Wave-induced sand ripples with drainage patterns

Natural sand ripple patterns with dendritic drainage channels

I thought this was an interesting phenomenon. Wave-induced sand ripples, that were merging into a tide pool at the base of the cliffs at Rhossili on the Gower Peninsula, had curvilinear and compressed dendritic drainage channels in the narrow valleys between the ridges.

Rocks at Ferriters Cove 10

This marks the final post in the series about the rocks at Ferriters Cove. I had spent a happy few hours on the beach and reached the limit of accessible shore at Ferriters Cove. Time to call it a day. At this northernmost part of the shore, the steeply sloping strata in the cliff, with the bedding planes facing outwards as a continuous sheet, at first seem to be buckling under their own weight, as seen in images 55a and 56 in the previous post. Then, just a few metres further on, the strata can be viewed side-on across the bedding planes with the sequence of individual layers revealed. The strata are curved concavely so that the cliff face is like the under-side of a huge wave, the crest of which is curving over and about to crash down and break. You can see this best in images 60 and 61.

There are also some enigmatic markings on that part of the bedrock on the beach which is covered each day by the tide. I wonder if these are fossils. Photo 73 has a number of rounded shapes that look like they might be gastropods; and Bembexia is a marine snail that is recorded in this locality.

More problematic are the plant-like patterns which occur on a number of rocks (see images 79 – 81). They seem to have a central stem with numerous branchlets along the length. I am not at all certain that these are fossils although they seem to be integral with the surface of the rock and to have a slightly different composition which is reflected in the fact that there is no black biofilm (maybe lichen) growing on them. I am fairly sure that the ‘plants’ are not grazing trails left by the feeding activities of the adjacent limpets and periwinkles. Plants are in fact recorded from the Silurian but I cannot find any illustrations that resemble these Ferriters Cove ‘plants’.

In an article about the Silurian Period on the website of the University of California Museum of Paleontology it says:

Perhaps the most striking of all biological events in the Silurian was the evolution of vascular plants, which have been the basis of terrestrial ecology since their appearance. Most Silurian plant fossils have been assigned to the genus Cocksonia, a collection of branching-stemmed plants that produce sporangia at their tips.

However, drawings of that particular genus show a very different branching system to that exhibited by the Ferriters Cove ‘plants’. Maybe I will get a clearer understanding when I have tracked down some of the specialist research papers on the fossils of this area such as those written by C. H. Holland:

Holland, C. H. (1969) Irish counterpart of the Silurian of Newfoundland. Memoir of the association of Petroleum Geologists 12, 298-308.

Holland, C. H. (1987) Stratigraphical and structural relationships of the Dingle Group (Silurian), County Kerry, Ireland. Geological Magazine 124, 33-42.

Holland, C. H. (1988) The fossiliferous Silurian rocks of the Dunquin inlier, Dingle Peninsula, County Kerry, Ireland. Transactions of the Royal Society of Edinburgh: Earth Sciences 79, 347-360.

Rocks at Ferriters Cove 8

Natural abstract design in Silurian rock at Ferriters Cove

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.

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Rocks at Ferriters Cove 7

Natural pattern, shape, and texture in Silurian bedrock at Ferriters Cove

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!

Rocks at Ferriters Cove 6

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.

Fossils at Ferriters Cove 1

Fossil brachiopod Leptaena in Silurian rock at Ferriters Cove

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.

Fossil brachiopod Leptaena in Silurian rock at Ferriters Cove

Fossil brachiopod Leptaena in Silurian rock at Ferriters Cove

Fossil brachiopod Leptaena in Silurian rock at Ferriters Cove

Fossil coral in Silurian rock at Ferriters Cove

Fossil coral in Silurian rock at Ferriters Cove

Could this be a trace fossil of animal burrows in the Silurian rock at Ferriters Cove?

Could this be a trace fossil of animal burrows in the Silurian rock at Ferriters Cove?

Could this be a trace fossil of animal burrows in the Silurian rock at Ferriters Cove?

Could these be trilobite tracks in Silurian rock at Ferriters Cove?

Could these be trilobite tracks in Silurian rock at Ferriters Cove?

Rocks at Ferriters Cove 5

Silurian rocks on the beach at Ferriters Cove

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.

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove

Silurian rocks on the beach at Ferriters Cove