The character of the rock changes as I continue my walk around the shoreline at Ferriters Cove in the Dingle Peninsula. Successive Silurian bedrock strata have different textures, colours, shapes, and sculpturings, each layer having originally been laid down on the bottom of an ancient shallow sea in varying environmental conditions that affected the chemical constituents and particle size of the sediments deposited, and the subsequent disturbance of each new layer.
The most readily available literature on the Silurian of the Dingle Peninsula does not provide enough details to enable me to understand what has specifically caused the different compositions seen in the sequence of strata in the Dunquin Group at Ferriters Cove (Cuan an Chaoil) itself. I can say though that these sedimentary rocks were deposited approximately 410 million years ago during the Silurian Period in a shallow sea with active volcanoes on its shore and hinterland. The sediments include pale brown, yellow, grey and red mudstones, siltstones and sandstones (frequently very fossiliferous) interbedded with volcanics such as lithic tuffs and lavas.
The sediments were no doubt laid down in this shallow sea in a series of episodes, each reflecting changes in that environment brought about in some part by increase and decrease in depth of the water. The polar ice caps increased and decreased in size during that time resulting in more or less water in the sea, and greater and lesser incursions onto the land. Volcanic ash and fragments would also have periodically rained down on the water and settled to the sea bed.
The photographs in this post show what I think is a particularly attractive group of rock layers. The colours are remarkable – though on another day and in a different light they might not look the same. I wondered if the polygonal pattern was dried cracks in the original soft sediment – but maybe not because the origin of the rock is from sediments laid down in a shallow sea – at this stage I don’t know how feasible an explanation drying out of the sediments by exposure to air would be.
I was only able to investigate a small part of this series of Silurian strata. Greater variations in composition and type are exposed further north along the shoreline in the locality. They include, for example, dark purple porphyritic lava, with large platy phenocrysts with flow alignment – the oldest unit of the Dunquin Group. I would have loved to have seen that. I really will have to go back to Ferriters Cove and discover more of its fascinating geology another time.
The parallel lines of thin strata look like sloping stacks of tombstones in some places on the beach at Ferriters Cove. Rocks at mid to low shore level tend to be superficially darker because of recent wetting, and encrusting biofilms of bacteria, lichen, algae and invertebrate organisms. The dry bare rocks at the top of the shore, however, reveal their true colours. The way that the sharp-edged and angular Silurian sedimentary rock layers project from the sand reminds me of the occasion when my front lawn was covered in broken slate tiles that had embedded themselves in the turf like so many thrown daggers after a violent storm had dislodged them from my roof.
Continuing my geological excursion around Ferriters Cove, the standing height of the jutting strata on the shore increases steadily from ankle height to shin, to knee, and then hip height. It is interesting to note that a few of the exposed rock layers have been eroded in a strange way, maybe because the sediments of which they are composed are softer than the other layers. The weathering of them has resulted in an irregular surface sculpturing, as shown in some of these pictures.
The patterning in this instance is vaguely reminiscent of the large trace fossil burrows that I have previously seen, for example, in Jurassic rocks on the Dorset coast at Winspit and Lyme Regis. I am not sure about these at Ferriters Cove. I think the texture is probably just a reflection of the uneven hardness of the rock. I have found definite “chondrites” fossil burrows in Silurian rocks elsewhere on the Dingle Peninsula at both Smerwick Harbour and Clogher Bay but those trace fossils were distinct and on a much smaller scale.
To continue the investigation of rocks at Ferriters Cove, I followed the curve of the cove to the right (or roughly northwards), where the Silurian strata become higher, exposing their sharp thin edges as they thrust through the beach surface. At high tide level they are bare – no seaweed or encrusting organisms. As the rock layers pass into the base of the low cliff, they retain their steep angle. However, immediately above the bedrock is a thick layer of broken shards lying almost horizontally but with some perturbations; I don’t know how they got like this but wonder if it has something to do with glaciation. This layer of broken pieces is overlain by softer, looser sediments – possibly more geologically recent aeolian or wind-blown accumulations. The clean, dry rocks have pleasing pale green-grey and orange colours.
The wide sandy beach that you see as soon as you get to Ferriters Cove, gives way to a broad expanse of olive green seaweeds at mid tide level that conceals an uneven pavement of low-lying and jagged rocks to which they are attached. These rocks are the first signs of the incredible ancient fossil-bearing Silurian strata for which this cove is famous. Walking around the cove in a northerly direction reveals, one-by-one, a series of strata, each with a character of their own in terms of colour, texture, and shape, and terminating in the massive flat shining slabs of rock that face the cliffs in the distance below Ferriters Castle.
This is the first in a series of posts illustrating the changing nature of the Silurian strata in the sequence exposed at this location.
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.
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!