The coastal section at Black Head, near Osmington, displays Jurassic rocks from the Kimmeridge Clay Formation of 152-157 million years ago. Fossils to be found include ammonites, brachiopods, coral, worm tubes and bivalves, as well as marine reptile remains, especially vertebrae and fish bones and teeth.. This site is where the huge skull of the famous Weymouth Bay pliosaur, Pliosaurus kevani was discovered but expect to find a less sensational specimen!
♦ The beach is best accessed by parking at the car park of the Smuggler’s Inn at Osmington Mills. The coastal path is accessed round the back of the pub, walk west (towards Weymouth) along the coastal path and then drop down onto the beach, via a series of metal steps, beneath the PGL Osmington Bay Centre and walk along the beach, west to Black Head.
♦ Alternatively, walk from Bowleaze Cove, heading east along the coastal path and taking in the lovely scenery before reaching PGL Osmington Mills and descending to the beach.
♦ PGL Osmington Mills is at DT3 6EG
FIND FREQUENCY: ♦ ♦ ♦ A good selection of fossils can be collected, from the Kimmeridge Clay Formation.
CHILDREN: ♦ ♦ ♦ Older children will enjoy the location. Younger children might find the walk to the site and the steps to the beach rather challenging.
ACCESS: ♦ ♦ ♦ ♦ ♦ Access is very easy, with parking at the car park at Osmington Mills, or in the lane leading to the pub.
TYPE: – Fossils are found in the small section of Kimmeridge Clay. Fossils are found with more frequency in the winter months, when coastal erosion is at its peak.
Fossil collection requires careful extraction of specimens from the Kimmeridge Clay, which is well-exposed at Black Head. This is the lower part of the Kimmeridge Clay succession and is not exposed at Kimmeridge Bay. The section here extends from the basal Kimmeridge Clay up to the Wake Stone Bed; a conspicuous white coccolith limestone. The Kimmeridge Clay is actually a mudstone, a sedimentary rock of shallow marine origin. The commonest fossil, seen on the ledges from Black Head to Bran Point are the bivalve, Myophorella clavellata. These can be notoriously hard to extract from the matrix but at Black Head, the shell banks are softer and specimens should be less difficult to collect, from the blocks that occur on the foreshore and close to the cliff base.
The oyster, Deltoideum delta, is common. Corallian rocks lie beneath the Kimmeridge Clay and from these Trigonia reticulata, Gervillia aviculoides and other bivalves occur, with occasional ammonites, particularly Rasenia cymodoce and coral.
The Kimmeridge Clay here yields occasional vertebrate remains of both fish and of marine reptiles, particularly ichthyosaurs and pliosaurs. Isolated vertebrae of the ichthyosaur, Brachypterygius extremus occur with some frequency.
The complex geology along this section of coast provides evidence of the Weymouth anticline: a huge dome-shaped structure, formed by the affect of earth movements that built the Alps some 20 million years ago. The outer ripples of this ‘Alpine storm’ have buckled and broken the strata in the coastal region of south Dorset, thus forming the Weymouth anticline, or upfold. Along its northern edge, the Weymouth anticline is affected by minor folds and a major fault system.The rocks rise out of the sea in the south, forming the wedge shape of Portland, and dive back into the landscape to the north. The result is a series of parallel limestone hills and clay vales that run east to west from the shores of the Fleet to Osmington.
From beach level, the Kimmeridge Clay show little evidence of marker bands. The black cliff face appears almost a continuous bed of rock. However, two beds of cementstone can b observed., One is the Virgula Limestone, composed almost entirely of the oyster, Nanogyra virgula. The other cementstone is the Nano cardioceras Cementstone. This nodular cementstone contains the ammonite Amoeboceras (Nanocardioceras) preserved in uncrushed calcite.
The Kimmeridge Clay often forms a crust of weathered clay in warmer weather, which can completely obscures the lower parts of the cliff. This can be removed by simply scraping away, to reveal fresher material beneath. Do not dig into the cliff, as this is an SSSI location.
A series of ledges run parallel to the beach, which are composed of Nothe Grit and the Upton Member of the Osmington Oolite Formation. No fossils can be extracted from these incredibly hard rocks.
The site is perfectly safe but be aware of incoming or rough tides, particularly during the winter months. Be aware also of any potential slippages on the cliff.
Fossils will need to be prised from the matrix of the Kimmeridge Clay. You will need a geological hammer and chisels to extract bivalves and ammonites from fallen blocks of the Cementstone beds or Myophorella clavellata blocks on the foreshore. and the Chalk. Leave sufficient matrix around the fossil, to enable finer preparation at home.
This site is an SSSI and forms part of the UNESCO World Heritage Jurassic Coast. This means you can visit the site, but hammering the bedrock is not permitted. For full information about the reasons for the status of the site and restrictions please download the PDF from Natural England – SSSI Information – South Dorset