Tag Archives: Carboniferous limestone

Rocks & Pebbles near Twlc Point

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Broughton Bay is a wide sandy expanse on the north shore of the Gower Peninsula in South Wales, facing the Loughor Estuary or Burry Inlet. A small promontory called Twlc Point at the western end of the beach has an interesting geology with an exposure of Hunts Bay Oolite from the Carboniferous Period. I have written about these strata in earlier posts such as:

Rocks on the west side of Broughton Bay Part 1

Rocks on the west side of Broughton Bay Part 2

Rocks on the west side of Broughton Bay Part 3

Brachiopod fossils in Hunts Bay Oolite at Broughton Bay

On this particular visit I was content to appreciate the way that pebbles of many types and colours on the upper shore were clustered around outcrops and boulders of the limestone which were often pink-tinged and sometimes fossiliferous.

Joggins Rock Textures 6

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Fractured and crumbling grey rock revealing red sediments

Joggins Rock Textures 6 – Fractured and crumbling grey Carboniferous rock, belonging to the Joggins Formation of the Cumberland Group, showing newly exposed red surfaces in the cliff face at Joggins on the Nova Scotia shore of the Bay of Fundy in Eastern Canada.

Joggins Rock Textures 5

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Calamites fossil replaced by siderite in Carboniferous strata

Joggins Rock Textures 5 – Fossil of the plant Calamites replaced by siderite in Carboniferous strata belonging to the Cumberland Group at Joggins Fossil Cliffs on the Nova Scotia shore of the Bay of Fundy in eastern Canada.

Joggins Rock Textures 4

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Calamites fossil replaced by siderite in Carboniferous strata

Joggins Rock Textures 4 – Fossil of the plant Calamites replaced by siderite in Carboniferous strata belonging to the Cumberland Group at Joggins Fossil Cliffs on the Nova Scotia shore of the Bay of Fundy in eastern Canada.

Joggins Rock Textures 3

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Calamites fossil in Carboniferous strata

Joggins Rock Textures 3 – More or less vertically aligned fossil stem of the plant Calamites (related to modern day horsetails) cutting through horizontally aligned Carboniferous strata at Joggins Fossil Cliffs on the Nova Scotia shore of the Bay of Fundy in eastern Canada. [Click to enlarge the image].

Joggins Rock Textures 2

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Rock texture and pattern in limestone cliff strata

Joggins Rock Texture 2 – Natural abstract patterns and texture in fractured cliff strata of Carboniferous limestone belonging to the Cumberland Group at Joggins Fossil Cliffs in Nova Scotia, Canada.

Joggins Rock Textures 1

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Fractured limestone rocks with an iron-stained stream

Joggins Rock Textures 1 – Fractured Carboniferous limestone rocks, belonging to the Joggins Formation of the Cumberland Group, at Joggins Fossil Cliffs on the Nova Scotia shore of the Bay of Fundy in Eastern Canada, showing the mouth of collapsed coal workings from which a deeply iron-stained stream is flowing.

Water-worn Limestone 3

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Natural sculpturing of limestone on the Worms Head Causeway in Gower, South Wales.

You would think that all the limestone strata on the Worms Head Causeway in Gower would be worn down equally to a smooth, flat, even surface – but not so. Upstanding at various points on what I suppose is really a wave-cut platform (albeit eroded by acid rain and seashore creatures as well), isolated areas remain standing. They look like giant teeth embedded in the worn surface strata. I do not know why these areas are more resistant, however, I have read that some parts of the limestone become harder by dolomitisation, a process in which the calcium carbonate is converted to magnesium carbonate by the intrusion of seawater (I think before the original sediments harden and compact). Maybe that is the explanation.

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.