Tag Archives: sedimentary rocks

Rocks at Dunquin on the Dingle Peninsula

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The Blasket Islands are deserted now but at one time the small pier at Dunquin was a lifeline for the small island community. It is hard to imagine the hardships of their existence and the way they would have navigated in all weathers across the short stretch of water to the mainland of the Dingle peninsula in tarred canvas-covered open boats called curraghs. For the islanders wanting to buy or sell goods, needing a doctor, having to attend church, confession, christenings, weddings, or funerals, or to visit mainland friends and family, Dunquin was an important landing place. The very basic, even primitive, life of the islanders is movingly and simply told in The Islandman by Tomás O’Crohan  who lived and died on Great Blasket Island (1856 to 1937). Nowadays, it seems to be mostly small boats that launch from the pier to ferry tourists to the uninhabited islands .

Dunquin harbour not only has this important historical association but it is also a noteworthy geological location. Walking down the steep, zig-zag path from the stone-walled green fields above to the beach and pier below, there are great views of the cliffs to the north and south of the harbour. It is a transition zone between two major geological periods – where a predominantly marine environment changed to a mainly terrestrial one due to vascillating sea level relative to the land. It is the location where yellow marine siltstones belonging to the Drom Point Formation of the Silurian Dunquin Group lie next to the reddish, purplish, and greenish sandstone strata of the Silurian/Devonian Dingle Group which are terrestrial in origin.

The cliff faces seem to be striped in contrasting subtle hues. The rock layers are steeply angled now following earth movements over the many millions of years since they were originally laid down in a horizontal position. Odd circular or spherical formations can be seen in some layers. The bedding plane of one outcrop next to the pier has a roughly polygonal pattern of drying mud cracks preserved in the stone. Curving veins of quartz cut across the strata to the south of the harbour.

Altogether a very good place for rock enthusiasts and well worth a return visit. Next time I would like to take a boat trip to the Blasket Islands where (on Inishvickillane at least) the rocks are mainly volcanic tuffs and lavas.

REFERENCES

O’Crohan, Tomás,  1937 The Islandman, Oxford University Press, ISBN 978-0-19-281233-9, re-issued 2000.

Horne, Ralph R., 1976, Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, Minister for Industry and Energy, Geological Survey Office. Reprinted 1999.

Fault Zone Rocks at Clarke Head Part 1

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A mélange of rock textures from the fault zone at Clarke Head, near Parrsboro in Nova Scotia, Canada. The geology here is extremely complex and I have only just begun to unravel what is going on. Key research papers with precise details are not easily accessible. Others are a bit too generalised to enable me to identify exactly each rock type that I photographed….for the moment. I will update when I can be sure I have accurate identifications. The variety was wide and included igneous, sedimentary and metamorphic rocks. It is the same place that I photographed the satin spar gypsum. The colours, textures, and patterns are amazing.

Water-worn Limestone 2

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Weathered limestone rock layers on the Worms Head Causeway in Gower, South Wales.

The limestone further east along the Worms Head Causeway shore, towards Tears Point, displays the results of a number of erosion agents leading to some curious formations. The clean, smooth surfaces of mounded and hollowed shapes result from mechanical abrasion where the rock is pounded by stones carried in the waves; by chemical and physical erosion caused by micro-organisms and marine invertebrates (bio-karst surfaces made by such organisms as lichens, limpets, and sea urchins); and acid dissolution by rainwater when the tide is out, especially around the edges of pools, in natural fissures like joints and bedding planes, and areas where water constantly drains – resulting in what is called karst topography. Small circular pits (image 48) of dissolved limestone readily connect with each other, soon enlarging into bigger pools that are known as kamenitzas – which in turn can interconnect with other pools as seen in the images below (particularly images 49, 50, and 51).

Water-worn Limestone 1

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Water-worn limestone rock layers on the Worms Head Causeway in Gower, South Wales.

The Carboniferous sedimentary strata outcropping on the landward shore of the Worms Head Causeway at Rhossili show differential erosion by the sea. Some areas of the Black Rock Limestone Subgroup are clean, smooth and worn down whilst others are sharp and jagged with encrusting biofilms and barnacles. This is partly due to the varying compositions and relative hardness of the different strata, and partly to the way in which the waves with their rock-bearing loads seek lines of least resistance in the shore with each tidal ebb and flow. Areas of weakness, for example, between bedding planes and in minor faults with veins of soft white crystalline calcite and red haematite, are more vulnerable to repeated abrasion. This has led to the formation of numerous channels, gullies, and basins among other more resistant rock outcrops. Rounded pebbles and cobbles frequently lying within the hollowed areas evidencing their role in wearing the bedrock away. Mechanical abrasion allied to varying rock resistance is not the only way that the limestone is altered. Elsewhere on the causeway, limestone acid dissolution and marine organisms are the most common agents of natural change in surface texture and sculpturing, creating karstic and bio-karstic limestone topography.

Beach Boulders at Charmouth (East) 2

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Natural fracture patterns in beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Here are some more pictures of the boulders at the eastern end of Charmouth Beach in Dorset, England, all exhibiting natural fracture patterns in sedimentary rock belonging to the Jurassic Charmouth Mudstone Formation. I’m not sure which particular layer they come from but it could be the Black Ven Marl Member. Perhaps someone can help me out with the identification? These images show the boulders at the foot of the cliff adjacent to the landslip or mud slide. In contrast to the dark boulders at the water’s edge shown in the previous post, these are dry and therefore lighter in colour.

I wonder if these boulders could have been the inspiration for an artwork in the sculpture park in Tout Quarry on the Isle of Portland featured in an earlier post.

Portland Stone sculpture at Tout Quarry, Isle of Portland, Dorset, UK on the Jurassic Coast - polyhedron (11)

Beach Boulders at Charmouth (East) 1

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View looking east at Charmouth Beach, Dorset, England.

The shoreline at Charmouth looked particularly dramatic on this April visit as storm clouds periodically burst and blue skies were only intermittent. Charmouth Beach lies on the World Heritage Jurassic Coast in Dorset, England. The rocks are mainly Jurassic Period Charmouth Mudstone Formation. The character of the cliffs changes as you walk from west to east because the sedimentary rock layers gently slope and disappear beneath the beach surface level while new rock strata are freshly revealed at eye level. The predominance of softer rocks has led to a great deal of cliff slippage, and this means that the chronological sequence of the layers is frequently obscured by fallen debris; it makes it difficult to distinguish which rocks are which. The numerous rockfalls regularly contribute to the boulders on the beach and in this post I feature some boulders that exhibit some interesting fracture patterns. Of course these are not the only rock type on the beach, and I will post some more photographs of other patterns and textures in boulders and in the cliff face on the eastern half of Charmouth Beach in due course.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach Boulders at Charmouth Beach (East) 3

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Detail of pattern and texture in a beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Close-up of pattern and texture in a beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Beach boulders at Charmouth on the World Heritage Jurassic Coast in Dorset, England.

Rock Textures at Little Tor, Gower

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Little Tor cliff at the east end of Oxwich Bay in Gower, South Wales, is made of Carboniferous Limestone of the Hunts Bay Oolite Sub Group. In common with beach outcrops of the same type of rock at Broughton on the north Gower coast, and Tenby that lies further west in Pembrokeshire, the surface is marked on a small scale with scalloped depressions and branching runnels that are the result of acid erosion and sand abrasion, giving rise to interesting textures and patterns.

The small sinuous etchings are called microrills (Ford and Williams 2007). They are typically 1 mm wide, round bottomed dissolution channels that are found close together. The pattern is reminiscent of rain running down a window pane. On gentle rock slopes they have curving paths and divide and rejoin in a network-like pattern. On steeper gradients the channels are straighter. Some microrills are made by slightly acidic water flowing down the rock surface but in other instances they are caused by the “water moving upwards, drawn by capillary tension exerted at an evaporating front. Capillary flow is believed to explain much of their characteristic sinuosity”.

REFERENCE

Ford, D. and Williams, P. (2007) Karst Hydrogeology and Geomorphology. John Wiley & Sons, Chichester, England. Revised Edition, p324.  ISBN 978-0-470-84997-2.

Great Tor, Gower

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The limestone formation of Great Tor on the Gower Peninsula

The magnificent limestone outcrop that separates Three Cliff Bay from Oxwich Bay on the Gower Peninsula in South Wales is known as Great Tor. It is composed of steeply dipping layers of Carboniferous Period sedimentary rock called Hunts Bay Oolite Sub Group.

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

The limestone formation of Great Tor on the Gower Peninsula

Rocks at Spencer’s Island

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There is an island called Spencer’s Island just off shore in the north Minas Basin in Nova Scotia, Canada. The same name also applies to the small community on the mainland opposite the island. It is the rocky outcrops along the beach of the mainland that feature here. While the island itself is mainly igneous basalt extruded in the Jurassic period, the steeply dipping strata along the mainland shore are composed of Carboniferous period Cumberland Group rocks (approximately the same type as are found at the famous Joggins Fossil Cliffs further north).

At the moment, I have not managed to find information to explain the details of these rocks but they certainly are interesting to look at. I know that they are composed of sandstones, siltstones, and shale (I didn’t notice any conglomerate or coal which are typical of this rock group). They are rapidly wearing away. They are severely impacted by strong tides and currents especially over the winter months. The outcrops reveal multiple layers of sedimentary deposits which occur in a cyclical way with harder more solid bands alternating with softer finely laminated ones. This reflects the depositional environment which was changing on a regular basis. All the rocks are subject to fine fracturing on a massive scale so that the slightest touch would cause the crumbling fragments to cascade to the beach. Sometimes the weathering process has led to falls of large boulders. Sometimes the outcrop is reduced to fine scree.

It was a very cold, dull, windy, wet day in late May when we visited Spencer’s Beach and we were very glad to discover a delightful beach cafe where we filled up on coffee and clam chowder. The beach was once a busy ship-building yard – famous as the place that built the Mary-Celeste.

Rock Formations at Langland Bay

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Carboniferous limestone rock formation at Langland Bay

This post provides a context for the earlier post of mostly close-up images in Rock Textures at Langland Bay 1. Langland Bay is a popular beach near Swansea in South Wales. It is located on the south coast of the Gower Peninsula. It has a wide stretch of lower sandy shore, and pebbles landward in the central part. There are also wide expanses of low-lying rock platforms with hundreds of shallow pools in which to hunt for seashore creatures. To each side of the bay low cliffs of Carboniferous period sedimentary rock are overlain with much more recent glacio-fluvial and later poorly consolidated deposits.

The distinct layers of Hunts Bay Oolite, High Tor Limestone, and Penmaen Burrows Limestone form the southwest limb of the Mumbles anticline that extends from east to west. Here at Langland the sea has breached the rocks to create the embayment. The strata are riven by numerous joints and minor faults that have allowed the sea to penetrate, eroding away the rock to form small coves, undercuts, caves, tunnels, and passages to explore. The photographs shown here probably do not do justice to the site, as it was a very dull and overcast day when I visited, but I hope they serve to illustrate that the geology of Langland Bay is interesting from many points of view.

REFERENCE

Bridges, E. M. (1997) Classic Landforms of the Gower Coast, Series Editors R. Castleden and C. Green, The Geographical Association and The British Geomorphological Research Group, page 17. ISBN1-899085-50-5.