West, Ian M. 2014. Anvil Point to Fisherman's Ledge to Blackers Hole Cave, Isle of Purbeck, near Swanage - Geology of the Wessex Coast (including the UNESCO Dorset and East Devon World Heritage Coast - Jurassic Coast). Internet site: www.southampton.ac.uk/~imw/Anvil-Point-Blackers-Hole.htm. Southampton University, UK. Version: 10th April 2014.
Anvil Point to Blackers Hole Cave, Isle of Purbeck, Dorset -  Geology of the Wessex Coast, by Ian West

Ian West,
Romsey, Hampshire

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.

|Home and List of Webpages | Field Guide Maps and Introduction | St. Aldhelm's Head, Isle of Purbeck |St. Winspit and Seacombe, Isle of Purbeck. | Dancing Ledge and adjacent cliffs, Isle of Purbeck | Anvil Point to Blackers Hole, Isle of Purbeck (this webpage) | Durlston Head, Isle of Purbeck |St. Aldhelm's Hd to Anvil Point - Geological Bibliography | |Portland Bill | Durlston Bay, Purbeck Fm. |

For TILLY WHIM CAVES immediately to the east, go to: Tilly Whim Caves section; in the Durlston Head Webpage.

Click here for the full LIST OF WEBPAGES

(You can download this educational website to offline downloading software keep a safe permanent copy; it is intended to run it from the present server but this is not necessarly guaranteed. Please note that at present there is periodic updating of the live version.)

Anvil Point Lighthouse, near Swanage, Isle of Purbeck, Dorset, 4 Sept. 2007

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The coastal footpath and slopes at the dry valley, Anvil Point, near Swanage, Dorset, 11th December 2013

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The dry valley at Anvil Point, near Swanage, Dorset, viewed from the sea, August 2007

The Subluminal rock climbing area on Portland Stone just to the west of Anvil Point, near Swanage, Dorset, 28 August, 2007 Rock climbers on the Portland Stone west of the Anvil Point Lighthouse, Isle of Purbeck, near Swanage, Dorset

Anvil Point Lighthouse seen from the hill above, with a capstan of an old Purbeck quarry, Isle of Purbeck near Swanage, Dorset

Anvil Point is in the eastern part of a dramatic stretch of the Portland limestone coastline of the Isle of Purbeck, stretching from Durlston Head to St. Aldhelm's Head. It has long been quarried at Tilly Whim Caves, Dancing Ledge, Seacombe and Winspit. It provides excellent exposures of the Portland Stone and the basal Purbeck Formation. The cliffs are superb for geologists and also for walkers. They are very well-known to rock climbers. The Portland Stone contains chert beds with sponge spicules, giant ammonites, serpulid worm tubes and shell-sand limestones, the quarried beds, with rich bivalve assemblages and the giant ammonite Titanites anguiformis. The coast has special geomorphological problems including a hanging dry valleys and an old coastal bevel. It also, incidently, has an interesting history. The Tilly Whim caves are old quarries of the Napoleonic War

This webpage is on Anvil Point and the adjacent coast.

Study this webpage - continue down - or -
Go east to:
Durlston Head?
or
Go west to:
Dancing Ledge?
or, beyond to:
Seacombe and Winspit?
or further west to:
St. Aldhelm's Head?



Location map and webpage limits for the coast between Durlston Head and St. Aldhelm's Head, near Swanage, Isle of Purbeck, Dorset

St. Aldhelm's Head to Durlston Head, Isle of Purbeck - Other Webpages

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

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INTRODUCTION:
Location and Initial Information.

An old topographic map of the coast west of Durlston Head and including Anvil Point and Dancing Ledge, near Swanage, Dorset

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INTRODUCTION:

Climbing Guides

The climbing guides are very useful for the names of coastal features on the cliffs. See Crewe (1977) or Coe (undated) These books are a source of much useful information, even if one is not a rock climber. Climbing, of course, should not be attempted, of course, without experience and proper equipment.

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INTRODUCTION:

Geological Maps

The 2000 edition of the 1:50,000 British Geological Survey Map, Swanage, Sheet 343 and part of 342, Solid and Drift - including the Isle of Purbeck and Lulworth Cove

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.

Part of the geological map for Swanage, 2000 edition, redrawn and showing the Durlston Bay and Anvil Point area

The old 1895 geological map of Swanage, Dorset, sheet 343, - see also the new 2000 edition BGS geological map of Swanage, sheets 342 and part of 343

Geology map, 1890

Geological map of the Swanage, Isle of Purbeck, area, based on an 1895 edition

Geological map showing the remarkable extent of erosion of Portland strata offshore from the south of the Isle of Purbeck, including Dancing Ledge, near Swanage, Isle of Purbeck, Dorset

Shown above are various geological maps of different dates. One is part of an old and simplified geological map of the Isle of Purbeck from Braye (1890). A geological map based on an 1895 edition of the Geological Survey map is also shown. The stretch of coast between Durlston Head and St. Aldhelm's Head is shown with the major coastal localities such as Dancing Ledge, Seacombe and Winspit. These maps provide a simple introduction to the geology of the area.

It is recommended, however, that you obtain the current geological map of the British Geological Survey (formerly the Institute of Geological Sciences), Geological Map - Swanage, Sheet 343 and part of 342. Solid and Drift Edition. 1:50,000. There is an accompanying explanatory memoir by Arkell (1947), which is good, but unfortunately now out-of-date in many respects.

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GENERAL INTRODUCTION:

General Topography of the Coast

Coast from Seacombe to Anvil Point

Aerial photograph, small-scale of the Dancing Ledge, Seacombe and Blackers Hole coast near Swanage, Isle of Purbeck, Dorset, courtesy of the Channel Coastal Observatory

The general features of the coast between Seacombe Cliff and Anvil Point are shown here. This is a superb rugged coast of considerable natural beauty. Particularly notable are the vertical Portland Stone cliffs with their natural caves, their quarry ledges with artificial galleries. Above is the interesting bevel or slope, based mainly but not entirely on Lower and Middle Purbeck strata. This will be discussed further. At the top is the old erosional plateau at about 120 metres, mainly cutting across Middle Purbeck strata just here. The photograph has been taken from the path from Seacombe to Winspit.

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INTRODUCTION:

Safety

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 the tops of the vertical cliffs. Slopes down to cliff-edges ledges vary from easy to very hazardous. Fatal accidents have occurred to rock climbers and, more rarely, walkers on this coast. Make your own risk assessment at the time of your visit, taking into account weather conditions etc.

Old galleries in abandoned quarries are not necessarily safe. For rock climbing by experienced climbers there is a climbing guide for this coast (several editions exist). There a risk of being swept of ledges by waves. Care should be taken in stormy conditions.

Adder at Seacombe Bottom

Adders are common but are rarely much of a hazard unless 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.

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INTRODUCTION:

Safety Continued; Supplementary Notes

On the Dorset coast in General (i.e. Jurassic Coast, eastern part), one cause of coastal accidents is that of rock fall. In the last 50 years about nine people have been killed by falling rocks, mostly in the Lulworth Cove area, but also at Burton Cliff and at Kimmeridge. About five people have been washed off ledges into the sea and drowned. Two boys were washed off the Portland Stone promontories of Lulworth Cove and drowned in recent years. Many years ago two rock climbers were washed off the Portland Stone cliffs west of Lulworth Cove. Another two rock climbers were washed off the bottom ledges of Portland Stone near the Anvil Point Lighthouse (probably just to the west) and, unfortunately, they too drowned. Sadly, there was a recent fatality due to a person being washed off the ledges at the foot of the dry valley, and just west of Tilly Whim Caves. References to published information regarding this are given below.

At Portland Stone cliffs west of Swanage, geologists are at only small risk of falling rocks, unless they enter old quarry mines. Geologists are not particularly likely to be risk of being washed away by the sea. However, at the few places on the coast here where there is some access to the lower ledges near the sea (Anvil Point, Dancing Ledge, Winspit and Seacombe) do not descend if waves are large or conditions generally stormy.

Go to the :
Lulworth Cove, Introduction Webpage for Lulworth Cove safety and accident information.

Go to the :
Burton Cliff Webpage for Burton Cliff safety and accident information.

[to be added - publications re Anvil Point]

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INTRODUCTION:

Access

Some of the coast belongs to the Durlston Country Park and some to the National Trust. Their regulations and safety guidance should be adhered to. The Durlston Country Park Information Centre is SZ 032773 and is easily found by driving from Swanage to Durlston Head. The eastern part of the stretch of coast discussed here is easily reached from here.

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GEOLOGICAL INTRODUCTION:

General Geological Succession

Comparison of Portland Group successions in the Isle of Portland and the Isle of Purbeck, Dorset, old scheme after Arkell

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.

Terminology of the Portland successions

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.

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GEOLOGICAL INTRODUCTION

Cliff Section - St. Aldhelm's Head to Durlston Head

A cliff section of the Portland Stone cliffs between St. Aldhelm' Head and Durlston Head, near Swanage, Isle of Purbeck, Dorset, including Dancing Ledge, Seacombe and Winspit

This cliff section is based on Davies (1935; 1956) and later work, including BGS maps. It shows the numerous north-south faults cutting the section and moderately displacing the strata in the cliffs. 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 (as at the western end of the Ragged Rocks). This Portland Sand is the uppermost part and usually consists of the Black Dolomites. Where this occurs large fallen blocks of uppermost Portland Sand and of Portland Cherty Series accumulate.

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STRATIGRAPHY

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PORTLAND STONE - STRATIGRAPHY:

The Ammonite Zones of the Portland Group

A shortened account, with emphasis on the Isle of Purbeck, is given here.

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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PORTLAND STONE - STRATIGRAPHY

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.

Origin of Portland and Purbeck facies

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.

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PORTLAND STONE - STRATIGRAPHY

The Portland Cherty Series

Chert in the 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.

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PORTLAND STONE - STRATIGRAPHY:

The Portland Freestone in the Eastern Isle of Purbeck

The Portland Succession in East Isle of Purbeck, Dorset, shown in a simplified succession

Portland Stone Succession - lateral changes

One diagram shows the general sequence of Portland Freestone and uppermost Cherty Series in the east of the Isle of Purbeck. Another diagram shows details of lateral variation in the limestones along the east Purbeck coast. The Under Freestone at the base of the Portland Freestone is the main unit that has been quarried along most of this stretch. Chert, however, increases towards the west in the lower part of the Portland Freestone and thus at St. Aldhelm's Head, it is the Pond Freestone that has been usually quarried. At the eastern end of this stretch of coast an interesting oyster reef is developed at Tilly Whim caves.

In terms of origin, the Portland Stone, the uppermost part of the Upper Jurassic Portland Group, is the result of a shallowing of the late Jurassic muddy seas (of the Kimmeridge Clay) to give way to clean white shoals of lime sand under shallow, clear, blue-green waters in a warm climate. In the Isle of Portland, this was an ooid sand producing an oolite, but in the Isle of Purbeck there were medium to coarse bioclast sands (both packstones and grainstones), producing a limestone that is not generally oolitic but may be rather similar in superficial appearance. It still has a granular surface and a light colour. It is harder and less porous than the oolite of the Isle of Portland.

As on Portland shell-beds are common in east Purbeck and there remains of the giant ammonite, Titanites. Good complete specimens are not common now, although I have long ago found a very large example near the base of the Shrimp Bed in the Cattle Trough area. Impressions or parts of ammonites can be seen at Dancing Ledge, Winspit and elsewhere.

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PORTLAND STONE - STRATIGRAPHY:

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 …tallon, 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]"

(Chert Member of the Portland Stone Formation or "Cherty Series" beneath)

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PORTLAND STONE - STRATIGRAPHY

Portland Stone of the Isle of Purbeck as a Building Stone

The Portland Stone from the east Purbeck cliffs is known as "Purbeck-Portland Stone". It is harder and more compact than that of Portland. The difference is largely because it is a calcarenite or shell-sand but not a true oolite and it is better cemented. The Portland Freestone at Tilly Whim may have been used for defences at Portsmouth. See the Tilly Whim section for more discussion of quarrying there. According to North (1930, p. 232) stone from here was used in building Corfe Castle. As mentioned in the "Localities" section, Dancing Ledge is said to have provided stone for Ramsgate sea defences. Seacombe Stone was suitable for landings, steps and in other situations for which normal Portland Stone would be insufficiently hard (North, 1930). At Winspit the Under Freestone was used for sinks and curbstones and the House Cap for breakwaters, it not being so workable as the other beds. The Titanites Bed (Blue Stone) was used for gate posts ( Woodward, 1895; Strahan, 1898). Purbeck-Portland Stone is worked at present at St. Aldhelm's Head and in Worth Quarry.

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GEOMORPHOLOGY

Data will be added.

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GEOMORPHOLOGY:

Prestwich's Raised Beach Theory

Prestwich raised beach map

To assist with the questions above, here is a map modified from a small part of a much larger map by Prestwich (1892). It is probably not totally correct. Some parts, however, seem to provide a sensible explanation for observed features. The original paper, although old, is a classic and should be read.

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LOCATIONS, EAST TO WEST:
FROM TILLY WHIM CAVES (NEAR ANVIL POINT) TO BLACKERS HOLE - 2.5 km.

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LOCATION:

Tilly Whim Caves (near Anvil Point)

Till Whim Caves, are old quarries in the cliff immediately to the east of Anvil Point and directly east the section of coast discussed here.

Please go to closely associated webpage at:

TILLY WHIM CAVES in the Durlston Head webpage.

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LOCATION:

Anvil Point - Introduction

See also:
Tilly Whim Caves in the Durlston Head Webpage. The Anvil Point exposures discussed here are directly adjacent and to the west of the Tilly Whim Caves, quarrying ledge.

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Portland strata and access location at Anvil Point, near Swanage, Isle of Purbeck, Dorset, 28 August 2007

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Dusk at the mouth of the dry hanging valley at Anvil Point, near Swanage, Dorset, November 2013.

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The Lower Ledge at the Southwestern End of the Tilly Whim Quarry

The southwest-northeast trending wall of Portland Chert Member of the Portland Stone Formation at Anvil Point, near Swanage, Dorset, with the lower ledge of Portland Chert Member

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Step-like ledges in the Portland Chert Member seen adjacent to the southwestern wall of Tilly Whim Caves, at the Anvil Point access location, near Swanage, Dorset, 12th December 2013

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Seawater running off the lower ledge in the Portland Chert Member at Anvil Point, near Swanage, Dorset

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A wave breaking against the wall of Portland Chert Member below the Tilly Whim quarrying ledge, and as seen from the Anvil Point, valley mouth, near Swanage, Dorset, 5th November 2000

Backwash of seawater from a breaking wave at the lower ledge, Anvil Point, near Swanage, Dorset, 5th November 2000

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(text to be added)

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LOCATION:

Anvil Point -

The Middle Ledges (Chert Member) with a Blowhole

Jointing and caves in the Portland Stone at the dry valley, Anvil Point, near Swanage, Dorset, as seen from the sea

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A view downslope towards the middle ledges of Portland Chert Member of the Portland Stone Formation at Anvil Point, near Swanage, Dorset, 4th September 2007

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A view from above the wall of Tilly Whim Caves towards the rocky ledges of Anvil Point, with the blowhole near the centre, Swanage, Dorset

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A view down onto the ledge with the blow-hole, at Anvil Point, near Swanage, Dorset, photograph 14th August 2009

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> A medium overview of the main blowhole, at the intersection of joints, and with beds labelled, at Anvil Point, near Swanage, Dorset, 11th December 2013

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The main ledge at the Anvil Point access locality, at the foot of the dry valley, near Swanage, Dorset, 11th December 2013

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The main blowhole in the a high ledge of Portland Chert Member at Anvil Point, near Swanage, Dorset, a closer view, 11th December 2013

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There is an obvious and notable blowhole, about half a metre in width, in the ledge at a middle level of the Portland Chert Member (Portland Cherty Series) at Anvil Point, where the dry valley descends to the coast. This blowhole is the result of enlargment of a joint system and is above a sea-cave. It is at present the most conspicuous blowhole. However, there are other ones nearer to the back cliff. One of these seems to have been covered up in recent years by a couple of rather rounded boulders. I may be wrong, but they looks as though they have been placed there artificially, perhaps, to remove risk of people falling in. I think that 50 or 60 years ago, this was the place where we used to view a fairly large blowhole. However, this cannot be confirmed because of the presence of the boulders now.

The blowhole shown in the photograph above is about 5m above the sea, and thus it may only operate as a true blowhole, with a blast of water and air, when the tide is high and the stormy sea conditions are severe. I have not been there at a time when it has been functioning.

A very sad accident happened here on the 3rd November, 2013. A lady was exploring the rocks with her brother. Both were washed into the sea. The man survived and got ashore. The lady was washed into the cave beneath the blowhole shown above, and did not survive. More details are in press reports, such as that of Marsden (2013) and Javinda (2013). Do not take risks on the sea ledges, which are easily accessible at the foot of the Anvil Point Valley, and at some other places.

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Anvil Point - Chert Diagenesis

Chert development around early carbonate nodules in the top of the Portland Chert Member, just west of Tilly Whim Cave, Anvil Point, near Swanage, Dorset, November 2013

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Anvil Point - Ammonites

Ammonite in the Prickle Bed, Portland Chert Member, at Anvil Point, near Swanage, Isle of Purbeck Dorset

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Giant ammonites in uncompressed form are present near the level of the Prickle Bed. The early cementation that this implies is of interest with regard to diagenesis. It is probable that the early cementation of the ammonites is related to the occurrence of carbonate nodules within, and therefore, earlier than the chert at this locality.

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LOCATION:

Ragged Rocks or Boulder Ruckle (Further West)

The Ragged Rock, at the western end of the Durlston Country Park, west of Anvil Point, near Swanage, Dorset

The western end of the Ragged Rocks or Boulder Ruckle, about 1 km west of Anvil Point, near Swanage, Isle of Purbeck, Dorset

The Ragged Rocks are a long stretch of large angular boulders at the foot of vertical cliffs west of Anvil Point. They consist of joint-bounded, Portland Stone debris. They are know to the rock climbers as "Boulder Ruckle" and details of many climbs here are given in the climbing guide books. The large angular rocks are particularly developed where the foot of the cliff is composed of uppermost Portland Sand. About a kilometre west of Anvil Point there is a major fault with a downthrow west of about 7.6 metres. From here westward the Prickle Bed, previously high in the cliff is now downthrown to just above sea level. This is "Amphitheatre Ledge" of the rock climbers.

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LOCATION:

Ragged Rocks - Alexandrovna Shipwreck

Apart from the geology and the rock climbing the stretch of the Ragged Rocks is also of other interest. This area is a site of wrecking of a sailing ship. The disaster to the Alexandrovna of Liverpool took place on the 29th April, 1882 during a hurricane which produced storm waves covering the sea with foam for hundreds of yards from the rocks. Rescuers ran along the cliff top to the spot but the ship was in pieces and all the crew were lost ( Legg, 1984). A few days later, the paddle steamer Empress (well-known for its numerous trips to Lulworth Cove over very many years) brought a large party from Bournemouth and Swanage round Durlston Head to see the remnants of the wreck. Some bruised and disfigured bodies were recovered.

Here is the full account of the Alexandrovna disaster at the Ragged Rocks, Swanage, as given by Robinson (1882).

Not many months after this description of a great storm at Tilly Whim had been penned, there actually occurred a catastrophe of the terrible nature imagined in the text; and happening as it did by day instead of by night, the event is not left to conjecture. The weather of Saturday, April 29, 1882, will long be present to the recollection of my readers living in the south of England. In the morning a fresh breeze and a falling barometer presaged a gale; but not even experienced seamen were led to expect such a hurricane as began suddenly to blow in the afternoon. To mention but two evidences of this, an emigrant ship with more than three hundred souls on board started earlier in the day from Plymouth (an act which would have been but courting destruction had her captain any means of forecasting the storm which soon burst upon him); and the great ironclad frigate Warrior was caught in the open Channel and shipped sea after sea, so that even her lofty decks are said to have been four times flooded with water. Both these vessels escaped fortunately unharmed; but many others were missing when the gale had subsided; and one of them was the Alexandrovna of Liverpool, a fine Scotch-built sailing ship of no less than 1250 tons.

Towards four o'clock that Saturday afternoon a large vessel was descried, approaching the shore in an apparently disabled state, by two persons who chanced to be spending their half-holiday on the cliff near Tilly Whim, watching the fury of the gale. It seemed as if she had been lying to, for her topsails were in ribbands, and she had but one staysail set. No crew could be seen, not even a man at the wheel, the atmosphere being too thick; and no endeavour to keep the vessel off the land was perceivable. The wind was then blowing almost directly on shore. In a very few minutes the fated ship was among the broken billows, which covered the sea with foam for hundreds of yards from the rocks. The two spectators at first made the best of their way in the teeth of the fierce wind towards the spot where she appeared likely to strike; but when she was clearly doomed, one ran back to Swanage, a distance of nearly two miles, to warn the coastguard there. Meanwhile the men at the St. Aldhelm's Head station, who had from a distance seen the peril in which the ship was involved, were hurrying down with their life-saving appliances. The vessel almost immediately struck with fearful force upon the perpendicular cliff, about half a mile to the westward of Tilly Whim. In ten minutes or less, before anyone could reach the spot, she was a mass of fragments, which had already begun to float away with the tide towards Swanage! Nothing was seen of the crew that day, nor could the coastguard effect anything when they arrived with the rocket apparatus; but a few feet of one of the topmasts, having still attached to it the yellow-painted truck, through which the signal halliards are rove, was found lying on the green slope above the cliff, showing that in the tremendous shock the mast must have been almost flung out of the vessel. Next day a life-buoy, having upon it the name Alexandrovna, was picked up ; and large pieces of wreckage were landed at Swanage and other parts of the coast. When the name of the unfortunate vessel was thus made known, it was ascertained that her crew consisted of more than twenty hands, every man of whom perished. Many of their bodies were afterwards found, either jammed in among the rocks, or floating in the waters of the channel, most of them bearing marks of frightful injuries-inflicted, it is to be hoped, after death. Those found near Swanage were buried in the parish churchyard.

The phenomenal violence of this gale may be judged from the fact that seasalt is recorded to have been blown by it more than a hundred miles inland; and that it completely stripped all the trees in exposed situations on the coast of their young green leaves, which the spring had just brought out. The elms at Swanage were not covered with leaf again till past midsummer.

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LOCATION:

The Amphitheatres, Cattle Trough Climbing Area
(just over a kilometre west of Anvil Point)

The amphitheatres or half-funnel shaped features in the Portland Stone cliffs, seen from a boat just over a kilometre west of Anvil Point,  near Swanage, Isle of Purbeck, Dorset, 2007

Details of an amphitheatres or half-funnel shaped feature in the Portland Stone cliffs, in the Cattle Trough - amphitheatre area west of Anvil Point,  near Swanage, Isle of Purbeck, Dorset, August 2007

Aerial photograph, small-scale of the Dancing Ledge, Seacombe and Blackers Hole coast near Swanage, Isle of Purbeck, Dorset, courtesy of the Channel Coastal Observatory

A general view of a rejuvenated landsliding coast of Purbeck marls and limestones above Portland Stone, the Cattle Trough climbing area, west of Anvil Point, Swanage, Dorset, aerial photograph courtesy of the Channel Coastal Observatory

Two of the amphitheatres in the Cattle Trough climbing area, west of Anvil Point, Swanage, Dorset, aerial photograph courtesy of the Channel Coastal Observatory

This is an area of at least three remarkable, semi-circular, erosional features in the cliffs of the Portland Freestone. These of an amphitheatre-like shape and eroded in their upper part into the Shrimp Bed. They are very unusual because most other coastal features in these cliffs are angular and controlled by joints. From the aerial photographs, courtesy of the The Channel Coastal Observatory , you can see that there are joints here and that they are very oblique to the coast trending NW-SE. In places they influence the erosion, but most of the curvature is not joint-controlled. The three features have been named by climbers, from east to west, as Amphitheatre (or Half Moon), The Pulpit and the Lecture Theatre. The slopes above them are rather irregular with small landslides. There are some small accumulations of loose rock.

These amphitheatres have been a mystery to me since I explored them in the 1950s. At that time aerial photographs of this coast were not available. Now, if you examine the aerial photographs above, courtesy of the Channel Coastal Observatory, you can see that they are situated within a landsliding coast has been rejuvenated (i.e. has had a new phase of landsliding at a site of ancient landsliding). The cliffs have cut back and a new, steeper slope with fresh landslides has developed to the south of the main slope. The amphitheatres see to have a relationship in terms of distribution to the area of landslides. Their peculiar curved shapes (like half a funnel) are very difficult to explain, though. Of course, the back slip-planes of landslides are curved. However they are usually fairly broad and the angle, in relation to the vertical, becomes less steep at depth and may flatten out completely (or even swing upwards. Thus the the amphitheatres might be relics of old landslides which penetrated the Portland Stone. However, they do not have exactly the form which might be expected. So far there is no proof of actual landslide movement within them.

Whatever their precise origin, the amphitheatres could have developed at a time of lower sea-level in the Pleistocene. It is very likely that they are related to the old Interglacial cliff line, which by comparison with the Isle of Portland, should have been just to the south of here (for more on this topic see Prestwich, 1892 and Brown (2000)).

The origin of the amphitheatres cannot be considered to be solved. They might be the relics of some type of late Pleistocene landsliding. They might be the remains of some type of stone chute, related to ancient cliff failure. They could be the result of some unusual type of wave erosion. The problem of the origin of the amphitheatres still remains unsolved. They are an interesting mystery!

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LOCATION:

Fault, High Cave and Possible Calcitisation;
East of Fishermans Ledge.

Fault, a high cave, and possible enhanced calcitisation of the Caps and Broken Beds, east of Fishermans Ledge, west of Anvil Point, near Swanage, Dorset, Jurassic Coast, 2007

Here, near the mile marker pylons is a peculiar place. There is a sea cave, normal for this area. However, there is also a high cave at a fault plane. Caves that are karstic and not sea caves are rare in this area, although there is a small one at Dancing Ledge. The feature shown has not been investigated. It does not look very deep, but I do not know. The fault here is very steeply dipping to the east, and is normal and thus, extensional. There are associated antithetic faults and possible a synthetic fault a little further east. The throw is small, as can be seen by the displacement of the Prickle Bed.

At the cliff top, above the fault there is a local anomalous area where the basal Purbeck Caps of replaced anhydrite are very strongly lithified. So too are the Broken Beds above. This is seen from the exposure in the grass above the cliff. However, I have not been to the cliff edge here and do not know whether it is accessible to the non-climber. Thus I cannot confirm the characteristics of this well-lithified patch. It is reminiscent, however, of the local area of strongly calcitised evaporites in the Caps and Broken Beds at Durlston Head celestite exposure. There there has been strontium mineralisation in addition to strong calcitisation. This site, above the high cave and the fault needs investigation.

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LOCATION:

Fisherman's Ledge and Conner Cove - Introduction
(Purbeck Cliff-top Exposure, West of the Measured Mile - look for the pylons)

Basal Purbeck limestones, just east of Fisherman's Ledge promontory and near the mile post indicator, west of Anvil Point, near Swanage, Dorset, Jurassic Coast, 2007

Aerial photograph, small-scale of the Dancing Ledge, Seacombe and Blackers Hole coast near Swanage, Isle of Purbeck, Dorset, Jurassic Coast, courtesy of the Channel Coastal Observatory

Conner Cove and Fisherman's Ledge, near Swanage, Isle of Purbeck, Dorset, aerial photograph

A view of the cliffs east of Dancing Ledge, near Swanage, Isle of Purbeck, Dorset, showing Blackers Hole, Connor Cove and Fisherman's Ledge, telephoto, 1st January 2013

The aerial photographs above and the cliff photograph from Winspit show the location and topographic details of Conner Cove and Fisherman's Ledge. It is a lower part of the cliff, with a fault at the east end and evidence of substantial Devensian landslide down to the former Ipswichian raised beach here which has since been destroyed by sea erosion. Conner Cove and Fisherman's Ledge are places where storm waves from the southwest hit a vertical cliff sending sea spray over the top. This causes erosion of Purbeck debris, and in turn initiates small modern landslides in a seaward direction. It has some similarities, as a storm-eroded cliff ledge, to the Fossil Forest Ledge, near Lulworth Cove.

A normal fault at Fisherman' Ledge and Conner Cove, near Swanage, Dorset, seen from the sea in 2007

Draping of anhydrite, since calcitised, over brittle-fractured Portland Freestone at the Fisherman's Ledge Fault east of Dancing Ledge, near Swanage, Isle of Purbeck, Dorset

The photographs show a cliff top exposure of the Purbeck Formation between Dancing Ledge and Anvil Point is conveniently marked by white pylons at the western end of the measured mile used for determining the speeds of ships. This is a rather hazardous cliff edge exposure. The location, nearly 2km west of Anvil Point, is known to the rock climbers as Fisherman's Ledge (Coe, 1998) and Conner Cove (to the west). It was reported by Strahan (1898) as: "Lower Purbeck Broken Beds, resting on tufa with "burrs", between which and the Portland Stone lies half and inch of brown earth or "Dirt"."

It was referred to in West (1975), in a paper describing the basal Purbeck evaporitic strata, as a ledge "West of Anvil Point" with the map reference SZ 011768. It is about half a kilometre east of Blackers Hole (a notable sea cave). The lower and vertical part of the cliff is of Portland Stone downthrown to the west (left) by a normal north-south fault, which can be clearly seen just right of the centre of the photograph. The ledge at sea-level in the eastern part of the image, where some white foam is breaking, is probably the Prickle Bed (a useful marker horizon in the Cherty Series). The basal Purbeck strata are clearly laminated (see the diagram in West (1975) for the succession. Above them is a slope of debris from higher parts of the Purbeck Formation. The basal Purbeck limestones and calcitised evaporites may be studied on the rough ledge above the vertical Portland wall, but the place is too risky to be recommended for general use.

The fault at the eastern end of Fisherman's Ledge is important. You can examine it from the sea (see photo). From the coast path it might be possible scramble down to the sloping ledge at the top of it and reach the Basal Purbeck strata, but much care needs to be taken and probably a safety rope is needed.

The fault is a north-south trending extensional fault (normal fault) like so many on this stretch of Portland Stone coast and those of the Kimmeridge coast. There is little doubt that such faults are the result of the Late Kimmerian extensional stresses that occurred as the North Atlantic was in the early stages of opening. Such faults are quite different from the compressional faults that are associated with the later Alpine (Tertiary) tectonic movements. This south Purbeck coast is a good area for studing the differences.

The extensional faulting at Durlston Head and associated celestite and calcitised evaporites have been a matter of interest to the present author since the the 1960s (see West (1960); Salter and West, 1965; West (1975)). The Fisherman's Ledge fault provides evidence of dating by analogy. The small scale step-faulting of the brittle Portland Stone is also present at Durlston Head. So too is the ductile draping and flowage of anhydrite (easily seen in thin-section as pseudomorphs).

If, as clearly seems to be the case, the two faults are similar and of similar origin, then movement of the Broken Beds, the evaporite breccia can now be dated as early as Late Kimmerian (i.e. Cretaceous - round about Aptian). Thus some, at least of the Broken Beds movement is quite early, and well prior to Tertiary tectonics.

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LOCATION:

Conner Cove (Fisherman's Ledge) - Basal Purbeck Strata

Basal Purbeck thrombolites on a cliff edge at Conner Cove and Fisherman's Ledge, between Anvil Point and Blackers Hole, near Swanage, Dorset, Jurassic Coast, 2007

Above the Fisherman's Ledge Fault, west of Anvil Point, near Swanage, Isle of Purbeck, Dorset

Thrombolites (

Basal Purbeck, Lulworth Formation, diagram showing correlation of strata in the Anvil Point to Blackers Hole clifs and adjacent region, near Swanage, Dorset, 2013

Here, thrombolites (or spongiostromata-type stromatolites) and the Lower Dirt Bed lie above the Portland Stone. Above this is much calcitised anhydrite. In the field this appears to be a crumbly and very porous limestone without fossils. The calcitisised anhydrite is discussed further below.

Silicified, Purbeck, coniferous tree remains found ex situ at Fisherman's Ledge (Conner Cove) between Anvil Point and Dancing Ledge, Isle of Purbeck, Dorset, and probably from the Middle Purbecks

Closely-spaced tree rings in part of a silificied tree trunk  found, ex situ, at Fisherman's Ledge or Conner Cove, west of Anvil Point, near Swanage, Dorset

Many years ago I found here the well-preserved, silicified remains of a coniferous tree, the rings of which have been described by (Francis, 1984). This tree has the best-preserved tree rings. The block of tree trunk was found loose, ex situ, on this ledge in the 1960s. The exact horizon from which it has come has not been determined, but no Great Dirt Bed seems to be present here. It could have come from the Lower Dirt Bed, but it is more likely that it is derived from the Middle Purbecks where tree remains are well developed in the Isle of Purbeck. It has probably come down the slope with landslide debris, which includes Cherty Freshwater limestone and fossils. Similar tree remains also occur higher in the Lower Purbeck of Bacon Hole ( Ensom ) and also in the Middle Purbecks of Durlston Bay, just above the Cinder Bed and also in the Purbeck Middle Purbeck limestone quarries. The tree rings in the Fisherman's Ledge or Conner Cove specimen are extremely closely spaced. This is compatible with an arid to semi-arid environment (Francis, 1984). That tends to suggest Lower rather than Middle Purbecks, but the palaeoclimatic change was not sudden nor consistent (and rare thrombolites occur quite high in the Middle Purbecks). Anyway no fossil trees have yet been found in situ in the basal Purbecks of the Isle of Purbeck (and the Great Dirt Bed is not even present here).

The Caps and Broken Beds here are strongly evaporitic. They consist of anhydrite replaced by calcite with some chert. Good pseudomorphs after anhydrite can be seen in the chert in thin-section (dilute HF etching helps). The Broken Beds seem less obviously brecciated than at Lulworth. Notice in the photograph how the anhydrite beds, now replaced by carbonate, drape over the fault. Underground, as seen in boreholes on the Isle of Wight and elsewhere this anhydrite horizon persists, unreplaced.

Amongst the debris of higher Purbeck horizons here is material from the Cherty Freshwater Member containing good remains of low-salinity gastropods. Ostracods from the Lower Purbecks, debris of the Cinder Bed with oysters and other fossil material is present.

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LOCATION:

Conner Cove (Fisherman's Ledge) - Calcitised Anhydrite

Cliff edge exposure of basal Purbeck calcitised evaporites above a thrombolite bed at Conner Cove, west of Anvil Point, near Swanage, Dorset, Jurassic Coast, 2007

The evaporitic basal Purbeck strata at Fisherman's Ledge and Conner Cove, on the cliff top between Anvil Point and Dancing Ledge, near Swanage, Isle of Purbeck, Dorset, with Adam Mears and another student in 1994

Calcitised anhydrite above a thrombolite bed in the basal Purbeck sequence at Conner Cove (Fisherman's Ledge) west of Anvil Point, near Swanage, Dorset, Jurassic Coast, after Mears, 1995

Broken Beds breccia over calcitised anhydrite, Fisherman's Ledge end of Conner Cove, west of Anvil Point, near Swanage, Dorset, Jurassic Coast, 2007

Soft, crumbly, calcitised anhydrite beds, at Fisherman's Ledge, Conner Cove, west of Anvil Point, near Swanage, Dorset, Jurassic Coast

Calcitised laminated anhydrite seen at Conner Cove, near Fisherman's Ledge, west of Anvil Point, near Swanage, Dorset, Jurassic Coast

Calcitised lath anhydrite from the basal Purbeck Formation at Fisherman's Ledge, west of Anvil Point, near Swanage, Dorset

A quartzine replacement of anhydrite in chert from the basal Purbeck evaporites of Fisherman's Ledge, west of Anvil Point, near Swanage, Dorset

Chert after anhydrite in thin-section from the basal Purbeck evaporites of Fisherman's Ledge and from Durlston Head, Swanage, Dorset

It is well known that evaporites are developed at the base of the Purbeck Formation in southern England. Underground, as in the Arreton Borehole of the Isle of Wight they are present as anhydrite, sometimes hydrated to secondary gypsum. This gypsum is mined in Sussex at the Brightling and Mountfield Mines. The evaporites have been calcitised (replaced by calcite), with some replacive chert, in Dorset and their distribution and origin have been discussed by West (1975, and other papers).

At Conner Cove near Fisherman's Ledge there are quite good exposure of crumbly secondary limestones formed by the replacement of laminated anhydrite. Some investigation of these has been made by West (1975) and Mears (1995). (For those who find this cliff edge exposure a little uncomfortable for detailed study, note that very similar calcitised evaporites, but with celestite, can be accessed more easily and more safely at Durlston Head.)

Some chert nodules replacing anhydrite also occur in the basal Purbeck evaporite beds here, as shown in a photomicrograph above. This is in contrast to the chert nodules of the evaporite beds of the Lulworth area which are after gypsum. Mears (1995) examined thin-sections with a sensitive tint plate (a standard accessory plate of a polarising microscope for determining the fast and slow directions of the speed of light in a crystal). The crystals proved to be length slow, whereas most chalcedonic quartz is length fast. Thus the chalcedony is quartzine, which occurs mainly in evaporites. Most Purbeck evaporites also contain the oblique-extinguishing variety of chalcedony - lutecite.

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The distribution of carbon 13 and oxygen 18 in the Caps and Broken Beds at Durlston Head, Dorset, and at Fisherman's Ledge or Conner Cove, west of Anvil Point, near Swanage, Dorset

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Footnote: Climbs at Fisherman's Ledge.

The cave on the right hand side of the photograph is, I assume, the Palace of the Brine, located near the pylon. Various rock climbs here include: Impending Gleam, Ringworm, Sue's Route, Aquascrotum 2, Limited Edition, Crackers, Squid, The Ritz, Paternoster, Clever Dick, Captain Blood's Cavern. The climbing guide comments: "Compact and sometime pocketed rock in Connor Cove, overhangs aplenty in the main area, and some ludicrous propositions in the eastern cove ensure that Fisherman's Ledge ranks as a major Swanage attraction. Although lacking the height of the main cliffs, the majority of the climbs here are on excellent rock and have sound, clean cut finishes. The sport of cliff-jumping has enjoyed a renaissance here, sometimes involuntarily combined with deep-water soloing." The climbers have many strange names for their climbs here, such as "Whack your Porcupine" and "Rufty Tufty Vivisects his Mummy"!

For more information see the rock climbing guide of Coe (1995) and other information in the St. Aldhelm's Head to Durlston Head Bibliography.
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LOCATION:

Blackers Hole Cave - Sea-Eroded Fault Zone

A view of the Portland Stone coast around Dancing Ledge and Blackers Hole, near Swanage, Isle of Purbeck, Dorset, from Winspit in 2007

Blackers Hole cave east of Dancing Ledge, near Swanage, Isle of Purbeck, Dorset, 28 Aug 2007, seen in the distance to the northeast

Details of Intra-Cretaceous (Late Kimmerian) faulting at Blackers Hole, west of Anvil Point, near Swanage, Isle of Purbeck, Dorset, 28 August 2007

Blackers Hole is one of the largest sea caves on the stretch of coast between Anvil Point and St. Aldhelm's Head. It is the consequence of extensional faulting. There are two major faults which trend obliquely to the coast in direction roughly NE-SW. Both are normal, with downthrows outward from the location of the cave; i.e. the cave is at a small horst.

The main fault is the southeastern one which downthrows the Prickle Bed to near sea-level on the southeastern side. It hades to the southeast. There an antithetic fault on the northwestern side of the cave. There is a surprisingly low angle, southeast hading, synthetic fault with a small displacement in the roof of the cave. At first sight the relationship of the basal Purbeck seems strange. They are draped not faulted! This is a similar circumstance to that at Fisherman's Ledge, where the draping is directly accessible to the geologist. The significance is clear. The Purbeck anhydrite drapes rather than shears, provided the displacement is not too great. With major vertical displacement it would, of course, fracture. At Durlston Head anhydrite, now replaced by calcite and celestite, has behaved with both plastic deformation (a downbulge) and with fracture (faulting). The potential economic application of this is that providing the fault has a small throw of probably not more than 5 to 10m the anhydrite may seal the fault completely. If the throw is greater there is potential upward path for transmission of fluids. In practical terms the anhydrite can act as a seal over small faults and may prevent escape of hydrocarbons (oil or gas). Larger faults may facilitate upward escape. There might be an implication with regard to gas fracking (hydraulic fracturing) of the Kimmeridge oil shale. Kimmeridge Clay extensional faults on the southeast Dorset coast are very similar to those in the Portland Stone. It is probable that these extensional faults continue up from the Kimmeridge Clay into the Portland Stone, which is then overlain by basal Purbeck evaporites. However, this is merely a local observation with regard to these Dorset coastal exposures. The situation is not necessarily the same in another structural basin such as the Weald basin.

Not followed in detail here is the interesting question of whether the local extensional faulting is Late Cimmerian (i.e. early Cretaceous stretching prior to the opening of the Atlantic Ocean). This outcrop of Portland Stone originated off Nova Scotia, shortly before the North Atlantic was formed. However, caution is needed because, although it seems unlikely in a compression phase, Tertiary tectonics cannot be completely ruled out.

[Footnote: Antithetic fault. Defined by Cloos (1928) and Hills (1940, fig. 41) as a minor fault that dips in the opposite direction to the dip direction of the beds they displace. Antithetic fault is now commonly used for a fault that dips in the opposite direction to a related dominant fault or fault set. From: Peacock, D.C.P., Knipe, R.J. and Sanderson, D.J. 2000. Glossary of Normal Faults. Journal of Structural Geology, 22, 291-305.]

The collapse of the cave after undercutting by the sea is because the limits are the fault planes and no strata below the Purbeck continues across these planes. Thus there is no lateral strength. The southern fault, the largest has, as shown, one obvous synthetic fault. However, there might have been others at a lower level. Collapse of these would have resulted in upward expansion of the cave. Another factor to be considered is that the cave is open to, and faces, the southwesterly storm waves.

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LOCATION:

Blackers Hole Quarry

Old quarry in the Portland Freestone at Blackers Hole, west of Anvil Point, near Swanage, Isle of Purbeck, Dorset, Jurassic Coast, 2007

Details of the old quarry immediately east of Blackers Hole, near Swanage, Dorset, showing thrombolites in the Hard Cap of the basal Purbeck - enlarged view

An ammonite, Titanites sp., from the Shrimp Bed of Blacker's Hole, west of Anvil Point, Swanage, Dorset, collected by Ian West

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Blackers Hole - to the west - Water Drainage

A run-off gully with infiltration into the Portland Stone, just west of Blackers Hole, near Swanage, Dorset, Jurassic Coast, 2007

Here is seen two run-off gullies, one of them much better developed than the other. They are descending from the Purbeck outcrop (Lulworth Formation). The upper part of these has not been studied, so it is not known whether they commence as springs in the Purbeck outcrop. This does happen elsewhere in the area as at Dancing Ledge. Within the Purbeck Group there are alternating limestone and mudstone units, so there bound to be small perched aquifers.

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GLOBAL WARMING

The Future Sea-Level Rise on this Coast

The stretch of coast from Durlston Head to St. Aldhelm's Head can be considered briefly in terms of relative sea-level rise over the next hundred years, although the information is sketchy. Most estimates of rise of sea level for this interval vary from about half a metre to about 5 metres. There is some additional local sinkage here due to isostatic rebound which makes the minimum realistic figure near 1 metre.

Whether the rise is small or large, this stone coast will be very little affected. The Portland Stone generally erodes at present at somewhere near 5 or 10 cm per annum. Thus only about 10 metres loss might be expected in normal conditions here in 100 years. A substantial rise in sea level might increase this to about 100 metres or even 200 metres. Caves, like Blackers Hole, will collapse and new caves will be formed. Dancing Ledge will go and there will probably be some minor advance of the sea into the mouths of the small valleys, but this may only produce very small coves at worst. The grassy, south-facing slopes on the Purbeck Formation which descend from the high plateau will begin to landslide on an appreciable scale. This has happened in the past (note the hummocky ground) and will happen again. The collapsing slopes will be of brown, clay and limestone debris with little vegetation.

In general this is one of the most resistant coasts in the region. It is a hard and high coast at the other extreme from the low ground of Weymouth and Sandbanks (which would get serious flooding during storms from a small rise and complete destruction with a five metre rise). The high and stony ground of Langton Matravers, Acton, Worth Matravers and Kingston etc may become popular places for safe housing development. On the upland, well away from the cliffs, there is little of the risks from flood and erosion that may affect low sea front land and cliff top areas. Of course the details of future sea-level rise are very much a matter for dispute and not at all certain; the reader should follow some of the relevant literature, which is rapidly increasing in quantity!

For more information see, for example, the following:

Environment: Sea special report: NASA's James Hansen on the IPCC forecast. 16 Aug 2007.
"By and large, critics of the IPCC fall into the sceptics camp. Not so NASA climate expert James Hansen, who claims the panelís predictions of rising sea levels are a stark underestimate. Professor John Shepherd explains."

IPCC - Intergovernmental Panel on Climate Change. A series of technical papers by panels of distinguished scientists on various aspects of climate change (Global Warming etc), including the rise in sea level. Papers can be downloaded as pdf files.

Melting glaciers will dominate sea-level rise. by Phil. McKenna. New Scientist News Service. 19:00 19 July 2007.

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ACKNOWLEDGEMENTS

I am very grateful to Ruth Brown for discussion in the field while she was studying the area for a Southampton University undergraduate project. The Fisherman's Ledge or Connor Cove cliff area was investigated in detail by Adam Mears in 1994-5, and his very good work is referred to here. I thank the members of the Probus society of Mudeford for their company on a boat trip to view this part of the coast. I am grateful to the help on Fisherman's Ledge of friends in the early 1960s, including Peter Roberts, Sally Liney, the late Arthur Waite, and Shirley Waite. My sister Dawn climbed down to the Blackers Hole quarry with me. Some early work here was with the late Valerie Bristol and this is certainly not forgotten.

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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 cancel 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.

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Dr Ian West, author of these webpages

Webpage - written and produced by:


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

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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.