|Durlston Bay - Peveril Point, Upper Purbeck Group
Durlston Bay, Swanage, Middle Purbeck
Durlston Bay - Lower Purbeck Group (Jurassic-Cretaceous, Lulworth Formation in part)
Durlston Bay - Central Zigzag Part and Coast Erosion
Durlston Head - Lower Purbeck Group and Portland Stone
Durlston Bay - Bibliography

INTRODUCTION:

Safety

Durlston Bay is a beautiful, extremely interesting and educationally and scientifically valuable place. Like other Dorset coast sections it can present some natural hazards. At times, it is rather more hazardous than most places on the Dorset coast. In 1975 a schoolboy was seriously injured by a rockfall at Swanage and the following year another was killed by a falling rock (Lee, 1992 ). It is particularly important not to go under the cliffs (except for the low parts) in wet weather or after heavy rain or when there is melting ice. There is a major risk of encountering falling rocks here in those conditions. Never loiter close to the cliffs or hammer or search under overhangs. Safety helmets are essential here and so too is sensible observation of the state of the cliffs to be sure that debris is not falling. Care must be taken before proceeding south of Peveril Point.

One of the main areas specifically to avoid when conditions are unsafe is south of the Cinder Bed Ledge in the northern part of the bay. Here the lower part of the cliff is vertical and there is loose debris above. Falling fragments achieve dangerously high velocities here. This is definately a place to avoid in late winter and early spring, particularly after or during heavy rainfall.

Safety Note - The area, to the south of the northern Cinder Bed Ledge has been subject to rock-fall and still has risk of rock fall! The cliff was undercut at the Mammal Bed and an open fissure has developed above.

Although the section is best studied at low tide, the tidal range is not great and unlikely to cause a major problem. You may, however, be obstructed at high tide at the north Cinder Bed Ledge. If cut off south of it ascend by the Zigzag Path. Ascent of the cliffs is dangerous except where there is a well-defined path and do not try cliff climbing. At low tide beware of slipping on slimy rocks and injuring yourself (be careful on the Upper Building Stones north of the Cinder Bed Ledge).

Note that the ledges near Durlston Head may be swept by waves and rock climbers using ledges in this region have been drowned. If the conditions are bad or the bay seems unsafe try an alternative and safer location.

INTRODUCTION:

Geological Maps - General

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. The map shown above is the new edition of the year 2000. It is different in some respects from older editions.

In addition to an old location map, above, some old geological maps from books are provided here to show the geological setting. One is from Woodward (1907), another is a hand-painted one from Braye (1890). This is also shown enlarged in part with details of Durlston Bay. Although the details of the town and roads have changed the geology shown here is basically correct. It is strongly recommended that you obtain the current geological map of the British Geological Survey, Geological Map - Swanage, Sheet 342 east and part of 343 (2000 edition. Solid and Drift Edition. 1:50,000. There is a much older accompanying explanatory memoir by Arkell (1947). The present map - sheet 343 and 342 is broadly similar but the details of the faults are corrected and some additional faults are shown.

---

INTRODUCTION:

General Geology

.

Please go to the:

Durlston Bed - Middle Purbeck Webpage for more details on the Middle Purbeck strata, including the Cinder Bed.

.

The central part of Durlston Bay provides good exposures of part of the Lower Purbeck Group, with gypsum. See the Durlston Bay - Lower Purbeck webpage for details of this. It also provides interesting sections in the Middle Purbeck. These strata can be studied in the central spur, shown in a photograph above, and some also in the vicinity of the southern ledge of the Cinder Bed (see photograph), which is just to the south of the 2001 Mudslide.

It is strongly recommended that the reader should have at hand a copy of Clements (1993), Type Section of the Purbeck Limestone Group, Durlston Bay, Swanage. Proceedings of Dorset Natural History and Archaeological Society, vol 114 for 1992, pp. 181-206 (there is also an earlier version). It is difficult to make any serious study without it and it is very easy to use once a number of key marker beds have been identified. Some bed descriptions and some other data here is based on it, but there is much more detail which cannot be given in this small webpage. The surveying work of Nunn (1992) is also recommended and it is particularly useful for this central part of the bay.

In general terms there are two obvious faults with a spur of limestone in the cliff between them. Both the faults downthrow to the south. The southern of the two seems to extend further inland in an east-west direction. The northern one appears to be of only limited extent and it is also oblique to the coast. This is shown in a diagram further down.

North of the faults are sea defences obscuring part of the cliff and beyond them to the north some Lower Purbeck exposures of the Hard Cockle and Soft Cockle Members, the latter with gypsum. In the central spur there is access to Cherty Freshwater, Cinder and Intermarine Members. To the south of the mudslide, as mentioned above, the Cinder Bed is very well exposed and associated strata, particularly below, can be examined.

.

COASTAL DEFENCES

Coastal Defences - Central Durlston Bay

In the 1960s it was unfortunate the construction took place of a block of flats at a site close to the cliff top that proved to be at risk from cliff retreat. They were placed above the middle of Durlston Bay, just north of the major faults and near the site of Beckles Mammal Pit. The cliff is steep and weak, particularly in the lower part. At the base (at beach level) is a breccia, the Broken Beds. Much of the cliff consists of the Lower Purbeck marls and limestones containing evaporitic beds with gypsum. It was once quarried here for manufacture of plaster. The gypsum is slowly dissolving in groundwater and this increases the instability of the marls of the Soft Cockle Member. These light-grey crumbly marls, with dolomite beds, form the lower part of this cliff, and do not provide a stable base for cliff-edge flats above.

Bird (1985) commented with regard to Durlston Bay that:
"It is a retreating coast: the cliff top above the protection works built in 1989 receded 25 metres between 1982 and 1988". If this is, as it seems to be, a reference to this particular site, then that is quite a major retreat since the flats were built. Perhaps the site seemed slightly safer at the time of construction, although the weakness of the cliff must have been obvious.

In due course the flats became too close to an unstable cliff edge. They were threatened by coast erosion and further collapse of the cliff. Dispute arose the construction of new sea defences extending from the shore up the cliff to the flats. The problem was that they would obscure part of the classic type section of the Purbeck Group (important not just in Britain but in Switzerland and France and elsewhere where the Purbeck strata outcrop). Conservationists and geologists opposed the construction of the stone rubble wall. Nevertheless in 1989 it was built and part of the Lower Purbeck Group was obscured.

In addition to the scientific and conservation aspect, the pile of Portland Stone rubble and rock armour at the base scarred the natural appearance of the bay. The defences appeared as a conspicuous white triangle of unweathered rocks; this is clear in the photographs. The large blocks of white Portland Stone contrast with the brownish and grey marls of the Lower Purbeck Group. Fortunately, as time has passed the Portland Stone is becoming greyer and with black patches, and will probably darken further. Vegetation has been slow to take hold because of the lack of soil but some has appeared and is slowly spreading. The scar is now less obvious and less of an aesthetic problem. The building is no less conspicuous.

Fortunately, the Durlston Bay situation has not resulted in the loss of the building. A very much worse case, very different in several respects, has occurred at a place called Jump-Off Joe, near Newport on the Oregon coast. Here the cliff was eroding very fast. Here a hotel or condominium was being constructed on a cliff top of soft Tertiary clay (see photographs above). It was suprising that its construction was permitted but it followed the preparation of a strange geological report that did not seem to explain the real and obvious risks. The cliff beneath the building began to collapse during the construction and the building was demolished.

(Footnote:

Here is an excerpt from Komar (1997:161): "In 1942, a large landslide in the bluff at Jump-Off Joe carried more than a dozen homes to their destruction (Sayre and Komar 1988). In spite of continued slumping, a condominium was built on the remaining bluff in 1982. A certified geologist had determined that the site was stable even though it was adjacent to the 1942 landslide and in the area with the highest rate of erosion on the entire Oregon coast, and the Newport city government gave its approval to the project. Within three years, before the construction was even completed, slope retreat caused the foundation to fail, and the city ordered the destruction of the unfinished structure. The developers, the contractor, a lumber company, and the insurance company that had insured the project against slippage went bankrupt. Creditors with claims of $1 million were paid between 18 cents and 1 cent on the dollar. The consulting geologist lost his certification. The debate over Jump-Off Joe was the most divisive land-use battle ever fought on the Oregon coast, and people still have strong feelings about the project."

See:
Newport Exposition and Event Center--What Has the City of Newport Learned From Their Failure with the Jump-Off Joe Development?)
(and other webpages that can be found from Google)

The Zigzag Mudslide of 2001

The central part of Durlston Bay is a notable place for large mudslides. A very substantial slide occurred in the late 1950s or 1960s and extended back almost to the road from Swanage to Durlston Head. It then ceased, became overgrown and forgotten about, and new building took place in the surrounding area. In early 2001 a new mudslide occurred fortunately stopping just at the gardens in front of some flats, as shown in photographs above. This 2001 mudslide was relatively small but it destroyed a stretch of forested cliff, immediately to the south of the Zigzag Path, making the flats more visible from Durlston Head. It cut the coast path along the previous cliff top. It was fortunate that it did not reach the buildings.

Later the mudslide dried out. By the 15th of July 2007 it was well-covered by low vegetation. You could walk on it but the ground is extremely irregular with rocks and banks here. Fissures, brambles and a small watercours prevent progress through it. Amongst the vegetation growing on the mudslide there are some garden plants which originated on the small parts of the gardens which collapsed. Dead trees are present, especially at the seaward base. It will take many years before it is a forested area once again.

A minor consequence of the 2001 mudslide is that the coast path was cut. A small diversion by La Belle Vue Road and Durlston Road takes the walker to the cliff top path continuation to the south. The steps to the coast path remain and can be used as access to the rough and rather slippery Zigzag Path. Incidently the building on the left of the steps is the block of flats that is partially supported by the sea-defence slope of Portland Stone. The cliff edge is quite close on the left hand side. As mentioned on the photograph caption on the right hand side of this path was the black, timber building of the Tilly Whim Inn, a tourist attraction in the early 1950s (and earlier?) and well-advertised in the town by a black taxi with Tilly Whim Inn painted over it in large letters. Occasionally I would have a drink on the veranda of the Tilly Whim Inn with its impressive views over the bay.

Part of the central Durlston Bay area has an instability or potential instability problem because of a large east-west fault (or faults). The regional dip here is to the north or northeast as shown on the maps above (17 degree in a NE direction on the latest map). The major east-west fault is extensional (i.e. normal) and downthrows south to the extent of about 30m. On the south side there are north-dipping shales and thin limestones of the upper part of the Middle Purbeck Group. Some of the limestones are permeable and may act as pathways for water flow. On the north, upthrow, side of the major fault there are strata from the Lower Purbecks to the lower part of the Middle Purbecks. These vary in character but they are mostly impermeable, especially the Lower Purbeck marls.

This interesting structure means that water flow through the north dipping strata can be blocked by the fault plane. The northerly dip and the extension of the fault inland (westward) probably channels groundwater into this area. More mudslides are expected here in the future, perhaps with long intervals between them. When the 2001 mudslide took place some pumping away of groundwater was quickly established. This might have prevented the head of the mudslide from extending landwards, but it might have just stopped naturally. On the previous occasion it extended much further landwards.

It should be noted that the exact trace of the fault inland and away from the cliff has not been determined precisely. This why it is shown as a broken line. The exact trace of the fault at the surface is complicated because it is an inclined plane intersecting a fairly complicated topographic surface. With detailed surveying techniques including geophysics and boreholes its position could be determined more precisely, were that needed.

Dinosaur Footprints, with a Long Toe

Dinosaur footprints occur in the central part of Durlston Bay, as they do in the northern part. Two photographs are shown above of natural casts in limestone of prints with usually long central toes. The impressions of claws can be seen. As is the case of dinosaur footprints in the Purbeck strata of the Isle of Portland the central claw is partially turned to the side in one of these prints. One of the dinosaur footprints is in situ in the Lias bed DB 118 of the Intermarine Member and this has been described by Nunn (1990). The other one is on the beach and has chisel holes in the block suggesting that an unsuccessful attempt has been made to remove it from the shore. The two prints are very similar but no detailed comparison has been made of the prints or of the limestone in which they occur. Thus it is not known at present whether they are from the same bed. This is quite possible as they are not far from each other.

For more on dinosaur footprints go to the Dinosaur Footprints of the Isle of Portland webpage.

ACKNOWLEDGEMENTS

This website is written and produced by Ian West but it kindly supported by the Head of School and staff of the School of Ocean and Earth Science, National Oceanography Centre, Southampton University. I am very grateful to the Director and Staff of The Channel Coastal Observatory , National Oceanography Centre, Southampton, for permission to use their aerial photographs. I particularly thank Dr. Travis Mason. I must appreciate the hosting of this website by Information Systems Services, Southampton University.

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

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

Webpage - written and produced by:

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

.

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

.