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.
This is the third in a series of photographs of Silurian rocks from Clogher Bay. A brief examination of the literature indicates that the rocks in these pictures belong to the Drom Point Formation which has accumulated to a depth of 300 metres and is part of the Dunquin Group of Silurian Period strata in Ireland. The Drom Point and Croagh-marhin Formations consist of shallow-marine, fossiliferous siltstones and very fine to fine grained sandstones.
By the time I had reached the northernmost edge of the beach at Ferriters Cove, the Silurian rocks had changed their appearance again. The cliff here is higher and composed of a wonderful patchwork of mainly yellow slabs with purple-grey markings. Many of these slabs have fallen to the shore in a thick loose layer. Among these pieces of stone I found some more fossils, internal casts and impressions of brachiopods, including different species to the one I found earlier (I thought that might be Leptaena sp.). Two particular brachiopods are mentioned on the sign at the entrance to the beach, Holcospirifer (bigugosus?) and Rhipidium (hibernicum?), and it is likely that the fossils in images 57b,c,& d belong to one or both of those species. I am wondering if the much larger regular rounded fossil in image 57a is a species of Atrypa.
Bassett, M. G., Cocks, L. R. M., and Holland C.H. (1976) The affinities of two endemic Silurian brachiopods from the Dingle Peninsula, Ireland, Palaeontology, Vol. 19, Part 4, pp. 615 – 625, pls. 93-95.
Water-worn, soft, and stripey Silurian sedimentary rocks make sporadic appearances through the sandy beach at Ferriters Cove and sometimes they can have a strangely sculptural appearance, or even of a landscape in miniature, depending on the perspective from which they are photographed. I also particularly like the pale blue-green colour contrasting with the muted yellow that contributes to the natural abstract striped designs.
You can click on an image to enlarge it.
The last part of the Silurian strata exposure of the small rocky promontory at Ferriters Cove, before the wide sandy strip with beach stones, is very abstract…. but not so sculptural in appearance as the patches of water-worn mudstones that emerge here and there through the sand – see the next post!
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.