West, Ian, M. 2020. Alum Bay: Geology of the Isle of Wight. Internet site: www.southampton.ac.uk/~imw/Alum-Bay.htm. By Dr. Ian Michael West, Romsey, Hampshire. Revised version: 27th August 2020.
Alum Bay and the Needles: Geology of the Isle of Wight

Ian West,
Romsey, Hampshire

a 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.
Website archived at the British Library

See associated webpage:
The Needles, Isle of Wight

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General Introduction and Name of the Bay


A general view of Alum Bay and Headon Hill, Isle of Wight from the slopes above the Chalk cliffs on the south and southwest of the bay, 25th June 2009, with Ian West


View from the ski-lift down to Alum Bay, Isle of Wight, September 2013


Distant view of the Chalk cliffs at Alum Bay, as seen, enlarged, from the top of Hurst Castle, 20th August 2020



Collecting coloured sands in Alum Bay, Isle of Wight, back in about the 1940s


View northward along the shore from the steps at Alum Bay to Hatherwood Point, Isle of Wight, September 2013


An oblique view, generally southward of Alum Bay, Isle of Wight, seen from the sea in June 2009


The coloured sands and clays and lignite beds of the Eocene, Bracklesham strata of Alum Bay, Isle of Wight, as seen from the Chalk cliffs


Alum Bay is a place located near the eastern end of the former, Eocene, Poole Delta. It has a wide variety of sediment types including sands, conglomerates, silty clays, some pipe clay and conspicuous lignite beds. The input of so much plant material into the margins of the sea with, of course, a major sulphate content, led to the development of pyrite, ferrous sulphide. The process presumably was the usual one involving sulphate-reducing bacteria. The oxidation of pyrite in the cliffs gives brown and reddish coloured deposits. The pyrite remains unoxidised below high-tide level. It is not conspicuous now, because of sea-level rise, and particularly the effects of coastal squeeze (inland movement of the low water mark). Removal during the industrial phase of Alum Bay is also a responsible factor. The similarity to the Eocene cliffs of Brownsea Island is obvious, and that locality too was important historically for attempts to produce alum. See the following with regard to the search for alum in the reign of Queen Elizabeth the First.

A problem of considering reports of "Alum" in the old literature should be borne in mind. True alum is a complex sulphate containing aluminium and iron. It is not normally found in natural conditions in Britain, but occurs in Italy. It has been manufactured artificially in Britain by a slow process that involves heat and ammonia (from urine). Alum of this type is of much value in fixing colouring dyes, so that the dye does not wash out.

Natural sulphates occur in the cliffs of Alum Bay as a result of oxidation of pyrite. The common products are ferrous sulphate - melanterite (green in colour) and a complex iron-bearing sulphate - jarosite (bright yellow in colour and in the past sometimes mistaken for sulphur). Melanterite has some alum-like properties. It will fix black dyes, but not coloured one. Melanterite has sometimes been referred to in the old literature as "alum". Alum Bay may have taken its name from the melanterite, and this green soluble mineral might be the alum ore referred to below.

However, a further complication is that pyrite (ferrous sulphide) was collected from the 16th century onward so as to make artificial melanterite. This is known as copperas. Nodules of pyrite were collected from Alum Bay and used for this purpose. Thus the question arises: is the alum ore mentioned below the melanterite in the cliffs, or is it the pyrite from which melanterite (copperas) can be manufactured?

Mineral Exploration in Alum Bay etc

[Pyrite like this, shown above, was exploited in Elizabethan times on Brownsea Island and was sought at Alum Bay.]

No. 11. Warrant to Mr. Richard Worsley to search for Allum Ore in the Isle of Wight.

AFTER my right harty Comendacons - Whereas the Queans Majesty [Queen Elizabeth the First] being infermyd that there is within that Isle certen Oure of Alume [i.e. Alum Ore at Alum Bay]. For Trial and Profe wherof Her Highness purtley sendith thither the Bearer herof One Bendall. There shall be in her Majesty's name to require you with your Authorite and favour so to assist him in that behalf, as he may revise such partes there as he shall thynk to be meete for the purpose and bring with him some part of the sayde Oure [said Ore - pyrite] to this end he therof make some Profe [proof] here in the Realme. In this part, as her Highness trusteth, You will give Order that no man there shal impede and resist him; So he hath Charge to use himself with such moderation and respect of behavior as shall apperteyne. And thus I byd You hartely well to fare - From the Court at Westmynster the 7th daye of Marche 1561.

Your Assured Frend,


[William Cecil, First Baron Burghly, chief advisor to Queen Elizabeth 1, Secretary of State and Lord High Treasurer.]


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The Needles Park

For more information on the history and geology of Alum Bay and the Needles, go to the separate webpage [not part of this system]:
[SEPARATE LINK] The History and Geology of the Needles.


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Alum Bay in the western part of the Isle of Wight, near the Needles, has impressive cliff faces of vertical Tertiary strata, with conspicuously coloured sands. It has been investigated my many geologists at different time and for different purposes. It is frequently visited by geological field trip parties.

This particular webpage was expanded to some extent because of an undergraduate study of the Barton succession by Rachel Helsby. The project was partly to help understand the occurrence of an unusual Barton limestone with Nemocardium and Xenophora that has been found offshore in Christchurch Bay by Dr Ken Collins. A previous project comparing the limestone with the Barton strata in the cliffs of the Hampshire coast has been undertaken by Sarah Snowdon.

Sailing westward past the Needles, Isle of Wight

Alum Bay provides fine views of the Chalk Cliffs which extend out to the Needles. The Needles are isolated stacks of Chalk which is steeply dipping towards the north as part of the Brixton or Brighstone Monocline. This is the uppermost part of the Chalk in this area and is of the Campanian Stage of the Upper Cretaceous.

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The car ferry from Lymington at Yarmouth Castle, Isle of Wight, 10th June 2009

The Lymington-Yarmouth ferry and saltmarshes - older photograph, 2002

It is easy to visit Alum Bay by car and you can drive directly to the Needles Pleasure Park and park on the cliff top. There are toilets, and in the summer, cafes, tourist shops and tourist facilities regarding coloured sands etc. In the winter the Pleasure Park and the chair-lift is not operational.

If you are coming from the mainland on a day-trip you may find, however, the ferry fare for cars is fairly expensive for a short visit. Should you wish to visit Alum Bay from the mainland at lower cost, you can drive to Lymington and park your car at the ferry terminal for a reasonable fee. You can then travel as a foot passenger on the Wight-Link Lymington ferry at a moderate price and this will take you to the interesting small town of Yarmouth. From here in the summer season you can easily catch a bus to Alum Bay, and (open-top) buses go in that direction about once an hour or even more frequently. In the winter the buses are less frequent, but it is not too difficult to get there. If you cannot find a bus to the bay soon after arriving you may be able to take a bus to Totland and walk a short distance to Alum Bay. You may also be able to travel by bus changing at Freshwater.



Generally the Tertiary sediments here are of soft sands and clays which slump or slide and there is generally a lower risk than at many other cliffs. However, there are some high projecting ribs composed of iron-cemented sandstone which could in places present a hazard of falling debris. Any of the sandstones are prone to fall to some extent from time to time, although the falls may not be frequent. There are also some thin pebble beds in places so it is possible that a flint pebble could fall from certain places.

Accidents have happened from the digging out of sand so as to produce sand-caves, although geologists are unlikely to be involved in this. In the summer the cliffs of coloured sands, mostly Bracklesham, are taped off for this reason. Note, then, that much of the Eocene strata is inaccessible in summer (although part of the Barton Clay and most of the Barton Sand, London Clay and Reading Formation can be reached)

A higher risk is presented by the cliffs of Chalk with flint nodules. This is dipping at a steep angle in high cliffs which are not stable. Large chunks fall from time to time. Apart from occasional major collapses of chalk there is risk that small pieces of flint or hard chalk may fall with high velocity at any time. Study of the Chalk cliffs should only take place with much care and preferably with the wearing of safety helmets.

Climbing over the boulders westward along the foot of the Chalk cliff has the risk of injury from falling debris and of slipping and falling over the rocks. There is also risk of being trapped by the tide or being washed off by a wave. Climbing on chalk cliffs is dangerous. There is obvious risk of falling from the top of the cliffs particularly the Chalk cliffs near the Needles.

There are some other risks. There can be mudslides in the Barton Clay and the London Clay and it is possible to become stuck in these. There is much soft mud on cliffs of Headon Hill to the north of Alum Bay. Flint pebbles or flint nodules should not be hammered because of dangerous splinters.

These comments do not necessarily cover all potential hazards and it is the responsibily of the individual or group leader to assess risks by direct observation at the time of a visit and to proceed in a safe manner. No liability is accepted here, and this webpage is merely a description of the strata and outcrop, and it is not a form of advice to go to a particular piece of cliff or coastline.

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[error here - no break] INTRODUCTION:

Geological Maps


An old geological map of the Isle of Wight, showing the structure and general geology, and still relevant for introductory purposes


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Cliff-Section at Alum Bay

Aerial photograph of the southeastern corner of Alum Bay, Isle of Wight, 8th June 2007, non-rectified, courtesy of Channel Coastal Observatory

A view from the sea of Alum Bay, Isle of Wight, showing the junction with the Chalk and the locations of the main units of Tertiary or Palaeogene strata, 10th June, 2009

A old section of the strata Alum Bay by Bristow, with terminology partially updated

Vertical Tertiary strata in the Alum Bay cliffs as seen from the southwest

Tertiary strata in the cliffs of Alum Bay, labelled

The various photographs and the cliff-section diagram given here indicate the main units visible in the cliffs of Alum Bay. It should be noted, though, that some of the succession here is not easily correlated with that elsewhere. The Reading Formation and the London Clay Formation do not present any major problems, although, of course, there can be argument about the exact position of the boundaries. The Boscombe Sands, Barton Clay, the Becton Sand are relatively straightforward. It is the Bracklesham Group which is more difficult to interpret. A number of coloured sand units alternate with heterolithic, laminated and lignitic sands and clays. Grey laminated beds like these occur elsewhere in the Bracklesham Group. They alternate elsewhere with greenish glauconitic sands that are quite fossiliferous. These green sands can be identified as specific formations by their fossil content. Alum Bay is unusual in that the sands are yellow or pink and are oxidised and generally without marine fossils. Correlations have been made and those of Insole, Daley and Gale (1998) are generally used here. There are complications, though, such as the upper and lower "leaf" of the Poole Formation and the Wittering Formation. Not every part is confidently tied in with the Whitecliff Bay section and some room for discussion exists. It should also be noted that older literature may use rather different correlations and that much of the Bracklesham Group may be listed as "Bagshot Sands". The Bracklesham Group will be discussed in more detail below.

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Chalk - Introduction

Please go to the web on the Needles for more information:


The chair lift at Alum Bay, Isle of Wight, over Tertiary sands and clays and  with vertical Chalk cliffs in the background, 2002


Alum Bay, view of weathered Barton Clay slopes in the foreground and Chalk cliffs in the distance, photograph from the steps down to Alum Bay, 2002


Seen in the distance here from the steps are the Chalk cliffs. This chalk is of the Portsdown Chalk Formation Belemnitella mucronata Zone of the Campanian Upper Chalk (Upper Cretaceous). It consists of chalk with flints steeply dipping to the north. The Needles peninula extends east-west so the cliff line of Chalk here is along the strike and almost represents the Chalk/Tertiary boundary. With this steep dip towards the sea, large chunks of chalk separated by curved slip-planes fall from the cliff from time to time. The scars can be seen in these photographs (taken in different light and weather conditions).

For a detailed and modern description of the Chalk at Scratchell's Bay and at Alum Bay (Chalk Cliffs to the south) see particularly the paper listed below which is in the Special Issue of the Proceedings of the Geologists's Association, 2011, The Geological History of the Isle of Wight, edited by Peter Hopson (and available online): Hopson, P. 2011. The geological history of the Isle of Wight: an overview of the 'diamond in Britain's geological crown'.

The Needles from Alum Bay, 1995

The Needles from Alum Bay beach, 1995

The two photographs of the Chalk cliffs immediately above were taken 1995. They show the details of some cliffs falls at the time. They can be compared to later photographs of the same cliffs.

Chalk cliffs at south side of Alum Bay, 2002

Notice the results of erosion of an old cliff-fall in the photograph above, from 2002. The pile of debris orginally sloped fairly evenly as a fan towards the sea. Recent wave erosion has cut a small cliff in the debris cone, leaving only a part with the original top surface slope. There are other small seaward slopes with grass, here and there on the cliffs. It is not clear as what it is the origin of these.

The Chalk cliffs from Alum Bay to the Needles, seen from above at the eastern end, Isle of Wight, 25th June 2009

The old photographs above, from 2009, shows the remarkable uniformity of the steep northerly dip in the Chalk here. There seems to be no suggestion of curvature, which would be expected in the monocline. No beach is developed for most of this stretch, but occasional cliff falls add small areas of chalk debris to the base. Future "Needles" will be formed from this stretch, as Scratchell's Bay erodes and moves eastward.

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Chalk Stratigraphy and Fossils

Stratigraphical sequence of the Chalk

Portsdown Member of the White Chalk Formation dips northward, near the Needles, 2002

Chart for the Chalk of southern England relating older Chalk Zones to the modern lithostratigraphic schemes of Mortimore and the British Geological Survey

Chalk belemnites of zonal importance, in relation to the Chalk of Dorset and the Isle of Wight, England

The common Chalk echinoids - Micraster cortestudinarium and Micraster coranguinum, both used for zoning the English, Upper Cretaceous Chalk

The Chalk in the southern part of Alum Bay is Campanian, of the zone of Belemnitella mucronata. At the end of the beach is the soft white chalk of the Studland Member. By walking about 50m along the base of the cliff from the Tertiary erosion surface there is a change to the harder chalk with thin marl bands which belongs to the Portsdown Member (Insole, Daley and Gale, 1998). There is no easy access to older parts of the Chalk succession here. To study older parts in the western part of the Isle of Wight go to Freshwater Bay and Compton Bay.

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Chalk-Tertiary Junction

The junction between the Upper Chalk and the Tertiary strata is seen at the southern end of Alum Bay. There is a well-defined corner where the cliff direction changes from roughly north-south to roughly east-west. The Chalk forms a high wall here and deeps steeply towards the north. The lowest Tertiary strata here is the Reading Formation of Palaeocene age and consists of reddish clays with a little sand and gravel. The Upper Cretaceous succession is not complete. The highest Chalk is of the Belemnitella mucronata Zone, of the Campanian, and the Maestrichtian strata has been eroded away. Thus the true K/T [Cretaceous-Tertiry) boundary, notable for the extinction of the dinosaurs, is not seen here. The Reading Formation lies on different parts of the Chalk succession in different places so that in broad terms there is an unconformity. However, within Alum Bay there is no obvious angular discordance between the Reading Formation and the Chalk.

Solution pipe in the Chalk-Tertiary surface, Alum Bay

Solution pipe in Chalk, closer view, Alum Bay

The top of the Chalk is rather irregular and has oval pipes containing grey, argillaceous sand. The pipes seem to be of early Tertiary age and predate the folding. In the past they have descended probably vertically down from the originally near-horizontal Tertiary surface into the Chalk; since then folding has steeply inclined the top Chalk surface. The actual junction is now a steeply dipping, nearly vertical face from which the Reading strata is partly stripped away by erosion to reveal the details. It is rugged and deeply furrowed, and the solution pipes attain greater dimensions than at the equivalent position in Whitecliff Bay. Their sides are often undercut and, in many cases, open laterally into sand-filled fissures, some of which follow bedding planes. White (1921) considered this surface to have the general aspect of a foreshore with potholes worn in an approximately horizontal limestone. However, if the pipes are solution features, as they indeed appear to be, then what is visible is a surface affected by early karst rather than mechanical erosion and potholing. Solution pipes which are inclined like this and not simply vertical are rare and interesting features.

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Reading Formation

The Reading Formation and London Clay in Alum Bay, Isle of Wight, seen from above, on the Chalk outcrop, 2009

Slumped Reading Formation at Alum Bay, 2003

Reddish clays, palaeosols in the Reading Formation, Alum Bay, 2002

Reduction patches in a reddish palaeosol, Reading Formation, Alum Bay, 2003

The Reading Formation is easily recognisable. It consists for the most part of clays and marls that are mottled with a reddish iron oxide stain. These are palaeosols that were originally gley soils of swampy, flood plain environments. Less obvious is a pebble bed and some sand. Slumping and collapse of the cliff obscures some parts.

On the basal surface of the Tertiary is a thin conglomerate containing a mixture of well-rounded flints and grey pebbly sand, but it may be obscured by slumped debris. This lowest unit is the Reading Formation Bottom Bed (Insole et al.,1998). Apparently this bed does not seem, in the field, to contain glauconite grains but there is a greenish tint in the deeper pipes. There are scattered dark green flint pebbles in addition to unworn flint nodules, eroded from the underlying Chalk according to (White, 1921) .

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London Clay Formation

London Clay, Alum Bay, with septarian nodules

The Venericor Septaria in the London Clay, Alum Bay

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Bracklesham Group


Vertical Tertiary strata with distinctive colouring seen in the Alum Bay cliffs from the southwest Chalk hills, 2003, image revised July 2016 [corrected 2016]

Tertiary strata in the cliffs of Alum Bay, 2003, labelled [corrected 2016]

Cliff-section at Alum Bay, by Bristow, 2002 photograph, updated [corrected 2016]

Eocene coloured sands of Alum Bay, 2002, corrected, 2016
[corrected 2016]


Coloured sands and silts, including red beds, in the Bracklesham Group, Alum Bay, 2002, corrected 2016
[corrected 2016]


Bracklesham strata viewed northward from the London Clay outcrop, Alum Bay, 2002, corrected 2016
[corrected 2016]


The uppermost part of the Bracklesham Group and part of the Barton Clay seen from a boat, Alum Bay, Isle of Wight, 10th June 2009

The coloured sands, lignite beds and leaf beds are interesting features of the Bracklesham Group. Because the succession is so varied and is seen in vertical orientation in a short stretch of cliff-line it is not always easy to find your place within it. It is also complicated by the fact it does not have the normal Bracklesham sequence with glauconitic marine beds that are easily recognised units at Whitecliff Bay.

Certain marker horizons are shown in the photographs here. The base of the Boscombe Sand is marked by a crimson bed in a photograph above. In older literature (eg. White, 1921) this sand is placed within the Bracklesham sequence. In the recent guide by Insole, Daley and Gale (1998) it is classified as the basal formation of the Barton Group. It is of course, debatable as to whether this sand formation should be placed with the Barton Clay Formation, but, however, note that the Becton Sand Formation (Barton Sand) is also placed in the Barton Group.

The yellow soft sandy part of the Branksome Sand Formation is very conspicuous and, as can be seen in a photograph above, prone to collapse. The formation is about 65m thick and corresponds to Beds 25 to 27 of (Prestwich, 1846). It includes not just the obvious sand but the lignite beds (lower No. 27 of Prestwich) and some whitish marly clay above. The sands are well-sorted with low angle cross-bedding or flat lamination. Burrows of the trace fossil Ophiomorpha occur in some parts Insole, Daley and Gale (1998).

Lignitic coal in Prestwich Bed - lower 27

The unit of lignitic beds is conspicuously grey in colour. The 11 metres of laminated muds and lignites, here, resembles the Marsh Farm Formation (Bed 24) lower down (that is south) in the succession and should not be confused with that. Lignitic coals occur in definate beds within bed - 27, lower part, and are quite conspicuous. Each of the lignite beds in No. 27 has a thin underclay penetrated by rootlets similar to those seen at Whitecliff Bay , but they are thinner (White, 1921) . The original peat beds have been attributed to an origin on coastal, slightly saline marshes by Insole, Daley and Gale (1998). I have found a small piece of amber from tree remains in one of the lignite beds, many years ago. The lignite or brown coal development in the Isle of Wight might be connected with the proximity of an early structural high developing on the Tertiary axis to the south of Alum Bay. These peat accumulations might have developed on a protected accretionary shore just north of early rising land.

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Poole Formation

Coloured sand in the Poole Formation, lower part of the Bracklesham Group, Alum Bay, Isle of Wight

The Poole Formation is the name for the lower part of the Bracklesham Group in the Poole, Dorset area. It consists of sands, grey clays with plant debris, lignite beds and pipe clay (ball clay) beds. The pipe clay is extensively worked in large pits in the Poole area. It is a kaolinite-illite clay, and more plastic than the kaolinite china clays of Devon and Cornwall. The Poole Formation is non-marine and has been deposited in the Poole Delta, the discharge area of a large river coming from the Dartmoor region. Although at a latitude of about 40 degrees north this was quite a hot delta, almost subtropical, because the Paleocene-Eocene was a time of greenhouse type, global warming. The almost tropical weathering beneath the dense forests led to much release of iron, and at times almost lateritic conditions. The abundance of iron in this relatively high latitude, but subtropical-type delta is ultimately responsible both the for bright colours of the ferruginous sands, and also for the pyrite from which Alum Bay (indirectly) takes its name. The source of the iron is not a controversial matter.

A major problem, does remain however. It is just when did reddish ferric hydroxide colouring originate. Was it due to penecontemporaneous oxidation or has it resulted from recent oxidation of contained pyrite (iron in reduced form as ferrous sulphide)? The matter is discussed with reference to Studland Bay, further below.

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Eocene Inversion (Basin Inversion or Uplift)

Circumstantial evidence regarding the Eocene uplift in the southern part of the Isle of Wight comes from several sources since the idea was put forward in 1964 (West, 1964) , and opposed at the time by Denis Curry. A relevant recent publication is that of Gale et al. (1999) and this provides specific evidence for uplift and erosion in the south (more information regarding this work will be given later). Other evidence include the Palaeogene warping in the northern Isle of Wight (Daley and Edwards, 1971) , the Bartonian unconformity in Dorset (overstep by the Creechbarrow Beds), the tilted iron pipes of the Poole Formation of Studland, the 'Bagshot' [Poole Formation?] overstep west of Broadmayne, Dorset(Arkell, 1947) , the gravels of Hardy's Monument, Blackdown, Dorset etc. This topic will be discussed more fully elsewhere.

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Bracklesham Group - Coloured Sands

Coloured sands used as a tourist attraction, above Alum Bay, 2003

Bracklesham strata, Alum Bay with coloured sands, 2003

Badlands-type erosion of coloured sands, Poole Formation, Alum Bay, 2003

Marsh Farm and Branksome Formations junction, Alum Bay

A rib of coloured sandstone projecting due to differential erosion, Brankstome Sand Formation, Alum Bay

Limonite-cemented vertical rib with some crimson sand above, Branksome Sand Formation, Alum Bay, 2003

Patch of crimson sand, Branksome Sand Formation, Alum Bay, 2003

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Coloured Sands - Origin of Colours

The coloured sands of Alum Bay are almost all within the Bracklesham Group. Some colours of sand are quite common. Yellow and brownish sands owe their colour to limonite (or goethite) which is the ferric hydroxide (effectively rust). This is usually produced by oxidation of dispersed pyrite or glauconite during recent or at least Quaternary weathering in wet, near surface conditions. Early oxidation similar produces the hydroxide in most cases but burial to significant temperatures often, but not always, converts the hydrated iron oxide progressively towards the anhydrous hematite. Red, as opposed to brown, colours often indicate early oxidation; this is a process which occurs in palaeosols, ancient soils. Regarding red palaeosols, particularly in the earlier Reading Formation of Alum Bay see the work of Buurman.

Red-stained sandstone, Redend Point

Thus the colours could be of palaeosol origin. However, there is some indirect evidence that the red and brown colours are the result of recent oxidation of pyrite. At Redend Point, Studland, on the opposite (western) side of Christchurch and Poole Bays, there are crimson-coloured sands like those of Alum Bay. These occur in the Redend Sandstones of the Poole Formation of the Bracklesham Group. They occur in sands beneath some steeply inclined ferruginous pipes which are not vertical but appear to be tilted in accordance with the 12° dip of the strata (Arkell, 1947). If the evidence of the dip of the pipes is correct then this suggests a penecontemporaneous origin. This is supported by the fact that they commence at and descend down from a lignite bed, probably once a peat deposit like the lignites of the Isle of Wight. The pipes have no relationship to present surface features or solution pipes. An example has been seen of a pipe at Studland Bay that is pyritic at low tide level. Pyritic pipes also occur in the Bracklesham Group offshore in Poole Bay. Thus the brown limonitic pipes there seem to be the result of oxidation of pyrite pipes. The associated red and brown colours in the sand at Redend Point are therefore also likely to be the result of oxidation of pyrite, particularly since colours occur where the pipes are abundantly developed. Just when this oxidation took place is an interesting problem, but if related to present tide-level then it is a relatively recent phenomenon.

There are, therefore, two alternative theories for the red and brown colours of the sands - a palaeosol origin or a recent origin by oxidation of pyrite. This clearly needs more investigation It should be noted, however, that Alum Bay has received its name because it was a place where pyrite was collected on the beach for the manufacture of alum. Not much pyrite is visible now (there is some) but perhaps pyrite is preserved, as at Studland, at and below low tide level. Perhaps the source of the pyrite of Alum Bay has been low-water exposures rather than the cliffs. We will leave the matter unresolved now for further study later.

A mention should also be made of other colours. Green is provided by glauconite, which is common in some more marine units such as the Barton Clay. It is usually, however, much contaminated with clay material. Pure quartz sands can give white. Lignitic material can provide a black colour. Bright yellow might be obtained from jarosite encrustations. It is difficult to think of a suitable source of blue.

In conclusion, it is the red-coloured sands of Alum Bay which make the place notable. There is much plant debris in the Bracklesham Group which would have caused local reducing conditions. In such circumstances iron can be trapped by sulphide ions (from sulphate-reducing bacteria) as pyrite. This was the source of alum from which the bay takes its name. It is probable that oxidation of pyrite either in the past or recently has produced the red and brown colours of iron oxides or hydroxides.

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Barton Group; Barton Clay and Barton Sand (Middle Eocene)

[Note that in older literature, the Barton Clay and Barton Sand may be listed as Upper Eocene.]

A general view from the sea of the Middle Eocene, Barton Clay at Alum Bay, Isle of Wight, 10th June 2009

The lower part of the Barton Clay, with the Boscombe Sand stratigraphically beneath it, at Alum Chine, Isle of Wight, September 2013


The succession of Barton Clay, Eocene, strata at Alum Bay, Isle of Wight, with a pebble bed containing Upper Greensand clasts in central part, after Gardner, 1888, modified by Helsby (2004)


Location of the Barton Clay in the cliff section at Alum Bay

Mud from the Headon Hill Formation trickling down over Barton Sands, 2002

Barton Clay cliffs at Alum Bay, with erosion

Fossils in the Barton Clay

Barton Fossil

The Barton Clay sequence in general resembles that at Barton-on-Sea and Highcliffe on the Hampshire Coast. It consists of grey clay with white aragonitic shells. It varies in content of quartz sand and of glauconite and has septarian nodules of argillaceous calcite (microspar) at certain horizons. The thickness at Alum Bay is greater than at Barton. The shells are more friable and because the strata are vertical there is only a limited exposure. As a result this section has been regarded as inferior to the Barton-on-Sea coast for fossil collecting. The mainland coast is not as good as in the past though because of sea-defences and accumulation of shingle. The Alum Bay section is interesting in the winter when it has to some extent been cleanly washed by storms. Only a limited part is obscured and that is near to the chair-lift, but even here more Barton Clay is becoming visible because of active coast erosion.

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Barton Clay Section - Mid-Barton Conglomerate

(or the "Steps Pebble Bed" or the "Chine Pebble Bed")


A  broad view of the vertical, Barton Clay Formation from the Steps at Alum Bay, Isle of Wight, northward and stratigraphically upward to the Barton Sand Formation, and showing the relative location of the Mid-Barton Clay Pebble Bed, photo 2002


Barton Clay with the Steps Pebble Bed

A general view of the Mid-Barton Conglomerate or pebble bed in the centre of the Barton Clay at the steps down to the beach, Alum Bay, Isle of Wight, about 2002


The Mid-Barton Conglomerate, or pebble bed, in the centre of the Barton Clay Formation, at the steps down, Alum Bay, Isle of Wight, about 2002


A Mid-Barton Conglomerate, resulting from reworking of older strata has been reported from Whitecliff Bay by Gale et al., 1999) . This bed was also found independently in Alum Bay by Rachel Helsby and Ian West in about 2003. They did not know about the Whitecliff Bay at that time. The bed in Alum Bay differs from that at Whitecliff Bay in also containing Upper Greensand Chert. This bed at Alum Bay is now discussed, with reference to a report by Helsby (2004).

Unusual pebble bed, the Steps Pebble Bed, within the Barton Clay, Alum Bay, Isle of Wight

Enlarged view of the Steps Pebble Bed, Barton Clay, Alum Bay, Isle of Wight

A bedding plane with pebbles and bivalve shells near the level of the pebble bed or Middle Barton Conglomerate, at the steps down to the beach, Alum Bay, Isle of Wight


Just north of the steps down to the beach is a remarkable pebble bed within the Barton Clay. To distinguish readily from the pebble bed at the base of the Barton Clay it is referred to here as the "Steps Pebble Bed". It is not known at other localities. The pebble bed is not like some other Eocene pebble beds, (such as the one at the top of the Boscombe Sands) either in terms of sequence or pebble content. There is clay above and below the thin pebble bed and it has a matrix of clay, albeit sandy. Some pebbles occur in the clay above and below. The bed has been described by Dr. Rachel Helsby while undertaking an undergraduate research project for me in 2004 (Helsby, R. 2004. Provenance and Sedimentation of a Bartonian Pebble-Bed and Associated Strata in Alum Bay, Isle of Wight. Undergraduate research project, unpublished, 51pp. plus diagrams and appendix data.)

The unusual pebble bed is notable for being extremely poorly sorted and apparently bimodal in particle size. Rounded, blackened and battered pebbles show that much material is clearly of storm beach origin and of a type that is common in the Tertiary marine deposits. Very unusual at this stratigraphical level are the other pebbles which appear to be flint nodules that have come from directly from the Chalk with very little if any abrasion. They are fractured in many cases but this has mostly occurred after deposition. They seem to have the original white and thin cortex and probably not a weathering patina. Laboratory study is needed, though. With the pebbles, the clay and some sand dispersed through the clay are many aragonitic bivalve shells. In cross-section many of these appear to be Cardita -like bivalves. There may be some similarity to the occurrence of Cardium shells in modern pebble beaches, like those of the present Solent.

Some initial ideas for discussion are put forward here. This should be seen in the perspective of the key publications on the Tertiary, Structures and other matters of Isle of Wight geology by the expert on the area Dr. Andy Gale. See also, the important Geologists' Association Guide to the Isle of Wight (Insole, Daley and Gale, 1998). In addition refer to Plint (1982; 1983).

The pebble bed is of small pebbles in sandy clay with more clay above and below. Application of Walther's Law [the vertical sequence of sediments is related to original lateral relationships of sediments] suggests a shingle spit with soft sandy mud on either side and no major area of clean sand. The flint-pebble beach was to some extent starved of clastics and so sharks' teeth and fossil wood occur in the bed. Such accumulations would be expected at the low-water mark of the beach, and the teeth may have been derived in part from reworking of earlier deposited marine clay.

The relative deficiency of sand implies that the Barton Clay sea could be very shallow in parts but still with a predominantly clay type of sedimentation. Reduction in rainfall (suggested because the Upper Eocene has an evaporite facies in the Paris Basin) might have been responsible for the reduction in the deltaic input from the west that so affected the underlying parts of the Eocene in this area. The Barton Clay, though, does shoal upwards into the well-sorted Becton Sand (Barton Sand) before the lagoonal Headon Hill Formation was deposited. It is possible that the Becton Sand is reworked, well-sorted sand from the remains of the earlier delta.

The Bartonian, in the Middle Eocene, was the time when the Alpine Folding was just commencing. The folding of the Chalk and other strata in the Isle of Wight and Isle of Purbeck was not, of course, a sudden process after the final deposition of the Tertiary sands and clays. It is hardly necessary to say that there was probably no mountain of soft Tertiary sands and clays on the southern part of the Isle of Wight (a comment made long ago by Ian West, 1964) . It was a structural high relatively early in the Tertiary and a place of gradual uplift. At times it would have been a low-lying island or promonotry. It would have been a place of condensed sequences and erosion, and the Eocene sands and clays would have thinned towards it. Overstep southward would be expected. This is observed in an analogous situation further west at Creechbarrow Hill near Corfe Castle in the Isle of Purbeck, Dorset At that locality there is an angular unconformity between the Creechbarrow Beds and the underlying strata. These beds overstep southward onto Chalk and contain large flint nodules derived from the Chalk. The Creechbarrow Limestone, a freshwater deposit at the top of the Creechbarrow Beds has been shown by Hooker (1977) to be of Bartonian age.

Just how much southward overstep of the Eocene strata occurred in the vicinity of Alum Bay is an interesting point for discussion. A key publication with specific evidence for Eocene overstep and erosion is that of Gale et al. (1999) . Detritus at Whitecliff Bay reveals clear evidence of eroded strata to the south.

Further aspects will be discussed later, and there are some problems with the Tertiary succession. One of these is the greater thickness of Barton strata at Alum Bay than on the mainland. It is also of interest that in the Alum Bay area there is an unusual development of freshwater limestones in the Solent Group that is stratigraphically above the Barton strata.

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Barton Clay Section - The Steps Pebble Bed or the Chine Bed - [Mid-Barton Conglomerate]

More on Reworked Upper Greensand Chert

Thin-section photomicrographs of Upper Greensand Chert pebbles and other rock types in the Steps Pebble Bed at Alum Bay, as studied by Dr. Rachel Helsby

The thin-sections show above clearly demonstrate the presence of chert that is very similar to Upper Greensand chert. Other material, including more spicule cherts, and even a clast of jasper are also shown [incidently jasper pebbles are common in the Eocene gravels of Dorset, north of Weymouth and just north of the Ridgeway Fault System. They also occur in the solution pipes in the Chalk in the area. [see Weymouth Relief Road webpage for more data].

Some of the supposed Upper Greensand chert found by Dr. Rachel Helsby in the Steps Pebble Bed of Alum Bay resembles that described by Dr. Chris Jeans in his Cretaceous silicifications paper of 1978. Notice from his maps that Upper Greensand Chert is present at no great distance from Alum Bay, being present at Compton Bay, only a few kilometres away and also in the Dorset area, further to the west.

[to be continued]

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Barton Clay Faulting

Faulting or incipient faulting in the lower part of the Barton Clay Formation, south of the steps to the beach, Alum Bay, Isle of Wight, 2002


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Bartonian Palaeogeography


Bartonian palaeogeography of the Isle of Wight and adjacent area showing the first stages of Eocene, Alpine tectonism, and its possible relationship to local oilfields

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Tertiary - Details

Clay Mineral Distribution

Clay mineral distributions in the Hampshire Basin, based on the work of Gilkes (1968), with some further palaeoenvironmental interpretation

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Headon Hill Formation

Headon Hill

The northern part of Alum Bay, Isle of Wight, shows the folding of the foresyncline and the change northward to the nearly horizontal Headon Hill Formation in Headon Hill, photo 10th June 2009

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Headon Hill Formation (Solent Group)

The Headon Hill Formation of the Solent Group at the western end of Headon Hill, Isle of Wight, seen from the sea, 2009

Late Palaeogene stratigraphy for the Isle of Wight, after Insole et al. (1998)

The Headon Hill Formation is exposed in the type locality of Headon Hill, northeast of Alum Bay. Several limestones are developed at this particular locality. They contain low-salinity molluscs. Headon Hill is not easy to study because of soft mud in places, minor landslides and some limestone cliffs.

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Some Fossils of the Alum Bay Strata

Lower Eocene Fossil

Some Barton and Bracklesham Fossils

Some Barton Fossils

Fossils of the Tertiary strata include the Eocene fossils of Alum Bay. The London Clay, the Bracklesham Group and the Barton Clay contain fossils (although Bracklesham fossils are rare at Alum Bay).

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Fossils of the Solent Group

Fossils of the Solent Group, Eocene - Oligocene

Solent Group Fossils

More Solent Group Fossils

A selection of just some of the many species of Solent Group (Upper Eocene to Oligocene) fossils are shown here. The left-hand diagram (coloured) is the more up-to-date. Most, but not all, of the common ones are included, together with a few rarer species. The centre and right-hand diagrams are very old with a few names updated. They may be useful supplements for identifying species not shown on the left-hand diagram. Treat the nomenclature on these with caution, though!

The molluscan fossils of the Solent Group are generally different from those of the marine Eocene strata beneath. They are most frequently found to be fairly small, white and thin-shelled and occur, often in abundance, in thin-bedded strata. They are preserved as aragonite, but usually without any preservation of shell colour. Most of them lived in lacustrine (lake) conditions or brackish, lagoonal water. Pond-snails such as Galba (Lymnaea ) or Viviparusare common in freshwater strata such as the Bembridge Limestone. Planorbis - type gastropods are also typical of freshwater strata, and charophyte algae may be present with them. Brackish water molluscs include Corbicula and various cerithid (ornamented and turreted) gastropods. There are some truely marine horizons, usually fairly thin and particularly recognised by the presence of oyster shells or "Barton Clay - type" gastropods such as Athleta.

The Solent Group is very well exposed on the Isle of Wight and this is the best place in the country to see fossils of Upper Eocene to Oligocene age. The Group includes the Headon Hill Formation, particular well seen at Headon Hill, near Alum Bay in the west of the Isle of Wight.

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Ian West is particularly grateful to the help of Dr. Rachel Helsby has investigated aspects of the Barton Clay at Alum Bay during an undergraduate project a few years ago. Dr Ken Collins, the discoverer of an offshore Barton outcrop, has helped initiate this work and has co-supervised the study by Rachel Helsby. We are very grateful to him. I much appreciate the arrangement of a geological field trip on a boat from Lymington to the Needles (Puffin boats) by the Probus organisation of Mudeford, and particularly the help of Peter Hollick. iSolution, the computer services organisation at Southampton University has hosted this webpage, and aerial photographs have been provided courtesy of the Channel Coastal Observatory.

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References and Bibliography

Please go to Isle of Wight Bibliography .

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See related webpages: The Needles, Isle of Wight
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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, Benedict Close, 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.