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.
The rocks at Fall Bay are arrayed like the riffled pages of a book. Layer after layer of Carboniferous Limestone is sequentially spread out across the west side of the bay. Each layer has an observably different texture; some are bioturbated with bioclasts and fossils such as fragmentary crinoids and corals. The bedding planes of some strata have deeply sculptured surfaces from weathering and bioerosion. Lichens, barnacles and limpets colonise the rocks and take advantage of the meagre shelter offered by cracks, crevices, and solution hollows.
These three galleries show pictures of rock texture and pattern in strata of Caswell Bay Mudstone Formation (CBMF) at Caswell Bay on the Gower Peninsula in South Wales. This is the type location for the CBMF – the place from which the rocks were first described and named. The rocks are part of the Pembroke Limestone Group of the Tournasian/Visean epoch of the Mississipian subdivision of the Carboniferous Period. The Carboniferous Period lasted from 359 to 299 million years ago (mya) but the Tournasian/Visean part only lasted from 359 to 326 mya. The CBMF were deposited around the middle of that period. The total thickness of rocks deposited during the Tournasian/Visean epoch was around 750 metres but the CBMF is just a narrow band – with estimates of its thickness varying from 0-14 metres (George, 2008) to 3 – 7.5 metres thick (Barclay, 2011). The CBMF is sandwiched between the Gully Oolite Formation limestone below and the High Tor Limestone Formation above.
Barclay (2011) says that: the Caswell Bay Mudstone Formation is composed of thinly bedded calcitic and dolomitic mudstones and micritic limestones (George, 1978; Ramsay, 1987). The basal bed is a calcrete (Heatherslade Geosol of Wright, 1987b), with beds of algal laminate and oncoid limestone. The rocks are pale grey to greenish grey, buff, brown and yellow, locally with some red staining.
Barclay says there are some but not many fossils. Also that: the rocks formed in shallow water environments referred to as “lagoon phase” by Dixon and Vaughan (1912). They are interpreted as shallow-water, peritidal deposits formed in a tidal flat lagoon complex behind a beach barrier in a humid climate, with abundant evidence of sub-aerial exposure in the form of dessication cracks (Riding and Wright, 1981; Wright, 1986; Ramsay, 1987).
The Caswell Bay Mudstone Formation lies unconformably on top of of the sub-aerially weathered palaeo-karst surface of the Gully Oolite Formation limestone. It means that there was a time lag between the deposition of the limestone and the next phase of deposition of the mudstones. The palaeo-karst surface is full of dissolved pot-holes. These pot-holes are a common karstic feature on Gower south coast beaches – with Mewslade Bay and the Worms Head Causeway exhibiting some good examples.
The Gully Oolite Formation limestone was deposited in warm tropical seas at a time when sea-level was standing still or slowly falling. The extended period of sub-aerial weathering that created the palaeo-karst surface occurred during a significant relative fall in sea-level (George, 2008, 85). The Caswell Bay Mudstone Formation was formed during a subsequent phase of slow sea-level rise.
Barclay, W. J. (2011) Geology of the Swansea District: a brief explanation of the geological map Sheet 247 Swansea, British Geological Survey, Natural Environment Research Council, ISBN 978-085272581-8, pp 4-6.
Dixon, E. E. L., and Vaughan, A. (1912) The Carboniferous succession in Gower (Glamorgan) with notes on its fauna and conditions of deposition. Quarterly Journal of the Geological Society of London, Vol. 67, pp 477-571.
George, G.T. (2008) The Geology of South Wales: A Field Guide, published by email@example.com, ISBN 978-0-9559371-0-1, pp 82- 86.
George, T. N. ((1978) Mid Dinantian (Chadian) limestones in Gower. Philosophical Transactions of the Royal Society, Vol. B282, pp 411-462.
Ramsay, A. T. S. (1987) Depositional environments of Dinantian limestones in Gower, 265-308 in European Dinantian environments, Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Sons Ltd.)
Riding, R. and Wright, V. P. (1981) Palaeosols and tidal flat/lagoon sequences on a Carboniferous carbonate shelf: sedimentary associations of triple disconformities. Journal of Sedimentary Petrology, Vol. 51, pp 275-293.
Wright, V. P. (1986) Facies sequences on a carbonate ramp: the carboniferous Limestone of south Wales. Sedimentology, Vol. 33, pp 221-241.
Wright, V. P. (1987b) The ecology of two early Carboniferous palaeosols, 345-358 in European Dinantian environments. Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Son.)
In an earlier post about rock textures and patterns at Tenby in South Wales I said that some rock surfaces reminded me of elephant hide. So, shown above are a few photographs that I took of the elephant skin on a prepared specimen exhibited at the Natural History Museum in London to show you what I meant – while below are a couple of examples of the textured limestone from Tenby for comparison.
Many of the large boulders that have fallen from the crumbing cliffs down to the shores of Chapmans Pool in Dorset, England, show signs of being weathered by exposure to the elements. Over long periods of time, rain, sun, frost, chemical weathering from acid water and from biogenic elements, mechanical weathering by wind-blown sand, and abrasion by wave-transported gravel and pebbles, have removed the surface of the rocks and revealed an amazing array of textures and patterns.
Softer rock matrices have been eroded to reveal harder structures normally hidden from sight within the boulders. These structures include networks of crystalline veins, as well as trace or ichno-fossils (where the hollows of burrows excavated by crustaceans in the sediments of ancient shores have been preserved, as the sediments were consolidated into rock, and eventually the spaces have been in-filled with harder and more resistant materials). It is interesting to see how small periwinkles frequently taken advantage of the dips and hollows of these eroding rocks to find shelter at low tide.
Barton, C. M.; Woods, M. A.; Bristow, C. R.; Newell, A. J.; Westhead, R. K.; Evans, D. J.; Kirby, G. A.; Warrington, G. (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast, Special Memoir for 1:50 000 geological sheets 328 Dorchester, 341/342 west Fleet and weymouth and 342/343 Swanage, and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 339 Newton Abbot, Compiled by M. A. Woods, British Geological Survey, Keyworth, Nottingham.
Levin, H. L. (1990) Contemporary Physical Geology, Third Edition, Saunders College Publishing, Chapter 5: Weathering and Soils, pp 109 – 120 & 128, ISBN 0-03-031139-x.
Nichols, G. (2009) Sedimentology and Stratigraphy, Second Edition, Wiley-Blackwell, 6.4 Weathering Processes, pp 89 – 92, ISBN 978-1-4051-3592-4.
West, Ian M. 2013. Chapman’s Pool (Chapmans Pool), Houns-tout and Egmont Bight, Kimmeridge region, Dorset; Geology of the Wessex Coast (Jurassic Coast, World Heritage Site) of southern England. Internet site. http://www.southampton.ac.uk/~imw/Chapmans-Pool.htm. Ian M. West, Romsey, Hampshire. Version: 14th December 2013.
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You don’t have to be that adventurous to find out about the geology in a given area. You can discover a lot by exploring the roadsides and urban environments. Very often the local rocks are used for buildings and sea defences; or quarries and cuttings can be seen from the car as you travel along the highway.
In Lunenburg, Nova Scotia, most of the buildings in the old part of town are wooden. However, the odd garden boundary wall is made of stone, using the local rock type. The photographs in this post, though taken on a very dull wet day, show a wall of rocks that are particularly colourful with rusty stainings from the iron minerals they contain; and they also display patterns of the thin layers or beds in which the fine sediments were laid down. The rocks are most likely to belong to the Halifax Formation, dating from the late Cambrian to middle Ordovician Periods, i.e. 499 – 470 million years ago. The original deposits of sedimentary rock have undergone metamorphosis and been converted into slates and metasandstones.
COPYRIGHT JESSICA WINDER 2013
All rights reserved