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

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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.

Ferriters Cove on the Dingle Peninsula

Layers of upstanding Silurian rock on the beach

The mountains look down on the golden crescent of sand at Ferriters Cove. It is isolated and peaceful – where the sound of gently lapping waves is only occasionally broken by raucous calls when flocks of oyster catchers or herring gulls suddenly take flight.

Rock layers here stand up like stacked tombstones with wide knife edges, or stumps of strata with sharp points protrude from the surface like nails on a fakir’s bed. The rocks are fossiliferous marine Silurian sediments, from the Ferriters Cove Formation in the Dunquin Group, dating from between 423 and 395 millions of years ago. They are composed of pale brown, yellow, grey and red siltstones, mudstones, and sandstones. They were deposited in a shallow sea with active volcanoes on its shore and hinterland, which produced volcanic deposits such as lava and tuffs.  The character of the rocks changes as you walk along the beach. Fossils such as brachiopods, corals, and trilobites are found in the mudstones.

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

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Beach Boulders at Eype 3

Another gallery of rock pictures from Eype beach. I hope the photographs in this and preceeding posts on the same subject will prove a useful resource for students of geology as well as those interested in the aesthetics of geology.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Beach Boulders at Eype 2

Sedimentary rock boulders on the seashore extending into the sea

Another gallery depicting the amazing rock boulders on the seashore at Eype in Dorset, England, and the contexts in which they are found on the beach.

The cliffs at Eype, and the boulders on the beach below them, are made of Middle Lias sedimentary rocks from the Jurassic Period. It is difficult to judge the scale of the cliffs from looking at the photographs, so you might like to know that the silty sand and hard sandstone bands of the Three Tiers layers at the base can reach a maximum of 9m thick (although much of that is buried in this location); the clays and sandstones of the Eype Clay Member above that are 60 metres deep; on top of that the soft sands of the Downcliff Sands Member are 30 metres deep; and above that the soft sandstones of the Thorncombe Sands Member are 21 metres thick. This makes for a sequence of rock strata measuring a potential 120 metres (390 feet) in height – the measurements are approximate as the depth of the layers varies a lot.

In their book Classic Landforms of the West Dorset Coast, Brunsden and Goudie state that

The coastal cliffs of West Dorset owe their shape to the relief and orientation of the coastline, the variable properties or lithology of the rocks, the geological structure, the history of relative land and sea movements, the sequences of environmental change, the difference in erosional energy of the sea between the more exposed and sheltered parts of Lyme Bay, and to the complex sub-aerial processes which currently act on the cliffs themselves.

At Eype, the boulders on the beach demonstrate not only the variable lithology of the rocks in the cliffs above but also their structure. The alternating bands of different rock composition and colour are evidence for the cyclical nature of changing environmental conditions throughout geological history, including changes in sea level relative to land; while their presence on the seashore illustrates the ongoing erosional processes affecting the coastline to this day.

COPYRIGHT JESSICA WINDER 2014

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Beach Boulders at Eype 1

Stratification in a beach boulder

As the soft rocks of the exposed Eype Clay Member mudstones in the cliffs at Eype are eroded, the harder sandstone and limestone rocks (that were laid down at a later date above them) are under-mined. The unsupported rocks then break under their own weight, slide down the cliff and eventually come to rest on the orange gravel and pebbles of the shore. At the western end of Dorset’s Eype beach, part of the Jurassic Coast World Heritage Site, the boulders extend continuously from the base of the cliff out into the sea. Elsewhere along the shore, the boulders are clustered at the cliff foot adjacent to the soft micaceous mudstone layer. Most of the images shown here were photographed in a stretch of shore just a couple of hundred metres long.

The boulders tend to be very large, some about two metres in height and width. Their shapes are endlessly variable and their colours are generally complementary shades of yellow and grey. They can be patterned by layers of contrasting width, colour, and composition; or by scattered fossils and iron nodules. This is the first of several posts showing the fascinating variety of composition and form of these boulders, and the contexts in which they can be found. The boulders in these photographs are the same ones from which I photographed the rock textures in an earlier post. Every boulder seems unique in its appearance.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved