West, Ian. M. 2014. Portland Bill, Isle of Portland. Geology of the Wessex Coast (part of Jurassic Coast, Dorset and East Devon World Heritage Site). Internet field guide. http://www.southampton.ac.uk/~imw/Portland-Bill.htm. Version: 4th July 2014.
Portland Bill 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 by courtesy of The Channel Coastal Observatory Southampton.

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Walking to Portland Bill, Dorset, 2011

An old aerial photograph of the Isle of Portland, Dorset, from the south, showing some details particularly in the Portland Bill region

The present and third lighthouse at Portland Bill, Dorset, 11th September 2009

The Trinity House obelisk at Portland Bill, Dorset, as seen from the top of the lighthouse, 11th September 2009

Sea breaking on ledges of Portland Cherty Series, South of Pulpit Rock, Portland Bill, Dorset

Red Crane Blowhole in action, 29th November 2009, Portland Bill, Dorset

Wave breaking on Portland Cherty Series, Whitehole, Portland Bill, Dorset

Horizontal spray from waves breaking at Whitehole, just north of Pulpit Rock, Portland, Dorset, 2nd December 2007

Storm waves at the Red Crane,Portland Bill, Dorset, produce a blow-hole effect in a fissure or enlarged joint, 2nd December 2007

Click or double-click on images for full-size high resolution versions!
(Browser zoom will not produce good photographs with sharp captions)

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INTRODUCTION:

Safety on Field Trips at Portland Bill

Some specific hazards with regard to Portland Bill are mentioned here. Most are so obvious that they hardly need mentioning. It is important not to hammer the chert in the Portland and Purbeck strata because dangerous splinters can penetrate the body or eyes. Take care with the vertical cliff edges and the fissures in the Portland Stone which are common in this area. Do not enter fissures or caves without the normal caving safety precautions. The low ledges around Portland Bill can be dangerous in rough sea and should be avoided in such conditions. There is real risk of being swept off a ledge in stormy weather. The upper slopes of the cliffs, particularly on the west side of Portland are at a moderate angle but are above vertical Portland Stone. These upper slopes are hazardous and should be treated with great care. Some celestite exposures on the cliffs from the Old Higher Lighthouse northward are not of easy access and are not recommended. In general take much care on the rocks, cliffs and quarries of Portland.

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INTRODUCTION:

Maps and Aerial Photographs

A general aerial view of Portland Bill, Dorset, for comparison with detailed map

A topographic map of Portland Bill, Dorset, showing various geological and coastal features, by Ian West

The map provides location information and some geological data for Portland Bill and adjacent areas. Many of the topographic features can be seen in the photograph alongside, which was taken from the lighthouse and shows the view in a northwestward direction.

More information on specific localities shown on this map is provided below in Location sections below. The map, shown here is in some respects a simplification of topographic features and for full details one should consult the Ordnance Survey map, 1:10,000 - SY 66 NE. Certain coastal features and landmarks are emphasised here, though, and many additional place names are added from the useful book by Bruce (1989) . The map is intended for general introductory purposes and also, more specifically, for fixing the locations of various geological details in the photographs of this stretch of coast.

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INTRODUCTION

Geology - Introduction

Portland Bill, block of Portland Roach, part of the Portland Freestone, in the old quarries

Pulpit Rock viewed towards the southwest, Isle of Portland, Dorset

The Bill consists of gently south-dipping Portland Freestone, the base of which is at about sea-level here at the extreme southern point of the promontory. Because the dip is southward this junction is a little higher in the cliffs just northwest of the lighthouse, as at Pulpit Rock, seen here. The oolitic limestone has been much quarried, modifying the coast in places, and where it is near sea-level it been much undercut by sea erosion into broad caves. The Portland Cherty Series lies underneath the Portland Freestone. It is at sea-level downwards at the Bill itself but gradually rises northward to form resistant vertical cliffs. At Pulpit Rock it forms the lower part of the cliff. Above the Portland Cherty Series and the Portland Freestone are the thinner-bedded limestones and shales of the Purbeck Group. Only the basal part of this unit is present at Portland Bill, although more is seen to the north. A broad raised beach platform cuts across the Purbeck strata and this unconformable sediment is Pleistocene in age. It has been classified into two raised beaches, both largely consisting of shelly and sandy pebble deposits.

Jurassic limestone and Pleistocene raised beach, Portland Bill

Geological processes in action during a storm at Portland Bill. The view is of Red Crane, used for lowering fishing boats, with the old house inland of it (see the Portland Bill map above). The oolitic limestone of the cliffs, the Portland Stone, which contains bivalves, gastropods and giant ammonites, was deposited in a late Jurassic sea. Now it is being eroded vigorously by the modern sea to produce a wave-cut platform. At the top of the cliff is a Pleistocene raised beach on a similar, but older, wave-cut platform. Notice how the storm waves have forced water into widened bedding planes and this is now pouring out as the waves recede. The breaking out of the well-jointed limestone produces caves beneath the raised beach and further along the cliffs a cave has broken through to the surface.

The Portland Stone is almost horizontal but actually dips at a low angle in a seaward direction. It is well-jointed, porous and easily carved and was used by Sir Christopher Wren for rebuilding St Paul's Cathedral and other great buildings in London after the Great Fire in 1666. Most of the early quarrying was in the northern part of the island. It seems to have been later that much stone was quarried from the cliffs at Portland Bill. One section is known as Whitehall because of its use in London according to Bruce (1989, p. 98), and in calm weather stone was loaded by crane into sailing barges bound for the Thames and London.

The interglacial raised beach is about 125,000 years old and has abundant mollusc shells such as species of Patella and Littorina and small bivalves that lived on seaweed. It has been much disturbed by cryoturbation (freezing and thawing) during the late Pleistocene ice age. Unlike the older Boxgrove Raised Beach further east no human remains have yet been found here.

Portland Bill is a very interesting place. There are silicified trees and celestite in the Purbeck Group in quarries and cliffs nearby.

Old lighthouses at the back of the raised beach platform

The upper surface of the raised beach is a gentle slope towards the northwest. The old cliffline is represented by the steeper slope between the two old lighthouses shown here. The cliff has been much subdued by solifluction of calcareous debris from what were once more extensive hills to the northwest. The Old Lower Lighthouse is now a bird observatory (because Portland projects so far south into the English Channel, northward migrating birds, including rare ones, land here). The Old Upper Lighthouse was once the home of Maries Stopes, well-known as a popularising of the use of contraception. She was also a palaeobotanist, expert on Carboniferous coal petrography. While on Portland she did curating work at the Portland museum.

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. INTRODUCTION: Portland Stone - Portland Bill

Portland Stone at Portland Bill with the effects of former quarrying

Portland-Purbeck junction northeast of Pulpit Rock, Isle of Portland

The oolite of the Portland Stone is well-exposed at Portland Bill. The ooid shoal was formed on a shallow marine shelf when water depth was particularly low (Townson, 1975). The oolitic limestone in the lower part of the cliffs is conspicuously cross-bedded with most depositional dips in a generally southerly direction. The Portland Roach is present beneath the Purbeck Transition Bed but the "Portland Screw"Aptyxiella is not normally present here (although abundant in the northern part of Portland).

The sea cliffs provide excellent exposures. In addition there are some old quarries. Strahan (1898) commented as follows: "Close to the Bill of Portland, but still on its west side, some quarries were worked about 1840 and were reopened in 1889. The measurements here are:-

Lower Purbeck (part of )
Laminated limestone ("slate") .. 6 feet, 0 inches (1.8 m.)
Carbonaceous layer ("lower dirt-bed") .. 1 inch to 2 inches (2.5 cm. to 5 cm.)
7. Tufaceous limestone ("skull cap") .. 2 feet ( IMW notes - stromatolitic limestone - 0.6 m. but Transition Bed not referred to)
Portland Stone (top bed of)
6. Limestone full of empty moulds of Trigonia etc. ("roach") .. 6 feet ( 1.8m)
5. Oolitic limestone ("whit bed")

Portland Stone at White Hole or Whitehall, Portland, view north, unlabelled

Portland Stone at White Hole or Whitehall, Portland, view north, labelled

Erosion of Portland Cherty Series at White Hole, Portland Bill, Dorset

Portland Stone, Purbeck Caps and debris associated with raised beach, north of Portland Bill, April, 2003

Shown above is an unlabelled view of the cliffs north of Pulpit Rock, Portland Bill, looking northward; top right image: labelled view of the same area north of Pulpit Rock. Also shown is a closer view of the promontory with the old cliff of the raised beach north of Pulpit Rock; another image shows the continuation to the right (to the east) showing the raised beach deposits and loam and head above, with the fence of the old research establishment.

This stretch of cliff with old quarries is shown as White Hole on some old maps but may have the name Whitehall, according to Bruce (1989, 1996) who commented that stone was quarried here for Whitehall in London. The lower part of the cliffs are composed of Portland Cherty Series. The Portland Stone is conspicuous as massive limestone, generally without chert. Only the basal part of the Purbeck Group is present as a few thin beds of the Caps (with the Lower Dirt Bed). The remainder of the Purbecks has been eroded away just here. Above these Jurassic strata is an erosional surface marking the base of the late Pleistocene raised beach. The western raised beach with pebbles is present here, together with debris that has slumped southward in Pleistocene periglacial conditions.

Portland Stone at White Hole or Whitehall, Portland, view south, unlabelled

Portland Stone at White Hole or Whitehall, Portland, view south, labelled

Images above show the view southward of the White Hole or Whitehall area, towards Pulpit Rock. In the foreground is carbonate cemented raised beach. This is part of the western raised beach believed to been deposited approximately 210,000 years BP (before the present). The rounded pebbles of the storm beach are mainly of chert. Some molluscan debris can be found near here. At this point the beach pebble deposit is steeply dipping because it has partly collapsed into a fissure (an enlarged joint) which has probably opened in the Pleistocene Devensian Stage, the last glacial phase, when sea-level was low. Note that the Portland-Purbeck junction is just below the top of Pulpit Rock (once part of a natural arch). On the top of it thrombolitic limestone encrusts the site of a fallen Purbeck tree, like those at the Fossil Forest.

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INTRODUCTION:

General Introduction - Quarrying History

An old photograph of Portland Bill, Dorset, from the sea, showing quarrying operations in progress in the early part of the 20th century

An old photograph, probably from the 1940s, of Pulpit Rock and Portland Bill, Dorset, and with the last of cranes that were once west of the lighthouse

Portland Bill was once a rather remote place of rocky cliffs and open grassland, disturbed by quarrying. Of course there was no car park, no Admiralty buildings and wire fences, no cafes, no recreational huts. Only the large lighthouse and associated buildings, seem intrusive; they was built in 1905. It is still possible to see what seems, at first sight, to be a similar natural and undamaged coast by walking about a kilometre north east to the Cave Hole area.

The rocky coast looks natural and, in part, it is. However, there has been much coastal quarrying in the past. The cranes have mostly gone. However, it is easily recognised by limestone which cut with straight faces, and particularly by the presence of rectangular quarried blocks. The top photograph above shows that in the early part of the 20th century the coast on the southwestern side of the Bill from Pulpit Rock to the Land Mark (at the end of the promontory) was still being quarried. It is interesting to see the presence of cranes in the slightly inland quarries that, now abandoned, can be seen to the west of the present car park and toilets. Pulpit Rock is a quarrying relic, a piece of Portland Freestone which has intentionally been left in place (similar to the Monolith at St Aldhelm's Head and Nicodemus Knob on the northeast coast of Portland). The quarrying around Pulpit Rock seems to have been much earlier.

Between the present lighthouse and Cave Hole there has been much quarrying at the cliffs in the past. There does not seem to be any photographic record of this phase of quarrying. It may have been before about 1850, but the details are not easily found. Some place names, though, such as Mugley's Plain come from old quarry sites.

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RAISED BEACHES:

Portland Raised Beaches at Portland Bill - Introduction

Raised beach at the old cliff line north of Pulpit Rock, Portland Bill

At Portland Bill there is an obvious and well-developed raised beach (or two raised beaches) of shingle with marine mollusc shells. It forms the lower ground on which the lower two lighthouses, the car park, the pub and the cafes are situated. The slope up from this to the area of the upper lighthouse is the remains of the old cliff. The original landward (northern) margin of the raised beach at this cliff is visible in a modern cliff section north of Pulpit Rock and adjacent to the western fence of the old research establishment on the cliff top. A panoramic view is shown here with more details in other images below.

Raised  Beach - location map

The general orientation of the raised beach is shown on this map. Conspicuous Budleigh Salterton quartzite pebbles, liver-coloured and grey, are notable features of the Chesil Beach, the raised beaches and also occur on the present beach at Lulworth Cove. As a result a speculative link between the Portland raised beaches and the Lulworth coast is shown here, the Fossil Forest having perhaps once been at the back of similar raised beach, on similar bedrock to that at Portland. Study of pebbles on present beaches in this area might reveal whether there is any evidence of this. Incidently, Budleight Salterton pebbles are common on the southwest coast of the Isle of Wight and elsewhere.

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Prestwich raised beach map

This map is 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 good explanation for observed features.

The raised beach (or as been suggested - two raised beaches) is in general about 7.6m above O.D. and trends in a southwest - northeasterly direction, in contrast to the northwest - southeasterly trend of the present Chesil Beach. It extends along the coast for about a kilometre. De La Beche in 1839 first reported marine shingle here. It was later described by various authors including Prestwich (1875).

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RAISED BEACHES - Access

The raised beaches of Portland Bill are for the most part in a public area. However, the classic exposure of the back cliff of the raised beach on the west side of Portland is within a private enclosure with a high and secure wire fence. This was formerly part of the Admiralty land and has never been directly accessible for more than half a century. Now it is no longer under Naval control but now belongs to Qinetiq, a multinational technology defence company. For most, non-specialist geologists there is sufficient view for general purposes, outside of the fence. However, a new fence is very close to cliff edge now and a party may find the location too hazardous now to approach closely. There is risk of falling from the rocky edge. From the outside a general view can be seen but the site cannot be accessed for detailed study.

The company, Quintiq, however are very cooperative and have issued the following statement (forwarded to me by Richard Edmonds, Earth Science Manager of Jurassic Coast).

Accessing the Western Raised Beach, Portland.

"The site lies within the Qinetiq compound but we are willing to accommodate groups whenever possible. There are some days it might not be safe to come onto site due to pre-arranged work. We will try to accommodate same day visits but you might be disappointed. Some days it might even mean limited access to the outside of the fence too. We will always endeavour to accommodate site visits if safe to do so. There are four ways to arrange access, first by contacting the guardhouse and requesting a visit at that time, this can be done in person on the day or by phoning 01305 861130 beforehand, second by calling the site manager Mr Barry Joplin on 01305 862023 or by writing to Barry at QinetiQ Portland Bill, Portland Bill Road, Portland, Dorset, DT5 2JT or by e-mail: bjoplin@qinetiq.com"

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RAISED BEACHES - continued:

Portland Raised Beaches - Ages

Aminostratigraphy has been used to date in approximate terms the Portland raised beaches (Davies and Keen, 1985). This method depends on the theory that organic matter degrades with time in some consistent manner. The organic matter is analysed so as to give a ratio of two amino acids (D-alloisoleucine and L-isoleucine) and the ratio obtained is compared with that from shells from deposits which have been dated by radioactive decay. Errors can occur, as a result for instance of temperature differences, and the dates are only approximate and should not be misinterpreted as precise and absolute. The relative dates, however, should be consistent.

Davies and Keen (1985) distinguished an eastern raised beach dated at approximately 125,000 years and a western raised beach dated at about 210,000 years.

The approximate date of 125kyr BP (thousand years before the present) for the Eastern Raised Beach corresponds well with Marine Isotope Stage (MIS) 5e, the last interglacial period. The height above sea-level also corresponds well. MIS-5e was characterised by a high sea level with levels 4 to 6 metres above O.m between roughly 124 and 119 kyr according to Rohling, et al. (2008). These authors have discussed the rapid average rate of sea-level rise at the time of about 1.6m per century. The rate of rise above 0m at the onset of MIS-5e around 123.5kry was 1.5 to 2.5m per century. They attribute this to orbital forcing, rather than greenhouse forcing. The rapid rise is of local interest because the Portland Stone is now eroded at a rate of round about 0.15m per annum. At the time of the high rate of sea-level rise in the last interglacial the retreat rate could easily have been about 1m per annum. This could account, at least in part, for the great loss of Portland Stone land between St. Aldhelm's Head and the Isle of Portland.

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RAISED BEACHES contin.

Eastern Raised Beach Fauna

The eastern raised beach is the part well seen to the east of the main lighthouse. Davies and Keen found 62 species of molluscs. Crab claws and carapaces, echinoderm debris and annelid debris also occur. Eight species of foraminifera and several molluscs were found in the western beach.

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RAISED BEACHES - continued.

The Old Cliff of the Raised Beach

Raised beach deposits on basal Purbeck limestone and Portland Stone, north of Pulpit Rock, Portland Bill

The raised beach funnel at Portland, Dorset

Above the raised beach funnel at Portland, Dorset

The old cliff of the raised beach is seen on the western side of Portland Bill at the seaward fence of the small Admiralty research establishment. This is not a safe place and should be treated with care and only approached if at all with full appreciation of the risk. There are many interesting features here, including the pebbles and flint-chip shown above.

The raised beach deposit is 2.1 m. thick at this point and is overlain by head, much of which has been eroded or quarried away. The beach pebbles lie on eroded basal Purbeck limestone with Portland Stone beneath.

Of special interest is the funnel of raised beach debris which has fallen into a fissure. This feature has been observed and sketched since Victorian times.The fissure is about 15m in depth and 7 degrees, almost north-south. The pebbles are cemented by crystals of bladed or subspherulitic low-magnesian calcite. The west side of the hollow dips at about 50 degrees and the east side at 45 degrees. Both the sloping surfaces are borded by about 0.15m of slumped pebbles bedded parallel to the inclined surfaces. These borders are particularly well-cemented but away from the fissure there is local cementation for only about a metre on each side. On the west side the cemented region closely parallels the inclined wall of the collapse hollow and the loose material has been eroded from beneath forming an overhanging cliff. In the upper part of the fissure some of the beach debris has been sufficiently cemented so as to remain as a festoon above the open cavity. Some deposition of travertine has occurred on the walls of the fissure below.

Had the fissure existed before the beach was formed it would probably have been filled with debris and the beach levelled off by wave-action. The date of formation of the fissure is thus later than the date of formation of the beach. The raised beach deposit is overlain by loam and angular head. Although there has been much recent erosion, remains of the loam and head are present in the bottom of the collapse hollow. Thus, originally, it probably filled the depression. Had the hollow existed as an unfilled depression at the time of formation of the head the appreciable prolonged solifluction responsible for the formation of the head would have destroyed or levelled the hollow. Thus it is probable that the collapse occurred and the funnel -shaped structure was formed after deposition of the head. Thus the fissure which caused the collapse is probably of Postglacial - Flandrian - origin.

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RAISED BEACHES contin.

Portland Raised Beaches - Pebbles and Boulders

Flint chip laminae at the top of the raised beach, Portland Bill, Dorset

Details of pebbles at the top of the raised beach, Portland Bill, Dorset

Bored boulders of limestone with shells of Patella and Littorina in the Interglacial raised beach, Portland Bill, Dorset, 11th September 2009

The pebbles consist of Chalk flint, Upper Greensand chert, Portland and Purbeck chert and limestone with rarer pebbles of granite, porphyry and the distinctive Bunter quartzites from the Budleigh Salterton pebble bed (Arkell, 1947 ; House, 1993). The limestone pebbles have often been bored by small marine organism. Siliceous pebbles usually show well-developed percussion marks resulting from impacts on a storm beach. Similar percussion marks are present in pebbles on the Chesil Beach. The pebbles are not well-sorted like those of the Chesil Beach and are very variable in size. The presence of the limestone pebbles is another difference. These do not occur on the Chesil Beach except at the end on Portland adjacent to limestone blocks on the shore.

A distinctive feature is that there seems to be less Upper Greensand chert pebbles than in the Chesil Beach, where they are so abundant and conspicuous there. This needs further study because it may reveal the extent of the very recent contribution to the Chesil Beach made by the Lyme Bay chert, which was presumably not such a major source for the Ipswichian raised beach or beaches.

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RAISED BEACHES contin.:

Portland Raised Beaches - Shells, Molluscs - General

Students from London South Bank University examine gastropods from the raised beach, south of the cafe, at Portland Bill, Dorset, 26th March 2010

A closer view of gastropods from the raised beach at Portland Bill, Dorset, south of the cafe, 26th March 2010

Shells of Littorina in the Portland raised beach

Shells in the interglacial raised beach of Dreamers Bay, Akrotiri, Cyprus

Interglacial litoral mollusc shells are abundant in the eastern raised beach. Examples of Littorina littorea are particularly numerous. They resemble modern examples but are worn and the shell-colour has faded, but not completely. The photograph shows that there are small and fragmented shells present.

The following extract from a field trip report by Brokenshire (1977) which refers to some of the common molluscs. " A short walk eastwards took us to the other raised beach near the cafe, where its various structures were examined. We noted how it sits on local limestones with limestone debris produced from the erosion of Portland's coast and laid down in a rocky near-shore environment some 120,000 years ago. The group collected numerous fossil shells washed out of the beach deposit (remembering that this is an SSSI and in situ material was not tampered with). The shells consisted of the following which can all be found on a modern rocky shore: common limpet; blue-rayed limpet; edible periwinkle [Littorina littorea]; sting winkle; thick-lipped dog whelk and a few broken cowries ( Tririas ). Someone managed to find a small crab claw. "

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RAISED BEACHES - contin.

Portland Raised Beaches - Mollusca - Faunal List
The following table of mollusca is from Keen (1987), based on Davies and Keen (1985).

GASTROPODA
Archaeogastropoda:

Haliotis tuberculata
Patina pellucida
Margarites helicinus
Gibbula magus
G. umbilicalis
Diodora apertura
Patellioda tessulata
P. virginea
Gibbula cineraria
Tricolia pullus
Patella vulgata
P. depressa
P. aspera
Calliostoma zizyphinum
Skenea serpuliodes

Megagastropoda:
Lacuna parva
Cingula semicostata
C. cingillus
Aclis spp.
Trivia spp.
Turritella communis
Littorina littorea
L. littoralis
L. saxatilis
L. neritoides
Omalogyra atomus
Lamellaria perspicua
Acmea subcylindrica
Rissoa parva
Rissoa spp.
Skeneopsis planorbis
Aporrhais pes-pelicani

Stenoglossa:
Trophon truncatus
Ocenebra erinacea
Lora spp.
Nucella lapillus
Buccinum undatum
Nassarius reticulatus
N. incrassatus

Bullomorpha: - Retusa retusa

Basommatophora: - Leucophyta bidentata

CHITONIDA: - Species as yet unidentified

BIVALVIA
Protobranchia:

Nucula sulcata
Glycimeris glycimeris
Mytilus/Modiolus spp.
Nuculana minuta
Anomia ephippium
Musculus marmonatus
Mytilus edulis

Pseudolamellibranchia
Ostrea edulis
Chamys distorta
Chlamys varia

Teleodesmacea
Astarte borealis
A. sulcata
Venus ovata
Turtonia minuta
Lucinoma spp.
Mysella bidentata
Macoma balthica
Solen marginatus
Neolepton sulcatulum
Montacuta furruginosa
Cerastoderma edule
Hiatella arctica

Foraminifera from the Portland East Beach
(from Keen, 1987 and Davies and Keen, 1985)
Elphidium crispum
Miliolina seminulum
Quinqueloculina seminulum
Q. intricata
Massalina secans
Miolinella subrotunda
Polystomella striatopunctata
"Rosalina " parisiensis

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. RAISED BEACHES - contin.

Portland Raised Beaches - Head

Raised beach deposits on basal Purbeck limestone and Portland Stone, north of Pulpit Rock, Portland Bill

Devensian (late Pleistocene) head deposits of terrestrial origin overlie the raised beach and have been investigated by Keen (1985). He found land-snail shells indicating grassland with areas of marsh and small pools. A cool, but not cold, climate is indicated.

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RAISED BEACHES continued:

Middle Purbeck Limestone Blocks

Portland Stone at Whitehall or Whitehole, Portland, view north, labelled

Portland Stone, Purbeck Caps and debris associated with raised beach, north of Portland Bill, April, 2003

In the head debris at the back of the Western Raised Beach at the old cliff-line and just above the northwestern margin of the eroded platform there are large blocks of Middle Purbeck limestone. These include material from the Cinder Bed and the Cherty Freshwater Member (presumably the Flint Bed). These strata are no longer present in situ on the Isle of Portland (although the Soft Cockle Member is present). The blocks have moved downslope, perhaps in periglacial conditions, during the late Pleistocene when the Middle Purbeck strata had not been eroded away. The blocks are present on the small promontory projecting westward (left in the image) below the metal fence and above the raised beach near the top of the cliff in this White Hole or Whitehall area. This is not a very safe place to study the strata and visiting it is not recommended.

Purbeck Fossils - modified from Arkell

Purbeck Fossils - old illustration

Fossils in these blocks include Praeexogyra distorta in the Cinder Bed material and various Purbeck pondsnails such as Viviparus in the Cherty Freshwater blocks. The images here show various common Purbeck invertebrate fossils including some species present here.

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RAISED BEACHES continued:

Cryoturbation beneath the Eastern Raised Beach

Cryoturbation of basal Purbeck limestones beneath the Eastern Raised Beach

Beneath the raised beach deposits on the south-east coast of Portland (Broad Ope) cryoturbation features are well-developed. They are at first obvious as contortions in fragmented limestone of the basal Purbeck Group and are the result of freezing and thawing of the permofrost in the subsoil during the late Pleistocene (Devensian), the final phase of the Ice Age. The main ice sheet did not reach this far south but the conditions were cold, periglacial (like Siberia or northern Canada). The cryoturbation features have been described by Pugh and Shearman (1967). They found the effects of the ice has affected the Purbeck Hard Cap (Top Cap), the Lower Dirt Bed and the Skull Cap. The cryoturbation features include small diapirs, slurried dirt beds and frost wedges. Notice the dish-shaped contortions in the photograph and particularly note that they occur beneath, as well as above, an almost undisturbed limestone bed.

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LOCATION:

Pulpit Rock, Portland Bill, Introdoction:

(map reference SY 674684)

Pulpit Rock viewed towards the southwest, Isle of Portland, Dorset

Pulpit Rock looking towards the south

Pulpit Rock from the landward side, Portland Bill

Pulpit Rock, Portland Bill, Dorset, a close view, with an AAPG Student Chapter, 2011

Pulpit rock, Portland Bill at sunset

Pulpit Rock forms a small promontory northwest of Portland Bill. It is not clear as why this is a projecting point. The tradition is that there was once a natural arch at this small headland. The succession is Portland Cherty Series from sea-level (and below) to about the level of the flat platform. Above this was Portland Freestone, the oolite. This is still preserved in Pulpit Rock but has been quarried away inland of this. The steps up to the rock are in a sloping slab left by the quarrymen and not in a natural position.

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LOCATION:

Pulpit Rock continued - Thrombolites:

(Microbial carbonate precipitation around fossil tree remains, taking place in a hypersaline lagoon -
of the basal Purbeck Group - see also:
Forest Forest, Lulworth Cove.)

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The sequence at the Portland to Purbeck junction in Pulpit Rock, Portland Bill, Dorset, showing cross-sections of thrombolites around tree branches, 7th September 2012

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The Portland-Purbeck regression seen at the top of Pulpit Rock, Portland Bill, Dorset

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A small elongate and cyclindrical thrombolite around a hollow mould left by a small tree or branch, Pulpit Rock, Portland Bill, Dorset

At the top of Pulpit Rock are some thin-bedded limestones. These are part of the basal Purbeck Group, specifically the Transition Bed or Skull Cap, the Basal Dirt Bed, the Lower Dirt Bed and parts of the the Hard Cap.

Beneath these the the lithified shell beach of the Roach; the aragonitic shells of this have been lost to produce mouldic secondary porosity. There is a gentle passage into the Purbeck Skull Cap, also known as the Transition Bed. This has no large marine molluscs, but just ostracods and small lagoonal gastropods (Hydrobia and Valvata). It is usually peloidal (pelletoidal) and shows small-scale cross bedding in this section. There is the Basal Dirt Bed, just above the Transition Bed, then the lowest part of the Hard Cap and then the Lower Dirt Bed. These dirt beds are rendzina palaeosols, carbonaceous and calcareous clay. They are now recessed in the cliffs by erosion. Trees were rooted in the Lower Dirt Bed. Small tree holes have been left by the decomposition of small fallen trees or branches (probably young trees).

Visible on the top of Pulpit Rocks is thrombolitic (stromatolitic) limestone that has coated a flat-lying tree trunk, lying in an east-west direction and associated with the Lower Dirt Bed. This blown-down or washed down tree has decomposed but the limestone preserves its shape as a mould, which resembles a dug-out canoe.

Higher parts of the Purbeck Group, such as the Great Dirt Bed, which should follow next, have been eroded away.

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LOCATION:

Pulpit Rock continued: Erosion

Bioerosion and solution seen at low tide, Pulpit Rock, Isle of Portland

Because Pulpit Rock is standing in water, even at low tide, there is no abrasion of the stone taking place by impacted sand or gravel clasts. Breakage can occur along bedding and joints during storm wave action, as shown by the partial disintegration of the ledge to the northeast. Less dramatic but progressive destruction of the carbonate rock is taking place by bioerosion and solution. There are attached molluscs and other organisms which can gradually damage and cut back the rock surface, although barnacles may possibly give a limited degree of protection.

Here is the coast south of Pulpit Rock to the Trinity House sea marker at the very end of Portland Bill. Quarrying in the 19th Century has damaged this stretch. Most of the Portland Freestone has been removed, leaving lower ledges of Portland Cherty Series with some non-oolitic Freestone.

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LOCATION:

White Hole, Whitehole or Whitehall

(map reference: SY 675685)

This is a shallow embayment north of Pulpit Rock. Quarrying has taken place here so that platforms or ledges of unworked stone are left. At one time there was still the last remains of a wooden crane of the typical Portland type on one of the platforms. At the landward (eastern) side of the stone platforms is a succession up to the basal Purbeck Caps with the Portland Western Raised Beach overlying. This is mostly within an Admiralty research establishment and closed off by a wire fence. Around the margins of the closed areas the abundance of rounded, beach-battered pebbles is obvious, but it has to some extent been disturbed by quarrying. An important section of the raised beach is preserved behind the fence at the northern end of this small stretch of cliff.

Comparison of photographs showing erosion at White Hole, Isle of Portland

Erosion and collapse of the ledge at White Hole

The Portland Stone seems very resistant to erosion and one might think that there is little retreat of the Portland Stone cliffs. This is not the case. Fissures, particularly the dominant NNE ones, are enlarged by continual wave action and become caves. Attack directly against the ledges eventually loosens blocks and they fall into the sea. This process can be studied by examining the photographs above. Part of the ledge north of Pulpit Rock is collapsing into the sea. Some of it has already gone and waves are breaking on the fallen rocks. Of course this ledge has been quarried and the Portland Freestone (oolite) removed. If the full sequence had been left probably this part of the coast might have been a little more resistant.

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LOCATION:

Broad Ope, Northeast of the Lighthouse, a Cave-Collapse Coast

The cave-collapse coast of the northeastern part of Broad Ope, near Portland Bill, Dorset, seen from the top of the lighthouse

The cave and ledge on the northeast side of Rudge Poryx or First Beach, Broad Ope, near Portland Bill, Dorset, 11th September 2009

Cave-collapse coast with raised beach or Plio-Pleistocene marine deposits at Dreamers Bay, near Cape Zevgari, Cyprus

Cave Collapse Coast at Dreamers Bay, near Cape Zevgari, Cyprus. Edited image of limited resolution.

Portland Bill, view from lighthouse

These photographs show erosion of limestone by undercutting, cave formation and collapse. This is very well-seen at Portland Bill in the area of the last Interglacial raised beach. It is interesting to note that on the Akrotiri Peninsula (Cape Zevgari to Cape Gata etc) on Cyprus a stretch of very similar coast has been produced. This is where Plio-Pleistocene or perhaps raised beach deposits occur above almost horizontal limestone of the Tertiary Pakhna Formation. The location, Dreamers Bay is near the southern end of the "Isle of Portland" of Cyprus. For more on this go to:
Salt Lake and Coast of the Akrotiri Peninsula, Cyprus.

Coast erosion by sea-cave collapse

This diagrams shows in a schematic manner a suggested process of coast erosion near Portland Bill by cave-collapse. The southeastern coast of Portland, at Portland Bill is known as Broad Ope. The eastern stretch of this section of coast has numerous large sea-caves cut into the Portland Freestone, the oolite, above the Portland Cherty Series. The roofs of the caves are the upper part of the Portland Stone and the lowest limestones of the Purbeck Group. Above that comes the raised beach. The sea can easily erode the relative soft oolite which is split by major joints and bedding planes. The cave pattern is probably controlled by these. The trend of the major joints is NNE which is oblique to the coast. Eventually the caves collapse in part or completely.

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LOCATION:

Red Crane

Nearly Horizontal Portland Stone at Red Crane promontory, Portland Bill, Dorset

A wave enters the major fissure or enlarged joint adjacent to Red Crane, Portland Bill, Dorset, March 2010

Red Crane Blowhole in action, 29th November 2009, Portland Bill, Dorset

Storm waves at the Red Crane,Portland Bill, Dorset, produce a blow-hole effect in a fissure or enlarged joint, 2nd December 2007

There is a major joint just to the southwest of Red Crane. This has been widened by erosion of the sea. At high tide during storms waves surge into this fissure trapping compressed air ahead in the narrow landward end. This immediately afterwards blasts out the top of the narrow slit as a blowhole (or blow hole). Although mostly inactive, this is an impressive feature during major storms with large and high waves

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LOCATION:

Robinson Crusoe Island and Pom Pom Rock

A sea stack of Portland Freestone, Robinson Crusoe Island, Broad Ope, Portland Bill, Dorset, 11th September 2009

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Pom Pom, a limestone stack, at Portland Bill, Dorset, in 2009, before its destruction by a storm in January 2014

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Loss of the small stack, called Pom Pom, at Portland Bill, Dorset, in the storm of 6-7th January 2014, as seen on the 17th January 2014

A small, vertical, stack of Portland Stone, named Pom Pom, was washed away by the severe waves of the storm of the 6-7th January, 2014. Robinson Crusoe Island was not affected

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LOCATION:

Butts Quarry

Butts Quarry at Broad Ope, northeast of Portland Bill, Dorset, 11th September 2009

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LOCATION:

Rudge Poryx (First Beach) and Adjacent Area

The cave and ledge on the northeast side of Rudge Poryx or First Beach, Broad Ope, near Portland Bill, Dorset, 11th September 2009

A beach probably formed by cave-collapse, Rudge Poryx, Portland Bill

Foresets in carbonate sand of the Portland Freestone, southwest of Rudge Poryx, Broad Ope, Portland Bill, Dorset, 11th September 2009

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LOCATION:

Cave Hole and Adjacent Area

(SY 687691, northeast of Portland Bill)

Cave Hole represents the first stage in cave collapse. This is a depression 12 m inland from the sea-cliffs where the roof of an underlying sea-cave has collapsed (Falcon-Lang, 1998). The map reference is SY 6865 6903). Steel rails and stone blocks have been placed over a hole down into the cave. First Beach (which used to be known as Rudge Poryx according to Bruce, 1989) is the probably largely the product of complete collapse of a similar sea-cave. It still overhangs on its eastern side. Note the cross-bedding in the Portland Stone on the western side. Other embayments may be formed, at least in part, by collapse of caves. The promontories are in some cases eroded to so as to leave stacks or small islands.

There has been much quarrying at various places along the whole stretch of Broad Ope. Much of it probably started in the 19th Century or earlier. Some seems to have continued into the 1930s because a photograph from this date in Morris (1990) shows many old cranes with piles of shaped stone blocks nearby and at the quarry near Cave Hole, a tall metal crane of later type. In some cases old quarrying has left level platforms. Stone was loaded into boats by the cranes on the cliff top. Some of these still exist, although more recently they have been used for lowering fishing boats. Broad Ope Crane is on the cliff edge near Cave Hole.

Cave Hole, Portland Bill

Cave Hole where a cave has broken through to the surface

Down Cave Hole

Golden samphire with butterfly, near Cave Hole

At Cave Hole the collapse of a cave to form a new narrow beach is in progress. A major cave has extended back some way from the cliff. At the top of the cave there has been some collapse of the overlying, thin Purbeck succession. The unconsolidated debris of the raised beach and loam has then funnelled down into the cave. The result is a shallow pit with a hole in the centre and a view down to the sea. This hole was presumably very dangerous until some iron bars were fitted but they are rusty and it is not safe to walk on them. Some years ago some Portland Stone blocks presumably from the nearby quarries were placed on top of the iron. They do not give security but if the iron bars were to fail then the stone blocks would fall before a person would. I suppose they thus would give a warning of a sort. The iron bars and the stone blocks will, of course, all fall in sooner or later and the hole will enlarge. Eventually the hole will join through to the cliff and the new embayment will be created. Notice that the rough ground with calcareous brown loam seems to favour the growth of golden samphire. Many butterflies can be seen on this in the summer.

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LOCATION:

The Stream, Waterfall and Waterfall Cave

(SY 688691, northeast of Cave Hole)

(image to be added)

At Waterfall Cave, near Cave Hole, also shown on the map above, a small stream trickles (or tumbles in flood) over the cliff. It may be completely dry, though, in dry summers.

There was a shipwreck in the cave here; there is a book about the shipwreck - Davison, 1953 - Last Voyage, Heineman and Attwooll (1998) provides a picture and more information. In 1949 Frank and Ann Davidson sailed from Fleetwood in the ketch Reliance intending to make a fresh start abroad on an island. They had difficulty with stormy weather since the start of the voyage. The ketch was driven half way into this cave on the 3rd June. Frank Davidson died but wife survived. The ship broke up, leaving only the remains of the engines on the seabed at the cave mouth (Bruce, 1989).

When in flood the stream splashes over a ledge formed by the Transition Bed at the base of the Purbeck lagoonal succession and above the Portland Stone. The top of the hard laminated limestone of the Transition Bed forms a flat ledge at the top of the vertical cliff along most of this stretch of coast. It is interesting that this little stream should have a waterfall over a cliff, rather than having a small valley graded to sea-level. Now the stream considered here is very small but there are other larger examples such as Osmington Cascade at Osmington Mills and Freshwater Steps Waterfall between Kimmeridge and Chapmans Pool. It is an example of a special and peculiar feature of the Dorset Coast. As Arkell (1947) pointed out "at first sight the last movement might be taken to be elevation. At Chapman's Pool, Winspit, Seacombe, Lulworth Cove, and at many small " gwyles" elsewhere, broad valleys end off abruptly half way up the cliff, as if cut by a knife, and in the floor small rejuvenated streams tumble rapidly to the sea through narrow gorges like the chines of Bournemouth, or end at the cliff edge in a waterfall." ..." But these valleys are all very short and have a rapid fall seawards. They are, in fact, only the heads of much longer valleys which formerly extended southward to join some lost trunk stream flowing down the Channel. The surviving heads are too high to have been affected by the submergence of the mouths of the streams. Instead they furnish a measure of the coast erosion that has taken place, for they are the last remnants of a valley system almost entirely consumed by the sea, which is advancing faster than the dwindled streams can cut down to reach it." The conclusion that we can draw with regard to this little rivulet is that even the Portland Stone is being eroded by the sea, presumably here by cave-collapse, faster than a stream can make a large knotch, let alone cut down to sea- level. Of course it may be argued that this little flow of water is too small for much erosion. Most streams do little most of the time. Flood, however, will occur occasionally and then erosion should be significant.

When you are on the coast have a further look around the cliff margin here. The barren edge of the cliff around here may due to quarrying or to the effects of sea-spray in fierce storms.

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LOCATION:

Northeast of Cave Hole (Godnor in local terms)

Part of the coast between Cave Hole and Sand Holes, Portland Bill, Dorset

Old photograph of old cliff quarry between Cave Hole and Sand Holes, Portland Bill, Dorset

Exploring the cliff top near an old crane at Sand Holes or Sandholes, Portland, Dorset, 2011

Northeastward from Portland Bill towards Sand Holes the Portland Group rises gradually up-dip. The Cherty Series is at sea-level here. Caves are formed and some of them are quite large. Because there is more Portland strata above sea level, these caves have more substantial roofs and do not collapse so readily. Thus, much of the coast here retreats in the normal manner by cliff erosion. As shown in the photographs there is an area of former quarrying where much debris has been tipped to the sea. It is not clear whether there was once a beach or small embayment at this particular place. Notice that on the cliff top where the photograph was taken there are many blocks of stone from the quarries. Since they have been chiselled to a rectangular shape and were presumably were ready for use, it is surprising that they were never transported away by sea. Notice, too, that the crane was situated immediately over a cave with enough depth of water for a boat to approach, and not dry even at low-tide.

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OFFSHORE OF PORTLAND BILL:

Seafloor Geology

Geology of the seafloor around and south of the Isle of Portland, Dorset, after Donovan and Stride (1961)

Portland Bill is situated at a rather unusual site in terms of the offshore geology. It is at the northern end of a plunging syncline. The present Portland lighthouse is almost on the axis. The geology of the seafloor of the area around Portland was investigated by Donovan and Stride (1961). There was an acoustic survey supported by coring, the observations of divers and sample collection. This map is based on their work with some minor modifications (the geology of the offshore area is also shown on the British Geological Survey 1:250,000 - Portland Sheet 50N 04W, solid geology). Later more information has become available from seismic studies and multibeam bathymetry studies. These have not changed the general pattern but provided important detail.

The major offshore feature is the Shambles Syncline. This plunges towards the east. The geomorphology of the Isle of Portland is influenced by this fold. The progresssive easterly swing of the dip from the north to the south of the island, as noticed by Strahan (1898), is because the northern part of the Island is situated clearly on the north limb whereas Portland Bill on the axis is an area with a dip that is controlled by the low angle of plunge of the axis to the east.

The Kimmeridge Clay occupies a considerable area of the seafloor and Portland Harbour lies over these easily eroded material. West of the Chesil Beach ichthyosaur and other vertebrate remains from the Kimmeridge Clay have been found on the seafloor by divers.

The Portland Stone forms prominant ridges on the seafloor but this and the Purbeck Group proved difficult to core. Ostracod limestones of Purbeck type were found in the eastern part of the submarine Purbeck outcrop shown on the map. The middle Purbeck limestones form a slight ridge on the seafloor, just east of the area shown on the map (Donovan and Stride, 1961).

What is shown as Wealden is not firmly proven to be such but it is very likely that it is. Donovan and Stride cored grey and faintly pink marl with lignite and pebbles of Purbeck limestone at one site but the material was disturbed and contained no diagnostic microfauna. Grey sand with carbonaceous matter was obtained and further east typical Wealden mottled clays and marls were found.

The Shambles Bank is a pronounced ridge rising about 20m above the seafloor. Donovan and Stride (1961) considered it to be a sandbank because Admiralty Charts and surveys record only sand, shingle and broken shells on its surface, and there are sand waves along the southern side. Two ridges at the eastern end are oriented differently from the remainder of the bank. The southern and larger one is assumed to be Portland Stone , whilst the northern one is a resistant bed in the Portland Sand.

Pingree (1978) has discussed the tidal origin of the Shambles Bank. More recent work has been done by Bastos, Kenyon and Collins (2000). They have used bathymetric, hydrodynamic and side-scan sonar to investigate the sediment processes associated with the headland of the Isle of Portland. They have produced maps which show a fairly symmetrical pattern of submerged sand banks on both sides of the peninsula with erosion taking place at Portland Bill. Of the sand banks on the eastern side, the Shambles Bank, is in much shallower water than the western ones, and is therefore more well-known. There is greater influence of waves on the exposed west side of the headland and this produces some asymmetry of pattern.

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OFFSHORE:

The Portland Race

Portland Race with rising tide and westerly wind, 29th November 2009, seen from near Red Crane, Portland Bill, Dorset

A telephoto view of the spindrift coming off breaking waves in the Portland Race off Portland Bill, Dorset, 29th November 2009

A modified aerial view of Portland Bill, Dorset, showing the western Portland Race and part of Portland Ledge

A fishing boat sails northeast against the Godnor Race, part of the Eastern Portland Race or strong current, from the cliffs near Cave Hole, Isle of Portland, Dorset, 11th September 2009

Strong currents of ebbing or flowing seawater rounding Portland Bill form the Portland Race. The currents can be seen from the shore as stretches of broken water. The Admiralty Chart of 1863 describes the Portland Race as "a periodical commotion of the sea which rages with great violence ( (Bruce, 2001). Apparently in fog or darkness it can be heard some miles away like the rumble of a distant train. The Race has been said to have prevented four galleasses of the Spanish Armada, vessels using sails and oars, to deal with Frobisher's ships, which may had been seen anchored or possibly becalmed off Portland Bill (Bruce, 2001). It has probably been involved in many shipwrecks. The wrecking of the Earl of Abergavenny on the Shambles Bank was the result of intended avoidance of the Race, and it is discussed briefly below.

Spring peak tidal currents, in the vicinity of Portland Bill and near the water surface, range from 0.4 metres per second up to 3.6 metres per second (about 13 km per hour or 8 miles per hour) (Bastos et al., 2002).

During the tidal cycle, the development of tidal eddies has been observed on both sides of Portland Bill in response to ebb- and flood- dominated flows respectively. The dominant wave approach over the area is from the southwest with significant wave height of 0.8 metres. Significant wave height exceeds 2 metres for only 10 percent of the year (Bastos et al., 2002).

The conditions on the race are not favourable for small boats. ( Bruce (2001) advised:

"Although a well-found and skilfully handled boat is unlikely to sink if caught up in it, there is always a danger of losing someone overboard. The best course of action within The Race is to attach safety harnesses to something really solid, close all hatches and point down the seas at the minimum speed to ensure steerage way. The tide, which may be running at seven knots or more, will carry a vessel through The Race in about ten unforgettable minutes."

There is an inshore passage clear of the western part of The Race up to about a mile wide at times. Boats often use this and pass quite close to the rocky shore.

The cause of The Race is of course the consequence of tidal currents passing round Portland Bill, which projects a long way to the south from the local coastline. More specifically there is about one kilometre of shallowly south-dipping Portland Stone south and southeast of Broad Ope descending to about 6 fathoms (roughly 12 metres) and lower (see map above). The broad ledge has a projection called the Shambles Ledge at the northeastern end and a wider projection for more than 2 kilometres directly south of Portland Bill. These projections to some extent control the locations of the separate eastern and western parts of The Race. The eastern Race has a branch, the Godnor Race, approaching the shore between Cave Hole and Godnor (although note there is some variation about the use of the name Godnor or God Nore - I refer here to Breston - see ( Bruce (2001) for more information on this point of nomenclature)

As shown on the map above, the western edge of Portland Ledge (of Portland Stone) is a steep scarp perhaps about 20 metres or more from top to bottom. Divers have reported caverns as large as a cathedral here ( Bruce (2001) but I do not know whether this is an exaggeration.

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OFFSHORE:

The Shambles Bank and the Portland Bank

Book on the wrecking of the East Indiaman, The Earl of Abergavenny, on the Shambles Bank, near Portland Bill, Dorset, in 1805

The Shambles red warning light in the upper part of Portland Bill Lighthouse, Dorset, 11th September 2009

Banks of shelly sand and gravel have accumulated east and west of The Race. The most well-known is the Shambles Bank because it is very shallow and a hazard to shipping. It is famous regarding a particular shipwreck.

Hayter (2002) wrote an account of the sinking of the East Indiaman, the Earl of Abergavenny after hitting the Shambles Bank in 1805. She summarises the story as below:
"In February 1805 The Earl of Abergavenny, one of the largest of the East India Company's ships set sail in convoy from Portsmouth for a voyage round the world to India and China. Her captain, John Wordsworth, was the younger brother of the poet William Wordsworth. On board were more than 400 passengers, troops and crew and a rich cargo of luxury goods and silver dollars. Only three days later, separated from the convoy by the stormy weather, the ship struck the notorious Shambles Shoal in Weymouth Bay and sank, drowning 260 souls including her captain."

The accident was apparently the result of the ship returning to Weymouth from the west of Portland. The pilot, a Portlander, sailed a league south to avoid The Race and the dreaded Shambles. The intention was to swing round the southeastern end and head north to safety of Portland Roads, near Weymouth. When the ship was at the eastern end of the Shambles, but really too close for safety, the wind dropped and the ebb tide carried the East Indiaman onto the breakers on the shoal. Captain John Wordsworth was heard to cry "Oh pilot! pilot! you have ruined me. The ship hit the Shambles at 5 o'clock on the evening of 5th February 1805. The sudden blow threw the ship on the side and the sea poured down the hatchways and companionways. However, she righted herself, but efforts to sail northward off the bank towards Weymouth sands were not initially successful and the ship was holed. She drifted northward into Weymouth Bay and sank during the night. The depth was such that the upper part of the mast and rigging were above water. Initial some of the ship's occupants survived in the rigging. Thomas Gilpin, a survivor, wrote about the captain - John Wordsworth:

"I don't think that he was wash'd overboard. I see him on the Poop, less than a minute before she went down. In the act of her going down I ran to the Poop, looking out which way to save myself, and she sinking so rapidly that as I ascended the Mizzen Shroud the water catch'd me, before I cd get up into the top. Sum minutes after this I see several men hanging by ropes, fast to the mizzen mast amongst which was Capt. Wordsworth. I went down into the Mizzen Rigging to see if I cd render them any assistance, I go within 10 or 12 Feet of where he was, I hailed him as loud as I cd and threw him a rope, he was motionless and insensible and he did not katch the rope or answer.

He was swept away by the sea. The above extract is from Hayter (2002), and the book is recommended for interesting reading.

The details of the Shambles Bank are best explained in the paper of (Bastos et al., 2002). It is classified as a Headland-Shelf Deposit, an HSD. It is the most prominent sedimentary deposit of the area. It is an elliptically shaped bank, with a maximum height of 22 metres, about 5 km in length and 2 km in width. The bank is aligned in a NE-SW direction, subparallel to the eastern coast of the Isle of Portland, showing a strongly asymmetrical profile, steeper towards the coast (NW) (Bastos et al., 2002)

Most people may be unaware that there is another bank on the west side of The Race. This is the Portland Bank; it is not conspicous because it is an accumulation of sediment in fairly deep water, with no obvious effect at the surface and no hazard to shipping. These banks have been investigated, using sidescan sonar and high-resolution shallow seismic processing, by (Bastos et al., 2002) and their work should be consulted for details (it is available online).

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APPENDIX 1.

- Comparisons with Raised Beaches of the Oregon Coast

View down to coast from Cape Perpetua, Oregon

Spouting Horn, a blow-hole, at Cape Perpetua, Oregon

Devil's Churn at Cape Perpetua, Oregon

Raised beach pebble bed near Devil's Churn at Cape Perpetua, Oregon

The Oregon coast of the U.S.A. is very different from the Wessex coast in certain respects, particular in the presence of mountainous forest adjacent to the coast in places, and because Tertiary basalt is very widespread. In some aspects, though, there are similarities. One feature in common is the presence in places of Pleistocene raised beaches or marine terraces above the bedrock.

An example of a Pleistocene raised beach deposit occurs above a raised wave-cut platform at Cape Perpetua (the name was given by Captain Cook who sailed past here). Tertiary basalt has been eroded at a time of higher sea-level in the Pleistocene and beach gravels and sands deposited above it. The present sea, now at a lower level, is now cutting back slowly into the rocks below the platform. Fissures have been opened by the sea and small caves formed. Spouting Horn is a blow-hole caused by large Pacific Ocean waves building up pressure in a cave with a small opening at the top. At Portland Bill the feature known as Cave Hole is a cave with an opening in the roof; in normal wave conditions, though, it does not at present operate as a blow-hole.

Also shown is the Devil's Churn at Cape Perpetua which is nearby. Here again there is a raised, wave-cut platform of basaltic lava. A joints has been opened to form a large fissures, as at Portland Bill. In this opened channel the seawater is churned to foam like the cream of milk. Notice that although there is direct wave action from the Atlantic Ocean to Portland Bill, foam is not developed at the Bill in any quantity. The foam from the Pacific Ocean at the Devil's Churn is the result of abundant algal plankton in the seawater. The foaming is increased too because large waves persistantly beat on the coast at that locality. It is interesting to note that warnings of tsunamis from earthquakes are frequently given on the signposts on the Oregon coast. There have fatalities due to such large waves. Even though it is not totally absent, the risk from tsunamis is small at Portland because the headland is not near an earthquake zone (although anomalous rise and fall of the sea has been recorded at Lyme Regis and unusual large waves have struck the Chesil Beach).

Raised beach at Seal Rock State Park, Oregon

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ACKNOWLEDGEMENTS

I would like to thank Tom Williamson, who is writing a book on the use of Portland Stone in London buildings, for helpful information on the early quarrying of Portland Stone. I am particularly indebted to the members of the Open University Geological Society and the members of many other visiting geological groups for discussion in the field relating both to this page and other web pages. Field parties have included the Delft Chapter of the AAPG and the Wessex Lapidiary and Mineral Society. Dawn and Tony Denyer very generously provided facilities and guidance for field work in Oregon, for comparison, and I am very grateful to them for their kindness and helpfulness. . I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.

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BIBLIOGRAPHY AND REFERENCES:

Go to Bibliography and References relating to the geology of the Isle of Portland.

Go to Bibliography and References relating to the Chesil Beach

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Copyright © 2014 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.