This guide is for a spectacular and quiet piece of coast that is a short walking distance to the west of Lulworth Cove (and east of Durdle Door ). It is a less-used part of the coast that provides good exposures from the Jurassic Portland Sand, through the Portland Stone, Purbeck Formation, Wealden, Gault and Upper Greensand through to the Chalk. The succession is very sheared and faulted and it is of interest to structural geologists. From Lulworth Cove proceed on foot to the short private road that runs uphill westward at the back of Stair Hole and past the large houses in the trees. Follow this to its end where you will find a narrow footpath sloping down to beach. Alternatively take a path to the left round to the top of Dungy Head for an exellent view. There is yet another choice too; as you walk up the hill on the road, you might notice an inconspicuous path between gardens of houses. This leads south to the cliff top. From here there is an interesting short cliff-top path to Dungy Head. In places this path is above fairly steep cliffs. By whichever route you take, sooner or later you will arrive at the beach just north of Dungy Head. This is nearly always accessible at all tides, but if you wish to walk to Durdle Door note that Man O'War Head can only be passed at low tide.

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INTRODUCTION:

General

To go to Dungy Head and St. Oswald's Bay walk up the short hilly road westward of Lulworth Cove, past Stair Hole and past the large red-brick houses shown in the image here. At the end of the tarmac road there is a footpath ahead leading down to the beach at St. Oswald's Bay.

If you prefer to see a high overview first, take one of the two paths to the left (south). There is a more hazardous route (shown in photographs here) on the rocky outcrop above Dungy Head, from which there is a view down to St.Oswald's Bay and on the Durdle Door. You can then descend a steep path to the right down the dip slope. To get round to the coast to the south go down by the usual route to St. Oswald's Bay. Then scramble southeast over the rocks around the foot of Dungy Head (but beware of falling rocks).

INTRODUCTION:

Topographic Maps

INTRODUCTION:

Geological Maps

The British Geological Survey map, 1:50,000, Solid and Drift, 2000 Edition, Swanage Sheet, 343 and part of 342, is well worth purchasing. It can be obtained from the British Geological Survey website and is very inexpensive, costing only 12 pounds sterling.

This map is based on Nowell (1997).

INTRODUCTION:

Strata of the Lulworth Region

This image shows the simplified succession of strata in the Lulworth Cove region. The thicknesses given are only approximate and most units thicken in an eastward direction. FF - Fossil Forest horizon, LGS - Lower Greensand (very thin or absent here), PL - plenus marl, a grey marl marking the boundary between Lower and Middle Chalk.

This simplified geological map of the Lulworth Cove area is based on Townson (1975b) and the British Geological Survey map of Swanage (sheet 343 and part of 342). The dip in the Portland, Purbeck and Wealden strata is steep and to the north. The synclinal axis can be recognised by the long narrow Tertiary outcrop north of the cove. This axis runs east-west. The main axis of the very asymmetrical anticline is under the sea, at the Lulworth Banks, south of the map. The steeply dipping strata is part of the north limb which runs along the coast. The locations of Dungy Head and St. Oswald's Bay are shown.

Lulworth Geological Cross-Section

Diagrammatic geological cross-sections through Lulworth Cove. The left one is a new version incorporating aspects of cross-sections of West (1964), Bevan (1985) and House (1989) but with appreciable modifications. It is intended to be illustrative of the general structure, rather than being the best calculated cross-section, and it intentionally incorporates some simplications. It can be considered in different ways, either as a simple hypothetical explanation of the structures or a basic crude model for correction and modification.

A modified version of a very good cross-section by House follows. This is more accurate in that it shows more faults, but it does not deal with the Lulworth Crumples.

The Chalk is folded so as to produce the Purbeck Monocline. This is a very asymmetrical anticline with a steep north limb and a very gentle south limb bringing the Chalk down to the sea-floor nearly 30 kilometres to the south. Note that the fold in the Chalk has a very acute foresyncline (the sharp bend at the base). Note also that there is an unconformity under the Chalk, Upper Greensand and Gault. That is to say, the Jurassic strata underneath are not parallel but were folded before the Chalk was deposited. Note also that the fold is situated over a fault, and this is believed to have originated as a thrust fault in Upper Palaeozoic strata at depth during the Hercynian Orogeny (Carbo-Permian - round about 300 million years ago). Stress in the basement rocks, conveyed across the English Channel from continental Europe has reactivated this in the Late Eocene to Miocene. Northward movement of Africa is probably the cause of this.

The fold is situated over what was once a fault downthrowing southward (in the Cretaceous). Now the southward side has risen; the direction of movement has reversed.

The early fault movement was progressive and related to the development of a basin to the south. This basin of thick sediments is a source area for oil. Because it was subsequently uplifted it is known as an "inversion". The Purbeck Monocline is a structure over a fault and the boundary between the English Channel Inversion and a high to the north (the high is an originally relatively uplifted area with thin or missing stratigraphical sequences). The high is a branch of the Wytch Farm structure that is so important for oil resources.

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Dungy Head - Detailed Geological Map

This map is based on Nowell (1997). Please consult the original paper for further details. It is particularly useful in not only showing the general geology but also revealing the details of faults in this area. The north-south distance of the Portland Stone from the Chalk varies greatly along this stretch of coast and the reasons are clearer if the details of the fault pattern here is studied.

Dungy Head - General Topography and Geomorphology

Dungy Head - Portland Group

Here we are looking eastward from St. Oswald's Bay to Dungy Head, the best place to see the Portland strata. The lowest part, the Portland Sand (actually siltstones, shales and dolomite with some sandstone beds) can be seen further on, at the base of the cliffs, round the corner to the east of Dungy Head. It is an interesting place and some ammonites can be found. The location of the image is at the eastern end of St Oswald's Bay with the small Pinion Rocks on the right and larger rocks, including Ball Stone at Dungy Head. The succession at Dungy Head is described below.

Dungy Head, is a good location to see the Portland strata. The lowest part, the Portland Sand (actually siltstones, shales and dolomite with some sandstone beds) can be seen further on, at the base of the cliffs, round the corner to the east of Dungy Head. Some ammonites can be found.

The Portland succession in the Lulworth area, particularly with regard to Dungy Head has been described by Arkell (1947), Townson (1975) and House (1993). It can be summarised as follows - in descending sequence:

The Portland Freestone

The Portland Stone (or Winspit Member) is mostly an oolite or ooid grainstone at Dungy Head becoming an ooid packstone east of Lulworth Cove (Townson, 1975).

Portland Cherty Series

The Portland Cherty Series is well seen at Dungy Head and both the upper part or Dancing Ledge Member of Townson (1975) and the lower part, the Dungy Head Member, contains much sponge spicule (Rhaxella ) wackestone, the silica from which has given rise to the chert. Most of the chert is early, as is usually the case; some is late and related to faulting. The conspicuous thin marker bed know as Puffin Ledge (Puffins used to nest on in the Isle of Purbeck) is easily recognised as a thin bed in the middle and is usually characterised by Thallassinoides burrows. Giant ammonites, Titanites occur at about this level.

Portland Sand

(At the top) Fine grained dolomites known as the Black Sandstones and part of the Gad Cliff Member. These contain the ammonite Glaucolithites . (4.1 m)
The Cast Bed with abundant bivalves, also part of the Gad Cliff Member. Glaucolithites occurs here too. (1.4 m).
The Exogyra Beds with Nanogyra nana, the serpulid Glomerula gordialis and Plicatula boisdini . (1.2m). (Notice how similar the facies is to the Upper Greensand facies seen a short distance to the northwest!)
Black siltstones and shales , the Black Nore Member (21.3 m)

For more information on the Portland strata see Arkell (1947) and Townson (1975a).

Purbeck Formation - Dungy Head

(Ready for additions)

WEALDEN - St. OSWALD'S BAY

Introduction

The Wealden succession has been drastically reduced in thickness at St. Oswald's Bay. Only 48 m. are left from a succession that measures 701 m. at Swanage and 428 m. at Worbarrow Bay. Thinning and unconformity are probably major factors in this area which was structurally high in Cretaceous times, before the formation of the English Channel Inversion. Strike faulting is probably a major factor and the Wealden strata exposed are very much sheared.

WEALDEN:

Wealden Jarosite and Chalybeate Spring

There is a conspicuous chalybeate (ferruginous) spring emerging from pyritic, plant-bearing, Wealden sandstone at the eastern end of St. Oswald's Bay. This precipitates some jarosite, and the water are undoubtedly high in sulphate ions, as is usual in the case of such springs.

Jarosite also occurs on a small cliff face of pyritic, argillaceous sandstone in the Wealden Group at the eastern end of St Oswald's Bay near Dungy Head. Oxidation of the iron sulphide pyrite can produce sulphuric acid and iron oxides. Reactions lead to the formation of the hydrated iron-bearing sulphate jarosite (with either sodium, potassium or ammonium). This yellow surface encrustation is often mistakenly referred to in old literature as " sulphur ". It is slowly soluble in water and is washed away to some extent by heavy rain but develops again in fairly dry weather.

St. Oswald's Bay - The Gault Clay

The Gault is a blue-grey clay which is of marine origin and contains marine gastropods, bivalves and ammonite. Unfortunately, it is prone to cause landslides and because of this is known as the " Blue Slipper " in the Isle of Wight, where it causes serious problems. Here, in the Lulworth area it causes small landslides and not large ones because the strata are almost vertical and it does not lie under large units of Upper Greensand and Chalk. In the cliffs it is usually much weathered and good fossils are difficult to find as a consequence. It has a distinctive pebble bed at its base and at its top it becomes sandy and passes transitionally up into the Upper Greensand.

The Gault - Wealden junction at Dungy Head is shown in the photograph, with alongside it a section also showing a drastic change in clay mineralogy. The pebbles are subrounded and consist of resistant material such as lydite (black Palaeozoic chert) and vein quartz. They represent a marine beach which was the start of a marine transgression over the fluvial Wealden sediments. The smectite clays are common features of marine sediments in this region but the Wealden sediments beneath usually lack such clays because of acid soil conditions but kaolinite and illite are stable. The plastic, expandible smectite clays are a major factor in the behaviour of this unit as a cause of landslides.

In terms of palaeoenvironments the marine transgression which the Gault represents is a major one, over the Wealden fluvial clastics. There is a major unconformity at the base of the Gault. It is not clearly visible here but it is at White Nothe, not far to the west. The compositions of the pebbles reflect the rocks eroded by the Gault sea.

St Oswald's Bay - General

Left image: St. Oswald's Bay is shown here looking westward from above Dungy Head towards the Durdle Door peninsula. This is the remains of a former cove, similar to Lulworth Cove, but which has been opened up by the sea. The offshore rocks show the outcrop of the Portland Stone. The nearest is Norman Rock. Beyond it is the larger Man O'War rock which , swings in suprisingly close to the Chalk in the middle distance (there is an interesting problem regarding this proximity). The smaller embayment beyond (Man O' War Cove ) was also once similar to Lulworth Cove, although smaller. This has as its western boundary the peninsula of Durdle Door (the arch is on the far side - the west). The brown-staining in the Chalk cliffs on the right is caused by dolines or solution pipes discussed below.

Central image: A view westward from the cliff top just west of Red Hole. This shows St. Oswald's Bay at the foot of the cliffs, and, beyond, Man O'War Head, Man O'War Rocks and and Man O'War Bay with the Durdle Door Promontory.

Right Image: View southeast from above Red Hole, showing Chalk with flints in foreground, part of the Red Hole solution pipe with content of Tertiary debris, low slumped ground in the area of the Wealden strata, vertical thin-bedded Pureck, and Portland Stone to the south.

The faults and other structures in St. Oswald's Bay are very interesting. There are complications of strike faulting in steeply dipping and overturned strata.

Macrofaults and mesofaults in the Chalk at the eastern end of St. Oswald's Bay. This classic section has been figured by Arkell (1947) and Phillips (1964) and the structures of this area interpreted in terms of maximum compressive stress by Bevan (1985). A more or less horizontal Sigma One is responsible for the S3 Shears (mesofaults) which are south-dipping with small thrusting northward. Sigma 3, the minimum stress or extension direction is near vertical.

The classification of mesofaults used here is from Arkell 1938; 1947. The S3 - the shallow south dippers - are the easiest to see and remember because they are conspicuous in the Durdle Door area and are a cause of the caves beyond the promontory. The S3s are part of a conjugate pair with north dipping group 6 and are regarded as younger than some other structures according to Bevan)

The macrofaults, strike faults, are interesting and require some thought to gain an understanding of them. The south-dipping F4 has the same orientation as the hypothetical major fault under the monocline (like the faults under the Abbotsbury-Ridgeway structure). The Chalk north of it is overturned as in the case of the F4 of the Lulworth Cove cliffs.

To understand this look again at the cross-sections which I provided for the start of the Lulworth Cove field trip. You can see how the F4 separates overturned Chalk from north-dipping Chalk at the back of the cove. No F5 is shown on this simplified section and the F5 at St. Oswald's Bay requires special explanation. It may be the same fault that cuts Man of War Head, a short distance to the west (Dr Jim Andrews - personal communication).

In the case of the F5, as Phillips (1964) pointed out, the disposition of younger beds on progressively older beds can most easily be explained by thrust displacements from the north. He suggested that an oblique strike fault, less steeply inclined that the bedding, developed in the lower part of the Chalk syncline due to extension below the neutral surface during an early stage of folding. (In simple terms - an early low angle fault, later folded during the formation of the monocline).

Slickensides on Upper Chalk with flints, St Oswald's Bay. These are north-dipping Group 2 shears of Arkell (1938; 1947), with a south-dipping Group 1 shear at the base and cutting through the middle (see Bevan, 1985 for explanation of the geometry and kinematics of these mesofaults. Sigma one, the principal compressive stress, dips at a low angle northward into the cliff. The very striated, Group 2 surfaces are Reidel shears. Group 1 are the result of flexural, bedding-plane, slip. ).

At a suitable exposure you can test the direction of movement on the mesofaults by running your hand up and down the striations, and noting in which direction it will jam. Note that in some places the tectonic effects are so severe that the flint has been ground up.

St. Oswald's Bay - Solution Pipes or Dolines

The photograph is of St. Oswald's Bay seen from the cliff edge near Man O' War Head, and looking towards Dungy Head. In partially overturned Chalk to the left of the picture can be seen brown-stained solution pipes. The large, funnel-shaped one, furthest from the camera is Red Hole. At the top of this the bedding in the Chalk appears to dip south, but in fact this is an overturned dip and the strata young in a northward (landward) direction, having been pushed past the vertical. The funnel-shaped top is probably largely the result of modern erosion washing out the fill of soft Tertiary loam and flint debris. This solution pipe extends to a short distance above the present beach, as shown by the iron staining.

Iron-cemented flint debris is present in the funnel-shaped cone of Red Hole, high above the beach of St. Oswald's Bay.

The base of a deep solution pipe in the Chalk is present near the western end of St. Oswald's Bay, approaching Man O'War Head (outcrop D of House (1996)). Solution pipes or doline like this are features of special interest in St. Oswald's Bay and have long been a geological mystery (House, 1965). These pipes extend from the top of the 120 metre high cliffs almost to the base through slightly overturned Chalk. Arkell (1947) mentioned that the cliffs here show large solution pipes in the chalk, filled with rusty sands and ground-up flints, and in places the wider pipes (eg. Red Hole) contain in addition masses of grey loamy sand. He said that it is not clear whether these formed before the Chalk was tilted into a vertical position and have been tilted with it, or whether they post-date the folding. If the deepest ones started from the top of the present hill and the filling is presumed to be Pliocene, they must have penetrated at least 120 metres. He stated that if, as is more probable, the filling is Eocene and they were started from the top of the Chalk, probably as the fold began to rise, they must have penetrated at least about 180 metres.

There are three red dolines and there is doubt about which one was named " Red Hole " or whether it refers to a red cave which no longer exists (House, 1996). The name is now normally used for the conspicuous and eastern of the solution pipes. Nine individual exposures of solution pipe material have been recognised by House (1966) and most of these are inaccessible, except by abseiling.

Clay mineralogy of a sample clay in the cliff above this site consisted of illite, kaolinite and degraded illite-smectite. The bulk rock consisted of very fine quartz with traces of illite, kaolinite, jarosite and k-feldspar (House, 1996).

House (1996) has described the particular exposure shown in the photographs above. This is a narrow solution pipe that almost reaches beach level (east of the F4 fault near Man O' War Head). It has been discussed previously (House, 1965; Cope, 1974). The base of the pipe is about 4 metres above beach level . The base of the pipe is about 4.5 metres wide. It was probably circular in cross-section. The upper part of the pipe is filled with chalk rubble in blocks. An almost horizontal lens of orange silt has been partly eroded out leaving the dark hollow in the middle of the image. This is rounded at one end and feathers out at the other; bedding is roughly horizontal. Below this marker bed the structure of the filling is contorted. There is chalk rubble and lenses of rusty and orange silts. House found lignite in this lower part, immediately below the marker hollow, but no Tertiary pollen was found.

Initiation of the Dolines

It is most likely the solution pipe fills had an immediate source from the Poole Formation overstepping southward onto the early rising monocline in the Chalk. There is a marked late Eocene unconformity at Creechbarrow Hill, further east along the strike. The Tertiary Creechbarrow Beds overstep southward. If a similar unconformity once existed here above Hambury Tout (and it would now be eroded away) then the Eocene sediments (Poole Formation) would have been at no great height above the solution pipes.

After this early initiation the pipes may have continued to descend from time to time, particularly in the relatively extreme conditions of the Pleistocene. The partial fills of chalk breccia may have originated at this time.

Man O' War Head

The interesting cliff section at Man O' War Head at the western end of St. Oswald's Bay is shown in this old photograph of Rowe (1902). I have added Rowe's annotations together with the position of the F4 fault from Arkell (1947). The notable feature here is the reduction in thickness of the planus zone by strike faulting. Note that at this locality the Man O'War Rock with Portland and Purbeck strata is anomalously close to the Chalk cliff. This can only be explained by more strike faulting of the F4 type, but in this case in the Wealden and Purbeck units.

Incidently, Rowe referred to this locality in the caption of the plate as " Red Hole, West Side of St. Oswald's Bay ". As mentioned by House, this brings up the problem of what really was and where was - Red Hole.
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The Lower Chalk here, of Cenomanian age, consists of cyclical, alternating marly chalk and harder purer chalk. It is shown in the illustration looking east (along strike) on the eastern side of Man-o'-War Head (near Man-o'-War Rock, east of the Durdle Door promontory). Note that flint is generally absent and the bedding is very clearly visible (nearly vertical - actually slightly overturned and younging to the left of the photograph). The softer marly beds, much burrowed, have been eroded out by the sea. The area covered by shallow water on the landward side to the left (to the north) consists partly of the relatively soft Plenus Marl, 2 or 3 metres thick. This is a conspicuous marker and useful line of division between Lower and Middle Chalk in the cliffs but its characteristic belemnite, Actinocamax plenus, is not often seen. If you search you might find examples or possibly see the echinoderm remains (Crateraster quinqueloba ) which can also occur in this bed in Dorset (Arkell, 1947). Usually, though, it looks rather barren.

There has been much discussion about the origin of cyles like those in the Lower Chalk here. Astronomical causes affecting climate and sea-level are usually invoked but whether they are the approximately 20,000 or 40,000 years cycles of Milankovitch is often debated. See the work of Gale et al. (1999) for the latest information. This paper also provides palaeogeographic maps for the Cenomanian and Turonian. For the Mid-Cenomanian Dorset is close to the southwestern landmass of Cornubia and receiving sandy detritus from that source. Whether Cornubia existed as land in the Turonian is uncertain.

The Beach of St. Oswald's Bay

These images of St. Oswald's Bay have been discussed above in relation to solution pipes and other aspects of the bay. They are also good photographs for consideration of beach features. The beach consists of granule-size flint, derived from the Chalk and coarse sand. You can see some features of a bay-head beach (although not necessarily typical), beach cusps, and you can see the berm crest move away from the vertical or undercut foot of the Chalk cliff so that it is seaward of the more sloping cliffs beyond (middle distance). In other words, at high water with some wave action the sea reaches the foot of the cliffs in the nearer part of the image, but rarely reaches the foot of the further cliffs. This only happens in major storms.

Acknowledgements

I would like to thank Dr Jim Andrews for helpful comments on the structures of this section. I gratefully acknowledge permission from the Geologists Association for permission to reproduce the plates of Rowe (1902).

Durdle Door

If you have time and if the tide not too high you can walk westward toDurdle Door , where there is much of interest. Alternatively if it late then return to Lulworth Cove by taking the easy sloping path up eastwards from the Wealden exposure of St. Oswald's Bay.

References See the Lulworth Geology Bibliography for the main references for Dungy Head. An additional one, for a comparative feature, is given below.

Allen, M.J. and Green, M. 1999 (for 1998). The Fir Tree Shaft: the date and archaeological and palaeo-environmental potential of a Chalk swallowhole feature. Proceedings of the Dorset Natural History and Archaeological Society, 120, 25-38.