Rock textures at Kimmeridge Bay on Dorset’s Jurassic Coast where water runs down the cliff face. Deep orange-red iron deposits on the surface of the grey limestone are revealed by recent rock falls; and granular calcium precipitation coats rock where water falls persistently.
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What is a septarian nodule? Well, basically, it is a big boulder containing a three dimensional jig-saw of smaller angular pieces of the same rock – and all the pieces are bound together with white crystalline calcite. I can’t do better than to quote the definition given in The Oxford Dictionary of Earth Science:
A concretion, roughly spheroidal in shape, usually of clay ironstone, and characterised by an internal structure of angular blocks separated by radiating mineral-filled blocks. The mineral filling the cracks is usually calcite. The structure results from the formation of a hard exterior to the nodule due to the development of an aluminous gell on the exterior, followed by dehydration of the colloidal mass in the interior, leading to cracking and subsequent infilling of the radiating pattern of cracks.
The British Regional Geology Series for the area indicates that the Ringstead Waxy Clays, which are virtually at the top of the Corallian Beds of the Upper Jurassic strata, comprise about 5 metres of clay with thin seams of clay ironstone that are nodular in places. It seems very possible that the septarian nodules are from this source. The Ringstead Waxy Clay is also the deposit in which numerous fossil oysters, Deltoideum (Liostrea) delta, are found [mentioned elsewhere in Jessica's Nature Blog and also on the sister site Oysters etc.]
References
Oxford Dictionary of Earth Sciences, Edited by Michael Allaby, Oxford University Press, first published 1990, third edition 2008, ISBN 978-0-19-921194-4
The Hampshire Basin and adjoining areas, R. V. Melville and E. C. Freshney (1982), British Regional Geology Series, Fourth Edition, Institute of Geological Sciences, HMSO, ISBN 0-11-884203-x.
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Click here for more pictures and Posts on Pebbles at Budleigh Salterton in Jessica’s Nature Blog.
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I love Chesil Beach. It’s so exhilarating. I love the pebbles, millions and millions of pebbles, different sizes of pebbles, different types of pebbles, the way they glisten wet after pounding by the waves, bank upon bank, terrace above terrace, the deafening roar of the pebbles as they are jostled and dragged on the water’s edge. I relive the experience every time I look back at my photographs. I’d like to share with you these pebble pictures from my last visit.
Click here for more Chesil Beach posts – or posts about Pebbles in Jessica’s Nature Blog.
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Opelet or Snakelocks Anemones – Anemonia viridis (Forskål) – exhibit quite a bit of variation in form and colour. Generally they are described as having “a smooth column, usually wider than high, up to 50 mm in diameter. Disc wide, with up to 200 long and sinuous tentacles; the tentacles rarely retracted. Column and acrorhagi (a ring of specialised warts) brownish, tentacles brown, grey or bright grass green, usually with purple tips. Contains zooxanthellae. On the shore in pools and other places open to the light, and down to about 20 m” (Hayward and Ryland 1998).
The bright green tentacle colour is caused by the tiny algae (zooxanthallae) that live within them. It is said that these organisms live in the specimens of anemone that live in well lit areas while the grey-brown anemones, without the symbiotic algae, live in areas without much light. However, that cannot be true because grey-brown and green individuals can live side by side. In the images below, Snakelocks Anemone (3) and (4), in the top left corner of the photographs you can see a plain grey-brown Snakelocks Anemone next to the featured green specimen with the pink- purple tips. And the grey-brown specimens in Snakelocks Amenone (5) – (8) were found within a few metres of the green ones.
The literature says that the tentacles of the Snakelocks Anemone are rarely retracted or withdrawn. This could be a bit of a problem for the anemone when the tide goes out. All the green anemones shown in this post were photographed in very shallow water pools in which the columns of the animals were flattened or compressed against the rock floor of the pool to ensure that the whole animal with its fully extended tentacles remained beneath the water surface. The brown specimen attached to a loose rock was fully exposed to the air and had partially retracted its tentacles – the first time I had seen this – presumably to minimise dehydration.
Typically, the Snakelocks Anemone has slender, delicate tentacles with fine gently tapering tips. The anemones from Lyme Regis in Dorset (1-4 & 8) have this characteristic. Recently, on a trip to the Worms Head Causeway, Gower, South Wales, I noticed some specimens with tentacles that were markedly stouter than usual. Additionally, a large number of the tentacles had blunt tips or had one or more constrictions towards the end. I think that the difference in tentacle shape is due to the particular conditions of the habitat. The Worms Head Causeway is a very high energy environment where the impact of wave action is accentuated; while the habitat provided by the shore at Lyme Regis tends to be a less extreme environment. The different shapes of the Worms Head Snakelocks Anemone tentacles could be the direct result of physical damage.
Reference
P. J. Hayward & J. S. Ryland (Eds) (1995 reprinted 1998), Handbook of the Marine Fauna of North-West Europe, Oxford University Press, pp 120-125.
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The spectacular yellow cliffs at West Bay near Bridport in Dorset are soon to form the back-drop for the new television series called Broadchurch. They really are a remarkable geological phenomenon – like so much of the landscape on the World Heritage Jurassic Coast.
The East Cliff (1) is largely composed of the Bridport Sand Formation with 43 metres depth of it exposed out of an overall depth of approximately 49 metres. It has been calculated that each metre of rock would have taken about 20,000 years to accumulate on the seabed where it originated. So it would have taken about 860,000 years for the depth of the rock exposed in the cliff to accumulate. This all happened in the Upper Lias phase of the Early Jurassic Period (about 176 – 205 million years ago).
The sediments of which the cliff is formed consist of alternate beds of soft fine-grained crumbly sandstone, and harder sandy limestone (2). The striped nature of the cliff is one of its most notable characteristics. The depth of each bed of soft sandstone gets progressively smaller with an increase in height above the beach. The sand beds vary in thickness from 30 centimetres to 3 metres. The carbonated-cemented sandy limestone layers are consistently narrower and range only up to 75 centimetres and have irregular upper and lower surfaces.
The harder calcareous sandstone layers stick out as horizontal parallel jagged ledges (3 – 5). They are more resistant to erosion by wind-blown sand than the softer sandstone. They are also typically bio-turbated, meaning that the original soft sediments on the sea bed were churned up and burrowed into by marine invertebrate animals. The tunnels or burrows were subsequently filled in with a calcium-rich sediment – forming trace or ichno-fossils of the burrows and tunnels. These are the hardest part of the rock, and when weathered and sculpted by the elements, they remain like a network of inter-connected cords. From a distance, they have a general honeycomb appearance and this type of erosional feature is known as tafoni (6).
Vertical cracks or joints, known as gulls, extend at intervals from top to bottom of the cliff (7). These were mostly created a very long time ago as part of a cambering process which affected the whole hill of which the cliff forms the eroded seaward edge. As far as I can understand it (and I may have got the wrong end of the stick here), this cambering was due to softer substrates lying beneath the Bridport Sand Formation rocks in river valleys to the east and the west of this BSF structure. Lack of support for the rock in these locations led to a kind of subsidence that opened up vertical cracks and led to the development of a sort of fanning of the strata and a curving of the top of the hill (21 – 22). This is one of only a few places in Britain where cambering rock strata can be observed.
The gulls are usually in-filled with sand and debris. However, these joints are particularly susceptible to erosion and are worked at by the elements so that they gradually recede landwards, creating angular recesses between protruding columnar buttresses right along the length of the cliff. Vegetation can colonise the cliff in these places because the roots can take hold among the infill debris (8 – 9). Caves can form where wind or waves clean out and extend the cracks (10 – 11). This can happen high on the cliff but is more common at the base.
The friable or crumbly texture of the sandstone means that rock frequently falls from the cliff and ends up as boulders on the beach (12). Wind and rain work on higher strata while the sea undercuts the lower layers. At the base of the cliff where waves frequently work (14, 18), and in newly exposed rock after falls (13), the true colour of the Bridport Sand Formation is revealed as blue-grey. The Bridport Sand Formation comprises fine-grained sandstone, a quartz arenite, regularly alternating with hard calcite-cemented strata. Within this matrix is pyrite. When pyrite is exposed to air, it starts to oxidise, forming a rust-like thin layer on the rock (15). It is this process that converts the natural blue-grey to the wonderful yellow colour of the rocks in the cliff.
At the base of the cliff for some distance along its length, it is possible to see an anomaly in the rock layers (19 -20). Most of the alternating bands of rock are virtually level, horizontal, and parallel to each other. However, there is an exception to this. There is a bed of hard rock which undulates up and down in a discontinuous or broken set of curves. This is thought to be a scour structure where an extreme storm event disturbed the shallow water marine shoal sands on the seabed early on in the development of the Bridport Sand Formation, just after the layer have been deposited and consolidated.
REFERENCES
Bridport Sand Formation at East Cliff , West Bay by Ian West
Brunsden D and Goudie A, (1981,1997) Classic Landforms of the West Dorset Coast, Series Editors R Castledean & C Green, The Geographical Association, ISBN 1 899085 19 x.
Melville R V and Freshney E C (1982) The Hampshire Basin and adjoining areas, British Regional Geology, Institute of Geological Sciences, NERC, HMSO, ISBN 0 11 884203 x.
THE PICTURE GALLERY
If you click on any picture in the gallery below, the image will be enlarged and show the descriptive caption. From there it is possible to either view the photograph in full-screen size or to the use the arrows to scan through all the pictures in the gallery in sequence.
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The boulder-covered shore at Monmouth Beach in Lyme Regis is mostly famous for its fossils like ammonites and petrified wood. The stretch of coastline to which it belongs is a part of the designated World Heritage Site called the Jurassic Coast….but even the boulders themselves are interesting and can show intriguing designs of fractures and cracks that are in some way related to the sediment types, although I don’t know how. Here are some examples, showing first the natural fracture pattern in a close-up shot and then the picture of the boulder on which it was found, in context on the beach.
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Oar Weed kelp (Laminaria digitata) undulating gently in the shallow water of an ebbing tide at Lyme Regis, Dorset, UK, March 2012 – with the sound of water lapping on rocks and seagulls calling overhead.
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On a visit to Lyme Regis in March 2012, I was delighted to witness acre upon acre of glistening kelp beds as they were gradually uncovered by the receding tide. It was a particularly low tide that day. The seaweed beds that are usually under water were gradually revealed. The algae are attached to wide rock platforms that extend seawards from the base of the cliffs.
The most visible and easily recognisable kelp was Oar weed (Laminaria digitata). When submerged, the Oar Weed stem is erect with the large flat frond wafting to and fro with the waves. As the tide recedes and water levels drop, the stem is no longer able to bear the weight of the leathery blade with its strap-like divisions, and the stem bends over so that the frond lies on the ground. This habit distinguishes Oar Weed from the similar species Cuvie (Laminaria hyperborea). Cuvie stems have a rounded cross-section and are capable of remaining vertical when out of water.
At first the day was dull and overcast – but after a while the sun came out and its oblique rays shone through the fronds of the Oar Weed creating natural fields of glowing gold. It was a magnificent sight. The normal olive brown of the seaweed was transformed by the light.
It was also interesting to observe the wonderful colour and texture combinations of the different types of seaweed. Although the large Oar Weed predominated, there were many other smaller species of brown and red algae. Branched filamentous kinds of red seaweed formed vast low-lying carpets that complemented the olives and golds of the Oar Weed or Tangle. One particular type of branching, flat-bladed red algae – Dulse (Palmaria palmata also known as Rhodymenia palmata) commonly grows attached to the stem of Oar Weed.
Click the post titles below for more information about Oar Weed in Jessica’s Nature Blog:
Satin-textured seaweeds at Ringstead
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It’s that time again – for the Great Dorset Beach Clean. Sunday 16th September 2012 will see dozens of volunteers descending on some of our most beautiful Jurassic Coast beaches to clear away the huge volumes of flotsam that has accumulated over the summer months – most of it plastic but also fishing nets and ropes and SHOES!
Here are some photographs of trainers, flip flops, and sandals that I found on the beach at Chapmans Pool last weekend. It always makes me wonder how the owners walked home after losing their shoes – no mean feat (or should I say ‘feet’) to climb up the hill again barefoot after the trip to the beach!
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