Seatown Ammonites

Lots of serious fossil hunters go to Seatown in Dorset to find fossil ammonites that have fallen to the beach from the cliffs. The cliffs for the most part are composed of Green Ammonite Member which is part of the Charmouth Mudstone Formation laid down in the Jurassic Period. The ammonites that are most commonly found in this type of rock are Aegoceras, Androgynoceras, Liparoceras, and Oistoceras. I haven’t found any decent fossils of the type I could pick up and take home, but there are plenty of fossils and ammonite impressions to be seen lying in pieces of rock on the shingle beach where people with hammers have broken them open. These pictures show some of the specimens that I found on my last visit. I am not sure which species they represent but maybe some local geologist may be able to look at these images and tell me what they are.

Jurassic Coral Fossils at Ringstead

These photographs all show fossil corals found in boulders on the beach at Ringstead Bay in Dorset, England. They date from the Jurassic Period and are embedded, along with numerous other fossils, in the Ringstead Coral Bed which is the topmost layer of the Ringstead Formation (that in turn is at the top of the larger Corallian Formation).

Whilst I cannot say for certain exactly which fossils each image portrays, I can say that there are four corals known from this rock bed. These are Thecosmila annularis, Thamnasteria concinna, Thamnasteria arachnoides, and Protoseris waltoni. It is highly likely that the pictures show details of the colonial Thamnasteria genus and I think both species are represented here. I will being going to my local museum to check the identifications with their reference collections. Thecosmila is a larger solitary coral so can be excluded as a possibility for these particular fossils..

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Ringstead Bay Fossil Bivalve – Ctenostreon proboscideum

Most of the examples of this fossil bivalve, Ctenostreon proboscideum, were partial specimens embedded in the rocks at Ringstead Bay in Dorset, England. However, the large strongly-ribbed shell is unmistakable and easily recognised in the many boulders on the beach at the west end of the bay – at least they were easily seen when the pebbles had all been washed away after the storms. The photographs in the gallery above show Ctenostreon shells as they were found on the beach last week. The boulders had fallen from the Ringstead Coral Bed which is a narrow layer,  packed with fossils, of no more than 30 centimetres depth, and which can be seen in short lengths in the vertical section through the strata at the top of the beach.

The almost complete fossil specimen shown with the blue background (photographed at home) was found many years ago after similar severe weather. You can see that the two valves are still together and the space between them filled with marly limestone material, indicating that the original animal was already dead, with the two shells gaping open, when it was buried under new sediments.

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Ringstead Bay Fossil Oyster – Deltoideum delta

Cluster of attached fossil oysters from Ringstead Bay on the Jurassic Coast

Deposits containing this large fossil oyster, Deltoideum delta or Liostrea delta, were clearly visible on the shore at Ringstead Bay in Dorset after winter storms had rearranged and largely removed the normal thick layer of pebbles. They are recorded as being characteristic of several late Jurassic Period strata. I observed them in situ in the Ringstead Clay Member waxy clays at the top of the shore in the western half of the beach. I also noticed them in rocky outcrops on the water line of the lower shore where the different composition of the matrix makes me think the exposed strata were probably from the Sandsfoot Formation which lies beneath the Ringstead Formation and pre-dates it.

In the eastern half of Ringstead Bay, the part which was inaccessible on that particular visit of 1st March 2014, I had previously seen this species of fossil oyster shell in deposits of Kimmeridge Clay from the Kimmeridge Formation which were laid down after the Ringstead Formation layers. So this particular species of oyster was around for a long time, geologically speaking. Its appearance and disappearance in the various strata is due to the changing and cyclical nature of the environment in this location – meaning that very specific conditions were required for the species to thrive but changes in water depth, salinity, and temperature made the environment more or less suitable for their existence at different successive times.

There are more posts about Deltoideum delta in both Jessica’s Nature Blog and Oysters etc.

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Trace fossils in Winspit rocks

Trace fossil burrows in a limestone boulder, made by marine invertebrate creatures in ancient times, at Winspit, Dorset, UK - part of the Jurassic Coast (1)

Trace fossils are geological records of biological activity. The rocks at Winspit in Dorset have preserved evidence of the burrowing habits of invertebrate animals that once lived in the soft wet sediments on the seabed and seashore of shallow seas over 135 million years ago in the Upper Jurassic period.

These trace fossils include burrows and tunnels made by crabs and worms. In some instances, the holes made by the creatures remain visible – as pictured in the first three photographs below. They look remarkably similar to the holes made by boring bivalved molluscs and by mud-tube dwelling marine polychaetes that occur in present day calcareous stones and mollusc shells (see the earlier posts on these subjects). As I am not a palaeontologist, I concede that this may not actually be a parallel causation – just a coincidence.

General view of the trace fossil bearing boulders where the valley meets the sea at Winspit, Dorset, UK - part of the Jurassic Coast (2) 

Trace fossils in a limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (3)

Close-up of trace fossils shown in Photos 2 & 3, limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (4) 

In most of the examples illustrated by the photographs below, the spaces enclosed by the burrows have been infilled by other sediments so that instead of the original hollow tubes and tunnels remaining in the hardened rock, the burrows have become roughly cylindrical solids. The infill substance seems to be harder than the matrix surrounding it. This means that the softer rock has  weathered more readily and the burrows stand out like cords.

Limestone boulder with trace fossils at Winspit, Dorset, UK - part of the Jurassic Coast (5) 

A pile of large angular boulders, between one and two metres across, rests on the rocky ledge at the foot of the cliffs where the valley meets the sea at Winspit. On the flat surfaces of some of the rocks are intricate patterns made up of  horizontally branching networks of solid tubes about a centimetres or so in diameter. These are similar to the burrows made by decapod crustaceans (crabs) called Thalassinoides

Close-up of trace fossils in limestone boulder (shown in photo 5) at Winspit, Dorset, UK - part of the Jurassic Coast (6) 

Large Titanites giganteus ammonite fossil in a limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (7) 

Large fossilised remains are found in these strata as well as the trace fossils. One of the best known, because of its large size, is the ammonite Titanites giganteus as shown in the picture above. It is about 40 centimetres in diameter. Although well embedded in the massive boulder, and a feature of the location for as long as anyone remembers, someone has recently tried unsuccessfully to hack it out of the rock. It is a shame because it has left the wonderful fossil damaged and defaced. 

Trace fossils on the surface of a large limestone boulder (with walking pole for scale), Winspit, Dorset, UK - part of the Jurassic Coast (8) 

Detail of trace fossils on limestone boulder (photo 8) at Winspit, Dorset, UK - part of Jurassic Coast (9) 

Detail of trace fossils on limestone boulder (photo 8) at Winspit, Dorset, UK - part of Jurassic Coast (10) 

Trace fossils on a limestone boulder at Winspit, Dorset, UK - part of the Jurassic Coast (11) 

Detail of trace fossils in a limestone boulder (photo 11) at Winspit, Dorset, UK - part of the Jurassic Coast (12) 

Detail of trace fossils in a limestone boulder (photo 11) at Winspit, Dorset, UK - part of the Jurassic Coast (13) 

Limestone boulder with surface covered in trace fossils at Winspit, Dorset, UK - part of the Jurassic Coast (14) 

Some of the burrows, particularly the larger ones that may have been made by crabs, seem from these trace fossils to have passed through a series of layers in the sediments. Sediments from different layers are mixed up, and older sediments brought to the surface and deposited above more recent ones, as animals excavate their tunnels. This process is called bioturbation. Geologists, palaeontologists, and archaeologists are all aware of the implications of bioturbation for the interpretation of results from their research and excavations. 

Another name for trace fossil is ichnofossil. To read some more about ichnofossils click here.

Detail of trace fossils on a limestone boulder (photo 14) at Winspit, Dorset, UK - part of the Jurassic Coast (15) 

Revision of a post first published 9 December 2009

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Petrified Wood – digital art

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Fossilised or petrified wood can be found in Jurassic Lias Limestone boulders on Monmouth Beach at Lyme Regis in Dorset, UK on the World Heritage Jurassic Coast. In the images of this slideshow, close-up photgraphs of the naturally-occurring patterns and textures in this ancient preserved wood have been digitally colourised to emphasis the natural abstract art in the stone. 

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Fossil wood at Lyme Regis

Fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (1a)

As you walk along Monmouth Beach at Lyme Regis in Dorset, hopping from one boulder to the next, you might see strange brown markings on some of the larger stones. Although not as spectacular as some of the finds recorded from this beach, a closer look will reveal that, in some cases,  these are the fossilised remains of pieces of wood. Their presence demonstrates that the rock was formed in seas that were not too far from land. Some pieces have been discovered with the fossilised remains of marine invertebrate animals still attached to the undersurface – showing that the wood was free floating for a while before settling in the sediments.

In this post, three such boulders of blue-grey Lias limestone containing this dark brown textured petrified wood (or lignite) are shown – together with close-up images of the peculiar texture and pattern of the preserved timber.

For more information about the other incredible types of fossils that have been found on this beach, see Ian West’s website at http://www.soton.ac.uk/~imw/liasfos.htm.

Detail of fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (1b)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (1c)

Fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (2a)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (2b)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (2c)

Fossil wood exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (3a)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (3b)

Close-up of fossil wood texture exposed on the outer surface of a boulder of Lias limestone on Monmouth Beach at Lyme Regis, Dorset, UK on the Jurassic Coast (3c)

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Rock on The Cobb

Portland Roach on the Cobb: Detail of the Portland Roach limestone blocks from The Cobb, Lyme Regis, Dorset, UK on the Jurassic Coast (1)

Fossils are not only found in the cliffs, the rocky ledges of the beaches, and the boulders strewn across the shore at Lyme Regis in Dorset. Even when you take a walk around the harbour, on the top of the massive sea defence known as The Cobb, there are fossils beneath your feet – but these shelly remains have come from further afield.

The Cobb is a great sinuous, almost organic-looking, sea wall enclosing a safe haven for boats. It is constructed from huge blocks of fossiliferous stone. You can easily see in the cut surfaces of the stone vast numbers of  bivalve and gastropod shells preserved as fossils and moulds within the rock. The blocks are Portland Roach, or Portland Admiralty Roach, which is a shelly oolitic limestone coming from the uppermost part of the Portland Stone Formation of the Upper Jurassic Period on the Isle of Portland. These big blocks lie above smaller slabs of Blue Lias argillaceous limestone from the local reefs. The Roach stone itself is characterised by the presence of moulds left by the long  pointed gastropod locally known as ‘The Portland Screw’ – Aptyxiella portlanica – that has something like a turret shell shape.

The large number of holes that riddle the Roach stone mean that it is not really suitable for building construction in the way that the famous Portland limestone is used. However, Roach is frequently used for seawalls and sea defences.

The Cobb has been in existance for many centuries but what is seen today is a reconstruction from 1820. I think that was the first time in its history that the blocks of stone were mortared together. Mortar is most visible between the stones at the landward end but towards the seaward end some extra measures have been introduced to reinforce the structure. It looks as if the patchwork of blocks have been curiously secured together with giant iron staples.

Portland Roach blocks on The Cobb: The upper surface of the curving sea defence known as The Cobb, at Lyme Regis, Dorset, UK on the Jurassic Coast (2) 

Portland Roach: Blocks of Portland Roach limestone with mortar in the sea defence known as The Cobb around the harbour at Lyme Regis, Dorset, UK on the Jurassic Coast (3) 

Fossil rock used for building: Fossil shells embedded in the Portland Roach stone used in the building of The Cobb around the harbour at Lyme Regis, Dorset, UK on the Jurassic Coast (4) 

Giant iron 'staples' securing the blocks of Portland Roach limestone on The Cobb at Lyme Regis, Dorset, UK on the Jurassic Coast (5)

Giant iron 'staples' securing the blocks of Portland Roach limestone on The Cobb at Lyme Regis, Dorset, UK on the Jurassic Coast (6)

View looking along the top of curving harbourside sea defence of The Cobb towards Monmouth Beach, at Lyme Regis, Dorset, UK, on the Jurassic Coast (7) 

Revision of a post first published 10 May 2010

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Rhynchonella inconstans & Lopha gregarea

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 1

Two Jurassic Coast fossils from Ringstead Bay in Dorset, UK. Although there are superficial similarities between these marine shell fossils, they are in fact the preserved remains of two very different kinds of organism. One is a marine bivalved Mollusc but the other is a Brachiopod. The two intact specimens are stuck together in a stony matrix which contains other smaller and fragmentary shell fossils. The clump of fossils was found in the Ringstead Bay area which is part of  the Jurassic Coast World Heritage Site.

Externally, they both look fairly similar with their possession of two shell valves, and distinct natural sculpturing of sharply angled ribs radiating out over their surface. However, the two valves in a Mollusc represent the coverings for the left and right sides of the animal contained within. In Brachiopods, on the other hand, the shell halves represent the external protection for the dorsal (upper) and ventral (lower) parts of the animal’s body. The animals in the two separate kinds of organism are therefore always orientated in different ways within their shells – although the shells are formed by much the same sort of biological mechanism.

Brachiopods are, strangely enough, related to the minute, colonial animals known as Bryozoa (Sea Mats or Moss Animals). Bryozoa and Brachiopods uniquely possess a structure called the lophophore with which the animals filter food particles from the sea water flowing through their shells. In Brachiopods, the lophore is supported by long internal extensions of the hard shell – and therefore Brachipods can be said to possess an internal as well as and external skeleton, compared with the Bivalves with just an external shell.

We are much more familiar with the many species of bivalve Molluscs (like clams, cockles, and mussels) than with Brachiopods. Brachiopods still exist today but there are only a couple of hundred species of these ‘lamp shells’ now remaining – compared with the many thousands of now extinct species from the long distant past.

The Brachiopod is Rhynchonella inconstans – seen on the left in the picture above and from other perspectives in the photographs below. It has other synonyms. It is also called Torquirhynchia inconstans and Rhactorhynchia inconstans. It is found in Lower Kimmeridge Clay from Upper Jurassic deposits (beneath the Kimmeridge Clays from which Liostrea delta is frequently recovered). The Jurassic Period lasted from about 195 to 136 million years ago.

The most easily recognised feature of R. inconstans is the way that one half of the shell is stepped down from the other in a peculiar form of asymmetry which is thought to be an adaptation to life in tidal environments.

The bivalved mollusc is probably Lopha gregarea – seen on the right in the top picture (with other views in the lower photographs). It is also known as Ostrea gregarea, Ostrea gregaria, and Alectryonia gregarea. I say ‘probably’ because it would perhaps be more likely to be Lopha marshi from its association with R. inconstans but it just does not look like it to me. Maybe someone out there could advise me?

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 2

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 3

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 4

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Some impressions of Ringstead in March

Ringstead bay seashore: The serpentine curves of the water's edge at Ringstead Bay, Dorset, UK - part of the Jurassic Coast - 2 March 2010 (1)

Just imagine the tremendous forces of nature that work from the sea to transport tens of thousands of tons of pebbles across a beach, to separate or mix the sizes, and to develop new seashore shapes and contours. And of the onshore gusts that stunt, bend, break or trim the cliff-top vegetation into leaning living sculptures.

When I visited Ringstead in early March, the heavy seas had shifted the beach components to reveal surprising and rarely glimpsed expanses of sandy beach in some areas. In other places the shingle was heaped up into undulating banks lying at right angles to the tide line rather than the usual parallel ascending ridges up the shore.

At the water’s edge, the now merely-lapping waves followed the contours of the newly carved beach so that, depending on your perspective, the white wet line either formed a long serpentine curve winding into the distance or a neat regular scalloped border as if the sea was taking bites out of the shore.  

Atop the low, soft crumbling cliffs, the still bare branches and twigs of trees and bushes revealed their true structure leaning permanently away from the prevailing blasts; the tips snipped to create shapes offering less resistance. The long dried reed-like grasses lower down the slope remained standing; and being more pliable bent in the opposite direction and into the wind. Looking up from the beach, the result resembled a series of layers one on top of the other – and of screens one in front of the other.

On the more western reach of the shore, approaching the point where Ringstead becomes Osmington Bay, rock beds and boulders are exposed from a complex and interesting geology which includes many different, often fossil-laden, strata. There is a red uneven stone pavement at one point. The colour is, as far as I can make out, caused by the oxidation in air of a dull, green earthy iron and potassium silicate in the rock (glauconite). Many rocks and the rock platform that extends out from the shore are crowded with fossils – such as the Trigonia Beds that are easily recognisable with their numerous characteristic fossil bivalved mollusc shells (Myophorella clavellata).

Ringstead is a fascinating place, as is the entire Jurassic Coast, difficult though it may be to fully understand its geology, palaeontology and coastal processes. It is, however, always a joy to visit and will provide me with a lifetime of learning opportunities.

Seaside vegatation at Ringstead Bay: Reed stalks and bare branches shaped by the wind atop the low cliff at Ringstead Bay, Dorset, UK - part of the Jurassic Coast - 2 March 2010 (2)

Rock at Ringstead Bay: Red rock pavement revealed beneath the pebbles at the water's edge, Ringstead Bay, Dorset, UK, part of the Jurassic Coast, 2 March 2010 (3) 

Shingle beach at Ringstead Bay: Steeply undulating shingle banks at the edge of the sea, Ringstead Bay, Dorset, UK, part of the Jurassic Coast, 2 March 2010 (4) 

Pebbles at Ringstead Bay: Wet pebbles on the mostly flint shingle shore, Ringstead Bay, Dorset, UK, part of the Jurassic Coast, 2 March 2010 (5)

Scallop pattern on the seashore: The scalloped junction between the sea and the shingle on the shore at Ringstead Bay, Dorset, UK, part of the Jurassic Coast, 2 March 2010 (6)

Bivalve fossils at Ringstead Bay: Rock full of fossil Myophorella clavellata bivalve mollusc shells at Ringstead Bay, Dorset, UK, part of the Jurassic Coast, 2 March 2010 (7)

Shingle beach at Ringstead Bay : Wave-sorted natural borderline between areas of larger and smaller pebbles at Ringstead Bay, Dorset, UK, part of the Jurassic Coast, 2 March 2010 (8)  

Revision of a post first published 19 March 2010

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