West, Ian .M. 2013. Osmington Mills - Black Head: Geology of the Wessex Coast (part of the Jurassic Coast - World Heritage Site). Internet field guide. http://www.southampton.ac.uk/~imw/osblack.htm. Version: 18th December 2013.

Black Head, west of Osmington Mills, Dorset

Ian West,
Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,
Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.

[This webpage is old and will soon be updated!]

Home and Contents |Field Guides Introduction | Osmington - Introduction |Osmington - Osmington Mills to Ringstead |Osmington - Bencliff Grit |Osmington - Osmington Oolite |Osmington - Black Head |Osmington - - Corallian Fossils |Kimmeridge Clay Fossils |Osmington - Bibliography |

Black Head, Osmington Mills, viewed from near Redcliff Point

Go to another Osmington Guide?

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 .

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Black Head from Osmington Mills

Reefs of Corallian rocks at low spring tide, Black Head

View from near Redcliff Point eastward towards Osmington Mills

Osmington Oolite reefs west of Osmington Mills

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).

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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.

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The Route along the Beach to Black Head

Redcliff Point to Bran Point, near Osmington Mills, Dorset - geological map of Arkell

Osmington Oolite reefs west of Osmington Mills

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.

Aerial photograph of Osmington Mills, Dorset, with the  Osmington Mudslides of 1910-1914 shown

Map of the Osmington Mudslides after Arkell (1951)

Boulder arc from a mudslide, Black Head near Osmington Mills

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.

Reefs of Corallian rocks at low spring tide, Black Head

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.

Examining Corallian and Kimmeridge debris at Black Head

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.

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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?

Osmington Oolite reefs west of Osmington Mills

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?

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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 in the Red Beds of the Corallian at Black Head

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.

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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.

Worn specimen of Myophorella sp., from Corallian, Black Head

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.

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Kimmeridge Clay (Upper Jurassic) - General

Black Head, Osmington Mills, viewed from near Redcliff Point

Aulacostephanus eudoxus, Kimmeridge Clay, Black Head area

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.

Bivalve shells on a slab of Kimmeridge shale, Black Head

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.

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Kimmeridge Clay Dinosaur, Black Head

Dinosaur bone, Kimmeridge Clay

Dinosaur bone, cross-section 1

Dinosaur bone, cross-section 2

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.

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Sauroptygian Remains

Vertebrate remains from Black Head, Osmington Mills, Dorset

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.

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Kimmeridge Clay Ichthyosaur Paddle

Find of ichthyosaur bones, Kimmeridge Clay, Osmington Mills, Dorset

Nodule with bones - apparent top, Osmington Mills

Nodule with bones - apparent bottom, Osmington Mills

Difficulty in removing nodule with bones, Osmington Mills

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.

Ichthyosaur paddle of Ian Troth, specimen prepared by Mark Hawkes

Bone structures of ichthyosaur paddles

Limb bones of an ichthyosaur, Kimmeridge Clay nodule, Lyme Regis

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.

Ichthyosaur fossilisation analogue - decomposing remains of a seal

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.

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Kimmeridge Clay Pyrite

Pyrite nodules washed out of Kimmeridge Clay and now amongst beach pebbles, Osmington Mills

Pyrite nodules derived from the Kimmeridge Clay, Osmington Mills, Dorset

Pyrite, FeS 2 , is a common mineral in dark shales. It is mainly produced in the early burial zone, the sulphate reduction zone. This is usually of some metres in thickness extending down from just beneath the sediment-seawater interface. Sulphate ions are available in connate water within the sediment and may also be available from the seawater above. Organic-rich sediments particularly favour the development of sulphate-reducing bacteria. These, living on the organic matter, reduce the sulphate ions to sulphide ions. H 2 S may be produced. An available iron reacts with sulphide ions to produce pyrite (although not necessarily directly and simply). The sea-floors of the Kimmeridge Clay and Lias were often suboxic and occasionally anoxic. This favoured the preservation of organic matter (debris from plankton etc) necessary for the sulphate-reducting bacteria. Thus pyrite is particularly common in these formations. It occurs as minute dispersed crystals but also as nodules, like those shown here (these have been picked out from the beach and placed on a nodule). Some such nodules may be coprolites, although the examples shown here do not show the characteristic shape.

Recent coast erosion of the Kimmeridge Clay and the Lias releases pyrite nodules. They tend to be concentrated in groups near the low-tide mark. They are heavy and rather metallic in appearance with a little rust on the exterior. If broken with a hammer they will show crystals of a more brassy metallic appearance (although not as yellow as chalcopyrite). Such nodules are worth examining in cases pyritised fossils are preserved or coprolites are present.

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Kimmeridge Clay Nodules

Kimmeridge Clay nodule, West of Osmington Mills, Dorset

Kimmeridge Clay nodule with beef, West of Osmington Mills, Dorset

concretion growth model for birchi nodules, Lias, Charmouth

Cone-in-cone from exterior of nodule, Kimmeridge Clay

Cone-in-cone detail, Kimmeridge Clay

Cone steps in cone-in-cone beef, Kimmeridge Clay, west of Osmington Mills

The development of fibrous calcite or beef with cone-in-cone around the outside of septarian nodules is a subject discussed with regard to the Birchi Nodules of the Lias of Lyme Regis. It is the result of burial diagenesis probably involving the dissolution of aragonitic carbonate and its reprecipitation as a stress-resistant fabric. A diagram regarding the Birchi Nodules is reproduced here and applies to these nodules too. The Kimmeridge Clay nodules here are at stage 3 in the Birchi Sequence. See further discussion on this topic in the Lias webpage.

Core of Kimmeridge Clay, septarian nodule

Expansion of bivalve shells on septarian nodule periphery

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Upper Greensand (Upper Cretaceous)

Boulder arc of Upper Greensand debris from a mudslide, Black Head near Osmington Mills Upper Greensand boulders and reefs of Corallian rocks at low spring tide, Black Head Serpulids in an Upper Greensand boulder, Osmington Mills, Dorset

As noted above and shown again in the left-hand and central images, there are many Upper Greensand boulders on the shore between Osmington Mills and Black Head. The right-hand image is of a boulder with serpulid worm tubes - Rotularia concava (J. Sowerby).

The Upper Greensand is not seen in in situ in the cliffs here, although there have been poor exposures at the back (northward) of the large mudslides here. The Upper Greensand boulders on the beach are divisible into two types according to Arkell (1947) . The first consists of very hard rounded grey doggers with flecks of mica, apparently from a bed of dark micaceous sandy clay; they contain numerous large echinoids of the species Cardiaster latissimus , together with Rotularia concava (J. Sowerby) and a few bivalves. Cross-sections of the echinoids are more likely to be seen on worn surfaces of the boulders. The example shown in the photograph above with the worm tubes is of this first type, although without echinoids. The second type of boulder are softer, green, brown and bluish in colour, certainly from a higher horizon; they contain large ammonites including Deiradoceras, Goodhallites, and Anahoplites picteti . There are bivalves of Blackdown type present in these nodules Arkell (1947) .

Redcliff Point to Bran Point, near Osmington Mills, Dorset - geological map of Arkell

This old geological map is provided again here to show the general source area of the Upper Greensand and Gault north of the coast between Osmington Mills and Black Head. The boulders on the beach have been brought down from this outcrop in the past by large mudslides. This map is old but later resurveys have not significantly changed the pattern shown for the geology of the coast. The inland area is not well-exposed and thus different authors have different interpretations with regard to details, and later maps are not exactly the same as this (e.g. see House, 1989;1993 and the latest BGS map).

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Beach Features

Beach of boulders and large pebbles near Black Head, Osmington Mills Black Head, Osmington Mills, viewed from near Redcliff Point

The beach east and west of Black Head is shown for comparison. East of Black Head there is much debris from mudslides and from the Corallian outcrops on the shore. Flint in limited in quantity. West of Black Head the flint pebbles are dominant. They increase in quantity and to some extent in size towards Redcliff Point. Why should this be? Wave direction and fetch must be involved and flint pebbles are presumably transported towards Redcliff Point. Waves are no doubt affected by the proximity of the Isle of Portland, which is effectively blocking storm waves from the southwest, the prevailing direction. Just how has the flint been supplied to this part of the beach, though, west of Black Head? There is no major source of pebbles in the cliffs and no incoming river valley of any significant size.

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Questions and Exercises for Students using the Images of Black Head:

1. The coast here trends at about 103 degrees, true reading (that is east-southeast to westnorthwest). What is the direction and angle of dip? What is the strike direction?

2. Sketch out a rough sequence of strata here. Can you identify particular beds by comparison with the sequence east of Osmington Mills? It may not be easy from the image.

3. A student with a tape measure is about to make an approximate horizontal measurement of a clay bed from the top of the bed at the base of the cliff to a small exposure on the beach. How will he determine the vertical thickness? Draw a small diagram to determine the formula. If a is the angle of dip, h the horizontal distance and v the vertical thickness, does it involve Cos a, Sin a or Tan a? If you have worked it out write down the formula. (Note that if, like some people, you prefer not to use simple trigonometry then you can solve it with a simple scale diagram! ).

4. The bed at the level of the same student's head seems slightly iron-stained and very irregular. Look at the base of the bed. What is the reason for the very irregular appearance?

5. Comment as best you can on the direction of jointing. Is the direction that which you would expect from your knowledge of the structure and structural history of the area?

6. Do you consider this to be a stretch of coast that is being rapidly eroded, moderately eroded or only slowly eroded? Do you expect the erosion to increase or decrease here as the coast retreats.

7. What is the approximate angle of the cliff-face here (say from the horizontal)? It is general for much of the cliff above (i.e. it does not steepen significantly, but becomes more gentle near the top). How does this compare with the upper part of the cliff at Bran Point? What is the reason for the different angle here?

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I am very grateful to Ian Troth and Mark Hawkes for the photographs of the dinosaur bone and information on this discovery. Ian Troth has been kind enough to show many fine fossil specimens from this area that I could not find myself. These include excellent specimens of ammonites and vertebrate remains including not only one from a dinosaur but also many ichthyosaur bones. I am again much obliged to Ian Troth, Mark Hawkes and photographer Barry Marsh for the opportunity to show a photograph of the prepared specimen of the ichthyosaur paddle. Going into the field with Ian Troth is a particular pleasure because it always seems to result in some remarkable finds!

I am very grateful to Shirley Swaine of Puncknowle, Dorset for kindly notifying me about a further dinosaur bone discovery at Black Head. I much appreciate the opportunity to scan the photographs and reproduce them as above.

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To continue the Osmington Corallian Field Guide go to the section of interest:

- Osmington - Osmington Mills Introduction

- Osmington - Osmington Mills to Ringstead.

- Osmington - Bencliff Grit

- Osmington - Osmington Oolite

- Osmington - Black Head

- Osmington - Corallian Fossils

- Osmington - Bibliography

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References and Bibliography

Go to Osmington, Corallian, Bibliography

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Go to another Osmington Guide?

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 .

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Copyright © 2013 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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Dr Ian West, author of these webpages

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.


at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.