. They form an incredible backdrop to the expanse of sandy beach topped by colourful pebbles at high water mark. They make an ideal place for clambering around and exploring rock pools.

Up close, each layer of rock looks slightly different – darker or lighter; harder or softer; coarse-grained or fine-grained; smooth, rough or sculpted; homogenous or with inclusions. They are different colours – red, cream, orange, purple, and green. They sometimes have internal patterns. Every layer of rock represents a particular phase of activity during the deposition of the sediments in what is known as the Freshwater West Formation between approximately 416 and 410 million years ago in the Lower Devonian Period of the Upper Palaeozoic Era. These rocks are also referred to as the Lower Old Red Sandstone.

The 580 metre thick rocks of the Freshwater West Formation are made up of clastic sediments. Clasts are particles of broken-down rock, and the size of the fragments may vary in size from boulders to silt-sized grains and are invariably the products of erosion followed by deposition in a new setting. So clastic rock is consolidated sediment composed of fragments of pre-existing rocks (Allaby 2008). Examples of clastic rocks would include conglomerate, sandstone, siltstone and mudstone. Sediments for the rocks at Manorbier were derived from the newly uplifted Caledonian Mountains and subsequently deposited in a variety or arid to semi-arid continental environments, including estuaries, broad alluvial plains, ephemeral braided-meandering rivers and alluvial fans (George 2008).

The sediment size in the rock layers depends a lot on the speed and volume of the water transporting the fragments from the mountains and across the land. The greater the speed and volume of water, the larger the particle size that can be carried – but the greater weight means that the fragments are deposited sooner than the finer particles which can travel further even when the water velocity is decreasing. The Manorbier rocks illustrate the cycles of deposition during this period, following greater and lesser flows. Each phase is marked first by a layer of the coarsest sediments usually with cross-bedded sandstones which can be red-brown, purple or even green resting on an erosion surface (Howells 2007) followed by layers of increasingly finer sediments like mudstone in a phenomenon known as upwardly-fining. The fining-upwards cycles are interpreted as the fills of ephemeral fluvial (river) channels with sinuous profiles (George 2008).

Pale-coloured nodules often present in the red mudstones and calcareous siltstones are calcrete. Calcrete is a concretionary carbonate horizon formed in the soil profile in arid to semi-arid environments. Calcretes are a feature of a palaeosol or fossil soil in which calcium carbonate is precipitated as root encrustations (rhizocretions) and as small nodules (glaebules) from water flowing through the soil profile. The glaebules grow and coalesce to form a calcrete or dense layer of calcium carbonate near to the surface (Nichols 2009).

References

Allaby, M. 2008, Oxford Dictionary of Earth Sciences, Oxford University Press, 3rd Edition, 978-0-19-921194-4.

George, G. T. 2008, The Geology of South Wales: A Field Guide, gareth@geoserv.co.uk, 978-0-9559371-0-1, pp 22 and 132-135.

Howells, M. F. 2007, Wales, British Regional Geology, British Geological Survey, Nottingham, Natural Environment Research Council, 978-085272584-9, pp 99-108.

Nichols, G. 2009, Sedimentology and Stratigraphy, Wiley-Blackwell 2nd edition, Sussex, England, p148.

Porthmeor Rocks 2

These rocks are seen on the west side of The Island or St Ives Head in Cornwall, England. They form outcrops on the west side of Porthmeor Beach. I was fascinated by the sheer complexity of the colours, patterns and textures. As far as I can make out they belong to an un-named igneous intrusion (matagabbro or metamicrogabbro) composed of silica-poor magma which was later altered by low-grade metamorphism in the Devonian period between 359 and 416 million years ago.

The rock looks very different from that nearby which was featured in an earlier post. I was particularly drawn some outcrops which had an almost dark navy blue smooth surface but the majority of the rock surfaces were blue-grey and dirty yellow shades – some with smoother almost layered textures and others textured like lumpy porridge. The patterns were complex but included ancient splits and cracks that had been infilled with other materials of contrasting colours, giving paler or darker straight lines and angled cross lines – not to mention all the complicated later fracture patterns. There seemed to be large inclusions of other rock types too; this often occurs in granites. It is incredible that rocks of the same basic type can vary so much in their appearance within a few metres.

Rocks at Porthmeor Beach, St Ives

The rocks in Cornwall are certainly different from any that I’ve seen before. Generally speaking, they are mostly igneous and metamorphic in type. Often the igneous rocks themselves have been metamorphosed. The rocks shown here were seen on the beach at Porthmeor and adjacent to The Island at St Ives. They lie below a thick layer of rusty-coloured superficial deposit of glacial till. I am not absolutely certain what the bedrock is. There are a number of rock types described by the British Geological Survey in their Geology of Britain Viewer in close proximity to each other, and it is rather difficult for an amateur to decide which is which. However, this particular outcrop reminds me of basalt which I saw on the shoreline of Grand Manan in the Bay of Fundy in Canada. So, I am going to suggest that it is an un-named mafic igneous rock formed of silica-poor magma that intruded into the earth’s crust in the Devonian Period between 359 and 416 million years ago. When the magma cooled, it formed an intrusion of fine to medium crystalline rock, often as basaltic dykes and sills.

On the other hand, I could be wrong, and it might be an un-named igneous intrusion of Metagabbro and Metamicrogabbro; or even Mylor Slate Formation with Hornfelsed Slate and Hornfelsed Siltstone.

Rocks at Fermoyle on the Dingle Peninsula

Red Devonian sandstone rocks at the beach with fucoid seaweeds

The place where I took these photographs is marked on the map as an island but it is actually just a tiny promontory near to the village of Fermoyle, along the Dingle Way, on the north coast of the Dingle Peninsula in Ireland. I am sure that most people visit the location for its wonderful long unsullied sandy beach. However, I was drawn to this particular part, at the extreme western end of the beach, because of its fascinating geomorphology. The rocks are sandstones and conglomerates (mostly but not exclusively red) of the Glengarriff Harbour Group from the Devonian Period. The bright olive, lime, yellow and orange colours of the seaweeds, and the black, yellow and white of encrusting lichens, clash garishly with the red rocks. The rock strata are clearly defined: sometimes on-end, sometimes as flat bedding planes, and in one place a dome of strata lies cut-away and exposed. Beach stones rather than pebbles cover a portion of this area; and there are also occasional huge boulders composed of conglomerate scattered along the shore nearest the inlet from Brandon Bay.

Pebbles at Trabeg on the Dingle Peninsula

Pebbles of many colours on the beach

The sand looks black from a distance as you descend to the shore at Trá Chathail near An Trá Bheag (Short Strand) – otherwise known as Trabeg. The path cuts down deep through the stratified red rocks to get to the beach which is strewn with pebbles, mostly shades of red, maroon, green, grey, and white.

Trabeg is on the south coast of the Dingle Peninsula in Ireland, and is the “type section” of the Trabeg Conglomerate Formation which is exposed in the cliffs on the beach. This is place where that particular rock type was first described. The rock layers constitute part of the Dingle Group and were formed in the Devonian period between 345 and 395 million years ago. The conglomerates are composed of fairly well rounded pebbles of red sandstones and mudstones, with white vein quartz and chert. A few pebbles of volcanic rock and of grey limestone are also present.

The way in which the conglomerate rock has formed from the mass movement and subsequent accumulation of debris from terrestrial locations during, for example, river flood events, means that the pebbles are derived from a wide area covering many different geological types. The pebble beds or conglomerates are inter-bedded with layers of red sandstones and mudstones, the finer sediments of which were deposited normally by rivers during non-storm/flood times. The alternating layers are now tilted from the original horizontal orientation in which they were first deposited, and are clear to see dipping south at about 70 degrees.

As the cliffs at Trá Chathail are worn away by the action of waves and weathering, the pebbles contained in the conglomerate matrix are freed up and remain the shore below – an instant pebble beach. Added to these are pieces of other rock or matrix that became rounded into pebbles after they arrived on the beach. Some pebbles and rocks may have been transported by wave action from further along the coast were the geology is quite different: from the Eask Formation, West Cork Sandstone, Bulls Head Formation, and the earlier Silurian rocks of the Dunquin Group.

REFERENCE

Horne, Ralph R. (1976) Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, reprinted 1999.

Red pebbles with pattern of quartz veins

Beach stone with white quartz on the beach

Red pebbles with pattern of quartz veins

Beach stone with white quartz on the beach

Beach stone with white quartz on the beach

Beach stone with white quartz on the beach

Beach stone with white quartz on the beach

Beach stone with white quartz on the beach

Pebbles on the beach

View looking south from Tracahill Beach

Close-up of Trabeg Conglomerate Formation

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Smerwick Harbour on the Dingle Peninsula

View looking due east across Smerwick Harbour showing outcrop of Silurian rock topped by rip-rap boulders

Smerwick Harbour on the north shore of the Dingle Peninsula in Ireland has a wide sandy beach overlooked on one side by mist-covered slopes of hills  and mountains, with Ballydavid Head and Pointe Bhaile Na NGall projecting into the sea, and the village of Murreagh nestling at the water’s edge. While on the other side lies the scalloped horizon of the Three Sisters with Smerwick Village in their hinterland. From the parking spot close to Na Cluainte, the sand stretches for about three kilometres, forming part of the extensive Dingle Way footpath, and the length is delineated by a small slipway at the northwest end, and a small promontory called Traigh an Fhiona at the southeast end.

The geology is so varied in this area that the two ends of this sandy beach are composed of entirely different rocks, with older compact and fractured layers of green and yellow Silurian siltstones of the Clogher Head Formation belonging to the Dunquin Group to the north – and younger coarser-grained, purple and red coloured Devonian conglomerates of the Trabeg Member of the Trabeg Conglomerate Formation of the Dingle Group to the south.

The differences in the two types of rocks are very obvious. They make an interesting contrast visually, and they afford a variation of habitat for seashore creatures, seaweeds, and lichens that colonise them. Between the two kinds of strata at the separate ends of this beach, the wide and mainly yellow sandy shore is subtlly coloured in some areas with shades of purple or pale green, reflecting the constituent grains derived from the local rocks. Pebbles exposed in wet patches at mid tide level exhibit many petrologies of which bright red stones of jasper are the most remarkable.

Some pictures illustrating these features are shown below.

Don’t forget, you can click on any photograph to enlarge the image.

Dunmore Head on the Dingle Peninsula

View of Slea Head on the Dingle PeninsulaPhotographs of the amazing Dunmore Head on the Dingle Peninsula on the West Coast of Ireland where the swell of azure blue waves crashes in white surf against the steeply sloping beds of Devonian strata in the cliffs, and breaks on the pinnacle-sharp rocks below. A small sandy cove, incredibly accessible even by car, is where visitors are privileged to picnic, build sand castles, brave the surf on boards, play among the rocks, and marvel at the views.

Don’t forget, you can click on any photograph to enlarge the image!

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved