The second selection of photographs showing details of the rocks in the Upper Kimmeridgian Clay cliff mudstone and shale strata with iron staining on the east side cliffs of Kimmeridge Bay in Dorset, England on the World Heritage Jurassic Coast.
The first selection of photographs taken at Kimmeridge Bay in Dorset, England, yesterday (27 March 2015) showing details of the natural patterns, textures, and colours of the rocks. The rocks are described as rhythmically inter-bedded blocky, organic-poor mudstone and fissile, organic rich shale.
For information about the geology of this location look at:
John C. W. Cope Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Geologists’ Association, 2012, pp 159-167, ISBN978 0900717 61 1.
M. A Woods (compiler) Geology of south Dorset and south-east Devon and its World Heritage Coast, British Geology Survey, NERC, 2011, pp 61 – 67, ISBN 978 085272654 9.
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.
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.
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.
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.
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.