West, Ian and Harvey, D. 2013. Chesil Beach Pebbles: Geology of the Dorset Coast. Internet site: http://www.southampton.ac.uk/~imw/chesil.htm. Version: 14th December 2013
Chesil Beach Pebbles .

Ian West,
Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,

Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.

|Home and Contents |Chesil Beach - Storms, Floods |Chesil Beach - Magnetite |Chesil Beach - Pebbles |Chesil Beach - Geological Bibliography |Portland - General |Portland Bill |Portland Harbour |Portland Bibliography |Bridport - East Cliff| |Fleet Lagoon|
Selected external links: | Chesil Beach Nature Reserve and Chesil Beach Centre | Jurassic Coast (Dorset County Council)| Portland (Dorset County Council) | BBC Education - Rocks |

. Chesil Beach Pebbles of Chesil Beach southern  end of Chesil Beach .
Chesil Beach from Portland Typical pebbles Portland end of beach


---- Field Guides to the Chesil Beach and Isle of Portland ----
Chesil Beach - Introduction ..... ..... ..... ..
Chesil Beach - Storms and Floods ..... ...
Chesil Beach - Pebble Types ..... ..... .....
Chesil Beach - Magnetite from Shipwreck
Chesil Beach - Bibliography ...... ..... .....
Isle of Portland - General ...... ..... ..... ...
Isle of Portland - Portland Bill ... .... .......
Isle of Portland - Withies Croft Quarry ....
Isle of Portland - Bibliography .... .... .....

INTRODUCTION

The pebbles of the Chesil Beach are almost entirely composed of very resistant minerals, mostly quartz and chalcedony (with a hardness of 6 on Moh's Scale, i.e. harder than steel). The bulk of the beach pebbles, 98%, consists of flint and chert. This comprises grey to brown flint from the Cretaceous Chalk and light bluish grey chert from the Upper Greensand. The dominant flint and chert has been derived from Cretaceous rocks, perhaps by way of Tertiary gravels such as those at Blackdown and Bincombe. At the Portland end, there is some black chert from the Portland Cherty Series, the Portland Stone and the basal Purbeck Formation. In this area some limestone from the local Portland and Purbeck Formations also occurs. Chert from the Portland Roach can be recognised easily because of the large molluscs and particularly the gastropod Aptyxiella portlandica .

Striking constituent are the discoidal pebbles of quartzite, red, purple ("liver-coloured" or white) such as occur in the Triassic pebble beds at Budleigh Salterton in Devon. Much rarer are pebbles of porphyry of a type that can be matched in the Permian breccia of Dawlish. Various tourmalinised rocks, red and black chert, vein quartz, etc., have been identified with the Palaeozoic rocks of Cornwall; many of them may have come like the flints, out of Tertiary gravels ">(Davies, 1956).

The pebbles are well-graded, coarsest near Chesil (Chesilton) and diminishing in size towards Bridport, and it is said that fisherman landing on the bank at night can judge their position by the size of the pebbles.

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The Pebbles as Seen in the Beach

Chesil Beach - view NW

Chesil Beach pebbles

Consider the pebbles shown here. These are of medium size in relation to those on other parts of the beach. They are in dry condition and, as such, do not show the details well for identification. They are almost all extremely hard, harder than steel. If you try to scratch one of them look at the site of scratching with a handlens. You will see that metal has come from the knife but that the stone is unscratched. Notice that some show a conchoidal (shell-like fracture). The pebble shown are almost all of one of two types. Upper Cretaceous Chalk flints include the slightly brownish pebbles, most of the medium grey pebbles and also the less well- rounded pebbles. The Lower Cretaceous, Upper Greensand chert pebbles are not very easily identified in the photograph but they are often rather clear and translucent with a pink or bluish tinge. One of these is 6 pebbles to the left of the handlens. Not clear in this photograph is that the pebbles often show percussion marks. These are small curved cracks formed by the impact of one pebble on another on the Chesil Beach. Other, less common pebble types occur but are not shown in the photograph.

Although it is not ideal for the purpose, it is possible to make a limited study of the pebbles from the photograph. You can comment on their shapes. You can measure, in an approximate manner the distribution of their lengths. With some pebbles you can compare the short axis/long axis ratio. In some cases you can see the intermediate axis.

The pebble size above high water mark coarsens in a southeasterly direction with most rapid rate of change at the Chiswell end (Carr, 1981). The mean long diameter at Chiswell is of the order of 5 cm, rapidly falling to 3.5 cm and finishing up as pea-sized shingle at West Bay or Bridport Harbour (Carr, 1969).

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Beach Pebbles - Rock Types Present

Specific pebble types will now be discussed. This is based mainly on hand-specimen observations. The addition of thin-section data would be desirable and this may be provided later.

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Flint Pebbles (Abundant)

Liesegang rings in flint pebble

As noted above, a very large proportion of the pebbles are of flint from the Chalk. The photograph shows a special variety with bands which are the result of the abrasion of liesegang rings. Such rings occur around flint and chert nodules as a result of rhythmic precipitation of incoming silica in solution around the silica core. A piece of the ring structure has been separated and worn in this case.

Flint pebbles in general are recognisable by the hardness, the fine grain, the conchoidal (shell-like) fracture and relics of a cortex (brownish or white outer particially leached surface layer, and unlike the interior with some microscopic porosity). Some brownish colour or whitening is often a good indicator that the pebble is flint. A brown colour is not so often seen in the pebbles of Upper Greensand chert. That is coarser-grained and often with chalcedony. Flint can be distinguised from Portland chert because the latter is blacker and sometimes has typical Portland fossil shells. Flint pebbles may contain cavities left by fossil sponges occasionally there may be silificified chalk fossils such as the echinoids Micraster , Echinocorys or some Chalk bivalve or brachiopod.

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Flint Pebbles - Percussion Marks

The flint pebbles, and also chert and some other types of pebble, show conspicuous curved cracks or percussion marks. These are the results of numerous impacts on the beach, when the pebbles have been thrown by waves. The percussion marks are very obvious, especially with a handlens.

A flint cobble from the Boscombe Sands Cobble Bed, with very well-developed percussion marks from impacts on ancient, Eocene

A former "Chesil Beach" existed in the Bournemouth and adjacent region during Eocene times, about 40 million years ago. The deposits of it are known as the Boscombe Cobble Bed. The beach was at the western end of a sea and thus the wave direction was from an easterly quarter. The clasts are generally rather larger than those of the present Chesil Beach and they too have percussion marks. A photograph of one of these is shown above.

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Upper Greensand Chert Pebbles (Abundant)

These are equally hard, but white to grey and more granular and gritty than the flint. The chert often has some bluish white chalcedony. Fossils such as Pecten type bivalves can occur in it. A characteristic of it which does not normally occur in Chalk flint is the occurrence of elongate siliceous sponge spicule (like minute pointed tubes). Note that sponge spicules can occur in Portland chert, but there is usually distinctive.

The source of the Upper Greensand chert is interesting. It is similar to the chert in the Upper Greensand of West Dorset (particularly from the cliffs between Lyme Regis and Sidmouth according to Damon, 1884). It is abundant in the Upper Greensand of West Dorset, but occurs in smaller quantity in the same formation in East Dorset. Has it come directly from the cliffs of West Dorset or indirectly by way of Tertiary gravels such as those at Blackdown and Bincombe, as suggested by (Davies, 1956) and other authors, or from both sources?

Portland Chert Pebbles (Common)

Portland is common at the eastern end. If you go to near the Cove House Inn, at Chesil or Chesilton, you will easily find this. Some of the Portland chert has the turreted gastropod Aptyxiella and large bivalves and is this is easily recognisable as having come from the Portland Roach. Much black chert, though is probably from the Portland Cherty Series. This is not usually very fossiliferous and may seem black and relatively featureless. You might see some elongate sponge spicules with a hand-lens (Pachastrella). To positively identify apparently featureless Portland Cherty Series chert it is necessary to make a thin-section and to look for the kidney-shaped Rhaxella type sponge spicules that are so abundant in that part of the Portland Group (but because, unlike limestone, chert requires some hard work to thin-section this is not often done!).

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Purbeck Chert Pebbles

Chert from the basal Purbeck Formation is also black, but frequently a replacement of stromatolitic (thrombolite) limestone. It does not usually show fossils and is irregular or botryoidal. This is the usual Purbeck type. Silicified Purbeck shelly limestone should be looked for. The shells are smaller than those in the Portland chert. I expect that it is in the Chesil beach because, although it comes from the Middle Purbeck strata which are missing on Portland, it is common in the Tertiary gravels of Blackdown Hill (Hardy Monument), not far away. I have not seen chert replacements of gypsum in the Chesil Beach but it occurs in the western cliffs and elsewhere on Portland and should be present (in very small quantities) in the raised beach. It is difficult to recognise without thin-section petrography and, sometimes, special etching.

(It is not generally necessary to undertake etching on the pebbles, it is not recommended, and for safety reasons I am not saying here what with! Specialists will know what is needed - but it must be used only dilute and only with very special safety precautions. In general it is sufficient for identification simply make the pebbles wet or varnish them).

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Budleigh Salterton Quartzite Pebbles - General

. Budleigh Salterton Pebbles . Laminated Budleigh Salterton Pebbles

. Laminated Budleigh Salterton Pebble . cross-laminated quartzite pebble

The ovoid or discoidal pebbles of red, purple ("liver-coloured") or white quartzite which seem to have come from the Triassic Budleigh Salterton pebble beds of Devon are common and these are of special interest. Examples of such pebbles, found on the Chesil Beach in the year 2000, and of various colours are shown above. They are very hard, cannot be scratched with steel (they take off a streak of the metal) and have a granular surface, but in general the individual sand grains are not removable but firmly cemented in. There can be a few pits, though, from which sand grains have gone, and there may be some darker grains in amongst the light-coloured quartz. The pebbles are ovoid and often flattened in one plane. Notice that the lamination is the original bedding of the sandstone which was lithified by quartz cementation to a quartzite. If you look at the images carefully you will notice that in two cases there is some small-scale cross lamination. The quartzite from which they have originally come has had some small quartz veins developed and some shearing as shown by the small fault displacement of the veins in one of pebbles.

The colouring of the pebbles will depend on the amount of iron present and its state of oxidation. Red or purplish hematite (ferric oxide) and yellowish brown limonite or goethite (ferric hydroxide) are probably the main pigments. In one pebble shown here the pigmenation is strangly leaf-like. The original staining of the quartzite with iron-oxides has probably occurred in continental, desert conditions above the water table where oxygen is available.

(Note on the pebble in the lower right illustration: This pebble is not quite the normal colour for a Budleigh Salterton pebble, it seems too brownish so it is possible that it is of different derivation. The angle of the cross lamination foresets is very steep. Is the pebble the right way up or is it inverted in terms of original sediment?)

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The Budleigh Salterton Pebble Beds

The Triassic Budleigh Salterton Pebble Bed in the cliffs west of Budleigh Salterton, Devon, with pebbles from the deposit forming the beach

The Triassic Budleigh Salterton Pebble Bed in the cliffs west of Budleigh Salterton, Devon, with the Otter Sandstone above

There seems little doubt that pebbles like those shown above have come from the Budleigh Salterton pebbles beds or their continuation in the English Channel, but the method of transport is a problem, which is discussed below. Firstly, though, It is worth considering the "New Red Sandstone" conglomerates of Devon briefly. Budleigh Salterton (map reference SY065818) is a small coastal resort in Devon, southeast of Exeter and east of Exmouth. The Budleigh Salterton Pebble Beds are magnificently exposed in the cliffs west of the town and have been discussed by Durrance and Laming (1982). Some 30m of mixed conglomerate and sand lenses are seen above a beach composed of more-polished versions of the pebbles from the cliffs. These "Budleigh pebbles" include distinctive types of purple quartzite which are found on beaches in many parts of southwest England. The source of the pebbles within the Budleigh Salterton Pebble Beds is not certain although the quartzite has long ago been matched to Ordovician rocks in Normandy on the basis of fossils (Orthis budleighensis) found in some of the pebbles. Although a northerly direction of flow is shown by pebble imbrication, it seems doubtful as to whether a river could move cobbles of up to 40cm in size a distance of 250 km from one side of a desert basin to the other. A less distant source might have been a Palaeozoic ridge with Ordovician quartzites in the vicinity of Start Point. The Budleigh Salterton pebble beds are notable for wind-facetted pebbles or dreikanter, especially at the top of the conglomerates.

The Budleigh Salterton Beach with Pebbles

Spit almost blocking outlet of the River Otter, east of Budleigh Salterton, Devon

Pebbles of quartzite on the beach at Budleigh Salterton, Devon, gulls for scale

Pebbles of quartzite on the beach at Budleigh Salterton, Devon, foot for scale

The well-known beach of Budleigh Salterton is well-known. It is composed largely of quartzite pebbles, derived from the Triassic Budleigh Salterton pebble bed in the cliff. Discoidal liver-coloured quartzite pebbles from this are notable in the Chesil Beach and are found on other beaches as far east as the Isle of Wight. A short account of the history of Budleigh Salterton suggests that many of the larger pebbles were thrown up in the 1824 storm to form the foundations of the present beach (see eastdevon.net - Budleigh Salterton - history. Of course they can be eroded directly from the cliffs, but a stock of the larger and less-mobile pebbles may have been accumulated just offshore.

Another webpage states that in the 1824 hurricane beach pebbles were moved eastward to form a spit blocking the mouth of the river Otter: "Budleigh Salterton, known locally as Salterton (formerly Salterne), derives its name from the manufacture of salt which was once a precious commodity, when it was the main food preservative. Large salt pans were situated at the lower section of the river Otter, the monks at the Otterton Priory holding the rights to this important enterprise. The pebble spit, now blocking what was once an open estuary, was a result of the great storm of 1824" ( Budleigh Diary: community information for Budleigh Salterton ).

Note however that the partial blocking of the outlet of the River Otter has long been a problem. The Otter Valley Association (1984?) in an Historical Guide to the Lower Otter Valley refers to a report by Leland in 1540 - "On the west side is Budleigh, .. it (the haven) is now clene barred" [clean barred]. He said that a hundred years earlier it was a thing "of sum Estimation". Efforts were made to clear a channel and the main channel was navigable to 60 ton vessels until 1810. Thus it is unlikely that the spit was created from nothing in the 1824 storm; it was probably just significantly enlarged and extended.

It is interesting that a hurricane in the English Channel not only seriously damage the Chesil Beach, also seems to be able to move source material from the west in an eastward direction towards Chesil. The movement reported here is on a relatively small scale and a headland prevents further transport. In the Holocene past when sea-level was lower Budleigh Salterton pebbles may have been moved by hurricanes westward on a very much longer spit and moved significantly towards the predecessor of the Chesil Beach. Beach pebbles of flint and chert at Ladram Bay, East Devon, with a relative rarity of Budleigh Salterton pebbles, September 2009 ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Footnote: An unlawful bucket of pebbles or an innocent bucket of crabs:
In the year 2000, a painter and decorator, a Mr Robert Pearcey, was convicted of taking pebbles from Budleigh Salterton beach. The mayor of Budleigh Salterton said "I blame Charlie Dimmock and Alan Titmarsh from Groundforce. Children has always taken home the odd pebble from the beach but since we've had fancy gardens it has become a problem because they are disappearing in large numbers. Its got worse because of the number of gardening programmes that suggest making paths or ponds or seaside features out of pebbles." Pearcey, 41, was seen carrying buckets of pebbles from a beach. East Devon Council introducted bylaws three years ago under the 1949 Coastal Protection Act to stop people taking large pebbles from the beach. Pearcy denied taking a bucket of pebbles from the beach and said that he and his wife were, in fact, carrying a bucket of crabs. He was found guilty, conditionally discharged for 12 months and ordered to pay 250 costs. The mayor said "East Devon District Council brought this case because they wanted to make an example of somebody" (de Bruxelles, 2000).
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The Problem of Transport of the Budleigh Salterton Pebbles

The Budleigh Salterton (Bunter) quartzites are valuable indicators of provenance. They are very easy to recognise, at least the liver-coloured ones. There is one specific original source area. However, as always, the story is more complex than it first appears. Budleigh Salterton pebbles occur not only in the Chesil Beach but also in the Portland Raised Beach (or beaches). Occasional specimens are found on the land in the Weymouth area (Arkell, 1947) , and some, surprisingly, are found on the beach at the back of Lulworth Cove (particularly in the north-eastern part). They are also found as lag concentrates on the sea floor of the English Channel, according to Kellaway et al. (1975).

Any explanation for the origin of the Chesil Beach has to explain the presence of these quartzites. They could travel eastward by longshore drift (with prevailing southwesterly winds and waves). However, there are problems at Otterton Point (SY 079819), near Budleigh Salterton and at Beer Head (SY 228878) near Seaton. There are no beaches at these headlands and pebbles cannot pass them by longshore drift in an easterly direction. Beer Head is a 120 m Chalk cliff, with the sea at the foot of cliff on the eastern side and is an obvious obstruction. How could pebbles get past this?

The general explanation for the origin of the Chesil Beach as given above was partly based on some form of direct transport eastward from Budleigh Salterton in the past and partly on Strahan's (1898) argument that the Chesil Beach represents the winnowings of lost land in West Bay. Strahan argued that because the pebbles cannot escape around the cliffs of Portland the Chesil Beach represents the accumulated gravels of the whole of the land-surface that once occupied West Bay (Lyme Bay).

He commented more specifically on the Budleigh Salterton pebbles. " The occurrence of the quartzites has caused much speculation as to the power of the waves and tides to roll pebbles along the sea-bottom, for it was assumed that they must have brought directly from Budleigh to the Chesil Beach . Their presence, however, in the Plateau Gravels (i.e Pleistocene gravels) shows that they may have travelled at a far earlier date that was supposed and at a time when conditions were altogether different from those of the present day. The Plateau Gravels, it will be remembered, were not improbably contemporaneous with the Glacial Deposits of other parts of England and themselves show indications of the action of ice. The Bagshot Beds, moreover, which, of course, must have extended over much or perhaps all of the district, contain a variety of far-travelled rocks, and probably yielded much of the material of the Plateau Gravels.

The mapping of the Plateau Gravels show there must have been at least two extensive sheets at different elevations on the north side of the Fleet, while a still more important deposit overspread the high ground at Abbotsbury. We may reasonably assume that these gravels were no less plentiful on the south side of the Fleet, and formed a souce capable of having supplied both the raised beach of Portland Bill and its modern representative the Chesil Bank. "

Arkell (1947) did not like this theory. He said "Though it is true that the winnowings of lost land in West Bay are represented in Chesil Beach, Strahan's explanation is unsatisfying. The Tertiary and Quaternary gravels of the land that still remains adjacent to the beach, and that which will be next fodder for the sea, would be utterly incapable of supplying the millions of small and large Bunter (Budleigh Salterton) quartzite pebbles so conspicuous on Chesil Beach, or the large quantities of Permian porphyry. In the (Pleistocene) Plateau Gravels Bunter quartzites are extremely rare and none is known exceeding 5 inches (12.7 cm) in diameter. No porphyries have been found. Strahan's theory is only plausible if it be assumed that on the lost land far out in West Bay some ancient river flowed from the west and brough with it deposits of gravel differing entirely in composition from those of the early Frome or the streams of the Weymouth District. " (Arkell went on to discuss the floating ice theory)

Arkell noted that " there at present a much higher proportion of quartzite pebbles in the coarse-graded part of the Chesil Beach from Small Mouth to Chesilton than in the Raised Beach. This at first suggests a replenishment since Raised Beach times, but it may be accounted for by concentration of the largest and hardest pebbles here owing to their relative indestructability, from a much longer stretch of destroyed Raised Beach."

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Granite

. Two granites and a porphyry . Two granites from Chesil Beach

Granite seems to be an occasional component of the Chesil Beach. The photographs above show granite, except for the middle pebble in the left image (which is porphyry). Note the coarse grain size, the pink or white feldspar, the quartz of glassy grey appearance, and the black mafic minerals, normally biotite (mica) and/or hornblende (amphibole).

Carr and Blackley have recorded that thin-section petrographic studies have shown the presence of pebbles of granite, microgranite, granophyre and a fault breccia of microgranite and slate. A related igneous rock found was quartz diorite. Igneous pebbles occur in the Portland Raised Beach and granite was recorded by Prestwich (1875a) , although the rock type is not common there. A now-destroyed extension of the raised beach could have supplied at least some of granite to the Chesil Beach. Ship ballast or cargo from wrecked ships (see the website on magnetite pebbles on the Chesil Beach ) could have contributed some granite and other rock types to the beach but it seems unlikely that they could become as rounded as the normal pebbles.

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Porphyry

Two granites and a porphyry Porphyries are common in the Chesil Beach and the central specimen here is an example. Note the larger porphyritic crystals of feldspar, white or cream in colour with cleavage and roughly rectangular, in a finer, dark and very hard matrix (usually cannot be scratched with steel). Quartz phenocrysts are not so conspicuous. Compare the porphyry with the granites. The latter have crystals which are all relatively large so that they can be seen with the naked eye and examined with a hand-lens. The porphyry has large crystals (not as large) scattered throughout, but these are in very fine-grained dark greyish brown matrix. The feldspar crystals in the porphyry differ from most of those in the granite in that they show a euhedral form (i.e. the crystal shape with parallel sides and sharp angles can be clearly seen in section).

The name porphyry is an old one and not precise in term of composition and petrography. It was originally applied to a purplish rock quarried in ancient Egypt and characterised by feldspar phenocrysts (larger crystals). The name porphyry comes from the Greek porphura meaning purple, but the term now refers to an igneous rock with larger crystals in a finer matrix. Rocks referred to here as porphyry are simply placed in a convenient group of similar porphyritic appearance, without any systematic petrographic or geochemical study having been undertaken.

Various authors, including Baden-Powell (1930) whose comments are reproduced here, have noted that many of these pebbles have probably been derived from the Permian breccias of Devon. " A certain type of porphyry is found as pebbles both in the Portland Raised Beach and in the Chesil Bank, although its frequency in both these deposits is very low, not more than 1 in 10,000 of the pebbles. It was pointed out to Sir J. Preswich (1875, p. 67) by Dr. Vicary that these pebbles could be matched in the Dawlish Breccia [Permian, Devon] and the present writer has collected specimens of this rock which are identical in all three deposits."

Varieties of quartz porphyry and related igneous rocks are found in nearly all the Permian breccias of mid and south Devon (Durrance and Laming, 1982). Geochemical and petrographic evidence links these porphyries, many of which have rhyolitic texture, to a former extensive rhyolite capping of the Dartmoor Granite batholith. In other words this is debris of the old lava flows of the violently volcanic, Cornubian upland that existed above the southwest England granites in early Permian times (about 290 million years ago). This type of lava is very hard because of its silica (quartz) content and it is not surprising that some of it has survived as debris in the Permian desert breccias. Much later, during development of the Chesil Beach it has been reworked from the floor and coast of the English Channel and transported eastward into Dorset. After such a journey this tough volcanic debris still resists the continous grinding of the Chesil Beach.

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Tourmalinised Rocks

. Pebble - tourmalinised . Pebbles - one tourmalinised

Pebbles of hard black, finely granular material are common. The left image shows a somewhat irregular pebble of vein quartz, stained yellowish, and tourmalinised slate (probably), all sheared and partially brecciated. This is undoubtedly from a quartz vein in southwest England, probably associated with a granite intrusion. Such material may occur, for example, around Dartmoor. Tourmaline is a hard mineral (7-7.5), but brittle, and tourmalinised slate may be eroded more easily than resistant vein quartz. This explains the irregular shape of the pebble.

The right-hand pebble of the right-hand photograph shows black tourmalined siltstone or sandstone with some lamination. The rock has been deformed and veined with quartz. Black tourmalinised pebbles should not be confused with chert pebbles, many of which are frequently black, have percussion marks, and do not usually show lamination. The black laminated and rather gritty rocks may be tourmalinised siltstone or slate from southwest England.

Tourmaline is abundant associated with the Hercynian granite batholiths of Devon and Cornwall, such as Dartmoor, Bodmin Moor and Lands End etc. There has been much boron metasomatism and this is the main product of such chemical input. Because of this western source it is not surprising to have tourmaline-containing pebbles on the Chesil Beach. The presence of small quartz veins in the pebbles is an indication of an ancient (i.e. Palaeozoic) origin. They have probably been deformed during deep burial and movements connected with the Hercynian Orogeny. To identify tourmaline for certain it would be necessary to make a thin-section of the pebble. Tourmalinised pebbles of quite different appearance to the example shown here can be present on the beach.

Tourmaline is a complex sodium aluminium silicate with boron. In the handspecimen it is normally black in colour (schorl) but more rarely it can be pink or in other colours. It often occurs in southwest England and elsewhere as large elongate prismatic crystals (trigonal), often occurring in radiating clusters. In the battered Chesil Beach pebbles it is unlikely that good large crystals will be seen and in most cases the tourmaline has a blackening effect and is associated with quartz which gives the pebbles sufficient resistance for survival in this harsh environment. Tourmaline could occur in Chesil Beach pebbles of various types.

Under the microscope in plane-polarised light, tourmaline is very variable in colour. It may be colourless, blue, green or yellow. It is normally pleochroic and this is a distinctive feature. It has moderate relief (ordinary ray 1.610 - 1.630, extraordinary ray 1.635 - 1.655). In crossed polars the birefringe can be seen to be moderate (0.021 - 0.026) with second order colours, frequently masked by a strong body colour. There is rarely twinning but there may be colour zoning. The mineral has two poor cleavages which may be visible in thin-section.

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Breccias

breccia pebbles A breccia is a rock of angular fragments. Only breccias that have been cemented strongly in hard silica are likely to survive on the Chesil Beach. Two examples are shown here. Some breccias are probably from Palaeozoic fault breccias; others can be derived from brecciated and recemented Portland chert, but this is not a common rock type on the Isle of Portland. If brecciated Portland chert is suspected then one should look closely with a handlens to see if any Portland fossils are present.

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Kimmeridge Oil Shale

oil shale and peat On the left of the photograph is a pebble of Kimmeridge oil shale or Blackstone. This is a characteristic, light greyish-brown in colour, fairly hard (but can be scratched by steel) and laminated. Pebbles like this can on occasions contain some Kimmeridgian fossils. This and the adjacent specimens are all from the same, upper part of the beach southeast of the Cove House Inn, but have been placed together here on top of the usual pebbles. Note the size contrast between the oil shale and peat clasts shown here and the hard flint and chert pebbles beneath. Kimmeridge oil shale is of low density and thus it tends to be thrown up high on beaches. Such flat pebbles can easily be transported laterally.

The very bituminous Kimmeridge shale can burn and sometimes ignites spontaneously in the cliffs. This has happened in the past at Portland where there was once a cliff or beach fire on the shores of Portland Harbour. Some information was given by Damon (1884). He stated that: " A bed of the shale presenting the appearance of burnt tile is exposed at extreme low water on the shore at Portland, near the Castle and extending to the north side of the Chesil Beach from which the bituminous parts have been exhausted or burnt out..." It is not clear exactly where this intersects the Chesil Beach but because of the former presence of the lagoon and mud of The Mere (where it presumably could not be seen) it was probably near Victoria Square. The pebble in the photograph, which is unburnt, normal Kimmeridge oil shale, has probably come from an outcrop under water on the front of the Chesil Beach in the area of the Cove House Inn. Such a subaqueous outcrop could be eroded during storms.

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Peat at Portland End of Chesil Beach

Peat is not a typical pebble material of the Chesil Beach, but in storms, ssome peat eroded by wave action from below low-tide level is thrown up on the beach. It is convenient to discuss this clastic material here too, with the pebbles. An example is shown in the centre of the photograph above of peat found on the upper part of the beach in front of the sea-defences a little to the southeast of the Cove House Inn. This has been bored by marine molluscs. Also shown, on the right from the same locality, is some peatified (carbonised) tree material.

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The Beaver Peat of Abbotsbury

location of peat at Abbotsbury, 18.10.2000 . Peat on Chesil Beach, Abbotsbury, 18.10.2000 . Peat on Chesil Beach, Abbotsbury, with author, 18.10.2000

. Peat on Chesil Beach, Abbotsbury, closer, 18.10.2000 . Peat on Chesil Beach, Abbotsbury, closer, 18.10.2000

Boulders of peat on the beach at Abbotsbury, are also considered here. These peat blocks are much larger than the peat fragments seen at the Portland end. They have been found just southeast of the old tank defences. Blocks of two metres or more in length have been thrown up on the top of the beach, as shown in the photographs above, taken on 18.10.2000. The blocks were probably eroded from peat beds beneath the beach by the lower part of waves in storms of the winter 1999/2000. The peat is argillaceous, light brown where it is clayey and dark brown to black where there is plant material. Some of this seems to be small logs or twigs. The material has since degraded largely by drying out and distintegrated to some extent. Notice how this argillaceous peat has shrunk and broken into small polygonal cracks.

Beaver bones have been found in this by David Harvey during the year 2000. There is no doubt that this peat represents marshy peaty sediments from the Fleet Lagoon over which the Chesil Beach has been driven back. Beavers were presumably once common inhabitants of the Fleet Lagoon, at least at the this northwestern end where the salinity is lower.

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Jasper

Damon (1884) reported the presence of jasper as a rare constituent of the pebbles of the Chesil Beach. This red form of silica, which originates in volcanic rocks, is scarce towards the Portland end of the beach.

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Devonian "Madrepores"

Damon (1884) also mentioned that Devonian "madrepores", that is corals or stromatoporoids, occur as a sprinkling amongst the pebbles but these too are scarce towards the Portland end.

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The Rare Star Coral

Portland Star Coral This is a pebble of silicified coral from the Jurassic Portland Stone and it may be the only known specimen from the Chesil Beach. It was found by David Harvey in the year 2000 and is in natural, unpolished condition. It is similar to the 'Tisbury Star Coral' of Tisbury in the Vale of Wardour, Wiltshire, also of Portland Stone origin. The coral is Isastraea oblonga , recorded by Damon (1884) from Grove Quarry.

Clement Reid in 1903 commented that most specimens of the Tisbury Star Coral have been found in ploughed fields to the northwest of Tisbury. The village has long been famous as the locality for the Star Coral, Isastrea oblonga, which occurs in the Portland chert, and polished specimens of which were to be found in most collections. The exact position of the bed yielding this "Siliceous Madrepore" had been a matter of some doubt, but it occurs in position of growth in the Chalky Series above the Ragstone. It was described as long ago as 1729 by John Woodward as the "Starr'd Agate". He said: "This was found, amongst several others, lying on Floors, like the common black Flints, amongst Chalk ... Underneath these Floors of starred flints lay Strata of Sand stone, in a quarry in Tisbury Parish."

Moss Agates

These are another rare type of pebble mentioned by Damon (1884).

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Various Fossils of the Chesil Beach

Various fossils of the Chesil Beach Fossils found on the beach include the following as shown in the photograph. 1. An ammonite from the local Jurassic strata. 2. A silicified example of the echinoid (sea urchin) Micraster coranguinum . 3. Limestone with gastropods. The ammonite is worn but is calcitic and iron-stained and might be a Ludwigia or related genus from the Inferior Oolite or other Middle Jurassic strata of the Burton Bradstock area. The silicified echinoid may well have been reworked from Pleistocene flint gravels in which such fossils are fairly common.

A tooth of an elephant has been picked up on the Chesil Beach in the 19th century (Prestwich, 1875a).

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Origin of the Pebble Accumulation

The first and simplest explanation of this assemblage of pebbles was that they had been swept by the waves along the shore from west to east, the usual direction on the south coast, and the larger pebbles had travelled faster than the smaller ones. Then, it was suggested that the Bunter pebbles, for example, had been swept across Lyme Bay (West Bay), either from the Budleigh cliffs or from a submerged outcrop; that they had been diverted at Portland and carried by the waves north-westward along the shore, suffering attrition on the way. It was Prestwich in 1875 who first pointed out that the Raised Beach at Portland Bill contained these pebbles, which had therefore come into the district in Pleistocene times when conditions were very different from those of the present day. Baden-Powell stated (1930) that the Raised Beach marks the earliest known date at which pebbles from the Dawlish breccia, from the Budleigh Salterton pebble beds, and perhaps from the Tertiary gravels were assembled in one deposit in the Portland district. The Raised Beach is only the remnant of a much larger deposit, formed when the Portland and Purbeck rocks stretched far to the west and east of what is now Portland Bill. North of the extended Portlandian escarpment the Fleet river deposited gravels, remains of which are seen at Fleet Common, at Langton Herring, and near Wyke Regis. It is from the sweeping together of the materials of these ancient shingles and gravels that the Chesil Bank was formed.

At present the beach is practically stationary. The pebbles drift sometimes eastward, sometimes westward, as the storm winds blow from W.S.W. or S.S.W. There seems to be a general movement towards a point near Chesilton, where the bulk of the beach is greatest and the pebbles are largest. These larger pebbles can only be moved by large waves, while the smaller stones my travel before smaller waves; this may have a sorting effect. There is a tendency for the beach to be rolled over on itself, and so to retreat toward the northeast, and this movement is most marked at the Portland end. Near Chesil (Chesilton) the beach appears to have overwhelmed blown sand (Davies, 1956)

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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.

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. .