By Andy Tenny (neuropteris)
The Yorkshire Coast of England exposes a series of marine and terrestrial rocks ranging in age from the lower Jurassic at Cleveland in the north through to the chalk of the Upper Cretaceous at Flamborough Head in the South. Many of these formations are highly fossiliferous and yield a diverse range of fossils including marine reptiles, ammonites, belemnites, fish, nautiloids, bivalves and brachiopods, corals, crinoids, and in some of the terrestrial deposits, dinosaur foot prints and a wide range of plants. Even Pterosaurs have been found here. One of my favourite places for hunting is the beach at Ravenscar at the southern end of Robin Hoods Bay.
The Geology of Ravenscar
The rocks forming the cliff and rocky platforms exposed at low tide are of the Toarcian stage (the upper stage of the Lower Jurassic) dating from around 182 million years ago and comprise the following members of the Whitby Mudstone Formation (youngest to oldest) :-
Fox Cliff Siltstone (11m)
Peak Mudstone (13m)
Alum Shale (37m)
Mulgrave Shale (31m),
They are overlain by the gritty sandstones and pebble beds of the Dogger and Saltwick formations. The sequence represents a fairly deepwater basin (Mulgrave Shale) which became progressively shallower as time progressed (Alum Shale) until the area was finally filled with deltaic sediments brought in by the rivers running off an encroaching landmass to the north and west.
Collecting at Ravenscar
After parking at the lay by near the National Trust Center the way down to the beach leads through the Golf course of the Raven Hall Hotel. Standing at the top of the cliff, a rocky promontory extending 500m out to sea (at low tide) can be seen. This is Peak Steel and is made up of sandstones of the Staithes Formation. This is bounded on both sides by the Peak Fault, a major fault in the area which was active in the Jurassic and the beds to the east of Peak Steel are downthrown 153m compared with those to the west. The top of the cliff also provides a view of Robin Hoods Bay with its curving scars. The Bay is made up of the older Redcar Mudstone Formation of Sinemurian-Lower Pliensbachian age. Follow the path through the golf course and down the slope to a chair. Here the path becomes a steep track.which follows the line of the Peak Fault. At the bottom of the path it's a short scramble down the eroded shale to the beach. This brings you on to the dark grey Mulgrave Shale rock. The Mulgrave Shale is rich in Pyrite and was laid down in a deep water and fairly anoxic environment. It is fossiliferous though not as much as the overlying Alum Shales and yields flattened ammonites, Harpoceras and Dactylioceras, and the bivalve Pseudomytiloides dubius. As you follow the coast south eastwards you are walking up the succession and after a few hundred metres come to the Alum Shales.
These Alum shales were deposited in shallower water and have a far more diverse array of fossils. Bivalves Dacryomya ovum and Pleuromya are very common, often still found in their muddy burrows and there are a rich variety of cephalopods to be found. Belemnite guards are common and the ammonite Dactylioceras commune is extremely abundant. Most often only the body chamber is preserved in the shales with the center of the shell crushed but at many horizons the ammonite forms the nucleus of calcareous or pyritic nodules. These are readily identifiable as the outer whorl of the ammonite protrudes from the nodule but many of the nodules which don't show evidence of an ammonite also contain one. Thousands of nodules eroded out of the cliffs litter the beaches.
The next most common ammonite found is Hildoceras named after St Hilda of Whitby (two species are present Hildoceras lusitanicum and less commonly Hildoceras bifrons). Peronceras fibulatum is also very common and can be distinguished from the similar looking Dactylioceras by the tubercles which project from the whorls of the shell. Rarer species which can also be found include Porpoceras vortex, Catacoeloceras crassum, Psuedolioceras lythense, the large Phylloceras heterophyllum and several others. Nautiloids also appear but are rare. Marine reptile remains Ichthyosaurs, Plesiosaurs and crocodiles are also occasionally found on this stretch of coast.
It's better to hunt amongst the beach boulders and in fallen blocks than to hammer nodules out of the cliffs as large chunks of sandstone and shale do fall occasionally. Drifted wood can be found in both the Alum and Mulgrave Shale members and in the upper beds of the Mulgrave Shale is often preserved as Jet. Jet has long been prized by local craftsmen and is worked into jewellery and ornaments. It's a hard black material made up fossilized wood from Araucarian conifers (Monkey Puzzle trees). Where Jet seams have been found Adits and small mines can often be seen dug into the cliff faces along the coast by generations of workers.
By scrambling over the huge blocks of Saltwick formation sandstones which cover much of the beach (and often show ripple bedding and coalified plant remains) you will eventually come to Fox Cliff (seeing many more nodules on the way). Fox Cliff overlooks a small narrow bay and gives its name to the Fox Cliff Siltstone which makes up much of the lower cliff face here. This is the best locality on the coast for finding the ammonite Grammoceras. Along the rest of the coast the Fox Cliff siltstone is poorly exposed. Grammoceras is found both in fallen blocks and in nodules which are often packed with ammonites. Belemnites are also common and there are many species of bivalve to be found.
You can return to the top of the cliff either by retracing your steps or by following a narrow overgrown path which leads back up the undercliff and meets with the path down. This is slippery and very overgrown but cuts out a lot of the boulder scrambling. Collecting at Ravenscar has some dangers. Large cliff falls are infrequent but do occur as can be seen by the size of the debris on the beach. Smaller falls are more common and this should be born in mind if looking at the cliffs. It is possible to be cut off by the tide although you have around 3 hours at each side of low tide to examine the exposures. The rocky platform can be very slippery and it's easy to trip while boulder scrambling. Also it's isolated so go with other people or take a mobile phone in case something untoward happens.
Andy Tenny -- November 2004