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Lulworth Cove Heritage Centre
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Stair Hole, just to the west of Lulworth Cove, is a remarkable small cove with natural arches cut into steeply-dipping Portland and basal Purbeck limestones. Through these arches and a gap where one has collapsed the sea enters to erode the softer parts of the Purbeck limestones and shales. Soft multicoloured Wealden strata slump southwards towards the sea. The most remarkable feature of Stair Hole is seen in the eastern cliff. Here is a cross-section throught the Lulworth Crumple, where small folds in the Purbeck strata are present within the steep northern limb of the Purbeck Monocline.
This place is a classic site, painted by Turner in about 1811, visited, photographed and sketched by thousands of people and shown in many geology and geography textbooks.
For more information, when at Lulworth Cove, visit the Lulworth Cove Heritage Centre with pamphlets, books and displays of rocks, fossils and minerals.
This is a separate section of a guide to Lulworth Cove . Information on the stratal succession and a geological and location map will be found there.
Go back to the start of the Lulworth Cove Field Guide?
2. Safety
Some general information regarding safety on Dorset geological field trips is provided and you are asked to read this if going to Stair Hole.
View eastward in Stair Hole in late summer, 1997. Like vertical cliffs anywhere in limestone and shale, the high vertical cliff at the eastern end of Stair Hole presents a risk of high-velocity falling blocks, and the foot of this should be avoided. The photograph shows a narrow but obvious stone-shute down the centre of the cliff. Activity on this will vary according to the season and from year to year and at times it may be almost inactive. However, even very small rocks falling from the cliffs of the Dorset coast have caused death or brain-damage (as at Durlston Bay, Swanage) and this shute should not be approached. It is probably the most hazardous point for rock-falls in Stair Hole. As the signs warn, the cliffs are unstable and you should act responsibly. Geologists and geological parties should avoid places like this, particularly in wet weather when debris is particularly prone to fall. Safety helmets are needed for geological work here anywhere near steep cliffs. You can see that the risk is less, but not negligible, further to the north, on the left side of the photograph where the cliffs are semi-vegetated. The Purbeck strata above the Cinder Bed are visible here and the uppermost Purbeck is accessible. The mottled and reddish-coloured Wealden seen to the left is slumped, presents little risk from falling debris but can be very soft in places in winter.
In general, the central Purbeck and Wealden parts of Stair Hole are safer than the east and west ends. The west end is not as steep as the hazardous east end but even here pieces of rock can fall and there are overhangs and caves. It is not safe to climb the central ridge. Obviously the cliff edges must not be approached from viewpoints above Stair Hole, but safe places can be found to view and/or sketch the rock structures.
Stair Hole - Introduction - Erosion
Stair Hole is a small cove situated immediately to the west of Lulworth Cove. There is an outer barrier of steeply dipping Porland Stone and basal Purbeck limestones (the Caps).The sea comes through caves in the eastern and central part and an open gap at the western end. The Great Dirt Bed is an ancient soil, a palaeosol, resting on the basal lagoonal limestones. The Broken Beds are an evaporite breccia, and the Cypris Freestones are limestones of pellets and ostracods (the shells of water-fleas) that lived in a very large and rather hypersaline lagoon, almost at the end of the Jurassic Period. To the right of the cave, the soil, the Great Dirt Bed has been eroded away to show the Teepee Structures which are megapolygons developed in the early cemented limestone on which the soil formed. A better place to see all these features is at the Fossil Forest , on the other side of the cove (a broad ledge on the cliffs).
There is an open gap at the western end. This is a collapsed cave and eventually the other caves will collapse in the same way. The open gap here has caused increased erosion of the shales and limestones of the Purbeck Formation, landward of the beach. In storms the seas directly attack this. At the back, behind the Purbeck strata are the pinkish and yellowish mottled clays and sands of the Wealden Group. The sea has not yet been able to erode these so the small cove does not have the same structure as Lulworth Cove with chalk at the back. The Purbeck barrier has been lowered, though opposite the open gap. This cutting down of the vertical Upper Purbeck strata has enabled to Wealden to slump down over it, coating it with red marl.
It has been said that Stair Hole is a " Lulworth Cove in the making " but it is being formed in a different manner from caves being eroded through the Portland Stone, rather than the sea opening up a stream valley (Mottram, 1973).
At the western end of Stair Hole the sea has been most successful at cutting back and causing collapse of the Purbeck limestones. The slumping of the Wealden strata is conspicuous and there is the development of mud-slides. As support for the Wealden is removed these will increase and once the sea gets significant access to the soft clays erosion will accelerate. Perhaps a small new cove will be formed in the Wealden strata at the western end of Stair Hole. It will be a long time before this happens. Once the sea is through into the Wealden then the expanded Stair Hole would soon join Lulworth Cove.
Notice how the Pubeck limestones are exposed in the centre of the beach area. The east-west strike is obvious.
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Many parties visiting Stair Hole will find that the general features can be seen from this viewpoint at the back (north) of Stair Hole. From here the Lulworth Crumple, the folds, can be seen. The high eastern viewpoint provides good views of other aspects of Stair Hole, but the Lulworth Crumple is not so well-developed in the western cliff which can be seen from here; the best fold structure is, of course, underneath this viewpoint. The eastern viewpoint has a wooden safety fence and this should not be crossed. Specialists may want to see other parts of Stair Hole, which may not necessarily be good places for large parties or school groups. The high western viewpoint is suitable only for small groups of people experienced with regard to cliffs. It is not recommended as safe for school parties.
Footnote: Accident at Stair Hole on 27 August, 2002...
A report on television (Meridian News and also BBC) gave the following information. A 14 year old German student, studying English, on a day out with other students was walking on the beach at Stair Hole when rocks fell from the cliff. Sadly, she received serious head injuries and was airlifted to hospital. Police investigating the incident wanted to trace two climbers whom they believed were in the area at the time. The comments and pictures seemed to suggest that the accident happened on the beach at the western end of Stair Hole, below the high western view point, where there is an inclined rock in the water near the Portland/Purbeck junction, although I do not know the details. The Lulworth Cove Coast Warden gave an interview on television. This field guide is for geologists and, although the accident did not involve geology students, it is sensible that they take note of places where rock-fall accidents have occurred. These rarely happen again at exactly the same place but do tend to occur where the cliffs have certain characteristics. This is at least the third in the east Dorset region at the foot of Purbeck cliffs. In the photographs on this webpage the foot of the eastern high Purbeck cliffs (below the Lulworth Crumple and main and eastern view point) has already been indicated as a risk area because of evidence of recently fallen material. Clearly, now, the western high Purbeck cliff has risk in some circumstances, although this area is not under such active erosion as the more vertical, eastern Purbeck cliff. Of course rocks fall from all cliffs anywhere but more often in winter weather conditions and more often at certain places. Most of the Stair Hole beach (the long north stretch) is not subject to rock falls, other than some slow slumping, and only the eastern high cliff is more dangerous than is is usual for the Dorset coast. As is standard procedure, geological parties should wear safety helmets wherever there is any risk of rock falls and they should avoid approaching the foot of steep cliffs and not linger unnecessarily at the base. They must take extra care in or shortly after wet weather. Most parties will see Stair Hole from above or from the relatively safe Wealden slopes and are not likely to be at risk. The Lulworth Cove area is certainly not more dangerous than many other places on the Dorset coast, and much safer than some, but far more people, and particularly far more geologists, visit it.
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Stair Hole Panorama - by Barry Marsh
This panoramic view of Stair Hole by SOC photographer Barry Marsh allows you to scan round from east to west and view a large proportion of this unusual small cove.
Stair Hole - the Purbeck Formation
Although, as mentioned above, some of the basal Purbeck strata can be seen here, Stair Hole is a good place to see certain features of the Purbeck Formation from the "Cypris" Freestone Member upwards. As noticed, the basal Purbeck strata including the Caps, the Great Dirt Bed and the Broken Beds are present in rather inaccessible positions in the central bastion and can be seen to some extent in the cave at the western end of Stair Hole, providing the tide is low (but take care and do not approach if debris has recently fallen from the cliff). The Soft Cap in the central bastion contains a small quantity of celestite and also some small gastropods, but avoid climbing this narrow feature.
These photographs show the Cinder Bed and the Flint Bed of the Cherty Freshwater Limestone Member at the eastern side of Stair Hole. Information is provided here because it is useful reference section for the Pubeck Formation. It is not a safe place, however, and parties should not approach the cliff here. There is risk of falling debris and under no circumstances should the cliffs be approached here if the conditions look at all hazardous and the place is not suitable for hammering. At a safe distance you can try to recognise the units, but to study the strata closely use loose blocks of the same rocks well away from the cliffs. Some of the strata can be seen, as shown in a photograph above, in the central part of Stair Hole at the back. The upper Purbeck can be seen north of the main vertical cliff, away from the main area of risk. The strata are also visible at the western end of Stair Hole, where the cliff is not so steep or dangerous, although as with any cliffs this is not free from risk and safety helmets are certainly needed. The cliff should be assessed for risk before any approach is made.
Here we see the same beds in more detail. Try to find examples away from the hazardous cliff. The Cinder Bed can be recognised by the numerous small, blue-grey oysters. The Flint Bed is almost white with conspicuous chert ("flint") nodules. The chert cannot be scratched with metal. Do not hammer it because it produces dangereous splinters. Without safety goggles you could lose an eye!
If you can find safe access to blocks of the Flint Beds, of the Cherty Freshwater Member, you can look for charophytes, freshwater algae (stoneworts), which can be found here. With a hand-lens you can sometimes see the 12-celled cross-sections of charophyte stems in the Flint Bed, although you may have to search a while for a favourable block. They show that the water depth of the freshwater lake was not excessive and in the photic zone. In sea-washed blocks look also for ostracods and freshwater gastropods, which can seen in cross-section.
The bed of shale bed (SH 68) with some chert and some thin laminated argillaceous micritic limestone and which occurs under the Flint Bed contains planorbid freshwater pondsnails. This is reminiscent of a particular bed at Durlston Bed. The sequence in Stair Hole is very condensed, though, and has its own special characteristics; only certain beds can be followed through.
In the freshwater strata below the Cinder Bed in a quarry at Sunnydown Farm in the Isle of Purbeck Paul Ensom has found mammals, dinosaur footprints and dinosaur eggs. The particular unit in which they occur has not been recognised here, though, and this is not a good place to study the Cherty Freshwater Member. There are larger exposures of it elsewhere where it is nearly horizontal.
Incidently before leaving this part of Stair Hole notice the joint directions in the Flint Bed. Why do you think that the Cinder Bed does not show the same type of jointing? How does the jointing in the Flint Bed compare with the jointing in the central bastion of Stair Hole?
Bluish calcitic shells of the small oyster Praeexogyra distorta in the Cinder Bed of Stair Hole. The matrix is microsparite - fairly fine grained calcite - and the rock can be classified in Folk's classification of limestones as a biomicrudite (I will explain the reason for this rather cumbersome name. It is a limestone with organic components - bio, with micritic or microsparitic carbonate - micr, and with the allochems or shells larger than 2 mm and therefore - rudite).
The little oysters are quite different from the large oysters of the Jurassic marine clays beneath. There is evidence of truely marine water in the form of rare echinoid remains ( Hemicidaris purbeckensis ), but the oysters have some similarity to the small Crassostrea virginica of the coastal lagoons of the Gulf of Mexico. What used to be called " Ostrea distorta " can be regarded as a euryhaline lagoonal oyster. Its tolerance of salinities lower than seawater can be demonstrated by its occurrence in brackish water beds in the Intermarine Member.
Here, the Flint Bed is on our left and we see the junction of the Marly and the Cherty Freshwater Members. A peculiar jointed bed (bed 65 of Stair Hole log) is marked by the empty can. It is laminated in its upper part (the strata are vertical but young towards the north - that is - to the left in the left image) but micritic and slightly argillaceous in its lower part. Are the joints tectonic and a response to just how brittle the bed is or are they primary sedimentary structures, desiccation cracks? It is interesting to note that Paul Ensom, the fossil vertebrate specialist of the Natural History Museum, London, has described sediment-filled cracks at a similar horizon at Worbarrow Tout. What do you think about this?
Oil Traces
In 1937 geologists of the D'Arcy Exploration Company, later to become part of BP, investigated the Dorset coast in a search for oil (Lees and Cox, 1937). They said " The Purbeck Beds at their outcrops on the Dorset coast show a number of impressive oil indications; these include bituminous limestone, as at Peveril Point, and fractured limestones at Mupe Bay, Lulworth Cove, and Durdle Door which show oil residues on the faces of cracks and which ooze a little soft bitumen in warm weather. " Their plate 11 has an arrow pointing to this part of Stair Hole. Here in a crack or joint of a block of Purbeck limestone is one of these oil residues of the type which started the oil exploration in this region. Eventually it led to the discovery of the great Wytch Farm Oilfield, the biggest onshore oilfield in northwest Europe.
Stair Hole - the Lulworth Crumple
These images show the Lulworth Crumple as photographed from the high western viewpoint of Stair Hole. The right one is labelled to show stratigraphical marker horizons particularly of the Purbeck Formation (note, incidently that the centre and right images are of high quality but are very large file and may load slowly).
The Lulworth Crumples are minor folds within the north limb of the major monocline. There are several of these and although they are best seen in the Lulworth area, the place where deformation due to Tertiary tectonics is most severe. Similar small folds, however, occur along the outcrop as far east as Peveril Point, Swanage. Specific examples have been exposed near Blashenwell (in the Purbeck marble), at Herston (west of the school) and are complex at Peveril Point itself (Cosgrove and Hearn, 1966). Undoubtedly there are others. The relative relationships to the major fold are shown diagrammatically in the cross section in the main Lulworth Cove introduction . Here we can look at the most well-known and the best-exposed, which is in the eastern cliff of Stair Hole. Firstly, look at it broadly and then check the minor structures within it. Look for evidence of compression (Sigma One - the maximum compressive stress); look for evidence of extension or of collapse. The evidence is mostly in the form of small faults and shears.
Left: Although this fold in the east side of Stair Hole is very well-known it is not entirely understood and presents some interesting problems. Previous disputes about its origin have involved discussion about whether the overlying strata here have moved upwards or downwards. I have argued for upward movement a long time ago (West, 1964) and still have the same view. Upward drag during the major folding of the monocline has been suggested (Arkell, 1938), gravity collapse has been suggested (Lees in Phillips, 1964) and downward collapse against the planes of thrust faults during the final stages of uplift of the south side. A good indication of what has happened comes from the Durdle Door area discussed below. The higher Purbeck strata, the very beds which are ruckled at Stair Hole, are faulted out at Durdle Door which represents a lower structural level in the main monocline. There is an unconformity beneath and to the north. The missing strata at Durdle Door could only have gone up! It would be better to hear more than one structural view, though, and weigh the argument rather than accept the first. Whatever your level of structural knowledge, why not sketch the structure (from the central part or, perhaps, the western cliff top) and then make your own observations and draw your own conclusions. Look carefully but do not get too close to the cliff, especially in wet weather.
Right: A modified diagram from Phillips (1964), from which I have removed the sliding and collapse. Phillips' detailed work is well-worth reading . Also please read a more modern interpretation by Bevan (1985). Note that you can probably place Sigma One (the principal compressive stress) in the lower part without difficulty. In the upper area of the Lulworth Crumple it is not so simple!
This lower resolution image shows both the photograph diagram to enable comparison. The images are not as good as the separate ones.
Abbreviations: DD = Durdle Door, WSH = West Stair Hole, EL = East Lulworth Cove, BH = Bacon Hole. Purbeck Members: A = Caps Member, B = Broken Beds and Cypris Freestones Member, C = Hard Cockle Member lower part, D = Hard Cockle Member upper part limestones, E = Soft Cockle Member, F = Cherty Freshwater Member and Cinder Bed, G = Intermarine Member and Chief Beef Member, H = Shales not shown.
Stair Hole - More Structural Features
The Stair Hole Snakes Heads. These resemble on a small scale the snake's head structures of thrusts (see for example McClay, 1989, fig. 3.13, p. 56), although the strata here are vertical. They are occurring near the top of the Intermarine Member, with the Cinder Bed visible on the right. The strata are younging towards the left. This is in the east side of Stair Hole.
Copyright
© 2008 Ian West and Tonya West. 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. No permission can be given for reproduction of any images of the Lulworth Cove area in books or in other websites, for special reasons.
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.
Webpage - written and produced by:
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Ian West, M.Sc. Ph.D. F.G.S.
