West, Ian M. 2016. Durlston Head, Dorset - Geology of the Wessex Coast (including the UNESCO Dorset and East Devon World Heritage Coast - Jurassic Coast). Internet site: www.southampton.ac.uk/~imw/Durlston-Head.htm. Version: 25th February 2017.
Durlston Head, Dorset -  Lower Purbeck and Portland Freestone Formations - geological field guide

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 and bathymetry images by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.
Website archived at the British Library

..| Home and List of Webpages |Field Guide Introduction and Maps | Durlston Bay - start | Durlston Bay -Lower Purbeck | Durlston Bay Bibliography | Purbeck Group Bibliography | Purbeck Palaeoenvironments | | 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 | Durlston Head, Isle of Purbeck |St. Aldhelm's Hd to Anvil Point - Geological Bibliography |Durlston Bay - Peveril Point, Upper Purbeck Group |Durlston Bay, Middle Purbeck Durlston Bay - Lower Purbeck |Durlston Bay - Central Zigzag Part & Coast Erosion |Durlston Head - Lower Purbeck Group & Portland Stone |Petroleum Geology, South of England. |Durlston Bay - Bibliography |Paper - Evaporites and Associated Sediments of the Basal Purbeck Group (Upper Jurassic) of Dorset, by Ian M. West, 1975.

Click here for the full LIST OF WEBPAGES

(You can download this educational site to SurfOffline or similar software to keep an offline copy, but note that updating of the live version takes place periodically.)

See also the closely related Anvil Point to Blackers Hole webpage

Durlston Head, Dorset, aerial photograph courtesy of the Channel Coastal Observatory

Durlston Head, and Durlston Castle, Dorset, photographed from the Paddle Steamer Waverley by Dr Clive Boulter on 19 September 1997

This is part of a series of webpages on Durlston Bay and adjacent area.

Durlston Bay, - Upper Purbeck Group
Durlston Bay - Middle Purbeck (Cretaceous)
Durlston Bay - Lower Purbeck (Jurassic-Cretaceous)
Durlston Bay - Central Zigzag Part and Coast Erosion
Durlston Head - Lower Purbeck Group & Portland Stone
Anvil Point to Blackers Hole
Durlston Bay - Bibliography

Some other Dorset Geological Field Guides

Lulworth Cove, Dorset, Jurassic-Cretaceous
Isle of Portland, Dorset, Upper Jurassic
Lyme Regis, Dorset
Mupe Bay, Jurassic-Cretaceous
Studland and South Haven Peninsula
West Bay, Bridport
Worbarrow Tout and Worbarrow Bay

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CONTENTS LIST FOR THIS WEBPAGE (in preparation)

DUR-1. INTRODUCTION - Arriving at Durlston Head
DUR-2. INTROUCTION - Location and Initial Information
DUR-3. INTRODUCTION - Durlston Castle
DUR 4. INTRODUCTION, GEOLOGICAL - Geological and Bathymetric Maps
DUR 5. INTRODUCTION, GEOLOGICAL - Cliff Section, Westward
DUR 6. [contents data to be added]
DUR 7.
DUR 8.
DUR 9.
DUR 10.
DUR 11.
DUR 12.
DUR 13.
DUR 14.
DUR 15.
DUR 16a.
DUR 16b.
DUR 17.
DUR 18.
DUR 19.
DUR 20.
DUR 21.
DUR 22.
DUR 23.
DUR 24.
DUR 25.
DUR 26.
DUR 27.
DUR 28.
DUR 29. ACKNOWLEDGEMENTS£
DUR 30. REFERENCES£

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DUR-1 INTRODUCTION:

Arriving at Durlston Head

You can park a car at Durlston Head (pay and display). Alternatively you can walk along the cliff top from Swanage or Peveril Point to Durlston Head. Seen from a boat trip from Swanage, notice Durlston Castle, the Great Globe, the Portland Stone cliffs and several faults in the limestone.

Visitors to the Great Globe at Durlston Castle, Durlston Head, Dorset, with visitors of 1887 and 1998

The Great Globe has been carved out of Portland Stone in Victorian times. There has been little change in the scenery here from a hundred years back, apart from abundant growth of the wind-blown Holm Oaks. You can obtain something of the Victorian views from the notices and poems carved in stone slabs.

For more information about this globe see Lewer and Smale (1994) . If you look carefully at the globe (in the field) you may notice some mistakes.

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DUR-2 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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DUR-3 INTRODUCTION:

Durlston Castle

[section to be added]

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DUR-4 INTRODUCTION, GEOLOGICAL:

Geological and Bathymetric Maps

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Geological map, onshore and offshore of the east Dorset coast, including Durlston Head, completely redrawn and simplified and with additional notes, but based on BGS offshore maps

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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

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

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

Multibeam bathymetry of the sea floor south of the Isle of Purbeck from Durlston Head to St. Aldhelm's Head, Dorset, courtesy of the Channel Coastal Observatory

Shown above are geological maps of different dates, including one based on an 1895 edition of the Geological Survey map. A multibeam bathymetric map with some sea-floor geology is also shown. These maps provide an initial 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.

The most suitable topographic map for the area is the Ordnance Survey, Outdoor Leisure Sheet 15, Purbeck and South Dorset. Scale 1:25,000. For detailed topographic information, particularly names of coastal features, see also the map of Anderson, and the rocks climbers maps in Crewe (1977) and Coe (undated).

Melville and Freshney (1982) have provided a good brief introduction to the area, that is less detailed and technical than Arkell's memoir. The field guide by House (1993) is recommended.

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INTRODUCTION, GEOLOGICAL:

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

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

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STRATIGRAPHY - INTRODUCTION:

Basal Purbeck Stratigraphy

The petrographic lithology of the basal Purbeck Group in Dorset shown in a series of graphic logs

Distribution of replaced evaporites and lithology in the basal Purbeck strata of Dorset, redrawn version, showing detail but not palaeosalinity - Redrawn

The diagrams above, after West (1975) shows the basal Purbeck stratigraphy at Durlston Head, and, in particular, the extensive development of calcitised anhydrite in the cargneule (or rauhwacke). This is a brecciated decollement horizon on evaporites and has the historic name - "Broken Beds". The units of the basal Purbeck strata will now be considered in descending order from the Cypris Freestones Member. This section differs from those further west not only in possessing a greater thickness of evaporites (with celestite) but also in lacking the palaeosols of the shelf area (Lulworth). Thrombolites (stromatolites) are confined to the very basal part of the sequence and silicified trees have not been found here.

Revised diagram showing petrographic facies and inferred palaeosalinity for basal Purbeck evaporites and associated basal Purbeck strata, including the Broken Beds breccia, Dorset

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PURBECK STRATIGRAPHY:

"Cypris" Freestones Member

Cypris Freestone Member Log - modified from Clements, 1969, 1993)

This consists predominantly of ripple-lamined ostracodal and pelletoidal limestones. It mostly originated in water of about twice sea-water salinity (West, 1975). In the dry summers (the climate was very seasonal) the lagoon sometimes partially dried up leaving halite crystals in the fine carbonate sand flats. These were later dissolved away and filled with sediment so that now natural casts are preserved.

Archaeoniscus brodiei

The isopod ("sea-slater") Archaeoniscus brodiei occurs at certain horizons. A conspicuous 1.3 metre marlstone, the Archaeoniscus Bed (DB 11), lies above the Broken Beds and is easily seen at Durlston Head. Archaeoniscus was recorded in this by Fisher (1856) and it also occurs in the overlying shales.

Dragonfly larvae are uncommon features of this member (Jarzembowki and Coram, 1997).

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PURBECK STRATIGRAPHY:

Broken Beds Member (Cargneule)

The Broken Beds were formerly visible in Durlston Bay at two localities; just north of the Zizag Path and at Durlston Head. The northern locality formerly showed the ostracodal and pelletoidal limestone facies of the Upper Broken Beds. There are brecciated, thin-bedded limestones similar to those of the "Cypris" Freestone Member. The lower and main part of the Broken Beds at Durlston Bay consist of calcitised evaporites, often very porous and with some celestite, an insoluble residue of evaporite diagenesis. This evaporitic part was not seen in the northern exposure, being below beach level and cut off by a fault. The sea defences which have been built to protect the block of flats on the cliff edge have largely obscured the Broken Bed exposure in the north of the bay

In the southern part of the bay there are good exposures at Durlston Head and just to the north of the head. It is necessary to know the correct footpath to get access to the beach here (and it could dangerous to take the wrong one), unless one undertakes a long walk over the boulders of the shore from the Zizag Path. The location which has to be reached is just north of the normal fault downthrowing the Purbecks against the Portland Stone and clearly shown on the right hand side of the photograph here. The footpath is a hundred metres or so to the right (north) of this; you are warned that it can be a short rough scramble or climb which varies because of cliff collapse, and can be accessed from north of the Great Globe. Another longer path from a bridge further north is sometimes possible, but access on either is not easy and not recommended for a large party.

At the foot of the cliffs, close to the faulting (which is obvious) there is an interesting structure. The evaporitic basal Purbeck strata have been squeezed down in a "downbulge" and displaced northward to some extent resulting in an increased thickness of Broken Beds near the Jagged Rock (easily recognised). This is an important exposure of Purbeck evaporitic strata. Some photographs are given below and text details will be added.

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PURBECK STRATIGRAPHY:

Caps Member (Basal Purbeck Group)

The Caps at Durlston Head are mostly evaporitic, but have some stromatolitic limestone at the base. The Great Dirt Bed is missing and no tree remains have been found. The Lower Celestite Bed is part of the Caps. The diagram above gives the succession. For more detail see West (1960), Salter and West (1965) and West (1975).

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LOCATION:

Durlston Downbulge (a structure in the Basal Purbecks Cargneule or former evaporite breccia)

Durlston Head, Dorset, seen across the bay from north, with an oblique view of the area of complex faulting and of celestite occurrence, August 2009

The "Durlston Downbulge" is a small east-west graben, which has squeezed out and bulged down into the basal Purbeck anhydrite (now calcitised). It is very similar to downbulges (or down-bulges) in the Permian anhydrite of the Billingham Mine, County Durham (Raymond, 1960, figs 5-11). It is therefore named here as the Durlston Downbulge. Like the Permian downbulges it is fault-related.

(The photograph above also shows one of the three convenient routes of reaching the shore at Durlston Head. There is a path visible to the right of the cliff-top bushes and this commences over a stone wall north of the Great Globe. It descends, with a curve, a steep slope. It is quite easy for the most part and although there are some old ropes alongside, until the bottom is reached they may not be needed. The final descent to the beach, though, is eroded by the sea and can be steep (it may vary). Descent here can be aided by an old rope, if it is still here. When returning, ascent of the lowest part, just a few metres of steep mud, at present needs hand-over-hand pulling up on the old rope. This route is not guaranteed safe and the ropes may rot and break or be removed. The route is therefore not recommended, especially for large parties. Future cliff erosion may make this route more difficult or even easier. If in doubt do not use this. There used to be another easier path further north (near an old bridge), the original Victorian path to the beach, although it may now be overgrown with brambles. Another means of access is to descend at the Zigzag Path in the centre of the bay and scramble over the beach boulders to Durlston Head. This is safer but quite hard work. It is good, however, for fossil-hunting on the beach.

The geologists should not attempt to descend the cliff in the area of the Jagged Rock or further south. Grass slopes lead to a very dangerous cliff at the base. If a sensible path is used with reasonable caution, Durlston Head beach is not at all difficult to reach.)

A view of Durlston Head, Dorset, and the Durlston Head Downbulge into evaporites, as seen from a boat in 2007

A diagram of the Durlston Downbulge at Durlston Head, Swanage, Dorset, where celestite and calciostrontianite is present in Purbeck calcitised anhydrite

Marl downbulges (down-bulges) into Permian anhydrite at Billingham, County Durham, for comparison with the downbulge into Purbeck anhydrite at Durlston Head, Dorset

The Durlston Downbulge at Durlston Head, Dorset, old photograph, 4th September 1965

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Looking northwest over the faults and the downbulge at Durlston Head, Dorset, to the thickened section and the Jagged Rock from the faulted Portland Stone, 2010

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The Durlston Downbulge at Durlston Head, Dorset, where an evaporitic breccia, the Purbeck Broken Beds, has been squeezed out and moved northward, old slide, 1990s

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The Durlston Head Downbulge in evaporitic strata, Durlston Head, Dorset, with people, labelled version, old image, 1990s

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Brittle-fracture tectonism, shown in more detail, at the Durlston Downbulge, Durlston Head, Dorset, 2010, unlabelled

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A northward view of the Broken Beds with celestite, increasing in thickness northward at Durlston Head, Dorset, photograph 17th March 2010

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The Durlston Downbulge at Durlston Head, Dorset, as seen on the 17th March 2010

The Caps and Broken Beds at Durlston Head, Dorset

Plastic flow of anhydrite, since calcitised, just north of the Durlston Downbulge in the Purbeck Group at Durlston Head, Dorset, 17th March 2010

Expanded Broken Beds breccia with the basal bed of the Cypris Freestones above, Durlston Head, Dorset, from old slide

The cliff at Durlston Head, at the southern end of Durlston Bay, shows much complicated faulting. The details here have been described by West (1960), Salter and West (1965) and West (1975).

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

A general diagenetic classification of the Purbeck calcium sulphate evaporites of Dorset and Sussex, based on thin-section petrography

The basal Purbeck strata at Durlston Head are of particular interest in largely consisting of replaced anhydrite. Unreplaced anhydrite is common in the basal Purbeck units to the east, as for example in the Arreton Boreholes of the Isle of Wight. Some of the anhydrite has been replaced by celestite, strontium sulphate, one of the most insoluble sulphates, and a common indicator of "vanished evaporites" (cargneule). The celestite here was the first definate proof of the evaporitic origin of the Broken Beds (West, 1960). Fine-grained calciostrontianite, here, is an alteration product of the celestite (Salter and West, 1965). Now there is much other evidence for evaporites ranging from pseudomorphs, lutecite to isotopic compositions. The diagram here shows the distribution of replaced evaporites at Durlton Head and localities to the west.

Basal Purbecks, Bacon Hole to Durlston

This diagram shows in summary form the evaporitic beds and the celestite at Durlston Head. Underground, as at Arreton on the Isle of Wight there is still anhydrite at this level. The relationship of the evaporites to the faulting suggests that the brecciation of the Broken Beds took place close to the date of the faulting.

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LOCATION:

Jagged Rock at Durlston Head
(Relic of the Thurlestone?)

A series of photographs which follow show the location and a progressive approach to Durlston Head and the Jagged Rock. It is present close to the Durlston Head celestite exposures, and the former location of hydrocarbons beneath the Purbeck Anhydrite (here calcitised).

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An aerial view of Durlston Head, Swanage, Dorset, showing some key geological locations, modified after GoogleEarth

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An aerial view of the Jagged Rock and adjacent celestite and hydrocarbon-calcitisation area, Durlston Head, Swanage, Dorset

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A distant view of Durlston Head and southern Durlston Bay from the sea defences, just north of the Zizag Path, Swanage, Dorset, June 2010

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The approach to Durlston Head, Dorset, seen when rock-hopping along southern Durlston Bay towards it, June 2010

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Approaching Durlston Head, Dorset, with the Zizag Rock seen as a possible relic of the Thurlestone, which gave the headland its name, June 2010

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The Jagged Rock, a stack formed largely of the Purbeck Broken Beds, at Durlston Head, Dorset, and the adjacent cliff of Broken Beds, with celestite, August 2009

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Jagged Rock of the Caps and Broken Beds, Purbeck Group, Durlston Head, Dorset, seen from the cliff slopes above, June 2010

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The Jagged Rock, Durlston Head, Dorset, seen from the cliff slopes above in stormy conditions at dusk, 13th February 2017

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On the shore at Durlston Head between Jagged Rock and the cliff with Broken Beds above the calcitised anhydrite of the Caps, Durlston Head, Dorset, June 2010

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The Jagged Rock at Durlston Head, seen from the northwest, and showing the top of the Caps and the base of the Broken Beds, Durlston Head, Dorset, June 2010

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The breccia, the Broken Beds, seen in the Jagged Rock, at Durlston Head, Dorset in 1993

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The Jagged Rock, Durlston Head, seen from the beach on the southern side, about 1993

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The Jagged Rocks at Durlston Head, Dorset, consisting of calcitised evaporite breccia, the Broken Beds, above, with unbrecciated calcitised anhydrite below, photo - 17th March 2010

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Looking northward though the gap between the clifff and the Jagged Rock at Durlston Head, Dorset, 2010

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The Jagged Rock of breccia at Durlston Head, Dorset, in about 1815, etching after Webster

At Durlston Head, at the beach at the southern end of Durlston Bay is the Jagged Rock. It consists of evaporite-limestone breccia, part of the Broken Beds, resting on calcitised evaporites of the Caps (with celestite). It is shown in photographs above. Also shown is an etching of the rock as it was in 1815 as published by Webster (1816) . This may be the relict basal part of the original pierced rock (the "Thirlstone" or "Durdlestone" from which Durdlestone or Durlston Bay takes its name - cf. Durdle Door) but the arch must have collapsed long ago. The Jagged Rock consists of Broken Beds resting on evaporitic Purbeck Caps.

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LOCATION:

Durlston Head continued:

Celestite and Calciostrontianite Occurrences

A celestite-rich clast in the evaporitic breccia, the Broken Beds, at Durlston Head, Dorset, March 2010

Platey celestite in a clast in the Broken Beds, Durlston Head, Dorset, March 2010

a celestite specimen with the strontium sulphate having grown into anhydrite nodules, basal Purbeck Group, Durlston Head, Dorset

A thin-section diagram of the Upper Celestite Bed of the basal Purbeck strata of Durlston Head, Dorset

A thin-section of celestite and calciostrontianite associated with a small shear in the Upper Celestite Bed of the basal Purbeck strata of Durlston Head, Dorset

A thin-section of spherulitic calciostrontianite with some idiotopic celestite, Upper Celestite Bed of the basal Purbeck strata of Durlston Head, Dorset

XRD data for calciostrontianite from Durlston Head, Dorset, and elsewhere, with d-spacings in Angstroms

Celestite or Celestine

Occurrence in Vanished Evaporites:

Celestite is a very common strontium mineral in evaporite deposits, normally resulting from replacement of gypsum or anydrite by influx of strontium. This usually comes from associated limestones, because Sr is a minor constituent of most calcite sediments and is even more abundant in aragonite. Solutions can convey both Ca and Sr from associated limestones into a gypsum or anhydrite deposit (calcium sulphate). The additional Ca has only a major effect if there is surplus carbonate in which case calcitisation occurs - i.e. gypsum or anhydrite is replaced by calcite. The Sr, however, replaces the Ca or the calcium sulphate (gypsum or anhydrite), but it is very insoluble and certainly much less soluble that gypsum. Thus it remains after groundwater has leached away the original anhydrite or gypsum deposits. This relatively insoluble remains in so-called "vanished evaporites" West (1973). It is therefore useful for recognising their former presence.

Celestite is very common in the Purbeck Group of Dorset because limestones are present within the Purbeck Group and even greater quantities of limestone occur as the underlying Portland Stone. Thus a geologist can find celestite in Dorset in the basal Purbecks at Durlston Head (discussed below), in the Soft Cockle (Purbeck) gypsum at Durlston Bay and the west cliffs of Portland. It present in the Lower Purbeck at Worbarrow Tout (and replaces bivalve shells in the uppermost Portland), and occurs in the Soft Cap on the central ridge of Stair Hole, Lulworth Cove. Small quantities may be found in almost any Lower Purbeck exposure. Further west it is present in the Mercia Mudstone (Trias) of Devon, and is so abundant in the Mercia Mudstone of Yate, Somerset, that it has been worked economically there.

(further note: - If the strata associated with the gypsum or anhydrite has little if any calcite or aragonite, but much clay or weathered volcanic material, then then an availability of (related) Ba rather than Sr is common. In these circumstance the evaporite relic is less likely to be celestite and more likely to be barite or baritocelestite (barytocelestite).)

When examining "vanished evaporites" or cargneule deposits expect also to see some euhedral quartz, and particularly lutecite chalcedony (lutecite is oblique-extinguishing), and length-slow chalcedony (quartzine). Look for pseudomorphs after lenticular gypsum or after anhydrite. Small relics of anhydrite may be preserved within quartz crystals or possibly in chalcedony. Pseudomorphs (or moulds or casts) after halite may well be associated. Replaced gypsum or anhydrite sabkha nodules may be present, replaced by various minerals, particularly quartz (as quartz geodes). Brecciation may be present. Fauna is likely to absent or limited to ostracods. To prove, with confidence, the former of former evaporites more than one of the above features should be present.

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General Properties of Celestite

Sr 4 strontium sulphate (cf. barite - barium sulphate).

Cry. Syst. - Orthorhombic.
Com. Form - Tabular crystals like barite: basal pinacoid (001 - obvious, large flat face), prism, macrodome (101 etc. i.e. parallel to the macro-axis - b) and brachydome (011 etc. i.e. parallel to the brachy-axis - a). May occur in bulk as fibrous, granular or massive.
Cleav. - Perfect parallel to the basal pinacoid and good parallel to the unit prism (useful for field identification).
Colour - White, sometimes with a pale blue tint (usually white in the Dorset Purbecks).
Lustre - Vitreous, inclined to pearly at times; transparent to subtranslucent.
Fracture - imperfectly conchoidal, but usually breaks on the cleavages; very brittle.
Hardness - 3 - 3.5.
S.G. - 3.96
Tests - Does not effervesce with HCl or accept Alizarin Red S stain. Not easily distinguished from barite in the field. Distinctive crimson flame in the flame test, although rarely used at present. X-Ray Diffraction is nowadays the quickest and most reliable method for distinguishing celestite from barite. Geochemical methods (XRF etc) can be used.
Optical - alpha - 1.622, beta - 1.624, gamma - 1.631. Birefringence low - 009. Closely resembles barite. High relief, low birefringence, straight extinction, perfect 001 cleavage, etc makes this easily recongnised in thin section apart from the barite problem. Often found with gypsum or anhydrite or calcitised evaporites.)

Brecciated calcitised anhydrite of the Broken Beds, with the Lower Celestite Bed of the Caps, beneath, at Durlston Head, Dorset, Lower Purbeck Group, old slide

Celestite in the calcitised anhydrite of the Caps and Broken Beds, basal Purbeck Group, Durlston Head, Dorset, old slide

A celestite vug in the Lower Celestite Bed, Durlston Head, Dorset, old slide

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Geochemistry of the Celestite and Replaced Evaporite Sequence at Durlston Head.

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Diagram showing the petrographic sequence and some trace element geochemistry of the basal Purbeck Group (Lulworth Formation) at Durlston Head, Dorset, after Quest and West

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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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(notes to be added)

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LOCATION:

Durlston Head - Chalybeate Spring

Chalybeate spring in the graben of the Durlston Downbulge, Durlston Head, Dorset, 17th March 2010

In the graben of the Durlston Downbulge, above the basal Cypris Freestones bed there is now a conspicuous chalybeate [iron] spring. I have no recollection of this having been well-developed in the years long past (I studied this place in 1960). Has it increased in development recently? What is the source of the iron? Has it any connection with drainage from Durlston Castle which is now, temporarily, in rather derelict condition and awaiting repair and rejuvenation as a Jurassic Coast centre. Within the Purbeck Group the most likely source of pyritic strata, which could provide the iron, is the Shales-with-Beef. Middle Purbecks crop out high on Durlston Head, so it is not impossibe source. Nevertheless it is a little puzzling.

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LOCALITIES:

Durlston Head Downbulge - Comparison with the Fisherman's Ledge Fault (east of Dancing Ledge)

(because of its importance to the evaporite tectonics, Broken Beds, and structures at Durlston Head this section is given in both the Anvil Point to Blackers Hole webpage and in this Durlston Head webpage.)

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 fault at the eastern end of Fisherman's Ledge is important. It 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. Such faults are probably the result of the Late Kimmerian extensional stresses that occurred as the North Atlantic was in the early stages of opening. These faults are 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. Intra-Cretaceous, about Aptian). Thus some, at least of the Broken Beds movement seems to be quite early, and well prior to Tertiary tectonics. This is compatible with the structural interpretation of Selley and Stoneley (1983 etc).

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LOCATION:

Portland Stone at Durlston Head, South of the Fault
(at sea-level, south of the celestite exposure)

The easternmost corner of Durlston Head, Dorset, as seen from the sea in 2007, and with the foot of the cliffs  just accessible on foot from the celestite exposure and the Jagged Rock

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On sea-worn Portland Chert Member at Durlston Head, Dorset, south of the celestite exposure and the major fault, 12th June 2010

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View northward towards central Durlston Bay from the Portland Stone cliffs at Durlston Head, Dorset, 12th June 2010

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LOCATION:

Durlston Head and South of the Great Globe (above cliffs)

Durlston Head, and Durlston Castle, Dorset, photographed from the Paddle Steamer Waverley by Dr Clive Boulter on 19 September 1997

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View westward from above Durlston Head, near the Great Globe, towards Tilly Whim Caves, at sunset, February 2017

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The cliffs at Durlston Head, Dorset, seen from the south, and with Badger sets in the calcitised anhydrite of the Purbeck Broken Beds

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The Portland-Purbeck junction on the cliff top south of the Great Globe, at Durlston HeadDungy Head

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The cliff top south of the Great Globe, Durlston Head, Dorset, in misty weather, 17th March 2010, with a badger set, and with some collapse of Portland Stone

The end of the headland of Durlston Head, Dorset, and the cliffs south of the Great Globe

The cliffs south of the Great Globe are difficult for the geologist to access. The Prickle Bed in the Portland Cherty Series forms a ledge which can be followed round at low tide from the south of Durlston Bay. At the cliff top there is a rather hazardous exposure of the Broken Beds. This is on the upthrow side of the major east-west normal fault. An interest aspect of this area is that there is a well-defined coastal bevel (like that in Cornwall) which is present irrespective of whether the uppermost cliff strata are Portland Stone or basal Purbeck. The bevel may the remains of an old cliff associated with the Ipswichian Raised Beach, which was once presumably close to the present coastline here.

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LOCATION:

Durlston Head to Tilly Whim Caves

The seabird lookout at Durlston Head, Dorset, is situated on the upthrow side of an extensional fault

Purbeck evaporitic strata draped over extensional faults in the Portland Stone, between Durlston Head and Tilly Whim Caves, Swanage, Dorset, 2007 - unlabelled version

Purbeck evaporitic strata draped over extensional faults in the Portland Stone, between Durlston Head and Tilly Whim Caves, Swanage, Dorset, 2007 - labelled version

Two images, one the orginal and other labelled, of the south side of Durlston Head, immediately west of the seabird lookout are provided above. They show the usual extensional faulting of this part of the coast, and, in addition the draping of the former Purbeck evaporites (the Broken Beds) over the normal faults. These Purbeck evaporites were anhydrite under burial (probably primary gypsum originally) and have to some extent flowed plastically over the moving faults. This plastic flow of anhydrite associated with a fault is particularly well seen at the beach exposure of Durlston Head, at the southern end of Durlston Bay (see West (1960; 1975 etc) and other parts of this webpage).

The importance of this southern coast of Durlston Head is the evidence that the north-south trending faults show similar features to the large east-west fault at the beach of Durlston Head. These are all extensional and all show evaporite flowage. There are many similar faults further west at Blackers Hole, Green Point etc. and shown in associated webpages. The brecciation of the Broken Beds can be shown to closely connected with the development of the fault at Durlston Head beach (the main part elsewhere might be slightly earlier, though, because the laterally extensive brecciation with bed-over-bed sliding could only have occurred easily before major faulting caused partitioning). Thus the age of the faults is important, with regard to the history of the Broken Beds. As mentioned elsewhere this extensional faulting is probably Late Kimmerian (Intra-Cretaceous) but the date is not firmly proven as yet (the matter will be discussed further at a later stage).

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LOCATION

Tilly Whim Caves - Introduction

Note: For the adjacent coast to the west at Anvil Point and beyond, go to: Anvil Point to Blackers Hole, Isle of Purbeck

The old entrance to the Tilly Whim Caves tourist attraction west of Durlston Head, Isle of Purbeck, Dorset

Tilly Whim Quarry after Hardy, 1910, near Durlston Head, Swanage, Dorset

Here is a brief account of a visit to Tilly Whim Caves by the Geologists' Association on Sunday 20th May, 1934 ( Arkell, 1934) and through the entrance shown here:

"Climbing up the zig-zag path below Middle Durlston, the party walked to Durlston Castle for tea, and afterwards on to Tilly Whim Caves. At the end of the descent of the tunnel a short account was given of the succession of the Portland Stone displayed, special reference being made to the oyster bed and to the quarrying industry here and in Purbeck generally. In Grabau's "Text-book of Geology" (1920, pp. 812-3) are two photographs of Tilly Whim headed" Elevated sea-caves cut by waves in horizontal Jurassic strata." But we owe to Thomas Webster, who first explored this coast geologically on behalf of Sir H. Englefield ("Picturesque Beauties of the Isle of Wight," 1816, plate 33) an accurate engraving of the" caves" as they appeared in 1811, with the quarrymen still at work. Quarrying, in fact, has alone been responsible for the cutting of these and the many similar galleries and ledges between Durlston and St. Albans Heads."

Tilly Whim Caves (quarry) west of Durlston Head, Swanage, Dorset, in 1816 and 2007

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Tilly Whim Caves, an old quarry, west of Durlston Head, Isle of Purbeck, Swanage  Dorset, seen from the sea in August 2007

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A view down into the Tilly Whim Quarry from the Anvil Point, near Swanage, Dorset, looking southwest from the cliff top path, 11th December 2013

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The location of the Tilly Whim Oyster Bed, in rock face of joint origin, Tilly Whim Caves, near Anvil Point, and near Durlston Head, Swanage, Dorset, 11th December 2013

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An oblique view of the southwestern end of the Tilly Whim quarrying ledge, near Anvil Point, near Swanage, Dorset, on the 11th December 2013

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

The western part of the Tilly Whim quarrying ledge and caves, seen from a boat, near Durlston Head, Swanage Dorset, 2007

The Tilly Whim quarrying ledge from the cliff path to the east, near Durlston Head, Swanage, Dorset

Looking down at a field trip party on the Tilly Whim Caves ledge in September 1965, near Durlston Head, Dorset

Part of a field trip party, with Ian West leading, on the Tilly Whim ledge in September 1965, near Durlston Head, Dorset

Tilly Whim Caves are actually an old quarry from the time of the Napoleonic Wars where stone was particularly needed for military defence works. Later in 1187 it became a tourist attraction of the Durlston Head and Durlston Castle area, as set up by George Burt. As a boy I used to go down this tunnel after paying a small fee at the gate. It seemed an exciting place!

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LOCATION

Tilly Whim Caves - Closure

Unfortunately, after having long been a successful tourist attraction and a very good geological locality, Tilly Whim Caves were closed in 1976. There was some threat of rock fall. Surprisingly, after the closure of the cave access route no other entrance to the ledge was arranged and it was and has remained completely shut off. Access could, in fact, have been made from the Anvil Point valley but, instead, a high wall was built there as a barrier. Unfortunately, this closure almost coincided with the the setting up of the Durlston Country Park and thus a major tourist attraction was lost to use. I spoke informally to some rangers from the country park at about 1976 when I met them above the cave entrance just before the closure; they were not cheerful about the bad news.

The ledge, now inaccessible, shows a good succession of Portland Freestone with the usual sequence for the most part. At the top, though, there is the unusual feature of an oyster reef, which replaces the Pond Freestone and the Shrimp Bed. On the ledge there are many large fallen blocks containing interesting bivalve fossils of the Portland Stone.

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LOCALITIES:

Tilly Whim Caves - History

The history of Tilly Whim Caves has been discussed by Robinson (1882). An extract on Tilly Whim follows:

[Walking Down Southeastward to Tilly Whim]

"Ever since leaving the Newton quarry we have been in full view of the sea, and so high above it as to command a very distant horizon. The level blue expanse is dotted with every kind of vessel, with long, low, dark-hulled screw colliers, fast mail-steamers, barques, brigs, and schooners, and white-sailed yachts of many rigs, all quite toy-like and tiny in appearance. From under the [Anvil Point] lighthouse garden wall, close to the edge of the cliff, we can look down on the very decks of those that come closer in, and see the crews moving like little black insects about them. Westward, a few paces bring into view a wild desolate coast of grassy downs, their steep slopes terminating to all appearance in the sea itself, but really in Titanic cliffs of Portland stone, which underlies the Purbeck measures, and outcrops here. Four miles away, the bleak promontory of St. Aldhelm's Head juts out into the Channel, and beyond it nothing is visible but sea and sky. Looking eastward across the southern end of the narrow valley, the course of which we have been following, the view is not so extensive, as the coast retires immediately behind another foreground of grassy downs. But it is more interesting, for just at the valley's mouth, all at once the swarded hillside breaks into an abrupt and barren wall of brown-coloured stone, with a rugged seaward outline of tumbled fragments of the same rock. About the middle of the cliffside facing us are two dark, square-cut apertures in the rock, each about the same size and above the height of a man. They open on to a flat ledge, perhaps thirty feet above the boiling waves, and over them is an equal thickness of solid stone, above which again the green slope rises to a much greater height. Such is the entrance to the well-known cave-quarry of " Tilly Whim," like nothing on earth so much as an Egyptian rock-temple.

No lack of grandeur characterises at all times the large simple features of this lonely spot; but to see it in all its wild sublimity, watch for a fierce gale from the south, when the billows dash themselves, rank after rank, on the black relentless rocks, only to be driven backwards, torn and mangled, in countless eddies of seething foam, while their windcaught crests are flung up in ragged clouds of dazzling spray, with such heedless force as to fly a hundred feet into the air, before they are scattered like thunder-showers inland, over all the reeking slopes of the down. On such a day no man can stand unsheltered on the cliff, and the eye can scarcely bear the salt rain that continually assails it. Nothing is heard but the thunder of the breaking waves; at every blow the solid rock trembles under foot. The fate of any vessel overpowered by the gale, and helplessly involved in this fury of the elements, is too frightful for the imagination to contemplate; though possibly enough more than one, during the long course of ages, has been lost here in the night, and in the morning no traces have remained.

[Entering Tilly Whim Ledge from Anvil Point Valley]

The valley may be crossed [travelling eastward], and at the risk of being crushed by the fall of a thousand tons of rock, which overhang frightfully at the corner of the cliff-hard by the tumbled debris indicating the position of another cave, the roof of which has already foundered for want of support-the ledge of rock [Tilly Whim Quarry] may be gained, which extends for more than a hundred yards along the hollow coast to the eastward, called Howcombe Cove. This ledge has been artificially formed by the gradual removal of vast quantities of stone in the process of quarrying. From it savage-looking openings lead to yet other obscure caves, all silent as the tomb, except for the constant rush of the sea, and the casual dropping of small chips of stone from the roof, or the querulous cry of the kittiwake gull. But even this barren exposed spot is not entirely without vegetation; samphire grows in the creeks and crannies of the rock, out of reach, and so does a maritime fern.

[Origin of the Name Tilly Whim]

It is now more than sixty years since these quarries were regularly worked; and this fact, coupled with their queer collective name of "Tilly Whim," is enough to account for the origin of a kind of modern myth, which will be found gravely recorded in one of the Swanage guide-books which are sold to visitors. "Tilly Whim," we are told, "is so called from a person of the name of Tilly, who persisted in working it in spite of the advice of experienced quarrymen, who warned him of the increasing hardness of the stone." [actually the Portland Stone of the Isle of Purbeck is harder than the Portland Stone of the Isle of Portland] Now, it goes to my heart to knock a baby myth on the head; but to begin with, as a matter of fact, the stone does not increase in hardness: the exact contrary is the case. And then again, if the margin of the ledge to which the quarries open is examined, traces will be found of the attachments of rude cranes, such as are still used in similar workings more to the westward. These were the machines for lowering the wrought stone into boats, which in fine weather came alongside and conveyed their loads to a vessel anchored at a short distance away. No stone was ever removed from Tilly Whim inland, because there was formerly no road except a steep footpath. But in the west country "whim" is the name for any contrivance for raising or lowering the produce of mines; and such machines being rare in the immediate vicinity of Swanage, Tilly's quarry was known as Tilly's Whim, not as imputing to him any folly, but after his newfangled crane. Who Tilly was, or when he lived, are questions that cannot be answered; but before 1721 Tilly Mead was the name of a meadow in the town, formerly parcel of the manor of Eightholds, on which the larger part of Tilly Whim is placed, though the entrance is from the Common Wear, a large close of pasture, including the valley we have seen so much of, as well as the site of the lighthouse. This Common Wear was from time immemorial, and still is, subject to sheep rights attaching to different lands in Swanage parish, and there have consequently never existed any records to show when the first encroachment by quarrying was successfully attempted. The earliest instrument in which Tilly Whim is mentioned by name is a lease dated the 5th of April 1805, of that part which lay within the manor of Eightholds, granted to three quarrymen by the lord of the manor.

[Quarrying Industry]

In a letter written by,that gentleman, dated April 13, 1812, is an interesting description of Tilly Whim, from which the following are extracts :

"There is so much room in this quarry for any assignable number of men to work, and so great a facility, in summer, of shipping the goods, letting them down at once by a crane into the vessel, that men of industry and enterprise ought to command almost the whole market for the species of articles which this quarry produces, and to supply it from this spot only." . . . "The sort of goods which this quarry yields are of what is called the Portland Purbeck, a sort of freestone, much like the Portland, only harder, and much used for building in bridges, harbours, fortification walls, troughs, columns, rollers, staddle stones, etc."

Accounts of the stone" dug" at Tilly Whim from 1805 to 1812 are also extant. They overlap one another in such a manner as to make it difficult to ascertain what quantity left the quarry in 1809-10, unless as appears probable, the date 1808 in one account is a mistake for 1809, in which case the total shipment for the five years ending with Ladyday.

37 "setts of bigs and caps."
14 setts of rink stones.
83 pair staddle stones.
340 and a half - "pecks" of sinks, and troughs.
318 and a half feet of rollers.
2305 tons of backing.
97 tons of block. 1
33 tons of pitchers.

For which the royalties payable to the landowner amounted in all to the magnificent sum of £20, 7 shillings and 2 pence.

The real reason for the final abandonment of Tilly Whim was not, as will be seen, the "increasing hardness of the stone," which, on the contrary, was a quality for which it was valued, but the increasing slackness of the demand. In the two years ending Ladyday 1812 there was only occasion to ship away 110 tons of pitchers, and after that date the quarry ceased to be worked altogether.

At the close of the great European [Napoleonic] war, which occurred about this time, the sudden cessation of all public expenditure on fortifications, and the absolute necessity for general retrenchment, threw out of ,york the great Inajority of the Swanage quarriers, and caused the desertion of a large number of quarries. So many persons came on the parish, that in 1813 the poor-rates amounted to no less than thirteen shillings and fourpence in the pound, as evidenced by the farm accounts of Eightholds for that year! And it is still told that men would leave their work at noontide when the rest for lunch was taken, and walk away separately for a few minutes, each out of pride endeavouring to conceal the unpalatable fact that he had not even a crust of bread to eat. When, years afterwards, the demand again grew brisk, it was found that so many of the quarrymen had been forced to seek work elsewhere, in that trying time, that there were not enough to re-occupy all the old quarries, and so the workings at a distance from the town, near the sea, have never been resumed. Perhaps the statement in the letter above quoted may be taken as identifying Tilly Whim with the" cliffs near Durlstone Bay," which are mentioned in Hutchins as the place where the stone was got to build the old fortifications of Portsmouth. At any rate, there are no particular traces of any large quantity of stone having been removed from Durlstone Bay itself, whereas the great caves and platform of Tilly Whim have evidently been produced entirely by the quarrying of an immense mass of rock. The layers of good stone are three in number, together from 15 to 20 feet thick, and are of a light cream-brown colour. Above them is a homogeneous" ceiling" composed entirely of a conglomerate of silicified oyster-shells, varying considerably in thickness, of no use to work, but extremely hard and heavy.

[Smugglers at Tilly Whim]

In the times before Free Trade, it is said that the coast near Swanage was a regular resort of smugglers, with whom many of the inhabitants had leagued themselves to cheat the revenue for the purpose of getting their grog cheap. One of the hiding-places for their illicit cargoes was a cave in the cliff between Howcombe Quarry [Tilly Whim Quarry] and Durlston Head, to which there is no access except from the sea at low water. Hither would one of their fast-sailing luggers resort, when the revenue cruisers were out of the way, and in quiet foggy weather hastily deposit the contraband goods, to be afterwards fetched away by the people of the town, when a safe opportunity occurred. It happened one night, many years ago, that an unusually valuable cargo of brandy, in which half Swanage was interested, had been safely secreted in the smugglers' cave, and burning to convey it to their hidden stores in the town (one of which is yet extant in the shape of an old ivy-grown cottage on the hill, having under it a cellar as large as itself), the confederates had given false information to the revenue officers, in the hope of leading them off elsewhere, and in the meantime landing the spirits. Unluckily the trick was only half successful. The revenue men were drawn away, it is true, but before they had gone far, had received precise intelligence, which brought them back in haste before the landing had been half completed. However, the smugglers had decamped in time; and for hours the hiding-place baffled all their attempts to discover its exact locality. At last one of the officers, keener of nose than the rest, became accidentally aware of a strong smell of brandy, near the edge of the cliff. He snuffed, he was not quite certain; he snuffed again, and was positive; he went down on his hands and knees, and sure enough the grass was damp, with a mixture which was never of Nature's distilling. He guessed on the instant that here the barrels had been hauled up, and one had been broken and leaked on to the ground. But even then the cave was a puzzle to find, and when, entered at last it was quite empty! No, not quite, though, for in one dark corner the officer could just espy a harmless little wooden buoy with a line attached. And the eyes of this astute guardian of the revenue were no more, at fault than his nose, for when he pulled it up, this harmless little buoy proved to have attached to it quite a quantity of kegs, of very excellent cognac. N early an entire cargo was confiscated on this occasion, and it proved a fine prize to the finder. The Swanage people were furious, and went so far as to mob the revenue station, when most of the officers and men were away. However, no harm was done, for the appearance of one man with a cutlass put them to indiscriminate flight."

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LOCATION:

Tilly Whim Caves - Portland Stone Succession

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

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

The sequence of Portland Stone at Tilly Whim is broadly similar to that elsewhere in the cliffs of the south Purbeck coast, as at Dancing Ledge for example. The ledge at Tilly Whim is cut in the Portland Freestone, which has been extensively quarried away, leaving the top of the Portland Cherty Series as the base of ledge. The outer cliffs which remain are of the Portland Cherty Series. This is fairly standard for the area and the Prickle Bed or Puffin Ledge is very obvious as a marker horizon. It is a thin bed without chert at about halfway up the lower cliff. It has fucoidal structure and is riddled with Thalassinoides decapod (crab) burrows. It has some similarities to the burrowed hardgrounds with Thalassinoides in the Cenomanian Limestone or Beer Head Limestone of the Lower Chalk at Beer, East Devon. It may similarly represent a shallow phase with, perhaps, a pause in sedimentation.

The Portland Freestone (see diagram above) has the Under Freestone which has been extensively quarried here. It is not a true oolite, like the Portand Oolite of the Isle of Portland, but is an intrasparite. The other units are similar to those at Dancing Ledge, until the Titanites Bed is reached. This is represented by an oyster reef.

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LOCATION:

Tilly Whim - The Oyster Bank

Arkell (1935) briefly described the oyster shell bank (or oyster reef?) at Tilly Whim Caves and Quarry as follows when discussing the Titanites Bed:
"At Tilly Whim a lenticular oyster bed develops on this horizon. Locally about 8 feet (2.4m) of rock is almost entirely composed of Exogyra nana Sow. [Nanogyra nana], E. thurmanni Etall., Ostrea expansa Sow., and Isognomon listeri (Brown) [in old literature this such as Damon (1884), this might be listed as Perna mytiloides], with a smaller proportion of Lima rustica and Plicatula boisdini de Loriol. Sometimes the fossils are dissolved away, leaving patches of rock like the Roach of Portland."

Perna (Isognomon), Isastraea oblonga and other from the Portland Stone, Upper Jurassic, Dorset

Arkell (1935) further commented (in a footnote):
"The Isognomon usually known as Perna bouchardi Oppel, and one of the commonest fossils in the Portland Beds, was first named Crenatula listeri by Brown (1845-1849, Illustrated Fossil Conchology, p. 165), the type being the typical cast from the doggers of the Shotover Grit Sands of Shotover Hill, Oxford, figured in Parkinson's Organic Remains, 1811, vol. 3, plate 15, fig. 5. This commemoration of Martin Lister antedates Oppel's name by ten years"

Incidently the Isognomon is a relative of modern "wing oysters", which are usually attached by byssus. It is also a distant relative of modern pearl oysters, not suprising in view of the Portland warm waters with ooids like those of the southern Arabian Gulf.

Above the oyster bed is the Shrimp Bed, which normally occurs at this horizon in the Isle of Purbeck (but not on the Isle of Portland). The Shrimp Bed has the usual micritic appearance and the usual numerous small fractures, but it also contains numerous fragments of oysters, mostly fairly small pieces.

For more detailed information on bivalve-dominated reefs in the Portland Freestone, but on the Isle of Portland see Fursich, Palmer and Goodyear (1994), Growth and distintegration of bivalve-dominated patch reefs in the Upper Jurassic of southern England. They state that "Reefs are unknown in the Portlandian rocks of the mainland". It is not know whether the oyster bed at Tilly Whim is a reef or just a shell bank of oysters. Perhaps those authors regarded it as a shell bank.

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Durlston Head at Dusk

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PETROLEUM GEOLOGY:

Durlston Head Gas Field

The Durlston Head Gas Field in relation to source rock areas and oil fields in the East Dorset coastal region

There is a methane gas field beneath the Durlston Head area. It has been investigated many years ago by BP who put down a deviated borehole into this, from a quarry south of Swanage. Long before this exploration, a gas seep into the sea at Durlston Head had been found by divers.

A fairly old record of the Durlston Head gas seep was given by (Hinchcliffe, 1978).

"On the seabed directly beneath Durlston Castle, above whose cliffs is poised the giant stone globe of the world, a further curious phenomenon is found. Here the seabed is bubbling. in 10 metres of water, long columns of bubbles ascend. Some sources are continuous, some spasmodic. I recently collected a sample of this gas and it proved to be an inflammable natural gas. An even more curious phenomenon here is the large numbers of huge bass and pollock which tend to swim about near these bubbles. Do they mistake the hydrocarbon gas for oxygen?"

Later, the diver, Mike Markey, who drew my attention to this article, reported on the 7th Dec. 1990 that the gas seep was still bubbling at that time. He had also found a similar gas seep on the Lulworth Banks, near the axis of the offshore anticline. According to Judd (2004), gas bubbles lose methane to the water as they rise, so deep water seeps are unlikely to contribute to the atmosphere. However, bubbles break the surface above some shallow water seeps. I do not know whether this happens at Durlston Head or whether the bubbles disappear before they reach the surface.

For more on the petroleum geology of the region, including discussion of the proposed California No. 1 Borehole (and of the previous Southard No.1 of BP) into the the Durlston Head gas field see: Petroleum-South-Portland-Wight-Basin.

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ACKNOWLEDGEMENTS

I am very grateful to Dr Clive Boulter for use of photographs taken from the Waverley. I am very much obliged to the Director and Staff of the The Channel Coastal Observatory , National Oceanography Centre, Southampton, for permission to use their aerial photographs. Travis Mason has been particularly helpful with regard to the aerial photographs and this assistance is much appreciated. I must appreciate the hosting of this website by Information Systems Services, Southampton University. Mike Markey has kindly helped with the topic of the gas seep at Durlston Head.

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Other Purbeck Webpages

Bibliography and References for Durlston Bay
Durlston Bay, Peveril Point - Introduction, Fossils and Upper Purbecks (Durlston Formation)
Durlston Bay - Middle Purbecks and Building Stones (Lulworth - Durlston Formations in part

Durlston Bay - Bibliography and References
General Purbeck Group Bibliography
Purbeck Dinosaur and Other Vertebrates Bibliography

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Copyright © 2016 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

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

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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.


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