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!
Between rocky outcrops and promontories at Ferriters Cove lie stretches of sandy beach with patches of water-worn beach stones and smaller pebbles. The stones are derived from a variety of Silurian strata, not only from this cove but also from the coast further to the north which is also composed of Silurian Period rocks.
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.
It was tranquil at Fermoyle on the Dingle Peninsula. Hardly a soul on the long sandy part of the beach, and only a solitary angler on the red rocky promontory at the western end. Vast swathes of short fruiting seaweeds clung to the rocks at the water’s edge, where the only sound breaking the stillness was the water gently lapping on the shore, while the seaweed danced slowly to the rhythm of the waves.
There are some unusual mini-habitats in the shallow pools that occupy the cracks and crevices between bedding planes of the Silurian strata at Ferriters Cove. Only a few seashore creatures are seen grazing within them. A few common periwinkles (Littorina littorea) and limpets (Patella sp.) are the most frequent inhabitants. The pools have a few isolated branching seaweeds – some miniscule fine branching greens but mostly tiny clumps of Coral Weed (Corallina officinalis).
However, the rock surfaces beneath the water are also covered with a continuous coating, or small individual patches, of a variety of coloured algae such as the chalky red encrusting seaweeds like Pink Paint Weeds (Corallinaceae crusts); dark red non-calcareous encrusting seaweeds (Hildenbrandia rubra and Peyssonnelia sp. are examples in this group but not necessarily present at this location); encrusting brown seaweeds (Aglaozonia sp. and Ralfsia verrucosa would be examples of the type algae in this group); and the commonly occurring bright green algal films. Less obvious to the naked eye but probably also present in this kind of habitat would be the biofilms created by microscopic cyanobacteria, fungi, and lichens.
It is difficult to identify the species or even genera without taking samples to section and examine under the microscope. A complication with the identification or classification of these encrusting seaweeds, particularly the dark reds, is that the crustal form may represent a true species in its own right, or it may simply be only one life stage (tetrasporophyte phase) in the life cycle of a more familiar-looking foliose (branching) seaweed, or even an extensive attachment disc for a foliose alga.
The Silurian rocks at Ferriters Cove are well known for their fossils. I found a few easily recognisable ones as I walked round the cove, such as the brachiopod Leptaena and Favosites coral. The fossils can be seen on the surface of the exposed bedrock and also in the numerous broken pieces of rock that lie on the beach.
Some features I am not sure whether they are fossils or not – they look as if they might be trace fossils – evidence of animal activity in the original sediments rather than the remains of the animal itself. These included some some fairly obvious branching linear features that could conceivably be evidence for crab burrows; each “burrow” is a couple of centimetres wide. The other features are more obscure and much smaller and occur as a pair of parallel curving lines rather like miniature army tank tracks. I thought they might be trace fossils of trilobite tracks. You’ll need to click on the images to view the features close-up and make up your own mind.
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.