Rock Clefts in Rhossili Cliffs

Crack in rock with calcite and heamatite

Extreme low tides at Rhossili on Gower allow access to parts of the cliffs less frequently inspected. The distal north face of the Rhossili headland shows a complex mixture of natural and man-made features, on both a minor and major scale, often impossible to distinguish. The Carboniferous limestone was at one time quarried and this has resulted in changes of shape in the cliff face. However, most of the characteristic cracks, crevices and caves are the result of the waves and the weather working upon naturally-occurring fissures and cavities in the exposed bedrock surface.

On a small scale, it is possible to see where fractures in the Black Rock Limestone, sometimes infilled by white crystalline calcite and red haematite, are the lines of weakness along which erosion is progressing. These natural clefts are typically wider towards the outside, narrowing down to the width of the crack. The wearing away and widening of the break in the rock is initiated by mechanical and chemical action of waves and rain. Once the crevice has started to widen, it becomes a habitat for small organisms which add to the erosion process, again by both mechanical action and acid erosion.

In the photographs accompanying this post, it is possible to see some conical limpets, small acorn barnacles, and spots of bright orange sponge taking advantage of the relative shelter of these natural clefts; while minute blue-black mussel spat frequently settle (at least temporally) in the narrow confines of the crack itself. The clefts tend to retain moisture to a greater degree than the main cliff face, and may act as channels for water to drain downwards to the beach. These damp locations make ideal places for microscopic algae and bacteria to grow, and for mud-tube dwelling marine worms to thrive, especially low down, near the mobile beach sediments. Mostly, it is the former presence of the worms that is indicated. The level of the sand and pebbles fluctuates and affects the survivability of the worms. They leave a strange texture on the rock surface – composed of thousands of burrows or tunnels, just a few millimetres in width, that were incidentally etched by their acidic metabolic waste products.

Natural abstract image of a crack in the rock of a cliff face with worm burrows

Natural abstract image of a crack in the rock of a cliff face with limpets

Natural abstract image of a crack in the rock of a cliff face with sponge, worm burrows and barnacles

Natural abstract image of a crack in the rock of a cliff face with sponge and worm burrows

Natural abstract image of a crack in the rock of a cliff face with pebbles

 Rock cleft in a cliff face

Natural abstract image of a crack in the rock of a cliff face with surface texture showing worm burrows

Natural abstract image of a crack in the rock of a cliff face with surface texture showing worm burrows

Natural abstract image of a cracks in the rock of a cliff face with worm burrows

A recessed gully where a rock crack has eroded back

Rock cleft in a cliff face

Rocks at Redend Point in Studland Bay – 5

Natural iron pipes protruding from the rock on the floor of a cave

A strange phenomenon of the colourful sandstone at Redend Point in Studland Bay is the natural occurrence of hollow pipes or tubes running through the rocks. No-one seems certain about the way they have formed geologically but it is something to do with the way that water-borne iron minerals have settled out in the sandstone.

You can see the open ends of these intriguing features underfoot in the wave cut platform around the base of the cliffs – half concealed sometimes by seaweed and flint pebbles; on the floors of shallow caves where they can resemble small volcanoes; on the ceilings of the undercut rock where you can sometimes see daylight shining through the tubes from higher up in the cliff; broken open in fallen beach boulders and on the cliff face itself; and as strange pedestals at the base of the cliff where the sea has eroded away the softer surrounding rock to isolate the naturally occurring iron-lined hollow pipes.

Natural iron pipes protruding from the rock on the floor of a cave

The openings to natural iron pipes running through sandstone seen on the ceiling of a cave

Natural pipe at the base of a sandstone cliff isolated by the sea eroding the softer surrounding rock

Natural iron-lined pipes exposed by breakage in a beach boulder

Natural iron-lined pipes exposed by breakage in a beach boulder

Natural pipe at the base of a sandstone cliff isolated by the sea eroding the softer surrounding rock

Iron-lined cavities and pipes in Redend Sandstone at Studland Bay

Rocks at Redend Point in Studland Bay – 4

Orange cliff rocks on the north side of Redend Point in Studland Bay, Dorset, England.

I revisited Redend Point at Studland in Dorset yesterday for the first time in several years. Here are some of the pictures I took. I was only able to look at the north side of the Point because of the state of the tide. The colours seem different from my last trip there. This could be to do with how much rain there has been but also possibly to do with the weathering affect on the iron. [The part of the Point with the wonderful pink and yellow stripes and patterns was further on – to the south of the Point which I could not reach].

More rocks have fallen from the ferruginous sandstone and from the overlying clays. This has brought down a large tree which now lies across the beach. In some areas the sea has undercut the sandstone to produce small caves. These have floors composed of a mixture very fine pale sand, rust-stained flints from the nearby chalk strata around the corner, and bright orange sandstone with pot-holes and eroded channels draining seawards. In this northern part of the Point the colours manifest by the Redend Sandstone seemed less varied than four years ago, and the carved graffiti was much greater than previously noted. Such a shame that almost every surface was disfigured.

Beach boulder and pebbles on the north side of Redend Point in Studland Bay, Dorset, England.

Flint pebbles and boulders on the north side of Redend Point at Studland Bay in Dorset, England.

Flint pebbles on the north side of Redend Point at Studland Bay in Dorset, England.

Beach boulder and pebbles on the north side of Redend Point in Studland Bay, Dorset, England.

Cliff rocks on the north side of Redend Point in Studland Bay, Dorset, England.

Beach boulder and pebbles on the north side of Redend Point in Studland Bay, Dorset, England.

Lower cliff rocks on the north side of Redend Point in Studland Bay, Dorset, England.

Lower cliff rocks on the north side of Redend Point in Studland Bay, Dorset, England.

Lower cliff rocks on the north side of Redend Point in Studland Bay, Dorset, England.

 Boulder and cliff on the north side of Redend Point in Studland Bay, Dorset, England.

Beach boulder on the north side of Redend Point in Studland Bay, Dorset, England.

Boulder and cliff on the north side of Redend Point in Studland Bay, Dorset, England.

Rocks at Redend Point in Studland Bay – 3

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Several years ago I first posted some of these photographs of Studland Bay rocks but I think it is still worth posting some more now, as it is not every one who will have had the time and patience to burrow through the archives of rock postings on this web log. I never cease to be amazed by the stripe patterns, and the red, yellow, and purple colours of the Redend Sandstone (Creekmoor Sand) at Studland Bay. They are incredible.

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rocks at Redend Point in Studland Bay – 2

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Pink and yellow patterned sandstone with Liesegang rings resulting from the dispersion of iron minerals dissolved in river water percolating through the rock. Photographed at Studland Bay in Dorset, England, in outcrops of Eocene-dated Redend Sandstone (Creekmoor Sand) of the Poole Formation in the Bracklesham Group. These soft sandstones in the low cliffs at the south end of the bay seem to be an irresistible  “canvas” for graffiti artists.

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rock colour, pattern, and texture in Creekmoor Sand (Redend Sandstone) at Studland Bay

Rocks at Redend Point in Studland Bay – 1

Rock texture, colour, and pattern in Redend Sandstone  at Studland Bay

Examples of rock texture, colour and pattern in Redend Sandstone (also known as Creekmoor Sand) which is a basal member of the Poole Formation (formerly referred to as the Bagshot Formation), of the Bracklesham Group. The pastel almost rainbow colours are caused by iron staining. Hollow pipes (as in the shot immediately below), which can be up to 15 cm diameter and sometimes extend as much as 4 m through the strata, are of unknown origin. The sandstones were laid down in the Eocene.

REFERENCE

Cope, J. C. W., 2012, Geology of the Dorset Coast, Geologists’ Association Guide No. 22, 191-194, ISBN 978-0900717-61-1.

Rock texture, colour, and pattern in Redend Sandstone at Studland Bay

Rock texture, colour, and pattern in Redend Sandstone at Studland Bay

Rock texture, colour, and pattern in Redend Sandstone at Studland Bay

Permian Red Beds at Lord Selkirk Park, PEI

View along the shoreline at Lord Selkirk Provincial Park in Prince Edward Island, Canada, with lush green early summer vegetation and red Permian rocks.

Views along the shoreline at Lord Selkirk Provincial Park, on the south coast of Prince Edward Island in Canada, show the red Permian sedimentary rock layers in low cliffs, and as mud beneath the water of the Northumberland Strait. The lush vibrant green of the new season’s vegetation makes a striking contrast to the outcropping red beds.

The whole of Prince Edward Island is underlain by Permian rocks which also extend outwards to New Brunswick and Nova Scotia. They date from around 255 million years ago and were laid down between the Carboniferous period which came before it and the Triassic period which followed it. Towards the end of the Carboniferous, the climate started to warm up and dry out; and this marked the end of the vast wetland forests and swamps that had been so characteristic of the Carboniferous and that are recorded by numerous plant fossils and extensive coal measures.

The plants and trees that continued to grow in the Permian Period were mainly those which were resistant to drought, like conifers, although some plants such as tree ferns persisted in the wetter ares.  Remains of these plants have been preserved as fossils in the Permian rocks. The terrestrial red beds of Prince Edward Island have also preserved the bones of vertebrates, reptiles, that were washed down rivers in monsoon flash floods and became buried in pebbles and debris. At Point Prim, very close to Lord Selkirk Provincial Park, trace fossils of one of these mammal-like reptiles has been found. We were not lucky enough to find fossils of any kind while we were there as our visit was so fleeting.

Stephanian to late Early Permian rocks occur in and under PEI. The fields, so famous for their potato crops, are a distinctive red colour, indicating the rock type from which the soil is derived. The rocks themselves are best seen outcropping along the coast – in fact one of the first things you notice as you cross the Confederation Bridge to get to the island is the continuous thin red rocky line along the miles of southern shore.

The Permian strata on PEI are divided into different phases, and those shown in the photographs here, from Lord Selkirk Park near Point Prim, belong to the Wood islands Member, which together with the Malpeque Member to its west, comprise the relatively recently-designated Hillsborough River Formation, that belongs to the Northumberland Strait Supergroup. The rocks are made up of conglomerates, sandstones, and mudstones.

REFERENCES

van de Poll, H. W., 1989, Lithostratigraphy of Prince Edward Island redbeds, Atlantic Geology, 25, 23-35.

Atlantic Geoscience Society, 2001, The Last Billion Years: A Geological History of the Maritime Provinces of Canada, Nimbus Publishing, ISBN1-55 109-351-0, Atlantic Geoscience Society Special Publication No 15.