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.
The alternating dark and light rock layers of the Belemnite Marls (belonging to the Lower Lias division of the Jurassic Period) at Seatown in Dorset, England, are riddled with small trace fossil burrows. These are mostly tunnels that were dug into the soft seabed sediments by marine organisms such as marine worms and crabs before the sediments became lithified or converted to hard stone. The patterns of these trace or ichno fossils in the cliffs show a wide range of sizes in the burrows with cross-section and longitudinal section views. Some of the tunnels are branched, some are u-shaped, and many are irregular. The shape and size of the burrows, and the particular location of the stratum in which they appear, provide clues to the identity of the creatures responsible. The burrows include Rhizocorallium, Thalassinoides, and Chondrites (Woods 2011). Most of the burrows shown in the photographs here are easy to see because of their contrasting colour – they have been excavated in layers of the darker sediment and have at a later stage been in-filled with the lighter coloured sediments from the layer above. The opposite can also happen, with burrows in lighter sediment being infilled with darker material from above, as seen in a couple of the pictures. Not all the trace fossils are burrows. Some traces appear to be a breaking up of the semi-solidified surface deposits with inter-mixing of sediment from the deposit above.
Woods, M. A. (compiler) (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast: Special Memoir for 1:50,000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth, and 342/343 Swanage, and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 339 Newton Abbot. British Geological Survey, Keyworth, Nottingham. ISBN 978-085272654-9, pp 28-33.
The most common fossils at Seatown on the Dorset coast are belemnites. These are bullet-shaped internal hard parts of a type of extinct cephalopod (think cuttlefish, squid and octopus). For a great deal of the length of the beach, the rock strata are hidden by debris falling down from layers above. There are lots of minor mudslides and landslips. However, as you get nearer to the western extremity of the beach, approaching Golden Cap, a continuous kerb-like, harder, and more calcareous stone layer makes an appearance. This is the Belemnite Stone that has been raised to view by a small anticlinal flexure. Below it are many layers of Belemnite Marl that can be seen in cross-section in the vertical face at the base of the cliff; and also extending out horizontally beneath the gravelly beach and exposed at low tide. They alternate light and dark layers. Fossils are abundant with belemnites predominating but ammonites are also common. The huge numbers of belemnites are thought to have resulted from mass die-offs following mating frenzies.
Boulders on the back of the quarried ledge at Winspit contain fossils and trace fossils. A recent discovery of mine when I last visited were numerous very small worm tubes, frequently amassed in discrete areas or layers, around and below black chert nodules in the Portland Stone Cherty series rocks. It is difficult to be certain of the identification of these worm tubes (maybe someone can look at these pictures and tell me) but it is known that serpulid worms called Glomerula gordialis are found in this particuar geological and geographical location, and I am assuming for the time being that these are the same species.
Plant fossils are abundant in the Ward Point Member rocks at Cape Enrage in New Brunswick. You do not need to be an expert to find them in beach stones beneath the cliffs. You do need to be an expert to identify all the fragments accurately. I am not an expert. However, as far as I can make out, most of the fossils that I saw were the strap-like leaves of Cordaites, a primitive conifer from upland regions which according to the guide books resemble Amaryllis leaves or corn husks. There were also fragments of Calamites stems; this was a tree-like plant that could grow up to 10 m tall and is related to the much smaller present day horsetails or Equisetum plants (see images below). The stem is ribbed and jointed like bamboo with a diameter of about 10 cm and it would have had narrow whorled leaves at intervals along the stem. It formed dense thick undergrowth in lowland wetter areas. The diverse fossil flora at Cape Enrage represents dead vegetation washed downstream by rivers and stacked up in piles on the banks of many river channels about 320 million years ago in the late Carboniferous Period. The plant debris would become covered in successive layers of sediment brought down by the rivers as they wandered across the flood plain to the sea, and eventually preserved in sandstones and mudstones.
The Cape Enrage Visitor Centre has some excellent examples of fossils on display, and education officers are available to give advice and help with identifications. They are very helpful and friendly. I am sure that, time permitting, a professional guided tour would reveal many more in situ fossils of different types than those illustrated here.
The small cove at Pwll Du (or Pwlldu) on the south coast of the Gower Peninsula in South Wales has been created by two fault lines extending approximately northwest to southeast. They converge inland, and separate towards the sea, meeting the shoreline more or less at right angles, with one at the east and one at the west of the bay, running through Carboniferous Limestone strata. The fault lines displace the normal sequence of rock layers so that to the west the rocks are Oxwich Head Limestone, in the middle between the faults it is Hunts Bay Oolite, and in the east it is High Tor Limestone.
Images of the rocks at the east end of Pwll Du Bay are shown in the gallery above. Photographs with details of the rocks at the west end of the bay are given in the gallery below.
Overlying the central area of Hunts Bay Oolite is a massive, multiple-tiered shingle bank which blocks the head of the valley and currently dams the river known as the Bishopston Pill. The shingle banks of Pwll Du will be featured in the next post.
Chondrites are trace fossils or ichnofossils. They are small branching burrows or tunnels that were made while the sediments were still soft and have subsequently become preserved in the hardened strata. There is a great deal of uncertainty about which organisms created the burrows because no animal has ever been found within them – but they may have been some kind of small marine worm. There is evidence to support the idea that the burrows were formed in sediments with reduced oxygen or none at all.
The trace fossil Chondrites, a highly branched burrow system of unknown endobenthic deposit feeders, occurs in all types of sediment, including those deposited under anaerobic conditions. In some cases, such as the Jurassic Posidonienschiefer Formation of Germany, Chondrites occurs in black, laminated, carbonaceous sediment that was deposited in chemically reducing conditions. In other cases, such as numerous oxic clastic and carbonate units throughout the geologic column, Chondrites typically represents the last trace fossil in a biotutbation sequence. This indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters.
The Chondrites shown in these pictures occurred in Silurian rocks of the Dunquin Group on the Dingle Peninsula in western Ireland. Some were found in beach stones at the northern end of Smerwick Harbour, however, the majority were photographed in Clogher Bay on large boulders and in bedrock.