School of Ocean and Earth Science ,
National Oceanography Centre, Southampton
Southampton University,
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Selected external links: | Jurassic Coast (DCC)
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St. Aldhelm's Head
St. Aldhelm's Hd to Durlston Hd - Winspit and Seacombe
Dancing Ledge
Anvil Point to Blackers Hole
St. Aldhelm's Hd to Anvil Point - Bibliography
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 where rocks could fall from above. There is a danger of being swept of ledges by waves 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.
This is a superb coast provided careful attention is paid to safety on a very rugged, rocky coastline with precipitous cliffs.
INTRODUCTION:
Stratigraphical Succession
This 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 classic 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 correlation problems and arguments referred to in the section on Zones of the Portland Group, below.
Two alternative schemes for the terminology of the Portland and Purbeck successions are given here. Townson (1975) introduced a largely new 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, and note also that some correlation problems which affect this are discussed in the section on zones, below.
INTRODUCTION CONTINUED:
East-West Cliff Section
This cliff section is based on Davies (1935; 1956). It shows the numerous north-south faults cutting the section and moderately displacing the strata in the cliffs. The throws shown here are in feet (1 ft. = 0.3048m). The general pattern of the faulting is extensional and has the consequence of bringing up Portland Sand to the foot of the cliff east of the more major faults.
The Portland Freestone in the Eastern Isle of Purbeck - Details of the Succession
The following tabulated succession was measured at Seacombe by William Joscelyn Arkell in the 1930s and has been published in Arkell (1933;1935; 1947). Although the details refer specifically to Seacombe, in many respects it is representative for much of the stretch of coast between Anvil Point and St. Aldhelm's Head. The main differences are an increase in chert to the west and the occurrence of the oyster reef in the upper part of the succession at Tilly Whim. The Seacombe succession serves quite well for Dancing Ledge, Hedbury Quarry and Winspit.
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" V. Shrimp Bed.
White fine-grained sublithographic limestone, constant in appearance from Durlston Head to Lulworth. The 'Shrimp' is a small Crustacean identified by Mr. H. Woods as probably Callianassa [related to the hermit crabs]. Bivalves abound, especially Trigonia gibbosa, mainly var. damoniana de Loriol, Protocardia dissimilis, Chlamys (Camptochlamys) lamellosa, Isognomon (Perna) listeri, Pleuromya tellina (Agassiz), with Trigonia incurva, Isocyprina sp., etc. Worth Quarry has yielded from this bed a fairly complete specimen of Titanites. There are also fragments of coarsely-ribbed triplicate ammonites suggestive of Buckman's Glottoptychinites or Kerberites. - 10 feet [3m].
U. Titanites Bed.
Hard, greyish, shelly limestone ('spangle'), as at Worth Quarry, especially shelly in the basal 2 ft [0.6m]. Innumerable Trigonia gibbosa, T. incurva, I. listeri, Chl. lamellosa, P. dissimilis, Ostrea expansa, etc. This bed is the source of nearly all the giant ammonites of various species of Titanites (though at Winspit some occur in the House Cap). At Tilly Whim a lenticular oyster bed develops on this horizon. Locally about 8 ft [2.4m.] of the rock is almost entirely composed of Exogyra nana, E. thurmanni Étallon, Ostrea expansa and Isognomon listeri, with a smaller proportion of Lima rustica (J. Sowerby) and Plicatula boisdini de Lorriol. Sometimes the fossils are dissolved away, leaving patches of rocks like the Roach of Portland. - 10 - 11 feet [3m - 3.35m].
T. Pond Freestone.
Good oolitic [sic] freestone. Fossils nearly all comminuted. Occasionally spoilt by lenticles of white silicified oolite [sic]. At Worth 7 - 7.5 ft. [2.13 - 2.28m]. Here only 5 ft [1.5m].
S. Chert Vein.
Limestone with chert, dense and nodular below, sparser above. 5 ft [1.5m].
R. Listy Bed
Grey limestone with a ready vertical and horizontal fracture, strongly marked off above and below. So called ' because it breaks easily'. Not present at Worth. The 'Lisky' Bed of Buckman (1926, p. 35) and the 'Nist' Bed of H.B. Woodward (1895, p. 190). 6 inches to 1 foot [0.15 - 0.3m].
Q. House Cap.
Hard grey shelly limestone ('spangle'), resembling the Titanites Bed, and like it especially shelly in the basal portion. The under surface, which forms the roof of the galleries at all the cliff quarries, is covered with large shells of O. expansa, I. listeri, L. rustica, Ch. lamellosa, etc. At Winspit several giant specimens of Titanites can be seen embedded in the basal 1-2 feet [0.3-0.6m]. About 5 feet [1.5m] from the bottom of the bed is a band of thin lenticles of white chert and silicified oolite [sic]. 8.5 feet [2.6m].
P. Under Picking Cap.
Hard freestone, locally called spangle. It is cut to waste in order to get at the Under Freestone. - 3 feet [0.9m].
O. Under of Bottom Freestone.
Fine cream-coloured oolite [sic], an excellent quality freestone, the shells nearly all comminuted. the was the stone for which all the old cliff quarries were principally worked. Partly false-bedded. - 8 feet [2.4m].
Total - c. 50 feet [c. 15m]"
(Cherty Series beneath)
At Winspit there has been extensive quarrying. There are numerous galleries from a long cliff quarry ledge east of the valley. There are also quarries and galleries to the west.
This photograph shows the succession in a south-facing part of the abandoned quarries at Winspit, at the sea-cliff just west of the mouth of the small valley. Here the Under Freestone has been worked. Much of the quarrying is old and there is a photograph from the 1890s showing a quarry face on the east side of the valley in Woodward (1895) and Strahan (1898). The latter author mentions that " the line of old adits forms a conspicuous feature". The west side of the valley seems to have been worked relatively recently, as least compared to others on this stretch of coast. Parts of some quarry buildings remain here. The quarries of Winspit provided stone for the building of Allhallows School (Bruce, 1989).
The succession at Winspit is as follows (partly from Woodward, 1895). This is from the top downwards:
Purbeck limestones and clays, including stomatolitic (thrombolitic) limestone. - 4 feet (1.2m).
Shrimp Bed. Fine-grained white limestone with remains of a small crustacean. It was formerly burnt for lime. - 8ft (2.44m).
Titanites Bed ("Blue Stone"). Hard grey shelly limestone with Isognomon bouchardi. Durable stone used for gate posts, etc. The shells stand out in relief after long exposure of the blocks. - 9ft (2.74m).
Pond Freestone ("Upper Freestone" or "Top Freestone"). Calcarenite, the best stone. - 7 feet (2.1m).
Chert Vein ("Flint Stone"). White and grey chert nodules in limestone. - 4 feet (1.2m).
Listy Bed ("Nist Bed"). 2 feet - 4 feet (0.6m - 1.2m).
House Cap. Coarse shelly limestone with Titanites ammonites. The stone was used for breakwaters. It was not so workable as the other beds. 5 feet - 6 feet (1.5m - 1.8m).
Under Picking Cap. Hard calcarenite with lenticular white chert. It had to be blasted out. 2 feet - 3 feet (0.6m - 0.9m).
Under Freestone. Calcarenite, good stone used for sinks, curbstones etc. - 6 feet (1.8m).
Cherty Series (locally known as the "Cliff Beds" as opposed to the "Inland Beds", the Portland Freestone)
(Thoughout the above succession "Trigonia" gibbosa occurs here and there at various horizons as bands of shells.)
Notice that a pillar of Under Freestone and Under Picking Cap has been left to support the roof of a large gallery (please keep out of the galleries because of the danger of rock falls). Giant ammonites occur in the House Cap, and they should be present in the Titanites Bed, although they are not mentioned in the above list and this bed is not very accessible here at present. The Listy Bed is not marked on the photograph but is thin and at the base of the Chert Vein. The Pond Freestone seems recessed by quarrying. The Shrimp Bed is fine-grained and splintery. It is split by many small joints rather than just by a few large ones. The fracturing of this bed increases close to the surface, probably largely because of freeze and thaw activity in the late Pleistocene glacials. Notice in general the regular orientation of major joints in all the beds seen in the quarry face. The supporting pillar appears tilted because its boundaries are presumably joints which are not exactly vertical.
This photograph was taken just round the corner to the east (right of the other image), where there is a gallery in the Pond Freestone in the east-facing quarry face. This upper gallery is showing some partial collapse of the roof, but it is not accessible now.
For more details of the succession here see Arkell (1935, p. 301) or Arkell (1947, p. 100, fig. 20). Some chert in bed P, the Underpicking Cap, and in bed Q, the House Cap are features of this section. In general, the section at Winspit is similar to that at Seacombe, Dancing Ledge and Worth Quarry.
A giant ammonite, presumably a Titanites has been left as a hollow mould in the House Cap at Winspit. Other examples can be found by looking at the quarry cliff face above the level of the lower galleries.
The details of the succession of Portland Freestone at Seacombe have been listed by Arkell (1933; 1935; 1947). They are given here in the Portland Freestone Succession section. Seacombe Stone was suitable for landings, steps and in other situations for which normal Portland Stone would be insufficiently hard (North, 1930).
Localities - Halsewell Quarry and Halsewell Bars
Between Seacombe and Winspit there is a large, seaward-sloping, fallen slab of rock known as the Halsewell Rock. Near here on the 6th January, 1786 the East Indiaman, the Halsewell of 758 tons was wrecked. Only 82 of the 250 on board survived. The dead were buried in Seacombe Bottom.
She had sailed from the Downs bound for Bengal, with Captain Richard Pierce in command, the oldest captain in the East India Company's service. Not long after the ship had started the weather grew severe, thick falls of snow and frost impeding the working of her sails, while a desire to land the pilot led her, on the following Tuesday, into a dangerous position on a lee shore off Dunnose, Isle of Wight. A violent gale arising from the south compelled the captain to carry on hard for an offing; the tortured vessel sprung a leak and became all but unmanageable. With seven feet of water in the hold, on the Wednesday morning, no hope remained but to get her before the wind, nor could this be effected except with extreme difficulty - by cutting away first the mizzen and then the main mast. The ship got a distant site of Berry Head in Devonshire and the ship tried to make for Portsmouth. On Thursday morning the wind began to blow freshly from the south and she was caught on a lee shore. She just weathered the dreaded Portland Bill but before nightfall St. Aldhelm's Head was sighted just a mile and half away. She tried to anchor but was driven towards the ironbound coast near Seacombe (Robinson, 1892).
There now follows a shortened version of an account quoted by Charles Dickens and reproduced in full by Robinson (1882).
" See the Halsewell, East Indiaman, outward bound, driving madly, on a January night, towards the rocks near Seacombe, on the Isle of Purbeck. The captain's two dear daughters are aboard, and five other ladies. The ship has been driving many hours, has seven feet of water in her hold, and her mainmast has been cut away. ..."
About two o'clock in the morning of Friday the sixth of January (1786), the ship still driving, and approaching very fast to the shore, Mr Henry Meriton, the second mate, went into the cuddy, where the captain then was. Another conversation taking place, Captain Pierce expressed extreme anxiety for the preservation of his beloved daughters, and earnestly asked the officer if he could devise any method of saving them; on answering, with great concern, that he feared that it would be impossible, but that their only chance would be to wait for morning, the captain lifted his hands in silent and dreadful ejaculation.
At this dreadful moment the ship struck with such violence as to dash the heads of those standing in the cuddy against the deck above them, and the shock was accompanied by a shriek of horror that burst at one instant from every quarter of the ship. ... The ship continued to beat on the rocks; and soon bilging, fell with her broadside towards the shore. When she struck, a number of men climbed up the ensign staff, under an apprehension of her immediately going to pieces. .....
The Halsewell struck on the rocks at a part of the shore where the cliff is of vast height, and rises almost perpendicularly from its base. But, at this particular spot, the foot of the cliff is excavated into a cavern of ten or twelve yards in depth, and of breadth equal to a large ship. The sides of the cavern are so nearly upright, as to be of extremely difficult access; and the bottom is strewn with sharp and uneven rocks, which seem by some convulsion of the earth to have been detached from its roof.
The ship lay with her broadside opposite to the mouth of the cavern, with her whole length stretched almost from side to side of it. But when she struck, it was too dark for the unfortunate persons on board to discover the real magnitude of the danger, and the extreme horror of such a situation. .
In addition to the company already in the round-house, they had admitted three black women, and two soldiers' wives; who, with the husband of one of them, had been allowed to come in, though the seamen, who had tumultuously demanded entrance to get the lights, had been opposed and kept out by Mr. Rogers and Mr. Brimer, the third and fifth mates. The numbers there were therefore now increased to fifty. Captain Pierce sat on a chair, a cot, or some movable, with a daughter on each side, whom he alternately pressed to his affectionate breast. The rest of the melancholy assembly were seated on the deck, which was strewn with musical instruments, and the wreck of furniture and other articles.
Here also Mr. Merton, after having cut several wax candles in pieces, and stuck them up in various parts of the round-house, and lighted up all the glass lanthorns he could find, took his seat, intending to wait the approach of dawn, and then assist the partners of his danger to escape. But, observing that the poor ladies appeared parched and exhausted, he brought a basket of oranges and prevailed on some of them to refresh themselves by sucking a little of the juice. At this time they were all tolerably composed, except Miss Mansel, who was in hysteric fits on the floor of the deck of the round-house.
But on Mr. Meriton's return to the company, he perceived a considerable alteration in the appearance of the ship: the sides were visibly giving way; the deck seemed to be lifting, and he discovered other strong indications that she could not hold much longer together. On this account, he attempted to go forward to look out, but immediately saw that the ship had separated in the middle, and that the forepart having changed its position, lay rather farther out towards the sea. In such an emergency, when the next moment might plunge him into eternity, he determined to sieze the present opportunity, and follow the example of the crew and the soldiers, who were now quitting the ship in numbers, and making their way to the shore, though quite ignorent of its nature and description....
Mr Merton discovered a spar which appeared to be laid from the ship's side to the rocks, and on this spar he resolved to attempt his escape. Accordingly, lying down upon it he thrust himself forward; however, he soon found that it had no communication with the rock; he reached the end of it, and then he slipped off, receiving a violent bruise in his fall, and before he could recover his legs he was washed off by the surge. He now supported himself by swimming, until a returning wave dashed him against the back part of the cavern. Here he laid hold of a small projection in the rock, but was so benumbed that he was on the point of quitting it, when a seaman, who had already gained a footing, extended his hand, and assisted him until he could secure himself a little on the rock, from which he clambered on a shelf still higher, and out of reach of the surf.
Mr Rogers, the third mate, remained with the captain and the unfortunate ladies and their companions nearly twenty minutes after Mr. Merton had quitted the ship. ... The sea was now breaking in at the fore part of the ship, and reached as far as the mainmast. Captain Pierce gave Mr. Rogers a nod, and they took a lamp and went together into the stern-gallery, where, after viewing the rocks for some time, Captain Pierce asked Mr. Rogers if he thought that there was any possibility of saving the girls; to which he replied, he feared there was none; for they could only discover the black face of the perpendicular rock, and not the cavern which afforded shelter to those who escaped. They then returned to the round-house, where Mr. Rogers hung up the lamp and Captain Pierce sat down between his two daughters.
The sea continued to break in very fast, Mr. Macmanus, a midshipman, and Mr. Schutz, a passenger, asked Mr. Rogers what they could do to escape. 'Follow me', he replied; and they all went into the stern-gallery, and from thence to the upper quarter-gallery on the poop. While there, a very heavy sea fell on board, and the round-house gave way; Mr. Rogers heard the ladies shriek at intervals, as if the water reached them; the noise of the sea at other times drowning their voices.
Mr. Brimmer had followed him to the poop, where they remained together for about five minutes, when, on the breaking of this heavy sea, they jointly siezed a pen-coop. The same wave which proved fatal to some of those below, carried him and his companions to the rock, on which they were violently dashed and miserably bruised.
Here on the rock were twenty-seven men; but it now being low water, and as they were convinced that on the flowing of the tide all must be washed off, many attempted to get to the back or sides of the cavern, beyond the reach of the returning sea. Scarcely more than six, besides Mr. Rogers and Mr. Bremer, succeeded. Mr. Rogers on gaining this station, was so nearly exhausted that, had exertions been protracted only a few minutes longer, he must have sunk under them. He was now prevented from joining Mr Meriton by at least twenty men between them, none of whom could move without the imminent peril of his life.
They found a very considerable number of the crew, seaman and soldiers, and some petty officers, were in the same situation as themselves, though many who had reached the rocks below, perished in attempting to ascend. They could yet discern some part of the ship, and in their dreary station solaced themselves with the hopes of its remaining entire until daybreak; for, in the midst of their own distress, the sufferings of the females on board affected them with the most poignant anguish; and every sea that broke inspired them with terror for their safety.
But, alas, their apprehensions were too soon realised! Within a very few minutes of the time that Mr. Rogers gained the rock, an universal shreik, which long vibrated in their ears, in which the voice of female distress was lamentably distinguished, announcing the dreadful catastrophe. In a few moments all was hushed, except the roaring of the winds and the dashing of the waves; the wreck was buried in the deep, and not an atom of it was ever afterwards seen. "
Almost before daybreak, the quartermaster, Mr. Thomson succeeded in scaling the very treacherous cliff, which had killed others. He saw a distant light, probably at Worth Matravers, and procured aid. Eighty-two men were saved by means of a rope which the neighbouring inhabitants, particularly the quarriers let down into the cavern. (Robinson, 1882).
St. Aldhelm's Head to Winspit .
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.
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Please note that this place is dangerous and the travertine can only be glimpsed from the cliff footpath. On no account place yourself in considerable danger by approaching the cliff edge on the sloping grass! This can have fatal consequences. You have been warned!
Localities - Winspit to St. Aldhelm's Head - 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.
I am very grateful to Gareth Lloyd for the use of photographs of the coast taken from the paddle steamer Waverley.
Please go to:
St. Aldhelm's Head to Anvil Point Bibliography
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Webpage - written and produced by:
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Ian West, M.Sc. Ph.D. F.G.S.