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|Home and Contents | Field Guide Introduction | St. Aldhelm's Head, Isle of Purbeck (this webpage) |St. Winspit and Seacombe, Isle of Purbeck. | Dancing Ledge and adjacent cliffs, Isle of Purbeck | Anvil Point to Blackers Hole, Isle of Purbeck |St. Aldhelm's Hd to Anvil Point - Geological Bibliography |Chapman's Pool, Houns-tout and Egmont Bight, Kimmeridge Clay & Portland Sand. | Portland Bill | Durlston Bay, Purbeck Fm. |
Selected external links: | Jurassic Coast (DCC)

St. St. Aldhelm's Head, Isle of Purbeck (this webpage)
St. Aldhelm's Hd to Durlston Hd - Winspit, Seacombe
St. Aldhelm's Hd to Durlston Hd - Dancing Ledge
Anvil Point to Blacker's Hole, Isle of Purbeck
St. Aldhelm's Hd to Durlston Hd - Geological Bibliography

Go to the Cliffs East or West?

Go east to:

|Winspit and Seacombe ?

Go west to:

Chapmans' Pool, Houns-tout and Egmont Bight?

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Related Field Guides and Bibliographies

|Chapman's Pool, Houns-tout and Egmont Bight, Kimmeridge Clay & Portland Sand. |Portland Bill |Durlston Bay, Purbeck Formation |Purbeck Fm. - Bibliography

INTRODUCTION:

Safety and Risk Assessment

Some general information regarding safety on Dorset geological field trips is provided and you are requested to read this if going to localities described here.

Some specific hazards with regard to the coast from St. Aldhelm's Head to Anvil Point are mentioned. Care must be taken with regard to vertical cliffs and it is best to keep to the cliff path wherever possible. Old galleries in abandoned quarries should not be trusted to be safe. Climbing should not be done casually and only obvious paths used by the geological visitor. Rock climbing should be left to experienced climbers with proper equipment, and there is a climbing guide for this coast. There are some places, such as at Dancing Ledge where rocks could fall from above but this is not so often a hazard on this coast. There is a serious danger of being swept of ledges by waves. Great care should be taken on coastal ledges especially in stormy conditions.

Adders are common on the land above the cliffs but are rarely much of a hazard unless trodden on or attempts are made to handle them. Here is one at Seacombe Bottom, encountered during the early stages of the eclipse of the sun on 11th August, 1999. Arkell (1935) drew attention to their relative abundance in the undercliff of Emmit Hill and elsewhere: " It should be noted that the luxuriant undergrowth and grass here and under St. Albans and Gad Cliffs harbour abundant adders. They are common enough to be dangerous unless gaiters are worn."

The common adder or viper, seen only once or twice a year on geology field trips on the Dorset coast, is not normally agressive and rarely a practical problem. One should just look where one is walking when going through rough grass or on little-used paths, as they sometimes bask in the sun in such places. They should not be disturbed but be conserved and left in peace.

In general, this coast is excellent for individuals, small groups and parties of careful people. This is a superb area provided careful attention is paid to safety on a very rugged, rocky coastline with precipitous cliffs.

INTRODUCTION:

Access

To visit the dramatic headland of St. Aldhelm's Head, drive to Worth Matravers in the Isle of Purbeck. From here travel a short distance westward to the car park at Renscombe. Leave your car here and walk about a mile and half down the rough road south to St. Aldhelm's Head. It is an easy route almost entirely on one level (and used by visitors of all ages), but a significant distance to go and return. There are circular walks both east and west. Variations that involve descending to the undercliff or to Chapman's Pool are more strenuous. The cliffs need much care and can be very dangerous.

INTRODUCTION:

Maps and Aerial Photographs

INTRODUCTION:

General Geography and Topographic Features

GEOLOGICAL SUCCESSION:

General

A classic diagram, based on Arkell (1933) shows the general uppermost Jurassic/basal Cretaceous succession in the Isle of Purbeck , compared with that on the Isle of Purbeck. The Purbeck sequence follows. This sequence of clays and limestones has been described by Arkell (1933; 1947) and many other authors. Sedimentology has been discussed by Townson (1975), West (1975), Bosence (1987) and others. Note that there are some correlation problems.

The Ammonite Zones of the Portland Group

A shortened account, with emphasis on the Isle of Purbeck, is given here. There is a slightly more extensive account of Portland ammonite zones in the Portland Field Guide .

Wimbledon and Cope (1978) revised the ammonite zones of the Portland Group. Their ammonite zones are listed below with the oldest at the bottom.

Titanites anguiformis Zone.
The Portland Freestone mostly belongs to this zone, although the Portland Freestone of Purbeck may start earlier, in the kerberus zone.

Galbanites (Kerberites) kerberus Zone.
This is common in the Cherty Beds of Portland and Upper Cherty Beds of the Isle of Purbeck (note the complication - not Lower Cherty Beds of Purbeck). It occurs from the Basal Shell Bed of Portland and from the Prickle Bed or Puffin Ledge (J-J1) of the Isle of Purbeck according to Wimbledon and Cope (1978).

Galbanites okusensis Zone.
This is present in the Black Dolomites ("Black Sandstones") of Purbeck. The dolomitisation renders the fossils poorly preserved and difficult to extract.

. Note the complication that Wimbledon and Cope (1978, p. 187) stated that G. okusensis had not been found in the upper part of its zone. In Dorset the upper part of the zone was apparently established on the occurrence of Titanites (Polymegalites) polypreon because this ammonite is associated with G. okusensis in the Swindon reference section. Perhaps G okusensis has since been found in the upper part. If not, the placing of the Lower Cherty Beds of Dorset in the okusensis zone, that is older than the lowest Cherty Series on Portland, is not placed on direct evidence of the occcurence of the zonal index but on circumstantial evidence. Townson (1975), Wimbledon and Cope (1978) and Wimbledon (1987) have all correlated the Prickle Bed (J1) of Purbeck with the Basal Shell Bed of Portland which would support the theory. It is worth drawing attention to the fact, though, that there is a question at this point in the succession, and that Arkell (1933; 1947), as shown in the diagram, correlated the base of the Cherty Series of Purbeck with that on Portland.

Glaucolithites glaucolithus Zone.
The base of the Zone is defined as the first appearance of the species of the genus Glaucolithites. The type locality for the Zone is Hounstout Cliff, near Chapmans Pool, Isle of Purbeck. The White Cementstone with G. caementarius fixes the base of the Zone according to Wimbledon and Cope (1978).

Progalbanites albani Zone.
Wimbledon and Cope (1978) defined the base of the Zone as the first occurrence of the genus Progalbanites. The type section is at Hounstout Cliff, near Chapmans Pool, Isle of Purbeck. The Massive Bed is the lowest horizon with Progalbanites.

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The General Portland Facies (Sedimentary Environments etc)

(See also Portland Stone details, below)

The Portland Stone consists of carbonate-cemented lime-sands formed in shoal conditions at almost the end of the Jurassic Period. The shallowest conditions were during deposition of the uppermost beds, just before transition into the lagoonal Purbeck facies.

A broad view reveals a general regressive (shallowing) sequence from the relatively deep-water Kimmeridge Clay, sometimes almost anaerobic, through the shallowing Portland Sand. In spit of the name much of this is marl, but with influx of quartz sand and silt in the Black Nore Sandstone on Portland and the Massive Bed in Houns-Tout Cliff, Isle of Purbeck. Evidence of shallowing generally continues up into the Portland Cherty Series (Dungy Head and Dancing Ledge Members of the Portland Limestone Formation of Townson, 1975)). Considering it in more detail, there was a limited transgression (deepening) at the base of the upper part of the Cherty Series (Dancing Ledge Member of Townson) before the continued final shallowing.

The upper and final, Portlandian regression followed this transgressive episode and deposited the sponge-spicule rich lime muds of the Upper Cherty Series (Dancing Ledge Member). These coarsen up into shelly packstones with large epifaunal bivalves and sponges (epifaunal means living on the surface of the sea-floor, as opposed to living within the sediment). Continued shallowing brought in shoal water ooid grainstones (i.e. white oolitic lime sand) of the Portland Freestone or Winspit Member ( Townson, 1975; Bosence 1987). There were subaqueous dunes of white carbonate sand on the sea-floor. These were low-angle features and they were moved southward under the influence of currents or waves. Most of the movement took place during hurricanes and storms. In contrast during the quiet phases, which occupied most of the time, thick-shelled bivalves, sponges and shrimp-like crustaceans colonised these mounds.

The origin of the facies of the Portland Stone is clearly explained in this classic schematic diagram of Townson (1975). Walther's Law is a widely used principle of modelling vertical changes in lithology in terms of lateral changes in environment. It has been used to produce this diagram, which is a model rather than an attempt at an exact interpretation of the position of lateral facies. Not all these facies were developed to a near equal extent at any one time, of course. Note the main facies of the Portland Stone of the Isle of Purbeck. The Cherty Series is mostly of sponge-spicule wackestone facies and the Portland Stone of rather shallower shell-sand facies.

The Portland Cherty Series

The Portland Cherty Series is not oolitic but consists of fairly fine-grained limestone with nodules of black or grey chert, in some cases joined into an almost continuous beds. The photographs here show the Cherty Series at Dancing Ledge. This chert is replacive, mostly very early, but in some cases associated with tectonic features such as faults.

The limestone is of an unusual type which can be described as a "sponge spicule wackestone". It is a micrite with carbonate sand grains (matrix supported), most of which are the peculiar kidney-shaped Rhaxella sponge spicules usually preserved as calcite replacements ( Townson, 1975). The spicules were originally opaline silica but this silica has migrated in solution and formed the chert nodules by replacement, leaving in turn the opal replaced by calcite. There are other components in small quantities. They include the branching elongate spicules of the sponge Pachastrella, some bivalve shells, some shell sand and some very fine quartz sand (less than 5%).

For detailed succession of the Portland Cherty Series, based on measurements and correlations at Seacombe, Winspit, Worth Quarry and St. Aldhelm's Head see Arkell (1947, p. 101, or Arkell, 1933 or 1935). See also Townson (1975) for facies variations in this.

Two alternative schemes for the terminology of the Portland and Purbeck successions are given here. Townson (1975) introduced a different terminology. However, it has not been widely used and the traditional scheme of Arkell, shown on the left is still in more common use (diagram after Bosence, 1987, from Townson, 1975). See Wimbledon (1986) for some discussion of this.

GEOLOGICAL SUCCESSION:

St. Aldhelm's Head - Details

East-West Cliff Section

This cliff section is based on Davies (1935; 1956). It shows the numerous north-south faults cutting the section and displacing the strata in the cliffs. The throws shown here are in feet (1 ft. = 0.3048m, or as a rough estimate - one foot is about a third of a metre). The general pattern of the faulting is extensional.

In this photograph, taken from the paddle steamer Waverley in 1997 by Dr Clive Boulter, the upper cliffs of Portland Stone are seen above slopes with quarry waste and natural fallen rock. Old quarries are present below the coastguard hut seen on the cliff top with a Land Rover and flagmast. The quarry tips and natural rockfalls make this coast resemble the coast of the Isle of Portland which has similar geology. The quarry tips there are known as "weares". The tide produces a "race" here, where turbulent water flows from east to west across a submerged ridge of Portland Stone. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Footnote: Mystery of the Body on the Cliffs.
In June 1974, there was the sad discovery of a badly decomposed body on the cliffs near St. Aldhelm's Head according to Legg (1984) . It proved to be the remains of Mrs Jean Baxter, a scientist of the Chemical Defence Establishment near Salisbury. She had been missing for four months during which time there had apparently been press speculation. An open verdict was recorded at the inquest but it was stated that she had been depressed and no security aspect was involved.
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West Side of St. Aldhelm's Head

The west side of St. Aldhelm's Head and Emmetts Hill is seen here from the sea in a photograph taken by Dr Clive Boulter on 23rd September, 1997. The lower part of the cliff, partly covered by fallen debris consists of the uppermost part of the Kimmeridge Clay. Above this the Portland Sand occurs. This is mostly mudstone and marl in the lower part in spite of the name, but a sandstone bed, known as the Massive Bed marks the junction and can be seen in the photograph. Some way above is another good marker, the White Cementstone. Higher in the cliff, above the marls are the "Parallel Bands" and the "Black Sandstones", of Arkell (1947). These are actually beds of dolomite. The Portland Cherty Series follows. Notice the dissolution (karstic) features with yellow sandy debris at the top of the cliff on Emmetts Hill. The valley is Pier Bottom and has been truncated by marine erosion and left "hanging", that is with its base above sea-level. Large rectangular blocks of limestone seem to form a natural sea-defence or barrier except at Pier Bottom where this stone is not available. Here there is a shale cliff in the Kimmeridge Clay. More information on the Portland Group and an alternative set of names is available in Townson (1975).

This piece of coast has many similarities to the West Cliffs of the Isle of Portland. It differs in not showing such well-developed rotational landslides and not having the quarry debris weares of Portland.

(more information on the Kimmeridge - Portland junction here will be provided later)

St. Aldhelm's Head seen from the cliff path on the east side. The Portland Sand forms the lower overgrown part of the cliffs with the Black Dolomites ("Black Sandstones") of Arkell (1947) above. The Cherty Series forms the vertical part of the cliff. In the distance is the toe of landslides and fallen debris at the tip of St. Aldhelm's Head.

This monolith has been left by quarrymen in the 19th century. The stem is of Pond Freestone which was quarried here. The head is part of the Titanites Bed. The main quarried stone at this locality was the Pond Freestone because the Under Freestone, much used further to the east, here contains too much chert ( Arkell, 1947). In the distance is the race formed by the underwater outcrop of the junction of Portland strata (to the east) and Kimmeridge Clay ( Donovan and Stride, 1961). ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Footnote: Rescued from Cliff at St. Aldhelm's Head
" Seven soldiers, including five cadets from the Royal Military Academy, Sandhurst, clung to a cliff face for nine hours after their boat sank.. Their plea for help went unanswered because their Mayday message did not give a location. All seven were eventually winched to safety from St Alban's Head, Dorset, in a second Coastguard operation after the first was abandonned in the dark. They were spotted only after setting off distress flares. In February last year, two policemen and a relative drowned in the same area after they broadcast only the word Mayday." Coastline, The Official Journal of the Coastguard Association, vol. 1, Issue 4, Spring 1999, p. 13.

LOCATIONS:

Mr Haysom's Quarry at St. Aldhelm's Head

This small quarry, belonging to Mr. Trev. Haysom has long worked the Isle of Purbeck variety of Portland Stone. This limestone is usually an intrasparite with lower porosity than Portland oosparite of the Isle of Portland. As a consequence it is a harder limestone.

St. Aldhelm's Head to Winspit

(For Winspit details please go to the Winspit webpage)

The easterly or east-southeasterly dip of the Portland Stone at St. Aldhelm's Head results in underground waterflow towards the eastern side of the headland. Some seems to emerge west of Winspit but for safety reasons the exact location is not specified here. The "goat's head" seen here is a curtain of travertine presumably formed by dripping or trickling carbonate-saturated water. Green algal slime here is also the result of the trickling freshwater. The features in cliff suggest that the water is emerging from within the Portland Cherty Series just below a prominant chert band. Presumably there is a change in permeability at this level. I have not had close access so I do not know the details. The feature is probably best studied from a boat but I have not been able to do this as yet. If anyone has studied it more closely I would be glad to hear about it. Photographs from the sea would be very useful. Rock climbers may know it (is it "Crab Hole" listed in the climbing guide?).

Small solution caves would be expected around here. The small-scale karstic features seen at the St. Aldhelm's Head cliff-edge quarry have originated in a similar manner when the water table was higher. These will be referred to elsewhere.

Please note that this place is dangerous and the travertine can only be glimpsed from the cliff footpath. Do not place yourself in danger by approaching the cliff edge on the sloping grass.

LOCATIONS:

St. Aldhelm's Head to Winspit - Topple

The Portland Stone is almost horizontal in this area and except in the western part at St. Aldhelm's Head there is no clay near the surface. As a result, there are no rotational landslides along most of this coast. Erosion of the Portland Stone is instead largely by uncutting by direct storm action on jointed blocks and, to a lesser extent, by bioerosion. This causes collapse from above. The Portland Stone is jointed but not with the major open fissures that are an interesting feature of the Isle of Portland. Topples of blocks separated by such fissures are not as common as on Portland and smaller isolated blocks usually fall. A small incipient topple is shown here. It is being wedged out by fallen debris in the gap between the rock and the main cliff.

I believe that the base of the sliding block is Arkell's (1947) bed K and thin bed that it is sliding on is the Prickle Bed or Puffin Ledge (J'). This needs confirmation. Note the relics of previous topples as large blocks at the foot of the cliff.

SEA LEVEL RISE

Probably effects at St. Aldhelm's Head and Supplies of Rock Armour

As is well-known, relative sea-level is rising at present at a relatively high rate in this region, about 4 to 5 mm. per annum. This is not a disputable topic. How much the rate will increase in the future is a very controversial matter, not discussed here. Sea level is supposed to rise by a metre in at most 200 years and possibly about 100 years. We do not know exactly when for sure but sooner or later sea-level will be about a metre higher.

In addition to gradual sea-level rise this region is prone at relatively rare intervals of something like 1 in 250 years to severe hurricanes which cause storm surges in the English Channel. These temporarily raise sea-level by about 3 metres or more and cause serious sea-flooding. The 1824 event (the "Great Gale") was one of these. It is discussed in the Chesil Beach Storms webpage.

In the specific area of St. Aldhelm's Head the effects of general sea-level rise and of storm surges are likely to be fairly limited. There is, at present some erosion of clay strata at Emmetts Hill near the Kimmeridge Clay - Portland Sand junction. Sea-level rise and storm surges will increase this erosion quite significantly. This will lead to new cliff falls and landslides. This hard rock coast-line will retreat a little but not to a large extent. This is not an area at major risk of coast erosion.

Examine the geological map above and consider just what indirect effect sea-level rise and storm surges might have here. On the south coast of England to the east of this region there is no supply of hard rock for rock armour to give coastal protection. It is very obvious to the most casual observer that there are huge resources of Portland Stone, both Portland Freestone and Cherty Series in the St. Aldhelm's Head and Worth Matravers area. At present it has been quarried on a small scale at St. Aldhelm's Head and on a larger scale at Swanworth Quarry near Worth Matravers. Neither are conspicuous. It is very fortunate that at the present this region has not been despoiled by a large amount of quarrying like that on the Isle of Portland. The present quarries have little effect on the environment. However, in many respects the Worth Matravers area is similar to the Isle of Portland before the major phase of large-scale and deep quarrying which has now taken place. At some date in the future when the low ground of Sandbanks Peninsula, Weymouth and other coastal areas are threatened by sea-flooding there is likely to be severe demands for local rock armour. At that time the St. Aldhelm's Head and Worth Matravers area may need to retain strong protection against large-scale quarrying.

Go to the Cliffs East or West?

Go east to:

|Winspit and Seacombe ?

Go west to:

Chapmans' Pool, Houns-tout and Egmont Bight?

Acknowledgements

I am very grateful to Dr Clive Boulter and Gareth Lloyd for the use of photographs of the coast taken from the paddle steamer Waverley.

Select Bibliography

Please go to the Geological Bibliography on the coast from St. Aldhelms's Head to Durlston Head.

See also the:
Durlston Bay, Geological Bibliography.

and the
Purbeck Formation Geological Bibliography.

Copyright © 2007 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:

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

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at his private address, under the auspices of the National Oceanography Centre, Southampton (NOCS),and web-hosted by courtesy of the Information System Services (ISS) of Southampton University. The website is an unfunded, private activity, and does not necessarily represent the views of the National Oceanography Centre, Southampton or Southampton University.
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(Contact Ian West: Please use the email address given in the small image).