Dungy Head and St. Oswald's Bay, Geological Guide

West, Ian M. 2014. Dungy Head and St. Oswald's Bay, west of Lulworth Cove; Wessex Coast Geology. wessexcoastgeology.soton.ac.uk/dungy.htm. 30th May 2014.
Dungy Head, and St. Oswald's Bay, geological guide

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Dr. Ian West,
Geology Wessex Coast, Romsey
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.

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(You can download this educational site to SurfOffline, WebCopier or similar software to keep an offline copy, but note there is periodic updating.)

CONTENTS:

DH-1. INTRODUCTION

This guide is for an impressive piece of coast, with Portland, Purbeck and higher Cretaceous strata, 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 and if the route is free from rock fall debris cones.

DH2 - INTRODUCTION continued:

Safety and Risk Assessment

Take care on the cliffs above Dungy Head. The paths are easy but some are close to very dangerous cliff edges. Keep away from cliff edges and in particular do not take children or dogs near the high cliff edges. The main route to the beach is an easy footpath without obvious hazard, and sometimes with steps at lower part. The footpath starts from the short private road which climbs westward from the back of Stair Hole at Lulworth Cove. There is also a lesser-known, narrow, path (near some garages) to the south off the road which leads towards the cliff edges above Dungy Head (and the coast just to the east of it).

If the tide is low enough, and the route is not obstructed by rock fall, it is possible to walk westward to Durdle Door from Dungy Head by the beach. At middle to high tide it is not and the geologist or party will have to return to the main access footpath from Lulworth Cove.

There is risk of falling rocks from the Portland strata of Dungy Head and from the high Chalk cliffs of St. Oswald's Bay, and this risk should be recognised. There have been landslides and rockfalls, some very large, along most of this stretch of coast so care should be taken. If there are signs of recent falls keep clear of the cliffs as possible. The beach is fairly wide at the eastern end of St. Oswald's Bay but it narrows westward to Man O' War Head but, as mentioned above, at high tide the sea comes up to the Chalk. This tide problem does not often produce a major safety hazard because the tidal range is limited and it normally easy to retreat to the east. Direct access to Durdle Door by the beach may not be possible at medium to high tide. Occasionally chalk may fall in St. Oswald's Bay. Avoid obvious areas of rock fall, and everywhere look, of course, for the tell-tale signs of freshly impacted material on the beach. Wear safety helmets where appropriate.

Note that the Chalk cliffs cannot be climbed and are very hazardous. The chalk crumbles and a fall would be almost certain. The Portland strata of Dungy Head should be treated with care and cliff-climbing on this is not recommended. Do not try to climb along the cliffs to reach Stair Hole. This is not possible.

Take care when scrambling over the large Portland stone rocks on the shore. Do not hammer the Portland chert, which will produce dangerous splinters and is a threat to eyesight. If you view the cliffs from the top of Dungy Head, an excellent viewpoint, then it is obvious that care must be taken not to slip over the cliff edges. The common adder might be encountered on rare occaions on the grassy slopes and paths above Dungy Head and elsewhere on the hills of the area. Although poisonous, these snakes are not aggressive unless handled.

DH 3. 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 an easy footpath ahead leading down to the beach at St. Oswald's Bay, near Dungy Head. There are usually steps at the bottom, but these may not necessarily survive storm action. From time to time the path may be changed slightly in location in the lower part because of mudslides of cliff falls.

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).

DH 4. INTRODUCTION:

Topographic Maps (Offline)

A detailed map of the area is in a link above, but has to be held offline, and only available for use offline, for reasons of copyright.

DH-5. 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).

DH-6. 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.

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DH-7. 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.

DH-8. DUNGY HEAD - General Topography and Geomorphology

[For more on the Chalk rock fall in St. Oswald's Bay go to: Cliff Fall in St. Oswald's Bay, 29-30th April, 2013.]

DH-9. STRATIGRAPHY - PORTLAND GROUP

Introduction:

The Upper Jurassic, Portland Group is very easily accessed at Dungy Head. The lowest part, the Portland Sand Formation actually consists of siltstones, shales and dolomite with some sandstone beds. It can be seen to some extent 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 succession at Dungy Head is described below.

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 consisting of the Portland Sands, the Portland Cherty Series and the Portland Freestone.

STRATIGRAPHY - PORTLAND GROUP:

DH-10-Portland Sand Formation

(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).

DH-11. STRATIGRAPHY - PORTLAND GROUP:

Portland Stone Formation

Portland Chert Member

The Portland Chert Member, or "Portland Cherty Series" as it used to be known is well exposed at Dungy Head. This is a particularly good and easily accessible section, only one kilometre west of Lulworth Cove and thus it is important. It is 11m. in thickness here, little more than about half that at Gad Cliff (18m.)Arkell, 1935; 1947. Except by boat the Portland Cherty Series is not accessible at Durdle Door, to the west. It cannot be reached at Lulworth Cove (it is out on the seaward cliffs). It is similarly inaccessible at Bacon Hole.

The section shows some of the peculiarities that might be expected at this northern margin of the Portland - Isle of Wight Basin (offshore) or English Channel Inversion. It was greatly compacted by the renewed northward movement during the Tertiary ("Alpine Orogeny")on the underlying Variscan thrust planes. Of course they did not move far compared with their Variscan activity. However, this short failure in the basement placed enormous forces onto the well-lithified Portland Freestone.

The photograph below clearly shows Bed J to Bed N of the Portland Chert Member. The units are easily recognised by comparison with the diagram above, based on (Arkell, 1935). The thick black chert bed is in the middle of Bed N.

The perisphinctid ammonite "Behemoth" is shown in the above photograph, if you can find it. A part of the outer whorl with ribbing can be seen in Bed K of Arkell, 1935; 1947. The early lithification is shown by the the total lack of compaction of this ammonite. As is usual this is internal mould and the aragonite shell has been lost in solution. Leave the specimen where it is; it cannot be extracted (whole Portland ammonites are obtained in the quarries of Portland Stone on the Isle of Portland, and Swanworth Quarry might be another source location).

The Portland Chert Member (new BGS name, the old name is 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 is know as Bed J' or the Prickle Bed or Puffin Ledge (Puffins used to nest on in the Isle of Purbeck). It 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.

DH-12. STRATIGRAPHY - PORTLAND GROUP:

Portland Stone Formation

Portland Freestone Member

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

DH-13. STRATIGRAPHY - Purbeck Group

Lulworth Formation

The Portland - Purbeck junction is very easily accessible at Dungy Head (unlike Durdle Door). However, at the foot of the cliff this is a very tectonised area and as at Durdle Door, some part of the Purbeck Group may be missing here, having been faulted out. The Portland-Purbeck junction is in hard limestone and less affected. However, even so there has been some tectonic displacement between the basal Purbeck and the Portland Freestone. Thus a reasonable section can be seen, but there are some uncertainties. The Transition Bed is not seen, and probably is absent here. The possibility that it is faulted out cannot be completely eliminated, but seems unlikely. The Hard Cap is well-developed above the Lower Dirt Bed, and this can also be seen on the cliff top above Dungy Head and for a short distance eastward. The Hard Cap is thrombolitic in its lower part, immediately above the Lower Dirt Bed. The Soft Cap rests neatly on a well-developed Great Dirt bed with pebbles. It is thin, though, and laminated and does not seem to contain thrombolites.

The Broken Beds are not properly developed, but there is some brecciated limestone, with some chert. The evaporite-based breccia represents a weak horizon subject to tectonism. It is not suprising that it has been much affected by the structural processes and is very thin now, with uncertainties about the original thickness.

DH-14. STRATIGRAPHY - Purbeck Group

Durlston Formation (i.e. Purbecks - Cinder Bed upward)

The Cinder Bed is the base of the Durlston Formation (in the terminology of the BGS) and it is particularly thin and poorly developed at Dungy Head. It is remarkable to see it in this condition.

The details of the Durlston Formation, Purbeck Group at and near Dungy Head are not well-exposed and clear. However, there are some interesting features visible. Shown above is distorted beef of the Chief Beef Member. It is not easy to conceive of a hydraulic jacking origin (with overpressure) for this particular case. Crystal growth during Tertiary tectonic stress seems more probable. The source material is aragonite which has probably mostly been lost by diagenesis just here.

DH-15. STRATIGRAPHY - WEALDEN

Wealden in St. Oswald's Bay

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.

DH-16a. STRATIGRAPHY:

Wealden Jarosite

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.

DH-16b. STRATIGRAPHY:

The Upper Greensand (Albian)

[to be added]

DH-17. STRATIGRAPHY:

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.

DH-18. OSWALD'S BAY DH-18. ST OSWALD'S BAY - GENERAL

Chalk Cliffs

St. Oswald's Bay is shown above 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.

This is a view westward from the cliff top just west of Red Hole 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.

Above is are views down the cliff at above Red Hole, the major doline of St. Oswald's Bay. The overturned Chalk with flints forms the main part of the cliff. In foreground, part of the Red Hole solution pipe has a reddish content of Tertiary debris. Dungy Head lies to the southeast. To the west there is Durdle Door. Low slumped ground occurs in the area of the Wealden strata, the thin-bedded Purbeck is thin bedded, and Portland Stone occurs on the south side of Durdle Door.

ST. OSWALD'S BAY - MAJOR STRUCTURE

The faults and other structures in St. Oswald's Bay are particularly interesting. There are complications of strike faulting in steeply dipping and overturned strata. The structure has to be understood in relation to the nearby section at Lulworth Cove. There are major similarities, but differences in detail. Sections across Lulworth Cove are shown below

This section above incorporates aspects of cross-sections of West (1964), Bevan (1985) and House (1989) but with appreciable modifications.

The section directly above is after House with minor modifications. This is more correct in that it shows more faults, but it does not deal with the Lulworth Crumples. Neither section is claimed to be highly accurate, but to provide general explanations of the structure. Differences between Lulworth Cove and St. Oswald's Bay are partly because the thicknesses of the strata, particularly the Wealden are less at the latter, and partly because the whole structure pitches down to the east at a low angle.

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. 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.

ST. OSWALD'S BAY - THE INVERSION STRUCTURE

St. Oswald's Bay and Dungy Head are situated at the northern margin of a major structure, known in petroleum geology as the Portland - Isle of Wight Basin or as the English Channel Inversion Structure. In the eastern part of St. Oswald's Bay and at Dungy Head the angular relationships of the strata in the hinge area can be clearly seen. These are shown in a labelled photograph above.

Thus the fold and faults last downthrowing north are is situated over what was once a fault downthrowing southward (in the Cretaceous). It is because the direction of movement has reversed that it is known as an Inversion structure. Other Inversion Structures in the British region include the Weald of Kent and Sussex, and some in the North Sea area. The Weald Inversion Structure is most similar to the English Channel Inversion Strucutre

The earlier phase was one of downwarping and stretching in the Jurassic basin. The end of this phase took place by faulting on the northern margin in the terminal Purbeck and this faulting increased through to the late Aptian where a major unconformity is present.

The second structure, was much later and of Tertiary age (Bartonian through to Miocene). The Purbeck Monocline developed as a structure rising on the south side and bounded by a fold and a fault to the north. The boundary between the English Channel Inversion and the South Dorset high to the north, is, amongst other places, actually present at St. Oswald's Bay.

The importance of the English Channel Inversion is that deep burial caused thermal maturity (heating to more than 70 degrees C and leading to development of oil and gas) of organic-rich shales in the Jurassic, particularly those of the Lower Lias. The Kimmeridge Clay is not thermally mature in the coastal exposure at Kimmeridge, even though this is in the Inversion, so the matter is not simple. An additional factor is the thickness of the Cretaceous overburden, which is greater towards the Isle of Wight.

The topic of the Inversion Structure is dealt with more fully elsewhere. However, the angular difference between Portland Stone and Upper Greensand is very obvious at eastern St. Oswald's Bay and Dungy Head, as shown in the photograph. In addition and of relevance to this, a minor oil seep in the Wealden is present here, and at Stair Hole, Lulworth Cove and Mupe Bay near the eastern continuation of the structures seen at St. Oswald's Bay.

DH-East End of St. Oswald's Bay

Greensand - Lower Chalk Junction

At the eastern end of St. Oswald's Bay there is a vertical junction between the Upper Greensand and the Lower Chalk (Grey Chalk, or Cenomanian Chalk). One photograph shown above was taken shortly after the major Chalk rock fall on the 29-30th April 2013. There were other, smaller rock falls at this time. As a result much of the cliff is obscured by talus on the occasion when this photograph was taken. A second photograph was taken when the cliff was clearer and cleaner on the 27th March 2014.

Notice the boulder bed at the top of the Upper Greensand and chert beds lower in the sequence (i.e. to the right). The softer and lower part of the Upper Greensand is not shown in these photographs

DH-21-ST. OSWALD'S BAY

Major Structure at Man O'War Head

At Man O'War Head there is a good exposure of a major fault (F4 on Arkell's 1938 classification. A major fault, upthrowing on the south side, brings together vertical Middle Chalk and overturned Upper Chalk (Micraster Zones) with bedding dipping at 115 degrees to the north (i.e. appearing at first sight to dip south). Compare the photograph and the diagram above. Look also for the Group 3 shears, probably part of a system of conjugate shears. (Notice also the solution pipe or doline, clearly shown in the photograph)

DH-22. ST. OSWALD'S BAY - MINOR STRUCTURES

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.

[Phillips (1964)gave a different explanation. 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.

DH-23. Norman Rock - Offshore Rock near Man O'War Head

This offshore rock, Norman Rock, does not seem to align as expected with the Man O'War rocks. It seems to be composed of Portland Stone and/or Purbeck limestone. I have not examined it closely and as yet do not understand the details of its structure. There are obvious shear planes developed within it.

DH-24. 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.

DH-24a. St. Oswald's Bay - More on Dolines in 2014

Harder and more resistant rocks from cone of fallen debris in St. Oswald's Bay that resulted from the major rock fall of early 2013 can be seen in March 2014 on the beach, and in at least one case in the water. A particular resistant red rock shown above could not be directly accessed. It is clear that it is very hard and contain some black flints. It may be reddened chalk.

On the beach nearby are some interesting rock types that have fallen from the dolines in the cliff, during the major rock fall. One type of particular interest is discussed here.

At Bincombe, north of Weymouth, the Weymouth Relief Road cut through the steeply-dipping Chalk at the Ridgeway-Abbotsbury Fault. This is the equivalent in the Weymouth area of the northern boundary of the Portland - Isle of Wight Basin, or English Channel Inversion Structure. Solution pipes are Bincombe contain material from Tertiary gravels with much vein quartz and also jasper. The gravels outcrop over a small area and they have been quarried in the past. Red Hole is a doline in an equivalent position in vertical Chalk in the Lulworth area. This red set of solution pipes has not been accessible because of its situation at the top of a high Chalk cliff. The rock falls of early 2013 have dropped a significant part of the contained gravels onto the beach. They are mostly iron-cemented which is the reason for the red colour of the hole in the cliff. Now they can be seen on the beach. Some are of the Tertiary type with the vein quart and occasional jasper. The clasts are small, usually only a few millimetres across and they angular to subangular. They are fairly well-sorted but not well-rounded. The small pebbles are in a brown, porous sandy matrix. There is much irregular variation in the deposits with distorted bands of sand.

In addition to the gravels of distinctly Tertiary type, there are also breccias of flint debris. These tend to be grey and with a chalky matrix rather than a ferruginous matrix. Examples can be seen in the deep doline that is almost accessible from the beach at Man O'War Head (at the western end of St. Oswald's Bay).

[more to be added]

DH-25-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.

DH-26. CLIFF COLLAPSE

St. Oswald's Bay - Major Rock Fall - April 2013

Go to Sky News - Dorset Landslide - Cliff Collapses into Sea.

See Sky News for for an initial report, immediately after it occurred.

The following images result from photography on the 3rd May 2013. This was a few days after the event but the weather was fine and there had been no intervening storm action. Thus the rockfall had probably not been significantly modified by wave action or weathering since the event on the night of the 29th to 30th April 2013.

By the 19th July the basal part of the Chalk rock-fall had been substantially cut back. There is no obvious change in the upper part, and there has been no further fall.

A similar rock fall has taken place in the same month, April 2013, at Oddicombe Beach, Torquay. For more informaion please go to the Torquay website: Oddicombe Bay Rock Fall section.

The Oddicombe Bay collapse was in red, sandy breccia, the Permian Oddicombe Breccia, and also occurred at night. The occurrence of a similar major rock fall at this time of the year, is a consequence of unusual weather conditions. A long phase of exceptionally heavy rainfall and high water-tables in the winter of 2012 to 2013 has been followed by a dry spell in April 2013. These two rock falls, although in very different strata are similar in scale and in type.

The Chalk rock fall (or "landslide") in St. Oswald's Bay that occurred on the night of the 29-30th April 2013 took place in Micraster Chalk, part of the Upper Chalk in old terminology, and probably Coniacian. In current terminology this would be some part of the Lewes Nodular Chalk Formation to Seaford Chalk Formation, although the exact position within these units is not known. The Upper Chalk here is overturned and dips at 115 degrees to the north (thus appearing to dip south at 65 degrees. This approximately coincides in angle with Arkell's (1936; 1947) Group 1 south-dipping shear planes. These are conspicuous and deeply slickensided in a vertical direction. They are parallel here to the overturned bedding. Group 2 shears dip north at 60 to 70 degrees neither cut nor cutting the Group 2. In other words they are conjugate shears or almost certainly Alpine (Tertiary) age. The rockfall (or rock-fall) has taken place on the Group 1. The debris cone is as at a lower angle of about 33 degrees. This is what would be expected at this locality.

St. Oswald's Bay is fully exposed to southwesterly wind and wave action in storm. It was seen that in 2012 the beach at the site of the future rockfall had lost beach shingle (drifted eastward) and has had some exposed chalk wave-cut platform and some limited, small-scale, undercutting and caving (cf. Burton Cliff in 2012).

There is a similarity elsewhere in the Lulworth area. In the upper part of the cliff at Lulworth Cove there is overturned Upper Chalk (i.e, north of the major E-W trending reversed fault) and beyond the fault the structure is almost the same as here in St. Oswald's Bay. Fortunately, however, in the lower cliff there is Lower and Middle Chalk, which dips north and therefore gives more stability. There has been only a minor fall from the Upper Chalk at Black Rocks in the 1950s. Lulworth Cove, has, of course, a small entrance, a good safety factor, and this protects the cliffs from the major wave action that can directly affect St. Oswald's Bay amd some other places. Thus, Lulworth Cove does not seem to have the same level of threat. The upper part of the cliff, is very grassed-over (the back cliff was once very white; see old photographs). Recent erosion at Lulworth Cove has been limited to small-scale, new white erosion features, at the base of the cliff and the upper part, so far, has remained stable.

Swyre Head, west of Durdle Door has a near-vertical cliff is in near-vertical Micraster Chalk. Thus it might be at risk of a rockfall like that of St. Oswald's Bay, but these events are very rare, and nothing could happen for a very long time (even a 100 years). Prediction is not possible. Note, though, there have been minor rock falls at Swyre Head in recent years and a small debris cone can be seen at the present time. Relatively few people walk past there, in comparison with other areas.

A major Chalk rock fall from a cliff about 120m high in the middle of St. Oswald's Bay took place on the night of the 28th-29th April 2013. This was a very large rock fall. A photograph above taken in November 2012 shows that the cliff was already being undercut with a small wave-cut platform developed and small caves cut into the base of the cliff. The reason for this seems to be that the beach shingle has been moved by longshore drift (with prevailing southwesterly winds and waves) towards the eastern end where it has accumulated. Shingle was deficient in the central part. In addition the relatively hard Chalk of the cliff is very much sheared and jointed.

DH-26a. CLIFF EROSION - 2014

St. Oswald's Bay - 2014- Erosion after Rock Falls of 2013

The major cone of rock-fall debris from early 2013 has been almost completely eroded away by March 2014. Some material of interest left on the beach includes iron-cemented flint gravels from the red solution pipes or dolines (Red Hole etc) that are a characteristic and special feature of this bay. This is discussed in the doline section above.

DH-27-MAN 0'WAR HEAD

[Safety Note: Although your western route on the beach may be cut off at Man O' War Head, this is not in normal conditions a place which is very hazardous. You can usually retreat easily to Dungy Head, although you may get your feet wet. You should not be here in very stormy conditions, though, and it is not safe then. There is also some risk here of rock fall in St. Oswald's Bay. The standard cliff path route back to Lulworht Cove might be cut off by landslides, but the steeper Dungy Head paths may be useable.]

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.

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.

DH-28. BEACH FEATURES OF ST. OSWALD'S BAY

(Including Kimmeridge Oil Shale)

This images of St. Oswald's Bay concerns matters which 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.

There were several severe storms in the early part of the year 2014. One of these was exceptional and probably about a 1 in 60 year storm. A consequence of very large waves breaking on to the beach at St. Oswald's Bay was the occurrence of worn slabs of Kimmeridge oil shale. The oil shale or Blackstone does not crop out in St. Oswald's Bay or at Dungy Head, but it does occur on the sea-floor, a short distance offshore and to the south of the Portland Stone outcrop here. The slabs are approximately rounded in outline, normal to bedding. Some have been bored by modern marine organisms. In one case a crushed ammonite was seen in a Kimmeridge oil shale slab. Ammonites do not usually occur within the richest part of the oil shale at Kimmeridge, but can be present in the central part (with lower organic content) and can occur in other bituminous shales of the Kimmeridge Clay.

DH-29 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). I much appreciate discussion with various field parties on the coast at Dungy Head and St. Oswald's Bay. I thank Nikolett Csorvasi for kindly giving an excellent photograph of the rock fall at Oddicombe Beach, Torquay. Alan Holiday is sincerely thanked for kindly providing photographs of the St. Oswald's Bay Chalk rock fall, at various stages in its history. I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.

[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. ]

DH-30. 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.

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Webpage - written and produced by:

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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.