INTRODUCTION:
Old Name of West Cliff - "Clay Knapp".
The name "Watton Cliff" is not the local name. It was used by Buckman (1922), p. 380, footnote 2, and it seems from the text of his paper that he invented the name:
"The local name for the hill of which this cliff is the face is 'Fourfoot Hill' and for the cliff itself 'Clay Knapp'. E.C.H. Day has the name 'Fourfoot Hill' (III, p. 286). It may be suggested that the name is really 'Forefoot Hill', from the tumbled platform at the base of the cliff, although this seems to trespass dangerously near the usual error of folk-etymology. It seem advisable to distinguish it as Watton Cliff, from the name of the farm which lies behind it."
(I met a local person walking above the cliff and I was myself surprised to find that
he had never heard the name - "Watton Cliff". It just seems to have been a geological invention by S.S. Buckman in 1922. It is now very well-established in geology and fossils have been named after it.)
INTRODUCTION:
General
West Bay, near Bridport, Dorset is situated somewhat west of the centre of the English Channel coast of southern England. The British National Grid Map Reference is SY 467902.
The area is discussed in the new publication:
British Geological Survey Memoir (2011), Geology of South Dorset and southeast Devon and its World Heritage Coast (available from the BGS Bookshop online at 24 pounds sterling).
West Bay is small attractive holiday resort with a harbour built in 1722. In 1403 Joan of Navarre arrived here en route to marry King Henry IV. Shipbuilding in the River Brit around here dates from the days of Alfred the Great in the 9th Century. More recently the schooner "Speedy" was built here in 1853 to compete with American clippers. West Bay, at the present time is used for boats, fishing, beach activities, tourism and geology.
INTRODUCTION:
Safety and Risk Assessment
This website is not an itinerary or a tour guide and not, of course, responsible regarding the details of your field visit or trip. It is certainly not advising you to go any particular place or carry out any particular procedure. It is only a geological description of the coast. No liability regarding accidents is accepted. Whatever you do, you do, you do so at your own risk and you should assess the risk at the time.
In any case, the coast will have different levels of and details of risk on different occasions, according to cliff conditions, weather conditions and tide conditions. You should not necessarily copy any procedure shown in the photographs. No responsibility is accepted regarding your safety. Do not, of course, proceed any place that you consider hazardous.
By February 2018, the western cliffs of the West Bay area do not seem to have had a record of several accidents like that of Lulworth Cove, but that is probably mainly a statistical matter, because there are less visitors to the cliffs in the Bridport area.
Please note that although the photographs of various dates here are intended to show details of the strata, there may be people in the photographs (of various years) who may by chance be in specific places that are not necessarily safe, and may not necessarily be recommended for visits. Of course, coastal erosion changes details of the cliffs. Everything shown should certainly not be copied.
At the cliff top, as is obvious, keep back from the cliff edge, and preferably stay on the footpath.
West Cliff or Watton Cliff can, like many other cliffs, be dangerous and parts may be prone to rock falls. There is certainly some risk of falling blocks of rock, particularly from the Forest Mable and the Boueti Bed. Parties should ideally wear hard hats. In general and provided there is not a combination of high tide and stormy weather the pebble beach is fairly wide and it is possible to pass without proceeding very close to the cliffs. In wet weather these can be very muddy with soft mud slides and care should be taken not sink into them.
In certain stormy conditions this stretch of coast might possibly be unsafe from breaking waves. In quiet weather or generally normal conditions there is no serious problem of being cut off by the tide. Special precautions should be taken during storms. There is easy access away from the coast at both the Bridport Harbour or West Bay end and at Eype Mouth. There is risk of falling from the top of the cliff so keep away from the cliff-edge. If proceeding westward onto the beach from Bridport Harbour (West Bay), note that a scramble over sea-defence blocks of Portland Stone is necessary. Take care to avoid falling and receiving injury or being trapped.
People should be aware that flint or chert pebbles must not in any circumstances be hammered. They produce small, high velocity splinters which can penetrate the skin and are a serious danger to eyes. Swimming in the sea is not advised and is not a subject of this webpage.
Do not proceed onto mudslides or any potentially soft mud. Generally such places are an obvious problem. There have been mudslides particularly in the fault-corner area. Take a mobile phone in case of emergencies.
Please make your own safety assessment at the time, particularly if you have a field party. If the conditions seem hazardous, keep away from cliffs, from the sea or do not go. Decide whether or where you need safety helmets. All this is obvious; but this website is not directing or persuading you to do anything specific.
FOSSIL COLLECTING
The cliffs of West Dorset, particularly the clay cliffs are famous for their fossils. The section at West Bay described here has interesting fossil material but is not usually a place for large-scale collecting. If you are interested in fossil collecting from any part of the coast under National Trust and Charmouth Parish Council Ownership between Lyme Regis and Hythe Beach at Burton Bradstock, then the pilot fossil collecting code of conduct for that area should be consulted. Serious collectors, in particular, and especially anyone wishing to excavate, should study the details of the scheme and registration. If you are not involved in major excavations but simply picking up a few odd fossils from the beach you are not likely to be adversely affected by this. The leaflets should be seen and they contain useful safety guidance.
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GEOMORPHOLOGY AND STRUCTURE - Fault Corner,
General
Fault Corner, near Eype Mouth, is the location where the Eype Mouth Fault crosses the coast obliquely. There has been much collapse of the coast at this particular site which is on the eastern side of the valley.
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A major mudslide has occurred in 2012 at Fault Corner near Eype Mouth. This is the site of a former mudslide shown on British Geological Survey maps. It is not a new feature but it has been reactivated after very heavy rainfall, this year. The mudslide is almost entirely in Frome Clay, although probably bringing down some material from the Forest Marble above and nearby. The Eype's Mouth Fault is at the back of it (obliquely) and it does not seem to extend landward of the fault.
By 9th June 2013 the basal part of this mudslide has been eroded back and it is no longer an obstruction on the beach. Rather smoothed blocks of Forest Marble are the conspicuous relics on the shore. Mudslide debris is still present on the cliff slope, but has hardened by ths date, almost a year later.
By 2018, there was little sign of the mudslide at the beach. It had been eroded back. Debris from it could be seen in a low cliff at the foot of the former slide.
GEOMORPHOLOGY:
Fault Corner Mudslide, 2012 - Backscarp
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[EYPE MOUTH, OR EYPEMOUTH]
If you are walking westward (from West Bay) on the cliff top, after looking down o the landslide at Fault Corner, you soon descend to Eype Mouth or Eype's Mouth. Perhaps on another occasion you may wish to park your car here. There is a small valley, with a stream. The stream is entrenched into a small deeper valley at the end; the is the result of coast erosion occurring and the stream cutting down to meet the shore. It is a common process on the Dorset cliffs and is well seen at Kimmeridge, for example.
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GEOMORPHOLOGY:
Fault Corner Mudslide, 2012 [since then, eroded back by 2018]
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The photographs above show the state of a large mudslide of Frome Clay and Forest Marble at Fault Corner, Watton Cliff, in August 2012. By June 2013 the lower part of this mudslide had been destroyed by sea erosion. The only relics on the beach were beach-worn blocks of Forest Marble limestone. These are shown in the photograph below.
STRATIGRAPHY - INTRODUCTION:
Cliff Section and Succession - Introduction
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The general structure and major rock units present in West Cliff or Watton Cliff are shown in this diagram. It should enable easy recognition of the main divisions. Following House (1993) , the clay above the Boueti Bed is shown as the lower part of the Forest Marble. This is emphasised because it is easy to assume that the Fuller's Earth (Frome Clay) extends up to the shelly limestone.
Fault Corner is complicated because the major fault is east-west and oblique to the coast. This simplified cliff section does not bring out the details but does show the great extent of the throw. More information on this important fault will given later.
Left: The Middle Jurassic zonal sequence and strata of Dorset are shown here for introductory and reference purposes, and also for comparison with the strata of other regions.
Right: This vertical section shows the Middle Jurassic succession of Dorset in more detail. Note that only the upper part of the Fuller's Earth, the Frome Clay, can be seen in West Cliff or Watton Cliff.
This diagram of Bathonian strata based on Torrens et al. (1969) is very useful for understanding the relationship between the very argillaceous Dorset sequence and the limestones of the Bath and Cotswolds areas. Note, for example, how the the thin Boueti Bed of Watton Cliff correlates in general with thick limestone of the Great Oolite. Both are in the zone of the ammonite Opelia aspidoides , although this does not mean that they are exactly equivalent in terms of bases and tops of the lithological units.
Footnote: Oppelia
This ammonite is very compressed and involute with subdued ribbing. It belongs to the Superfamily Haplocerataceae. It shows marked dimorphism with ammonites of the two sexes having been placed in the past in different genera. Oppelia aspidoides (Oppel) has been figured by Callomon (1963).
This diagram gives more detail for Watton Cliff, but it is not accurately to scale and is intended only for interpretative use. Note that the Eypemouth Fault, in particular, is very oblique to the cliff and shown differently in different interpretative diagrams. The British Geological Survey geological map should be consulted to understand the three-dimensional structure. The cliff section above is largely based on Buckman (1922) . In addition to the general disposition of beds in the cliff, in particular it reveals the nature of the original exposures in the eastern part of the cliff which unfortunately have now been concealed by sea-defences and cliff-grading.
The succession of strata in Watton Cliff, and relating to this diagram follows. It is based on Buckman (1922) , but with some modification and updating. Metric conversions have been made. Note that the numbering of beds is in downward sequence as given by Buckman, not the usual upward sequence. The numbers correspond to those on the cliff diagram of Buckman, above. Of these Beds 1- 4 are in the middle part of the cliff and still exposed in 2004. Beds 5 to 12 are mostly obscured by sea-defences now. Beds 13 to 18 are exposed at the western, Eype Mouth, end of the cliff. The thicknesses given are approximate.
Watton Cliff - Lower and Middle Jurassic - Sequence of Strata.
(top downward)
1. Forest Marble: massive shelly blocks of limestone with clay-
partings. Brownish in the cliff, light-grey when sea-washed. 9m (30 ft).
Upper Grey Marls (part of Forest Marble sequence). 9 m (30 ft).
3a. Micromorph Ostrea Bed: a mass of small oysters rarely 5 mm long. Pedicle-valve of Dictyothyris; fragments of Acanthothyris (A. bradfordiensis Walker ?).
I b. Boueti Bed: about 40 cm (16 inches) thick, mainly brown, crumbly, but white and compact (limestone or marl) for about 5 to 7.6 cm (2 to 3 inches) from the bottom. Full of specimens of Goniorhynchia, often crushed; also occasional examples of Ornithella sp., crushed. Rarely Terebratula langtonensis
Walker.
c. Lower Grey Marl.
d. White Marl.
Total Thickness for 3a,b,c and d: 1.5 m (5 ft).
4. Frome Clay or Upper Fuller's Earth in part.
- "Large Conchoidal Bed" of Buckman: clays which break into large pieces. Measured up the cliff from 5 to 3d. 38 m (125 ft).
5. Liostrea hebridica Clays ("Ostrea acuminata" Clays of Buckman). 6 m (20 ft).
6. Wattonensis Beds (Brachiopod Beds of Buckman): stone bands 15 to 30 cm (6 to 12 inches) thick separated by clays 45 to 76 cm (18 to 30 inches) thick. [Are these cyclical?]. The contents of the stone-bands vary, and their sequence is somewhat supposititious, in the words of Buckman. 7.6 m (25 ft)
----- a. Ornithella Bed
----- b. Large Smithii Bed: Rhynchonelloidea aff.
smithii, rather large examples without other brachiopods.
----- c. Acanthothyris Bed: A. powerstockensis, Rhynchonelloidea aff. smithii, and a large ammouite (Parkinsonites?).
----- d. The Globata Bed or Terebratula Bed: Stiphothyris spp. = Terebratula cf. tumida Davidson (T. globata auctt. non Sow.) + T. cf. nunneyensis S. Buckman. Rhynchonelloidea cf. smithii
----- e. Shelly Bed: small forms of Rhynchonelloidea cf. smithii.
6. Liostrea hebridica Clays (Ostrea knorri Clays of Buckman): possibly Bed 6e is really a subdivision of this bed. 1.5 m (5 ft)
The following beds (7-9) were originally seen by Buckman in the eastern part of the cliff beyond the second fault, but this part is now obscured by sea-defences:-
7. Umber Bed: umber-coloured clays with a nodular
band. 6 m (20 ft).
8. Small Conchoidal Bed: clays with conchoidal fracture, breaking into small lumps. 12m (40 ft).
9. Ochre Band: a yellowish marly band about 0.3 m (1 foot) thick, seen near the top of the cliff, eastern end, resting on:
-- Laminated Clays: a thick sequence of laminated clays with light bands of somewhat calcareous rock. Occasional lumps of pyrites.
Thickness of all No. 9 (mostly the laminated clays): 15m (50 ft).
(Of the following beds, details for 10 and 11 are furnished by Burton Bradstock. There are Inferior Oolite rocks, the Black Rocks, just out to sea to the west of the third fault which is now concealed. The location is towards the western end of the modern espanade or sea-wall. Then part of the Bridport Sands (No. 12 of Buckman) was seen immediately east of the third fault at Battery Point (for details of the erosion of Battery Point see the postcards reproduced in Atterbury (2003) . This promontory has since been destroyed by coast erosion and extension of the esplanade. The basal part of the Down Cliff Clay (No. 13) is found with suceeding beds in the cliff west of the Eypemouth Fault at Fault Corner.)
10. Belemnite Clays. 4.6 m (15 ft).
11. Inferior Oolite Limestone (Aalenian and Bajocian). 4.9 m (16 ft).
12. Bridport Sands (Toarcian - Aalenian at the top). 40 m (130 ft).
13. Down Cliff Clay (Toarcian). 21 m (70 ft).
14. Junction Bed. Fine creamy-white lithographic limestone in paper-like laminae (Grammoceras of striatulum-thouarsense types) and yellowish conglomerate, somewhat sandy, with derived Tetrarhynchia thorncombiensis. 1.5 m (5 ft).
15. Thorncombe Sands: yellow sands with doggers (carbonate nodules). (Pleinsbachian). 10.7 m (35 ft).
16. Blue clay. (Blue Band). 0.6 m (2 ft).
17. Margaritatus Bed. (Pleinsbachian). a prominent and easily -
recognized datum-line. A bluish-brown, sandy fossiliferous limestone.
0.3 m (1 ft).
18. Down Cliff Sands. (Pleinsbachian).These are laminated and were once know as the Laminated Beds. They are grey and silty in the lower part becoming more sandy towards the top. The thickness is not given in Buckman's table but these are 26 m according to House (1993) and are underlain by the Eype Clay, visible at Eype Mouth)
Approximate total according to Buckman (excluding the Down Cliff Sands) - 190m (624).
STRATIGRAPHY:
Frome Clay (Fuller's Earth)
The Frome Clay (or upper part of the Fuller's Earth in old terminology) is well-exposed in Watton Cliff or West Cliff. It is a grey massive clay or ,probably, more strictly a marl because it is calcareous. It is lacking in obvious lamination or cyclicity and notable for showing a conchoidal fracture, a clear consequence of a homogeneous texture. It is not very fossiliferous in terms of ammonites but contains brachiopods. The massive, grey appearance contrasts with the laminated shaly characteristics of the Kimmeridge Clay or the lower part of the Lias. It does resemble in some respects the upper Oxford Clay, however, and is not very different in appearance from the Eype Clay or Green Ammonite Beds. In spite of the old name - the "Fuller's Earth" at this locality it is not suitable for fulling (the degreasing of wool) because it does not contain a major proportion of expandible clays (smectites and mixed layer). Elsewhere parts of the Fuller's Earth are true "fuller's earth" and contain volcanic ash degraded to expandible clays (like bentonite but less pure).
Buckman's description of the Watton Cliff or West Cliff Fuller's Earth is given above. Here is a later summary from Torrens et al. (1969). For information on the Wattonensis Beds see also Muir-Wood (1936) and Wilson et al., 1959.
(thicknesses in metres)
FOREST MARBLE
9. Boueti Bed - 0.40
UPPER FULLER'S EARTH
8. Blue shaly marl - 1.50
7. Laminated white cementstone -1.50
6. Blue-grey marl (main part of the cliff) -16.4
5. Thin pale cementstones and marls - 0.45
4. Marl - to beach level - 6.1
3. Gap of unknown thickness obscured by the beach - c. 12.0
(Buckman (1922) estimated 12 m. which is probably an underestimate)
2. Liostrea hebridica (Forbes) Oyster Bed, full of small specimens and broken fragents - 0.75
1. Wattonensis Beds. Thin fossiliferous limestone bands alternating with clays. (seen at very low tide) Estimated - 8.0
The Wattonensis Beds are only visible in a disturbed condition: a. in the cliff caught up vertically in the great shatter belt at the eastern end of Watton Cliff, b. beside the Eype Mouth Fault above the landslip in Fault Corner, and c. on the beach itself at the seaward end of the same fault.
The Wattonensis Beds are very rich in fossils and a varied fauna can be seen if the beds are adequately exposed along the shore when the
overlying shingle is removed by storms. The brachiopods are the most
characteristic element (Rugitela bullata, R. powerstockensis, Tubithyris spp. Wattonithyris spp., Acanthothiris powerstockensis, Rhynchonelloidella wattonensis etc. including much topotype material). Bivalves include Trigonia elongata, Parallelodon sp., Modiolus anatinus and Catinula knorri mendipensis Sylvester-Bradley MS). Ammonites are also occasionally found and include Procerites wattonensis Arkell (M-holotype), Procerites spp., and Choffatia (Subgrossouvria) sp. of the basal part of the 'Retrocostatum' Zone.
Some of the more common brachiopods of the Frome Clay, the Wattonensis Bed and the Boueti Bed are shown in this diagram. Note the grouping of the species in the diagram into the two orders - above - Rhynchonellida (with crura in the dorsal, brachial valve from which the soft lophophore, a nutrient-gathering apparatus, is supported; usually with conspicuous ribs and sulcus) and - below - the Terebratulida (calcified loops support the lophophore; often without large ribs). As noted above, there are others species present in this cliff and for identification of these specialist literature should be consulted.
STRATIGRAPHY continued:
Frome Clay - The Boueti Bed
The Boueti Bed with Goniorhynchia boueti is a well-known horizon. The brachiopods are very abundant. It is also well-known on the shores of the Fleet Lagoon. To find it in the cliff here the easy slopes east of the main cliff can be ascended or a traverse made from the coastal path up the hill. Avoid going onto the steep cliff slopes because the marl or clay does not give a stable footing. The photograph above shows where it can be found just above a light-coloured marly or marly limestone bed, pointed out in the photograph by Ian West.
Depending on the state of cliff collapse the Boueti Bed may also be accessible near the western end of
Watton Cliff at Fault Corner. Conditions vary from time to time and it may or may not be well-exposed when you visit there. Access might be difficult at times in the late wet season when landsliding is taking place.
STRATIGRAPHY:
Forest Marble
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The Forest Marble is well-exposed in the cliff top in the central part of West Cliff or Watton Cliff, West Bay, Bridport. It can be reached by a scramble up the Frome Clay in the central part of the cliff. Professor Dorrik Stow and M.Sc. student Ana Montesdeoca are shown ascending. Care is needed because there is some significant risk of limestone blocks falling. There have been major cliff falls in this area, so it is not, in general, recommended that climbing of the cliff takes place.
Much fallen material from the Forest Marble can be accessed on the beach, but, of course, the detailed stratigraphic relations cannot be seen here. Some of the fallen blocks are quite fossiliferous. In summer 2013 there was much fallen material available because of landslides resulting from the preceding very wet winter.
STRATIGRAPHY:
Forest Marble - Petrography
Here is a photograph of a thin-section from the Forest Marble limestone. The specimen is ex situ but the material is from the shelly limestone in the upper cliff.
As you can see the rock is very informative and it tells us the water depth, the salinity (i.e. suitable for ooids, echinoids and oysters). The lack of micrite envelopes is remarkable and if this is considered with the various other features present then the rock points to continuous wave action. This fits in with the mixed sediment and shallow water. The rock indicates that it originated as a shoal or beach sediment with scattered calcitic (not aragonitic) ooids, and that the climate was warm and probably fairly dry at times, and fairly windy. The carbonate sand was friable until cemented during burial, after the clay above had been deposited.
STRUCTURAL GEOLOGY:
Fault Corner and the Eypemouth Fault
Please also go to the Eype Mouth webpage.
A short distance east of Eype Mouth at the western end of Watton Cliff is one of the most celebrated faults in Wessex. This is the Eypemouth Fault described by Jenkyns and Senior (1977) and
Jenkyns and Senior (1991). An old map by Buckman (1922), above, shows the east-west orientation of this fault and the fact that it is oblique to the ESE-WSW coast. Note that the middle fault on this map (shown on a diagram in this webpage as the Clay Knapp Fault) does not necessarily have the orientation shown by Buckman. The BGS map shows a roughly north-south trend. Note also that the strata labelled as "Basal Bridport Sands" are the Thorncombe Sands, and the "Brachiopod Limestones" are the wattonensis Bed.
The Eypemouth (or Eype Mouth or Eypesmouth) Fault is an east-west trending, normal fault, that is a fault with a fault plane that hades (inclines from the vertical) towards the downthrow, which in this case is on the south side. It downthrows Middle Jurassic Fuller's Earth against Lower Jurassic Lias. The throw (vertical displacement) cannot be measured directly in the cliff but from the usual thicknesses of the sequences this is about 200 m. The east-west trend of this fault is typical of the Pre-Albian (i.e. Intra-Cretaceous or Late Kimmerian) faults in this area. It resembles in extent of throw, direction of downthrow and of trend the other major Pre-Albian fault - the Abbotsbury Fault, north of Weymouth Wilson et al. (1958) . It is a growth-fault because there has been progressive development or growth of the fault over a long period of time, movement having taken place in the Jurassic. This is shown by the variation in thickness of the Junction Bed and it is a topic which will be discussed further in this webpage, in due course. The drag (flexure of the strata adjacent to the fault plane) is in the direction that you would intuitively expect. Thus it is referred to as "normal drag" rather than "reversed drag".
Question BRW 1
for the reader (level - intermediate; topic - structural and basin-analysis):
In spite of the name, "drag" is not necessarily actually the result of a dragging process during faulting but it may represent a preserved relic of preliminary flexure which preceded the faulting. The Eypemouth Fault presents clear evidence of a Lower Jurassic phase of growth faulting. Obviously, therefore, the flexure of the Middle Jurassic strata, as shown in the diagram, must necessarily postdate the earliest phase of faulting. That initial faulting was on a small scale. Now consider the major phase of faulting which is thought to be Cretaceous in date. Does the "drag" flexure predate this major phase of faulting or is it, indeed, the consequences of a true drag effect in the Cretaceous and therefore contemporaneous with the major faulting? Discuss, giving reasons for your answer.
Question BRW 2
for the reader:
The Eypemouth Fault, being normal, is apparently of extensional type. Firstly, relate the Eypemouth Fault to the regional phase of extensional activity in the region that was responsible for basin-development. In this part of your answer give consideration to the Lyme Bay borehole. Secondly consider the possibility of later compressional movement at Eypemouth, connected with the Channel Basin inversion, as in the case of the Abbotsbury Fault. Could there have been a partially draped monocline in Cretaceous strata that once existed above, or is this unlikely? The apparent normal "drag" at the middle ("Clay Knapp") fault may be relevant and requires some interpretation. Discuss these points, giving reasons for your answers.
STRATIGRAPHY:
BEACON LIMESTONE FORMATION - JUNCTION BED
The Junction Bed is the old name for a well-known, thin Jurassic limestone. It has now been renamed by the British Geological Survey as the Beacon Limestone Formation. See the new (Jenkyns and Senior, 1977; 1991) and of Buckman and of Jackson for more information.]
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The remarkable and quite peculiar Junction Bed (Jenkyns and Senior,1977; 1991) can be seen at Fault Corner. There are some good fallen blocks on the shore and details can be investigated provided the way-up can be determined. The bed is generally about 4 m thick but has much local variation in thickness and lithology. It represents no less than 5 ammonite zones. These roughly represent about a million years each in round figures. A thin bed with something like a million years a metre is not normal; it is a condensed sequence (perhaps with other complications). Comparison should be made to the Inferior Oolite of Burton Cliff, Dorset , which shows another very condensed sequence. The Junction Bed is certainly the most condensed Lower Jurassic bed in southern England and has a very interesting and complicated origin.
This large block from the Junction Bed is the most seaward of the large blocks on the shore at Fault Corner. There are some smaller blocks associated with it, but the large block is a key example, because it has a very clear succession of lithologies and fauna, and it is sea-washed and clean on the seaward side. It is the right-way up on the basis of very clear geopetal structures. The photographs above are views, progressively moving in closer, of the fallen blocks on the shore. You can see these features there on the beach provided the tide is quite low and the shingle is not banked up against the block. To see some features you will need to crouch down low on the beach on the seaward side (if the tide and waves permit you to do this in safety!). You can see both the geopetal structures and the borings quite easily at low spring tide.
BEACON LIMESTONE FORMATION - JUNCTION BED -
Way-up Criteria
Fortunately, the fallen blocks of the Junction Bed on the shore have much evidence of way-up (Jenkyns and Senior, 1991). They are best seen on sea-washed, clean, vertical surfaces. Examine, once again, the top photograph of the above set. Small cavities are visible on the right hand side of the top photograph. These contain geopetal structures; that is they are partially filled from the bottoms. With practice you will be able to determine the way-up of most of the sea-washed blocks on the shore. This is essential before making further study.
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BEACON LIMESTONE FORMATION -JUNCTION BED
Neptunian Sills
BEACON LIMESTONE FORMATION - JUNCTION BED
Lithophaga Borings from Sill Cavities
Upward Lithophaga borings are into grey calcareous sediment are mostly filled with the buff limestone. This is the sediment fill of the cavities which became neptunian sills according to Jenkyns and Senior (1991). It was soft pelletoidal carbonate at the time.
The general matrix of the Junction Bed in the blocks is what appears to be grey limestone with some iron-staining in the photograph. This may vary in different parts of the block but they describe it as typically developed as: "grey-green to reddish-brown calcareous/ferruginous quartzarenite (i.e. quartz sandstone), locally conglomeratic and rich in shell material and brachiopods". The brachiopods appear as small calcite-rimmed holes in the darker sediment. The white specks in the photograph are largely crinoidal debris. Jenkyns and Senior (1991) regarded this as the original rock.
These authors have described the buff limestone more specifically as: "fine-grained parallel- and cross-laminated buff to pale rose
calcilutites and coarser milky limestones, both of which contain sparse quartz grains". They regarded this buff limestone as "fissure facies" and these near-horizontal bodies as "neptunian sills". They stated that the neptunian sills generally have smooth undulating bases and more irregular tops. This can be seen in the top photograph.
Most of the features shown in the photograph are quite compatible with the neptunian sill theory of Jenkyns and Senior (1991) . It is surprising, however, that the borings are apparently upward. It would seem that boring molluscs bored upwards from a sub-horizontal cavity, probably at a time when there was already some soft fill at the base unsuitable for borings. The former cavity was subsequently filled almost to top by incoming buff carbonate sediment (now limestone). Some of the borings have been completely filled, but others have only received in the lower part and sparry calcite above. The sparry calcite is filling what would have been a water-filled space.
BEACON LIMESTONE FORMATION - JUNCTION BED -
Origin of the Neptunian Sills - Discussion
We will now consider whether existing theories for the neptunian features of the Junction Bed at Eypemouth are probably correct, or whether they could be modified. The comments, here, of course, are tentative and merely for discussion; they are not based on any serious new phase of research and the complexities of the Junction Bed should not be underestimated!
Jenkyns and Senior (1991) summarised their interpretation of the sequence of events
leading to the formation of the Junction Bed:
"(1) Deposition of a ferruginous calcareous quartzarenite (Marlstone) during ?[Pleuroceras] spinatum Zone to early [Dactylioceras] tenuicostatum Zone times. Much erosional reworking and incorporation of older faunal elements (derived from the Thorncombiensis Bed of the upper [Amaltheus] margaritatus Zone); modest submarine lithification under conditions of minimal net sedimentation.
(2) Deposition and precipitation of lime mud locally rich in ammonites, thin-shelled bivalves, gastropods, brachiopods, belemnites, echinoids and foraminifera (Junction Bed sensu stricto), similarly in an environment of very slow sedimentary rate. Development of non-sequences marked by formation of concentrically laminated calcareous limonitic nodules.
(3) Various phases of submarine faulting leading to multiple intrusion of the Junction Bed, Marlstone and uppermost Thorncombiensis Bed by lime-mud variably rich in ammonites and gastropods. Injection of rapidly flowing sediment-charged liquids by vacuum suction into partially lithified sediment is suggested by grading, cross- and parallel lamination of the peloidal lime mud, the hydraulic sorting of fossils and the local gradational contacts between matrix and fill. That these fissures ever gaped partially open on the sea-floor with the cavities inhabited by ammonites (cf. Wendt, 1971
) seems unlikely. However, small voids filled with fibrous and equant calcite suggest that at least some cavities were incompletely filled, unless these represent a dewatering phenomenon; and the uppermost levels of some fissures contain sediment that may have entered by passive filtration. Demonstrable times of intrusion, assuming that the ammonites accurately represent this, were: [Harpoceras] falciferum, [Hildoceras] bifrons, [Haugia] variabilis and [Grammoceras] thouarsense Zones, spanning an estimated time of 4 Ma (e.g. Harland et al. 1982
).
(4) A further series of undated but pre-levesquei Zone movements resulting in intrusion of vertically oriented neptunian dykes cutting matrix and some fissures (Jenkyns & Senior, 1977
). Presumed deepening of water, due to regional tectonic and/or eustatic effects, leading to deposition of clay (Downcliff Clay) in [Dumortieria] levesquei-Subzone time (Sellwood & Jenkyns, 1975
)."
[end of extract]
The above account explains most of the faunal and sedimentological features of the Junction Bed at Eypemouth. A complication, however, is the presence of the upward Lithophaga borings. These were probably not visible at the time when their paper were written, because some of the beach shingle may have been washed away only quite recently. These borings show that bioerosion was a mechanism involved in at least the enlargement of the cavities into which sediment entered, so as to form the neptunian sills. Jenkyns & Senior (1991) considered that the cavities did not gape open on the sea-floor. The Lithophaga borings indicate that they did on at least one occasion. They also suggest that the matrix rock was lithified rather than just partially-lithified. Perhaps the Junction Bed took the form of a rocky outcrop or reef on the shallow sea-floor that was subject to bioerosion, and perhaps mechanical erosion and chemical dissolution. Thus very irregular cavities were formed, almost honeycombing the submerged rocky ridge. From time to time carbonate sediment was washed into the small karst-like cavities.
Of course, this argument for some erosional development of the sill spaces does not in any way negate the theory for penecontemporaneous tectonic fracture. Jenkyns & Senior (1977; 1991) have shown that sediment-filled fissures increase in number in proximity to the Eypemouth Fault. The problem is not whether there was only early tectonic fracture or there was only bioerosion. The features seen may be a product of both, with fissures first initiated by fracture and then subsequently subject to enlargement by bioerosion and other processes. It is an interesting matter and it is well-worth reading the works of Jenkyns & Senior (1991) and of earlier authors before forming a firm opinion. We leave the topic now as a future discussion point.
BEACON LIMESTONE FORMATION - JUNCTION BED -
Ammonites and Ammonite Zones
Some relevant ammonite illustrations are given below. Note that the specimens in the Junction Bed and associated strata at West Cliff are generally poorly preserved and not type of ornamental specimen that are seen on sale in fossil shops. Perhaps fortunately, they are not highly collectable!
The ammonite Pleuroceras spinatum characterises the upper part of the Middle Lias (Upper Pliensbachian). Here the strata of this age are at the bottom of the Beacon Limestone Formation (the Junction Bed - i.e. the Marlstone).
The genus Pleuroceras of Hyatt (1867), the "ribbed horn ammonite", has a subevolute to evolute shell that is quadrilateral in whorl section. It has strong radial ribs often with tubercles or spines. The venter is flat with a prominent keel bordered by flat areas or grooves. The keel ranges from corded with strong chevrons to smooth.
Ammonites of the genus Amaltheus occur in the Amaltheus margaritatus Zone of the lower part of the Middle Lias. This is represented by the Eype Clay, Down Cliff Sands, Margaritatus Bed and the Thorncombe Sands. All this is lower than the Junction Bed but visible in the Cliff west of Fault Corner and beyond.
The genus Amaltheus, like Pleuroceras, belongs to the SuperFamily Eoderocerataceae. The shells are typically subevolute to involute. Notice that in these specimens the whorls overlap in an involute manner. The ammonite is compressed to very compressed and oxyconic with triangular to circular whorl sections. The compression is obvious here and the whorls are almost triangular in section in these examples. The ribs bifurcate to trifurcate, passing forward to produce a prominent corded keel. Spiral ornament is present in some oxycones, as in the Amaltheus margaritatus shown here.
FROME CLAY FORMATION (BATHONIAN)
Wattonensis Brachiopod Beds
The Wattonensis Beds or Brachiopod Beds are alternating marine, grey limestones and clays with abundant brachiopods and other fossils. At present (August 2012) they are very easy to find. They are at top beach level, just west of the recent mudslide and more or less in line with Fault Corner. They are only a short distance to the east of Eype's Mouth and can be found by a short stroll along the pebble beach. They are a short distance west of the conspicuous white blocks of Beacon Limestone (Junction Bed) that have fallen on to the beach.
The beds are a cyclical sequence of marl and grey argillaceous limestone. Brachiopods are numerous in both, although sometimes a little crushed or distorted. Most obvious is the small rhynchonellid - Rhynchonelloidella wattonensis Muir-Wood, and this is abundant.
The bed is also at times exposed at low tides on the foreshore and it is probably from such a sea-washed exposure that the best brachiopod material has been found. Wilson et al. (1958) commented that the most westerly exposure is seen lower down on the foreshore between April and June when it is not covered by shingle. Muir-Wood (1936) noted that
"The 'Brachiopod Beds' were well exposed as black limestone reefs .. in June 1936, and extended almost parallel to the cliff face along the fault. About twenty reefs were observed projecting about 1 ft [0.3m] above the shingle and dipping steeply to the S.S.E. at an angle of about 75 degrees. Seven of these beds attained a total thickness 0f 15 ft [4.6m], contained numerous brachiopods, Wattonithyris, Rugitella, etc., but it was not possible to make out any sequence."
According to Wilson et al. (1958), the
sequence of about 25 feet [7.6m] was established on the foreshore at this locality by the well-known ammonite palaeontologist Sidney Savory Buckman (1922b, p.381). The terminology has been revised by Helen Muir-Wood (1936) of the Natural History Museum, London. The sequence is as follows:
(a) Ornithella Bed with Rugitella powerstockensis and R. cadomensis.
(b) Large smithii Bed with Rhynchonella smithii var crassa.
(c) Acanthothyris Bed with A. powerstockensis and Rhynchonelloidella smithii.
The globata Bed or Terebratula Bed with Stiphothyris spp. = 'Terebratula' cf. tumida, Wattonithyris wattonensis, W. nunneyensis and W. mifordensis Muir-Wood.
e. Shelly Bed with small forms of Rhynchonelloidella wattonensis.
The rich brachiopod fauna includes the following: Wattonithyris wattonensis, Rhynchonelloidella spp., Rugitella bullata, Tubithyris and others, monographed by Muir-Wood (1936, Monogr. Palaeontog. Soc.).
WATTONENSIS BEDS
Palaeoenvironment - Palaeosalinity and Temperature
The Bathonian is atypical of the marine Jurassic in that faunas are sometimes dominated by brachiopods as opposed to molluscs. Ammonites are also rare,
making correlations difficult. The Wattonensis Beds are an easily accessible and well-known example of this peculiar brachiopod dominated facies. The problem is the reason for such apparent anomalies in the Bathonian and this is a matter which has been much discussed in the literature. The topic is only considered briefly here in this field guide. See the easily accessible paper in the Journal of the Geological Society for a full discussion of the matter and for references to previous publications.
Price and Teece (2010).
The above-mentioned authors introduced the problem as follows:
"It has been suggested (e.g. Johnson, 1999) that low-diversity Bathonian faunas indicative of substantially reduced salinities occur in central and eastern England and even as far south as
Dorset, whereas a higher diversity of faunas in Normandy indicates relatively normal marine salinities (Bradshaw and Cripps, 1992). However, others (e.g. Palmer 1979; Hendry and Kalin
1997) have not identified any salinity reduction in England south of the Oxford area. The basis for Palmer's (1979) view is that stenohaline - marine groups (e.g. cephalopods, echinoderms, corals and articulate brachiopods) occur widely in southern England. In contrast, Johnson (1999) suggested that the presence of cephalopods, corals, echinoderms and particularly brachiopods should be regarded more sceptically as evidence of fully marine
conditions. This has significant implications, as many geological studies have used the presence of an articulate brachiopod fauna to indicate a fully marine environment. More recently Johnson et
al. (2007) noted that additional factors must be involved in size reduction of Bathonian bivalves and suggested reduced productivity as the main cause."
However, there is a contrasting view that the Wattonensis Bed contains a normal Jurassic fauna. The Wattonensis Beds of the Winterbourne Kingston borehole were studied by Penn (1982) who found a diverse typical marine assemblage. The distinctive fauna is dominated by epifauna and includes Pinna, Oxytoma and Astarte, in addition to the association of Wattonithyris wattonensis, Rhynchonelloidella
smithi and Acanthothiris powerstockensis, diagnostic of the Wattonensis Beds (Kellaway and Wilson, 1941).
The diversity recorded by Penn (1982) is, according to
Price and Teece (2010),
comparable with the typical diversity of the Bathonian of Normandy (Palmer and Fursich, 1981).
Bradshaw and Cripps (1992) have taken the high diversity of the Normandy faunas to indicate relatively normal marine salinities.
With this background discussion in mind,
Price and Teece (2010) undertook stable isotope studies to determine the palaeotemperature of the seawater at the time of deposition. They investigated the isotope del 18O [ie. the oxygen 18 isotope value as opposed to that of the normal oxygen 16] values in belemnites (Belemnopsis) from the Wattonensis Beds. They found the range is from -1.4 to -0.7 parts per thousand and carbon isotope values range from -2.3 to -0.5 per thousand VPDB [carbon isotope ratios are measured against Vienna Pee Dee Belemnite (VPDB), a reference belemnite standard - originally Belemnitella americana, from the Cretaceous PeeDee Formation in South Carolina.] Isotope values of well-preserved Rhynchonella from the Wattonensis Beds range from -2.4 to -1.3 parts per thousand for oxygen and -1.1 to 0.6 parts per thousand VPDB for carbon. The del 18O data suggest high palaeotemperatures (possibly up to 28 degrees C).
Thus the important work of
Price and Teece (2010) [available from the Geological Society]
shows that isotopic data from the Wattonensis Beds are consistent with fully marine conditions. The oxygen isotope data are also suggestive of high marine palaeotemperatures (possibly up to
28 degrees C) comparable with those found by previous studies from elsewhere in
Europe. They state that these are analogous with temperatures of modern Florida Bay. The investigation of
Price and Teece (2010) strengthens the case for greenhouse periods during the Jurassic.
From a local, Bridport - Eype area, point of view they show that the Wattonensis Beds, seen at the beach, are quite normal for a warm Jurassic phase, and the salinity was not abnormal. The conditions for the nearby Elongata Beds were the same regarding salinity, but slightly cooler. The Boueti Bed also had normal salinity and the conditions were warm. So, all in all, the Frome Clay seems to have originated in normal salinities, not reduced, and in typical warm Jurassic conditions. One must admit that the Frome Clay looks strange to the visitor because of relative lack of ammonites, but it was just a normal Jurassic sea-floor mud.
WATTONENSIS BEDS
Structure
Concerning the structure, an initially misleading aspect is that the Wattonensis Beds at the top of the beach are seen to dip fairly steeply towards the north. This is in spite of the fact that they are actually on the downthrow side of the east-west trending Eype's Mouth Fault. Obviously drag effects of the fault should have caused them to dip south. Obviously there is some complication at the back of the cliff, probably the result of landsliding, that is not clearly visible because of slumped debris and vegetation.
A more expected situation is the dipping of the beds in the foreshore exposure to the SSE at an angle of about 75 degrees Wilson et al. (1958). They further comment:
Between 'Fault Corner' and West Bay a large fault cuts the cliff, and on the east side of the fault are seen a number of bands of argillaceous limestone, twisted and broken and in places vertical. These limestone bands show by their fossils that much of the strata are wattonensis Beds, some of which are incorporated in a fault conglomerate some 30 ft [9m.] wide. When the cliff extended further seaward Solly and Walker (1890, pp. 119-120) observed ten or more beds of stone dipping at an angle of 34 degrees to the west; they recorded typical "Fuller's Earth" brachiopods from these beds.
SELECT BIBLIOGRAPHY
- on the cliffs of West Cliff, Bridport to Eype Mouth
.
Allison, R.J. (Ed. ) 1992. The Coastal Landforms of West Dorset. Geologists' Association Guide No. 47. 134 pp.
.
Arkell, W.J. 1933 (reprinted 1970). The Jurassic System in Great Britain. Clarendon Press, Oxford. 681 pp.
.
Atterbury, P. 2003. Just a Line from West Bay: the Story of a Dorset Harbour and Resort in Postcards.
Postcard Press, Whin Bridge, Eype, Bridport, Dorset DT6 6AL. 84 pp. With postcards from the collections of Keith Alner, Peter Cove and Christine Paull. Paperback, ISBN 0-9545372-0-3, price was 6 pounds, 50p. [A publication that is very useful in showing the state of the cliffs, and harbour, at various dates from about the end of the 19th century onwards. It is particularly informative about the changes at Battery Point, west of Bridport Harbour. It also shows storms and storm damage.]
.
Bradshaw, M.J. and Cripps, D.W. 1992. Mid Jurassic. In: Cope, J.C.W., Ingham,
J.K. and Rawson, P.F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoirs, No. 13, pp. 110-120.
.
British Geological Survey (BGS). (Compiled by M.A. Woods) 2011. Geology of South Dorset and South-East Devon and its World Heritage Coast.
Special Memoir for 1:50,000 geological sheets 328 Dorchester, 342 West Fleet and Weymouth and 342/343 Swanage and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 330 Newton Abbott. Compiled by M.A. Woods. By Barton, C.M., Woods, M.A., Bristow, C.R., Newell, A.J., Westhead, R.K., Evans, D.J., Kirby G.A., and Warrington, G. Contributors: Biostratigraphy - J.B. Riding; Stratigraphy - E.C. Freshney; Economic Geology - D.E. Highley and G.K. Lott; Engineering Geology - A. Forster and A. Gibson. British Geological Survey, Keyworth, Nottingham, 2011. 161 pp. This is the new version of the Geological Survey Memoir for the Dorset Coast etc. and replaces Arkell (1947) and the earlier memoir by Strahan (1898). It covers a wider area than these old memoirs, though, and includes all of "Jurassic Coast", UNESCO World Heritage Coast. It is a key reference work.
Available from BGS Online Bookshop at 24 pounds stirling (in Jan. 2012).
.
Bird, E.C.F. 1990 (for 1989). The beaches of Lyme Bay.
Proceedings of the Dorset Natural History and Archaeological Society for 1989, published June 1990, vol. 111, pp. 91-97. By Eric Bird, then at Department of Geography, University of Melbourne, Australia.
Abstract:
The beaches of Lyme Bay consist largely of flint and chert shingle, with some sand, derived from eroding cliffs and sea floor sources. They include Chesil Beach, which shows lateral grading from small pebbles in the west to large pebbles in the east. Several other beaches on the north coast of Lyme Bay also show lateral grading, low beaches of poorly sorted sand and shingle to the west becoming higher and often wider, coarser and better sorted to the east. Lateral grading is attributed to an alternation of eastward beach drifting by strong south-westerly wave action with westward movement of finer material by gentler southeasterly wave action. Whereas Chesil Beach is a relict shingle formation, the other beaches are still receiving small quantities of sand and shingle. Cliff erosion and slumping are more rapid behind low beach sectors than where a high, wide accumulation of coarse shingle protects the shore. It is suggested that artificial beach nourishment should be used as a method of coastal protection on the shores of Lyme Bay. [end of abstract - Bird, 1990]
.
British Museum (Natural History)
. 1962 and various editions onward. British Mesozoic Fossils. 207pp.
.
Bradshaw, M.J. and Cripps, D.W. 1992. Mid Jurassic. In: Cope, J.C.W., Ingham,
J.K. and Rawson, P.F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoirs, 13, 110-120.
.
Buckman, S.S.
: Sydney Savory Buckman, son of a professor of geology, was born 1860 at Cirencester and later lived at Bradford Abbas, Sherborne, Dorset, studying the adjacent area. He had written a paper on the Astartes of the Inferior Oolite by the time he was 18 and wrote a large number of publications subsequently. He was well-known for his interpretation of Jurassic stratigraphy based on ammonite studies, and particularly for his Type Ammonite monographs. There was some criticism of his excessive "splitting" of ammonite taxa. A French palaeontologist wrote: "Buckman has made 40 species of Proplanulites that only proves that he has handled 40 specimens!". Davies said, though, that he is more likely to have handled 400, and that the quarrel between "lumpers" and "splitters" will last as long as men study natural history. Buckman's his contribution to Jurassic palaeontology and stratigraphy was of very great value. He received the Murchison Fund and the Lyell Medal of the Geological Society of London and became a Fellow of the Royal Society. He died in 1929. See discussion of his work in Arkell (1933) , and see also Davies, A.M. 1930. The geological life work of S.S. Buckman. Proceedings of the Geologists' Association, 41, 221-240. Only a few of his many publications are referred to here.
Buckman, S.S. 1881. A descriptive catalogue of some of the species of ammonites from the Inferior Oolite of Dorset. Quarterly Journal of the Geological Society, London, 37, 588-608.
Buckman, S.S. 1883. The brachiopoda from the Inferior Oolite of Dorset and a portion of Somerset. Proceedings of Dorset Natural History and Archaeological Field Club, 4, 1-52.
Buckman, S.S. 1884. On the Trigonia bella (Lycett) from Eype, near Bridport, Dorset. Proceedings of the Dorset Natural History and Archaeological Field Club, 5, 154-156.
Buckman, S.S. 1886. Notes on Jurassic brachiopoda. Geological Magazine, 23, 217-219.
Buckman, S.S. 1889. Certain Jurassic (Lias-Oolite) strata of south Dorset; and their correlation. Quarterly Journal of the Geological Society, London, 66, 80-89.
Buckman, S.S. 1910. Certain Jurassic (Lias-Oolite) strata in south Dorset and their correlation. Quarterly Journal of the Geological Society, London,66, 52-89.
Buckman, S.S. 1909-1912. Yorkshire Type Ammonites. 1. London
Buckman, S.S. 1913-1919. Yorkshire Type Ammonites. 2. London
Buckman, S.S. 1919-1921. Type Ammonites. 3. London
Buckman, S.S. 1922. Jurassic chronology: 2 - preliminary studies. Certain Jurassic strata near Eypesmouth (Dorset); the Junction-Bed of Watton Cliff and associated rocks. Quarterly Journal of the Geological Society, London, 78, 378-436, with appendices (Jackson and also Spath, Pringle, Templeman and Buckman) and discussion 436-457. By Mr. S.S. Buckman, F.R.S. Read December 7th 1921. [This is an old but paper that is important in certain respects; it deals with Watton Cliff or West Cliff at a date when the eastern part was not obscured by a promenade and sea defences. It describes strata which is not visible now. ]
.
Buckland, W. 1837. Geology and Mineralogy considered with Reference to Natural Theology
. The Bridgewater Treatise on the power, wisdom and goodness of God as manifested in the creation. Treatise VI. London, William Pickering, Vols 1 and 2.
.
Cope, J.C.W., Getty, T.A., Howarth, M.K., Morton, N. and Torrens, H.S. 1980. A correlation of Jurassic rocks in the British Isle.
Part One: Introduction and Lower Jurassic. Geological Society, London, Blackwell Scientific Publications, Oxford.
Cope, J.C.W., Ingham, J.K. and Rawson, P.F. (editor). 1991. Atlas of Palaeogeography and Lithofacies. Geological Society of London. Geological Society Memoir No. 13. ISBN 0-903317-65-6. (see Jurassic section - Bradshaw et al., p. 107 et seq. )
.
Damon, R. 1884. Geology of Weymouth, Portland and Coast of Dorsetshire, from Swanage to Bridport-on-the-Sea:
with Natural History and Archaeological Notes. Weymouth. R.F. Damon. London, Edward Stanford, 55, Charing Cross, S.W. 2nd Edition.
.
De La Beche, H.T. 1822. Remarks on the geology of south coast of England from Bridport Harbour, Dorset, to Babbacombe Bay, Devon.
Transactions of the Geological Society, London, Series 2, vol 1, 40-47, Plates iii-viii.
.
Dorset Coast Forum
. 1999. Newsletter of the Dorset Coast Forum. 1999. Issue 4: November 1999. Dorset Coast Strategy
moves into implementation. Web Site: www.dorset-cc.gov.uk/dcf. Extract regarding West Bay: "West Bay Improvements - Exhibition. An architectural model of the proposed coastal defence and harbour improvements scheme for West Bay will be the centre piece of an exhibition that will go on display at West Bay between 29 and 31 October and at Bridport between 1 and 3 November 1999.
The two piers at West Bay have experienced major failures throughout their life; with there being seven extensive repairs in the last 50 years. They are an integral part of West Bay's coastal defences, as well as providing an entrance channel to the harbour. The piers are again in need ofattention in order to ensure they continue to protect West Bay from storm attack. The scheme includes the demolition of the existing West Pier and the construction of a 230 m long replacement pier on a new alignment, with strengthening work being undertaken to East Pier. The scheme will also include the construction of a newgroyne and shingle replenishment of West Beach; this will give protection to the western side of West Bay as well as providing an improved amenity beach. The East Pier will have a small rock extension which, together with a beach management plan, will maintain East Beach at a safe width. The scheme has been jointly developed by West Dorset District Council's engineers and Hydraulics Research Limited. A 1:45 scale physical model of the scheme has
been tested at Hydraulics Research, with the proposed scheme significantly increasing the number of days the harbour entrance can be safely navigated. Currently the entrance can only be used on average 50 percent of the year. The new scheme will allow access to the harbour for all but the worst storm conditions, as well as providing a safe haven for craft using Lyme Bay. [The new harbour piers and entrance was opened in March 2005.]
[end of Dorset Coast Forum reference]
.
Hannah, G.W. 1987. The evolution of Bridport Harbour.
Proceedings of Dorset Natural History and Archaeological Society for 1986, published 1987, vol. 108, 27-31. Abstract: For centuries boats have passed between the great cliffs at 'Bridport Mouth' into a harbour which today remains the smallest of ports. It is still difficult for ships to enter this extraordinary place. In 1912, Henry Symonds wrote an outline history of the development of the port (Symonds 1912). This article seeks to recount the evolution of the harbour emphasing different aspects of the story. It is a record largely of appeals for help to repair damage wrought by storms and choking accumulations of sand. [Fig. is a map of Bridport Harbour and the River Brit in 1890.]
[end of Hannah reference]
.
Harland, W.B., Cox, A.V., Llewellyn, P.G., Pickton, C.A.G., Smith, A.G. and Walters, R. 1982. A Geological Time Scale. Cambridge University Press, Cambridge.
.
Hendry, J.P. and Kalin, R.M. 1997. Are oxygen and carbon isotopes of mollusc shells reliable palaeosalinity indicators in marginal marine environments? A case study from the Middle Jurassic of England. Journal of the Geological Society, London, 154, 321-333.
Hendry, J.P., Perkins, W.T. and Bane, T. 2001. Short-term environmental change in a Jurassic lagoon deduced from geochemical trends in aragonite bivalve
shells. Geological Society of America Bulletin, vol. 113, 790-798.
.
Hesselbo
, S. 1992. Excursion A1. Tectonics and Sedimentation in the Lower/Middle Jurassic of the Wessex Basin.
Pp 20-30. BSRG 1992, Southampton, Field Excursion Guides. Department of Oceanography, University of southampton, 65 p. (Bridport Sands show changes between Thorncombe Beacon and East Cliff - see Fig 4.).
.
House, M.R. 1993. Geology of the Dorset Coast. Second Edition. Geologists' Association Guide No. 22. Burlington House, Piccadilly, London, 164 pages and plates. Paperback. ISBN 07073 0485 7. [see also new edition by Cope]
.
Jackson, J.F. 1922. Sections of the Junction Bed and contiguous deposits.
Quarterly Journal of the Geological Society, London, 78, 436-448. Appendix 8 to paper by Buckman (1922), by James Frederick Jackson, with remarks on ammonites and brachiopods by S.S. Buckman. [With nine measured sections of the Junction Bed, each with detail on lithology and ammonite content.]
Jackson, J.F. 1926. The Junction Bed of the Middle and Upper Lias of the Dorset Coast. Quarterly Journal of the Geological Society, London, 82, 490-525.
.
Jenkyns, H.C. and Senior, J.R. 1977. A Liassic palaeofault from Dorset.
Geological Magazine, 114, 47-52. (Both authors at that time at Durham University). Abstract:
Evidence is presented for Liassic (Toarcian) faulting at Watton Cliff in coastal W Dorset. This movement is inferred from the abrupt changes in thickness of the Junction Bed and contiguous strata given that the contact with the overlying shale and clay is horizontal. Further evidence for synsedimentary movement is manifested by the presence, in the palaeofault zone, of numerous calcilutite-filled fissures penetrating a coarser, more sandy matrix. These neptunian dykes and sills contain abundant ammonites of the middle to late Toarcian, presumably the time of fault movement. This onshore Jurassic palaeofault broadens the known extent of such features from Sutherland (Brora-Helmsdale Fault) and Yorkshire (peak Fault) to include southern Britain.
Jenkyns, H.C. and Senior, J.R. 1991. Geological evidence for intra-Jurassic faulting in the Wessex Basin and its margins. Journal of the Geological Society, London, v.148, part 2, pp. 245-260. (H.C. Jenkyns at University of Oxford; J.R. Senior at University of Durham). The work was financed by a grant from BP Research to study the Dynamic Stratigraphy of the Wessex Basin. The project was initiated by C.P. Summerhayes. Abstract: Geological observations on Jurassic outcrops close to major faults in the Wessex BasinMendip area reveal the local presence of ammonite- and brachiopod-bearing sediments penetrating underlying strata. Toarcian and Bajocian neptunian dykes and particularly sills are associated with the Eypemouth Fault and Bajocian sills with the Bride Fault and Mere Fault. In the Mendip area numerous neptunian dykes of Hettangian, Sinemurian, Pliensbachian and Bajocian ages, cross-cutting Carboniferous Limestone, are recorded, typically also associated with major basement faults (e.g. Cranmore and Leighton Faults). These periods of assumed sediment injection are taken as indicating times of displacement along the faults in question.
Variations in facies (Hettangian-Sinemurian, Toarcian, Bajocian, uppermost Oxfordian, Kimmeridgian) spatially linked to faults are documented from some areas, and boreholes reveal considerable fault-controlled thickness changes in Hettangian-Sinemurian, Bajocian and Kimmeridgian sediments. The timing of Jurassic faulting in the Wessex Basin-Mendip area thus polarizes into two intervals: Hettangian-Bajocian and latest Oxfordian onwards, correlating with the early rifting phases of the Central and North Atlantic respectively.
.
Johnson, A.L.A. 1999. Evidence and cause of small size in Bathonian (Middle Jurassic) marine bivalves of north-western Europe. Palaeontology, 42, 605-624.
Johnson, A.L.A., Liquorish, M.N. and Sha, J. 2007. Variation in growth-rate and form of a Bathonian (Middle Jurassic) oyster in England, and its environmental implications. Palaeontology, 50, 1155-1173.
.
Kellaway, G.A. and Wilson, V. 1941. An outline of the geology of Yeovil, Sherborne and Sparkford Vale. Proceedings of the Geologists' Association, vol. 52, pp. 131-174.
.
Lake, S. D. 1985. West Bay, Bridport,
In: Continental extension tectonics (ed. P.L. Hancock), Preconference excursion guidebook, Geological Society of London.
.
Legg, R. 1999. Bridport and Lyme Regis: The Story of Dorset's Western Coast
. Dorset Publishing Company. 192 pp. ISBN 0-948699-66-3. By Rodney Legg. [With 37 monochome illustrations at the front and 61 at the back, many of them of old photographs, paintings and sketches. Local events, mostly regarding the coast are recorded in chronological order from the year 774 to 1998. Although references to sources are not given (and can be searched for elsewhere), this book contains much interesting information and informative old photographs. It is a paperback and at low cost - 7 pounds,95 p. reduced to 4 pounds.99 p. in April 2004. It should be found in the shops of Bridport and Lyme Regis. It is not a geological publication but has points of geological relevance and is recommended for general and topographical interest.]
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May, V. 2005 (?). Coastal Form Processes. Part of: Physical Changes to the Coast.
Webpage:
Coastal Form Processes . By Professor Vincent May of Bournemouth University. Part of Dorset Coast Digital Archive - Dorset Coast Digital Archive, operated by Bournemouth University, Bournemouth Borough Council, Dorset County Council and Dorset County Museum. [A very good website with specific data. Recommended.]
Contents:
Introduction
General Information
Detailed Information
Waves
Tides
Sea Level Rise
Coastal Erosion
Beaches
Dunes and Estuaries
Example extract - Introduction:"The features of the Dorset coast between Highcliffe in the east and Lyme Regis in the west are the result of thousands of years of marine and sub-aerial processes acting upon a wide range of geological materials whilst climate, sea-level and the human use and modification of the coast have changed significantly. Coastal processes act on timescales that range from the few seconds of a wave breaking to the many millennia of sea-level change. Similarly, these processes also occur on spatial scales of a few millimetres to the scale of the English Channel and beyond. For example, a pebble falling from the cliffs at Budleigh Salterton in East Devon several thousand years ago and found today on Chesil beach has probably made a journey equivalent to the distance from Earth to the planet Neptune. During all of that time, the combined effects of waves, tides and currents have moved the pebble up, down and along the shore and buried within the beach as well."
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Melville, R.V. and Freshney, E.C. 1982. British Regional Geology: The Hampshire Basin and Adjoining Areas
. British Geological Survey (formerly the Institute of Geological Sciences), London, Her Majesty's Stationery Office. 146 pp.
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Morris, C. 2005. An investigation into Chesil Beach, Dorset, England and Eype Beach, Dorset, England with regard to providing quantitative evidence of the past arbitrary nature of the)Chesil Beach western limit.
Undergraduate Project Report by Charlotte Morris, Oceanography with Physical Geography (joint honours) BSc. School of Ocean and Earth Sciences, National Oceanography Centre, southampton, southampton University.
Abstract: The aims of this report are a quantitative study into the aspects of size grading, shape classification and pebble type at two compartmentalised pebble beaches, Chesil Beach, Dorset, England and Eype Beach, Dorset, England, with the intention of providing evidence as to the past sediment transport paths of Chesil beach material, as there are many visual comparisons between the beach material of Eype and Chesil Beach which denote possible similarities in origin and evolution. The initial 22 pebble samples were taken in November 2004 from the Chesil Beach main crest to Seatown extending over a distance of 26 Km of which 11 samples sites were studied. A further pebble study comprised of 4 additional sample sites in April 2005. In the initial study 200 pebbles were analysed per sample site for long and intermediate axis length, classification and shape. The data were inputted into a Microsoft Excel spreadsheet and presented graphically and as tables. The results show lateral grading from coarser well-sorted cobbles in the eastward margins of both Chesil beach and Eype beach with a rapid decline in pebble size and sorting westward at each beach. Pebble and cobble types remain fairly uniform with a few beach sections displaying a small amount of Jurassic limestone addition. Further investigation into the Jurassic limestone addition at Eype beach has identified it as originating from the Forest Marble bed of West Cliff [or Watton Cliff]. The further pebble study of April 2005 analysed samples for long, intermediate, short axis length and pebble type at Chesilton, Abotsbury, Eype and an unrelated beach in Hampshire [Lepe Beach]. The data were inputted into a spreadsheet and the oblate-prolate index of Dobkins and Folk (1970) was calculated. The results show Eype and Chesilton cobbles to be similar in size, shape and type. All these cobbles are well-sorted. The Abotsbury pebbles show a change in pebble shape with westward movement and less sorted beach material. Results of the Eype locality in West Bay have been quantitatively correlated to results of the Chesil beach locality suggesting that Eype beach was in the past
an active part of Chesil beach.
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Muir-Wood, H.M. 1936. Monograph on the Brachiopoda of the Great Oolite Series.
Part 1:The Brachiopoda of the Fuller's Earth. Monograph of the Palaeontographical Society, 404, p.11 plus 144 pages, Plates 1 to 5. London. By Helen Marguerite Muir-Wood, a palaeontologist, specialisting on brachiopods, on the staff of the Natural History Museum, London. Appointed part-time worker in 1919. Attained Deputy Keeper of the Natural History Museum in 1955, the first woman to do so, retired in 1961, visited until 1965. (Also wrote Muir-Wood, H.M. 1936. Brachiopoda from the Lower Lias, Green Ammonite Beds, of Dorset. Quarterly Journal of the Geological Society, London, vol. 92, pp. 472-487.)
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Natural History Museum
, London. (originally as British Museum (Natural History) 1962 and various editions onward). British Mesozoic Fossils. 207pp.
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Palmer, T.J. 1979. The Hampen Marly and White Limestone Formations: Florida type
carbonate lagoons in the Jurassic of central England. Palaeontology, 22, pp.189–228.
Palmer, T.J. and Fursich, F.T. 1981. Ecology of sponge reefs from the Upper
Bathonian of Normandy. Palaeontology, 24, pp. 1-23.
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Penn, I. E., Dingwall, R.G. and Knox, O'B. 1980. The Inferior Oolite (Bajocian) sequence from a borehole in Lyme Bay, Dorset. Report
79/3 of Institute of Geological Sciences, Natural Environment Research Council, ISBN 011 884093. Published London, Her Majesty's Stationery Office, 1980. Summary: A borehole drilled in Lyme Bay cored 19.30 m of ammonite-bearing Inferior Oolite limestones and proved all the Bajocian zones from Ludwigia murchisonae to Parkinsonia parkinsoni. Bajocian dinocyst, sporomorph, coccolithophorid, foraminiferal and ostracod assemblages are related to the ammonite zones and subzones for the first time. When the palaeontological and petrographical descriptions are combined with previously published data a reconstruction of the depositional environment and the delineation of the South-Dorset High as one of the several "swells" controlling Bajocian sedimentation in southern England can be made.
[with palaeogeographic and isopachyte map, and with ammonite distribution by C.F. Parsons].
Penn, I.E. 1982. Middle Jurassic stratigraphy and correlation of the Winterborne Kingston borehole. In: Rhys, G.H., Lott, G. K. and Calver, M. A. (eds) The Winterborne Kingston Borehole, Dorset, England. Report of the Institute of Geological Sciences, 81-3, 53-76.
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Periam, C.E. 1957. The Forest Marble Ridge at Rudge near Beckington, Somerset. Proceedings of the Geologists' Association, 68, part 3.
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Price, G. D. and Teece, C. 2010. Reconstruction of Jurassic (Bathonian) palaeosalinity using stable isotopes and faunal associations. Journal of the Geological Society, London. vol. 167, pp. 1199-1208. By Gregory D. Price and Christie Teece. School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK. (e-mail: g.price@plymouth.ac.uk).
Abstract:
The Bathonian is an interval from which salinity-controlled benthic faunas are particularly well known. This study examines Late Bathonian faunas from Dorset, UK, from the Wattonensis Beds (Fuller's Earth), of known marine origin, the overlying Elongata Beds, and the Boueti Bed marking the base of the Forest Marble. Del 18 O values of belemnites (Belemnopsis) from the Wattonensis Beds range from -1.4 to -0.7 parts per thousand and carbon isotope values range from -2.3 to -0.5 permille VPDB. Isotope values of well-preserved Rhynchonella from the Wattonensis Beds range from -2.4 to -1.3 permille for oxygen and -1.1 to 0.6 permille VPDB for carbon. The del 18O data suggest high palaeotemperatures (possibly up to 28 degrees C). A similar spread of data is derived from brachiopods (Goniorhynchia boueti) from the Boueti Bed. Given the marine origin of the Wattonensis Beds and overlap of data, the reduced faunal diversity of the Boueti Bed is considered unlikely to be the result of reduced salinity. Brachiopods therefore should not be regarded more sceptically as evidence for fully marine conditions, as has been suggested. The oysters (Praeexogyra hebridica elongata) and brachiopods (Rhynchonella) from the Elongata Beds have isotope values that also imply fully marine conditions and are interpreted as being indicative of slightly cooler conditions.
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Schnyder, J., Baudin, F. and Deconinck, J-F. 2005. A possible tsunami deposit around the Jurassic-Cretaceous boundary in the Boulonnais area (northern France).
Sedimentary Geology, 177, Issues 3-4, 209-227. By
Johann Schnyder, Francois Baudin and Jean-Francois Deconinck.
Abstract: An unusual succession of facies locally deposited around the Jurassic-Cretaceous boundary in the Boulonnais (northern France) is attributed to a tsunami event by comparison with recent tsunami deposits. This sedimentary succession includes basal erosion with reworked lithified blocks, soft-sediment deformations, an erosional conglomerate overlain by wood fragments and clays containing continental and marine fossils in one setting and conglomerate with mixed fauna in an other setting. The tsunami probably affected the coast of the Boulonnais area of the London-Brabant Massif. The origin of the event is unknown. It was most probably triggered by an earthquake, but other origins such as volcanic eruptions, a giant landslide, or even the impact of an extraterrestrial bolide into the ocean may also be considered. [Possibly relevant to the Junction Bed because neptunian sills were developed in this tsunami deposit.]
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Sellwood, B.W., Durkin, M.K. and Kennedy, W. J. 1970. Field meeting on the Jurassic and Cretaceous rocks of Wessex.
Proceedings of the Geologists' Association, 81, (4), 715-732. By Professor Bruce Sellwood of Reading University and co-authors. [This is a well-illustrated and informative paper, notable for a set of excellent diagrams of trace-fossils]. Example extract: "The Lias Junction Bed. This bed marks the junction between the Middle and Upper Lias. It is readily recognisable, for the upper part is an intensely hard, white, pink and yellow mottled porcellaneous limestone. The detailed stratigraphy of this complex condensed bed (Jackson, 1922, 1926; Howarth, 1957) was reviewed, and many of the characteristic fossils noted. Attention was, however, concentrated on the origin of this bed: Hallam (1967, 423, 436) makes brief mention of it, but many undescribed features remain. The well-known planation of ammonites and other fossils was demonstrated, and although conspicuous at the 'erosion' surfaces developed in the sequence, it also occurs elsewhere in the rock. This phenomenon is common in other, comparable, limestone lithologies (Hollmann, 1964) and is a pressure-solution effect which also occurs in the Portland Limestone (W. G. Townson, personal communication).. Undoubted pebble beds occur, and many ammonites are rolled and broken. There is no evidence that the formation of these beds required emergence above sea-level.. The most conspicuous feature of the Junction Bed limestones is the complex lamination developed at many levels. Many erosion surfaces are capped by laminated carbonate several centimetres thick. There are miniature unconformities, concentric laminae surround pebbles and fossils, which are in turn enveloped by further laminated sediment. All these features indicate that the bulk of the limestone is stromatolitic. Many features are identical with those seen in both fossil and recent algal laminites and oncolites (Hamilton, 1961; Wolf, 1965a, b)."
Sellwood, B.W. and Jenkyns, H.C. 1975. Basins and swells and the evolution of an epeiric sea (Pliensbachian-Bajocian of Great Britain). Quarterly Journal of the Geological Society, London, 131, 373-388. (By Bruce Sellwood of Reading University and Hugh Jenkyns, at that time at Durham University). Abstract: During Pliensbachian-Bajocian times northern Europe, including Britain, was covered by an epeiric sea. Sediments formed include clays, sandstones, limestones and ironstones, usually cyclically arranged; different facies were developed synchronously in different areas. There is very little evidence, by way of slumps or turbidites, that redeposition processes were active. The sequence clay, sandstone, limestone/ironstone is here interpreted as representative of a shallowing. Thus, at any one time, despite differing bathymetric zones, bottom slopes were apparently subdued enough to ensure that sediment displacement did not generally take place.
To reconcile the concept of Jurassic 'basins' and 'swells' with the subdued bottom slopes of the north European epeiric sea, we suggest that these structural elements were characterized by great and negligible subsidence respectively, but that sedimentation was always rapid enough to maintain a roughly level sea floor. We relate the Mendip, London Platform and Dorset coast 'swells' to early Jurassic positive fault motions in the basement. The Market Weighton 'swell' is ascribed to relative buoyant rise of a salt pillow or granitic body whose movement was probably triggered by the same motions. These, presumably extensional, tectonics were probably the driving force behind the formation of the whole north European epeiric sea and must be related, in turn, to the opening of the oceanic central Atlantic and Alpine-Mediterranean Tethys. [This is a well-known, key paper for setting the palaeogeographic and facies scene for the later early Jurassic. It is easy to access in libraries.][end of Sellwood, et al. 1975]
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Solly, H.S. 1890a. The geology of Bridport. Proceedings of the Dorset Natural History and Antiquarian Field Club, vol. 11, pp. 109-117.
By the Reverend Henry Shaen Solly, M.A. of Bridport.
[A coast excursions, a field trip, was led by Henry Shaw Solly
Solly, H. S. 1890b. Note on the fault in the cliff west of Bridport harbour. Proceedings of the Dorset Natural History and Antiquarian Field Club, vol. 11, pp. 118-121.
Solly, H.S. and Walker, J.F. 1890. Note on the fault in the cliffs west of Bridport Harbour.
Proceedings of the Dorset Natural History and Antiquarian Field Club, vol. 11, pp. 118-121.
By the Reverend H. Shaen [Shaw?] Solly and John Francis Walker, M.A. [The text of the short paper follows:]
[Location discussed - end of the Esplanade, west of Bridport Harbour, West Bay, Dorset. The fault is now obscured, so this is an important record. The paper discusses, amongst other matters, Battery Point, west of the harbour and this has now been completely eroded away.
Therefore for a photograph of this and other historic photographs go to the excellent webpage:
Dorset Photo Heritage - West Bay Esplanade from Battery Point, and associated photographs. Battery Point was composed of high Bridport Sand Formation, just under the position of the Inferior Oolite.]
In the Cliff west of Bridport Harbour there is an
interesting and complicated Fault, on which it
may be worth while to offer a note, especially as
this portion of our coast will before very long be
washed away by the land springs and the tides.
It is, however, necessary to caution the student
that the observation of the Cliff is far from easy on account of the very different appearance it presents at different seasons of the year, and the way the lower portions are obscured by the slipping
or washing, down of the higher beds. In wet weather the water falling on the surface of the cliffs cut deep gullies, while high tides undermine the base. Huge overhanging masses, therefore, fall from time to time, often in hot dry weather throwing up
clouds of dust. The fallen material is rapidly removed by the sea.
The cliffs [west of the harbour] begin with Midford Sands [Bridport Sands] of the same level as, and similar to, the well known Cliffs to the east of the Harbour [However, this short section of Bridport Sands cliff has now been eroded away, so there is now no Bridport Sands cliff here resembling East Cliff and Burton Cliff]. After 70 yards [metres][now at the end of the sea front esplanade] we reach the beginning of the fault, and find traces of a considerable downthrow. [The fault is now covered up under a sloping surface].
First we have a section of Midford Sands [Bridport Sands] 15 feet [4.6m] in height from the base of the cliff, but it is Midford Sand of a higher level, i.e. that which is found immediately under the Inferior Oolite, which has here all been removed by denudation. [i.e. a cliff of 4.6m of high Bridport Sand Formation, west of the harbour, and now completely eroded away].
Thirty-three yards [33m] further to the west the --- can be seen in position, about 2 feet [0.6m] thick. In 1888 Mr. Walker found in a bed of clay about 1J [?} feet thick, immediately above the Inferior Oolite, a flat variety of Terebratula phillipsii, also T. decipiens, and Ammonites which have been determined by S. S. Buckman, F.G.S., as Perispinctes sp. and Oppelia fusca. This Inferior Oolite clay occurs at the top of the quarries around Crewkerne and Broadwindsor, and was first noticed by J. F. Walker in his paper on T. morieri, Geological
Magazine, 1878.
These beds slope down towards the west below the Fuller's Earth, and at a distance of fifty-six yards appear to reach the level of the beach. Near this point the junction of the Midford Sands [Bridport Sands] and 1nferior Oolite consists of a rubbly broken conglomerate, composed of 1nferior Oolite
cemented together with iron-sand: the pieces appear to vary in size from a cubic foot to the size of coarse gravel, showing that extensive denudation occurred before the deposition of the Parkinsoni zone of the Inferior Oolite. The Inferior Oolite itself is a very hard rock containing masses of iron pyrites, and is of a greenish white colour, due to the iron being chiefly in a state of prot-oxide. [this is interesting as a probable berthierien occurrence. The well-known Burton Cliff section shows oxidised Inferior Oolite, with much goethite rather than berthierine, the green iron silicate mineral common at depth and offshore]. The greater part of the Inferior Oolite seems to belong to the Parkinsoni zone, as it contains the characteristic fossils T. spheroidalis,
T. stephani, A. parkinsoni, Waldheimia carinata, Acanthothyris spinosa. Some of these fossils are a dark blue colour, having been protected by the clay from oxidation. The top of the cliff above
the Fuller's Earth contains brown surface soil, probably derived from the decomposition and washing down of Forest Marble.
Beyond this point the base of the cliff at present shows no trace of either Midford Sands [Bridport Sands] or Inferior Oolite, but blocks of these are plentifully strewn along the shore and running out into the sea form the Black Rocks [probably covered by sea defence rock armour now!].
One hundred and sixty yards [metres]further west we arrive at another good point of observation. Here bands of stone, ten or more in number, belonging to the Lower Fuller's Earth, reach the level of
the beach. These can be traced more or less perfectly, according to the state of the cliff, rising eastward at an angle of 34 degrees. Up to
this point the clay composing the bulk of the cliffs is of a very much darker colour than that further west: this is seen best in dry weather, and was once specially noted by Mr. Walker when passing the coast in Mr. Thompson Stephens' steam launch, and
suggested to Mr. Walker that at this point we have an important, though hitherto unobserved, fault in the Fuller's Earth itself. Both the bands of the rock and the clay between contain abundance
of Rhynchonella smithii, also specimens of T. globata and Waldheimia ornithocephala, nearly always crushed, and more
rarely Acantliothyris spinosa, var. Poorstockensis. The rock also
contains a costate Trigonia, allied to Trigonia elongata, from which
it appears to differ by the costae being more numerous (we propose to call it variety stephensi), a large Pholodomya, Modiola giblosa var., Ostrea accuminata, which occurs sometimes in masses, and various bivalves too imperfect to determine ; also a small belemnite,
B. parallela. This section of the Cliff is clearly Lower Fuller's Earth.
Beyond this point the Cliff, stretching towards Eype, appears to be Upper Fuller's Earth [now Frome Clay Formation]. It is paler in colour, and contains very few fossils, except some imperfect bivalves, and these are far from plentiful. This stratum gradually passes into Bradford Clay [lower part of the Forest Marble now?] and Forest Marble, with which it is capped for a considerable distance.
The same bed may be observed in the Cliffs beyond Burton Bradstock, where it is more fossiliferous, containing gasteropoda as well as bivalves ; but neither there nor towards Eype does the
Upper Fuller's Earth contain any of the fossils which occur in the lower clay. Mr. Walker is of opinion that the Lower Fuller's Earth (with the stone-bands) belongs to the Inferior Oolite series, while the Upper Fuller's Earth, which appears to be faulted against the Lower, belongs to the Great Oolite series, and may represent that bed. It is not an unusual occurrence for a rock which is limestone
in one place to be represented in other districts by a band of clay.
For instance, Professor Seeley, T. Roberts, and others have shown that the Corallian series are represented by a bed of clay in the Fen district.
If this opinion is correct, an important line of division has hitherto escaped all notice on our Bridport coast owing to the faintncss of its lithological character, and we have one more
instance of the necessity of determining these lines upon a careful study of palaeontology.
[End of short paper]
Solly, H.S. - Field Trip of the Dorset Natural History and Antiquarian Field Club.
Geological part at Seaton was led by the Rev. Solly.
The walk from Stanton St. Gabriel will afford
an opportunity of examining the Belemnite and Green Ammonite beds of the Lower Lias (Zones of Am. Henleyi and Lataecosta), and by extending the walk a short distance east of Seaton to Down Cliff the
whole of the Middle and Upper Lias may be seen, including the Three Tiers, the Starfish Bed, and the Junction Bed. A fine view may also be obtained of the Midford Sands, Inferior Oolite, Fullers' Earth, Forest Marble, and Green Sand. At various points along the beach Mr. Solly drew attention to the features of these beds presented in the cliffs above.
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Sylvester-Bradley
, P.C. 1957. The Forest Marble of Dorset. Proceedings of the Geologists Association , vol. 68, pp. 26-28. By Peter Sylvester Bradley.
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Symonds H. 1912. Bridport Harbour through seven centuries. Proceedings of the Dorset Natural History and Archaeological Society, 33, 161-199.
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Thomas, J. and Ensom, P. 1989. Bibliography and Index of Dorset Geology. Dorset Natural History and Archaeological Society. 102 pages. See also the internet version - Bibliography and Index of Dorset Geology.
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Torrens, H.S. et al., 1969. International Field Symposium on the British Jurassic, Excursion No. 1 , Guide for Dorset and South Somerset, University of Keele, 71 pp.
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Wendt, J. 1971. Genese und Fauna submariner sedimentarer Spaltenfullungen im mediterranen Jura. Paaeontographica, A136, 122-192. [paper referred to by Jenkyns and Senior (1991) with regard to origin of the Junction Bed]
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Wilson, V., Welch, F.B.A., Robbie, J.A. and Green, G.W. 1958. Geology of the Country around Bridport and Yeovil (Explanation of Sheets 327 and 312). With contributions on: The Purbeck Beds by F.W. Anderson, Palaeontology by R.V. Melville, and Groundwater by S. Buchan. London, Her Majesty's Stationery Office, 239 pp. Department of Scientific and Industrial Research. Memoirs of the Geological Survey of Great Britain.
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Woodward, H.B. 1887. Excursion to Bridport, Bothenhampton, Burton Bradstock, Bridport Harbour and Eype. Proceedings of the Geologists' Association, London, 9, 200-209.
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© 2018 Ian West, Tonya West 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.
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Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.
Webpage - written and produced by:
Ian West, M.Sc. Ph.D. F.G.S.
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at his private address, 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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