Osmington Mills - Introduction ; ... Osmington Mills to Ringstead ; ... Osmington Mills - the Bencliff Grit ; ... Osmington Mills - the Osmington Oolite ;..
Osmington Mills west to Black Head ; ... Osmington - Corallian Fossils ;... Kimmeridge Clay Fossils ;... Osmington Mills - Geological Bibliography .

This field guide is specifically for the coast from Osmington Mills westward to Black Head and a short distance beyond (at Shortlake). Redcliff Point to the west will dealt with separately. The photographs above show the location in relation to local features and to Osmington Mills, where there is a car park and pub (Smugglers Inn). Black Head (map reference SY 726819) is a cliff feature about a kilometre west of the Osmington Mills slipway. It is not at present a very prominant headland. At one time it was probably larger and extended southward protected by a rampart of Corallian sandstones and limestones which have since been eroded to a series of low reefs. It takes its name from the black or really, dark grey, Kimmeridge Clay which forms the upper, rather slumped, parts of the cliff. It is easily reached by descending to the beach at Osmington Mills by the steps alongside the steep slipway and by turning right (westward) in the direction of Weymouth. There is a walk of about 30 minutes over the fallen Cretaceous and Kimmeridge boulders (fossiliferous and of interest) on the beach to reach the Corallian and Kimmeridge Clay cliff section at Black Head. There are interesting aspects of coast erosion, mudsliding, fossils and sedimentology in this area, and if the various features en route are studied in detail, it may take an hour or more to reach the headland.

With regard to the Corallian (of the Upper Jurassic System), the Black Head section is most useful for the good exposures of the upper part. The lower part is in the reefs and most of it is not accessible. Comparison of the upper part, however, can be made to the relevant section east of Osmington Mills (Bran Point etc).

Safety at Black Head and adjacent area

The moderate angle of the cliff here means that in dry weather, the risk of injury from falling debris is low. However, it is still possible and hard-hats should be worn. In wet weather clay and other material may slump down the cliff. There have been major mudslides to the east of Black Head and wet deeply boggy patches may occur in these in which you could get stuck. At low tide boulders on the beach can be slippery and at high tide some parts of the exposures on the shore may be submerged. Please read the safety during fieldwork guidance and bibliography for the Dorset Coast.

The Route along the Beach to Black Head

The map shown above provides information on the locations and general geology according to Arkell (1947) (although subsequently updated in detail by other authors). Descend from the car park of the Smugglers Inn (this is intended for customers so it is fair to have a drink or a meal in the pub at the end of the trip!). The original path down to the beach was at the ramp or slipway but this has been damaged by a landslide. Geologists should have no difficulty, though, reaching the shore down a moderate slope. At the beach turn right in a westward direction. The immediate area is complicated by faulting. There are reefs of Corallian strata visible at low tide. Proceeding onward in a westward direction for about 100 metres north dipping Corallian strata, in particular the Osmington Oolite, is seen in the cliff. Next on the beach, as shown in the right-hand image, is a short stretch where septarian nodules with beef have been washed out of Kimmeridge Clay debris that has been supplied to the beach by a fairly small mudslide. This mudslide is still active in part from time to time. This is a good location to study concretion development and beef (cone-in-cone). It is also the site of an ichthyosaur discovery, referred to below.

From here westward there is broad shallow bay with slumped cliffs of Kimmeridge Clay and numerous Cretaceous boulders on the beach. There have been major mudslides active here in the past (Arkell, 1951) . Probably the main cause has been a breach by the sea in a wall of more resistant, north-dipping, Corallian strata. This breach has allowed the weak and mobile Kimmeridge Clay to slump southward in landslides. Landward of this and retained by a fault is water-saturated Gault and Upper Greensand. This in turn has moved seaward in a system of triple mudslides on a large scale. In some respects the situation is reminiscent of the north side of Stair Hole, Lulworth . There the north-dipping Purbeck limestones and shales are for the most part still supporting incompetent Wealden Marls behind, but where this rock protection has been breached the marls have started to move seaward in mudslides.

Arkell (1951) commented that the glacier-like triple mudslides between Osmington Mills and Black Head were the largest and most remarkable features of their kind on the British coasts. Apparently, according to local recollections they orginated one night in early spring between 1910 and 1914. There was major movement up to about 1951 but since then there has been much erosion of the toes of the mudslides. The numerous boulders are dominantly of calcite-cemented Upper Greensand and are the large wave resistant residue of the debris of the mudslides.

Closer to Black Head there is a boulder arc. This marks the position of the seaward end of one of the three large mudslides. The Upper Greensand boulders were originally nodules or "cowstones". You will notice that they contain serpulid worm tubes (Rotularia concava (J. Sowerby) formerly known as Serpula concava ), cross sections of echinoids, many of which are probably Holaster , occasional ammonites, some molluscs and many burrows. Some small brown phosphate nodudles are sometimes present within the large boulders. Occasional Chalk boulders appear towards the western end of this boulder field. They generally are from the Lower Chalk and therefore without flints. You can see from Arkell's map that there is a limited amount of Chalk in the source area of the mudslides. Typical Lower Chalk fossils such as the sponge Exanthesis labrosus (T. Smith) (formerly known as Plocoscyphia labrosa) occur in these blocks. Notice the abundance of flint pebbles. These occur on both sides of Black Head, but are limited in number at the promontory itself. Note that in the photograph here you can see that the flint pebble storm beach is overlying the deposit of Upper Greensand boulders. Much of this flint debris may have been brought down by the mudslides, so that natural renourishment or replenishment (civil engineering terms) of the beach has occurred. The finer debris, the clay and sand has probably been transported away by wave and current action.

At low tide, and particularly at low Spring tides, reefs of Corallian rocks are visible running east-west along the shore at each end of the shallow embayment. They are best developed at Black Head itself, as shown in the accompanying photograph. They are also present, as shown in the photograph above, at the Osmington Mills end of the bay.

Black Head is beyond the stretch of abundant Upper Greensand boulders. In addition to the Corallian rocks or ledges at low tide level, there are also Corallian outcrops at the foot of the cliff. Here there are good exposures of the upper part of the Corallian succession. Loose boulders contain Corallian fossils. Here too there is much interesting debris fallen from the Kimmeridge Clay above which forms a moderately projecting and sloping dark grey cliff. The exposure of the Kimmeridge succession above is an important reference section. Details of the Kimmeridge Clay are described in a section below.

Corallian Group (Upper Jurassic System) - Osmington Oolite Formation

The Middle White Oolite of the Osmington Oolite (central part of the Shortlake Member), which has increased in thickness westwards. The general setting is also shown with the remains of a very large mudslide in the middle distance and Upper Greensand debris brought down to the beach by it in the past.

The rock is of Middle White Oolite, shown in the centre of one the photograph. There is cross-bedding and rip-up clasts or mudclasts. Both of these features are evidence of the the high-energy conditions in which this carbonate sand was deposited. Notice that the beach pebbles here, incidently are mostly of flint probably from Quaternary gravels which occur nearby. They are subangular and poorly sorted, unlike the pebbles on the Chesil Beach. The small-scale stack of oolite at Black Head show some interesting features of coast erosion. Compare the landward and seaward sides of these stacks. Notice where algae is present and absent. What types of erosional processes are dominant here?

Above is shown part of the Osmington Oolite Formation, probably the Middle White Oolite, seen in reefs exposed at low tide just to the west of Osmington Mills. These ledges seem to make visible a cyclicity or rhymicity in the oolites.

Above is a clean, sea-washed surface through an oolitic bed in the Osmington Oolite at Black Head, exposed on the beach. Left is west, right is east; the pen gives a scale; the pebbles are mostly of subangular flint.

Questions for students: Have a close look at this photograph of Osmington Oolite. What sedimentary structures can you see? Can you give a name to the trace fossils? What has been the sequence of events just here in the warm, almost subtropical Jurassic sea with its shoals of white lime-sand? The banks of ooid sand do not seem to have been very thick at Osmington Mills, only about 1 metre in general. Oolite banks in the Portland Stone are thicker than this. Why were the Osmington Banks so thin? Have you any comments on the cross-bedding? Why are there no burrows in the upper part?

Here are some questions about the pebbles. Why are the flint pebbles mostly subangular (with some rounding on the projections) and not well-rounded as on the Chesil Beach? Only one pebble in view is rounded. What would happen to this on the Chesil Beach? Comment on the sorting. Why are some flint pebbles brown and some grey to black?

Trigonia clavellata Formation (part of the Corallian Group - Oxfordian)

These three photographs (November, 1999) show limestones and clays of the upper part of the Trigonia clavellata Formation at Black Head with Sandsfoot Clay slipping down from above. The Trigonia clavellata Formation is high in the Corallian and above the Osmington Oolite Formation. The students shown here are making graphic logs for exercises involving poster presentations. Notice the brown ankerite (a trigonal carbonate of ferrous iron and magnesium) in the Red Beds and also some white calcite veining. Most veins in these strata are of calcite (No. 3 on Moh's Scale and therefore can be scratched with steel; it, of course, effervesces readily with dilute HCl ). Quartz veins are not normally present in the Dorset Jurassic and Cretaceous strata, but are widespread in siliceous rocks that have been more deeply buried to higher temperatures (in southern England that means Pre-Hercynian strata - approximately pre-295Ma).

Ooids are present not only in the Osmington Oolite but also in the Red Beds. Here the reddish-brown matrix of siderite makes the white ooids conspicuous. The Red Beds here represent a transitional environment from an ironstone facies to a clean carbonate oolite facies. They were probably deposited fairly nearshore where there was influx of iron from chemical weathering on the land. The Abbotsbury Ironstone of early Kimmeridgean age is an oolitic (berthierine and limonitic) ironstone that was developed at a short distance above stratigraphically. This occurs northwest of Weymouth and almost certainly closer to an original Jurassic shoreline.

Shells of Myophorella clavellata ("Trigonia clavellata ")

A loose block from the Red Beds of the Trigonia Clavellata Formation on the shore near Osmington Mills with the bivalve Myophorella clavellata (J. Sowerby), formerly Trigonia clavellata, and shown at two magnifications. Note the thick shell, presumably originally of aragonite, but now replaced by calcite. This bivalve, a "cockle of the Mesozoic" had a robust shell, probably well able to withstand storm wave conditions. They were shallow burrowers with muscular T-shaped feet so strong that they could jump. They were suspension feeders that lived nearshore in water only 10 - 15m deep. The shells are in a matrix of ooids originally mobile, aragonite lime sand, but now replaced by calcite. The carbonate sand was impure and contained some clay and iron content. There is now some siderite in the matrix.

Left: Here are two views of an isolated specimen of Myophorella clavellata. Unfortunately, it is somewhat worn but the tubercules show clearly. It belongs to the superfamily Trigoniacea and used to be called "Trigonia". The Myophorella shell is trigonally ovoid with anterior erect inward pointing beaks and obtuse posterior carinae; posterior slope on each valve is bipartite with little ornament. Myophorella is a genus which ranges from the Lower Jurassic to the Lower Cretaceous and is cosmopolitan.
Right: This worn specimen is of another species of Myophorella. It was found ex situ on the beach at Black Head, and has probably been eroded from the Corallian strata here.

Kimmeridge Clay (Upper Jurassic) - General

Black Head takes its name from the black or dark grey cliffs of Kimmeridge Clay. These cliffs slopes steeply back from the Corallian strata at beach level which protect the shales from erosion. Debris slumps and falls from these Kimmeridge Clay cliffs providing a variety of interesting and fossiliferous shale samples and nodules on the beach, including good fossiliferous material from the Aulacostephanus zones. The cliffs above contain the type section of the Lower Kimmeridgian (Ziegler, 1962) A very useful desription and drawn section has been provided by Cox and Gallois (1981) and reference should be made to this work for details. The cliffs require some minor scrambling up steep crumbly shale to view them and the exposures are incomplete or obscured by debris or weathered clay. Cox and Gallois (1981) revealed much of the succession by digging. There is in fact almost a full sequence in the cliff from the basal beds, just above the Corallian, up through the Lower Kimmeridge and extending up to the level of the White Stone Band (a conspicuous, white, coccolith limestone in the Pectinatities Zones). At the base is the interesting Inconstans Bed (Bed 1) with the well-known, asymmetrical, rhychonellid brachiopod Goniorhynchia inconstans. The Nana Bed (as the name indicates this contains something small, the little oyster Nanogyra nana) occurs just above, with Wyke Siltstone (Bed 5) and the Black Head Siltstone (8) following. The water deepened and siltstones give way to shales without silt. The Xenostephanus Beds are listed as Bed no. 14. (House, 1993) .

A specimen of Aulacostephanus eudoxus found by Ian Troth is shown here. This specimen is in its original unprepared state. The shell is of aragonite. Ammonite shells are only preserved as such in the argillaceous units of the Jurassic in Dorset, but even here they have lost most of the organic matter and the shell colour. The body chamber in this specimen may be present to the left although some of the shell is missing. It is nevertheless a remarkably good specimen. This ammonite is normally found only in compacted condition, not in solid, three-dimensional, state as is this one. The lack of compaction is the result of early cementation in a calcitic concretion. Notice the well-defined tubercles. The venter is not shown here but is of the usual and characteristic Aulacostephanus type. It is rather rectangular in section and flattened ventrally. The ribbing is coarser than in A. pseudomutabilis and this is a distinctive feature. Note that the eudoxus Zone is the fourth zone up from the base of the Kimmeridge and still within the Lower Kimmeridgian. The Lower Kimmeridgian zones are, from bottom to top: Pictonia baylei, Rasenia cymodoce, Aulacostephanus mutabilis, Aulacostephanus eudoxus, Aulacostephanus autissiodorensis. The importance of Black Head becomes clear when it noted that the succession in the cliffs at Kimmeridge descends no lower than about 10m into the eudoxus Zone. The zones beneath this are exposed at Black Head, and at Ringstead to some extent.

The higher Kimmeridge ammonites tend to be not of Aulacostephanus type with their smooth venters but are perisphinctids with relatively round whorl sections. An example, well-known to collectors, is Pavlovia rotunda from Chapmans Pool. The Black Head Kimmeridge section does not reach the pavlovia Zone, but pectinatitid-type Upper Kimmeridge ammonites should be present. They are probably mostly compacted and flattened. Look for them in the debris or in the upper part of the cliffs.

This typical slab of Kimmeridge shale is full of Lucine Clams. The modern ones are mostly between 4 and 7.5 cm but these Kimmeridgian ones are very small and named as Lucina minuscula Blake. The modern Lucine Clams are mainly warm-water species which burrow in sand or mud. They too are discoidal and they are also characterised by a long, narrow, anterior muscle scar and the absence of a pallial sinus. The discoidal or circular outline is obvious here with regard to these fossil examples, but the details of the interior of the shell are not visible in this photograph. Notice that the associated Protocardia is also small, actually unusually so for the Jurassic (contrast with examples of the common Portland Protocardia dissimilis (J. de C. Sowerby)). It resembles in size, though, the Purbeck Protocardia purbeckensis , which was clearly euryhaline and tolerant of relatively adverse conditions in terms of high salinities. Both these Kimmeridgian infaunal species were surviving in sea-floor conditions that were adverse in a different manner; oxygen deficiency occurred at times and seafloor was often suboxic and on rare occasions anoxic (during oil-shale accumulation). There are some traces of ammonites on the slab showing evidence of fallen nekton.

Kimmeridge Clay Dinosaur, Black Head

The photographs show a dinosaur bone from the lower part of the Kimmeridge Clay at Black Head, part-way up the cliff in the Blackhead Siltstone. Only a small part was initially visible and it was excavated. This bone was found in August 2001 by Mark Hawkes and Ian Troth. It is shown here in initial condition, before any treatment or restoration. It is in very shelly shale. There were no indications that other bones were present. The scale bar is in inches - one inch = 2.54cm. The specimen now resides at York Museum.

Sauroptygian Remains

Saurian remains were found at Black Head by Shirley Swaine in April 2004. This nodule, almost certainly derived from the Kimmeridge Clay above, was protruding from a mud slump on the beach around the Black Head ledges. Steve Etches, the Kimmeridge vertebrate specialist, considers that it is of sauropterygian (plesiosaur or pliosaur) origin. The blocks also contains the remains of small oysters.

Kimmeridge Clay Ichthyosaur Paddle

On the 22 March 2003 Ian Troth, postgraduate research student and fossil collector, was walking back to Osmington Mills with Ian West after studying the section at Black Head. Ian Troth noticed some small bones on one side of a nodule. On turning it over it was seen to contain much bone material, probably including ribs and vertebrae of an ichthyosaur. These are shown on the images above.

The specimen clearly needed removing from the beach to a safe place and undergoing professional preparation so as to reveal the details of the bones. Removing it from the beach proved very difficult. Perhaps containing pyrite (of high density) it was extremely heavy to handle, being about the weight of a person. Ian Troth carried it up the beach and then with a car tow-rope tried to drag it towards Osmington Mills. This did not prove successful. Later, assistance was kindly provided by a member of the public who was on the beach.

Later in the year (August) the specimen was seen after removal of limestone and careful preparation by Mark Hawkes of Stone Treasures. This well-preserved example of an ichthyosaur paddle remains in possession of the finder - Ian Troth. With a photograph of the prepared specimen there is also provided, for reference, a diagram showing the general skeletal structure of ichthyosaur paddles. Although this diagram shows the paddles of a Liassic ichthyosaur and the number and arrangement of phalanges varies between species, it should enable to reader to recognise, at least in part, the bone structure of the Osmington specimen. Compare the photograph with the diagram, and decide firstly which end is proximal and which is distal. Where should the humerus or femur be located? Which is the anterior side of the paddle? Can you tell whether this is fore-paddle or a hind-paddle?

In addition, to the right are shown two limb bones seen on the underside before preparation of the specimen. The humerus and the femur of the ichthyosaur are rather similar. However, the femur is likely to be smaller than the humerus. If, as you may consider, the prepared surface shows part of an upper limb bone, then these two have to be from the other pair of limbs. Do you think that they are femurs or humeri? These preliminary remarks are only based on the photographs. Ian Troth, no doubt, will provide more precise information later from a study of the prepared specimen and we await this with interest. He has already mentioned to me that there are also rib (costal) bones present and you can see parts of these in the right-hand photograph. It is an unusual specimen because so many bones are together in a relatively small septarian nodule.

Fossilisation processes like those which preserved the ichthyosaur remains are probably taking place at the present time. Here are shown the decomposing remains of a seal with the paddle or fin bones (phalanges) becoming visible together with part of the shoulder girdle. This is on a beach, at Pondfield Cove near Worbarrow Bay, and after the corpse has fully decomposed the bones will become scattered by the waves. If, however, this had sunk into suboxic or anoxic (black) mud, with some carbonate content, then the geochemical environment of the decomposing remains (producing ammonia etc) could initiate the development of a calcite septarian nodule.

For more on Kimmeridgian fossils please go to the Kimmeridge Clay Fossils webpage.

Kimmeridge Clay Pyrite