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Needles from Alum Bay cliff top

Needles from Alum Bay beach

Compton Bay

"In examining the natural productions of a country or district, one of the first objects that strikes the attention is that of the geological structure of its surface. What a field this opens for enquiry! What can exceed the interest naturally excited in the mind when it reverts back to ages which have no place in this world's history? Thought itself becomes bewildered by the speculation which oppresses it...

Few spots in Britain of similar size contain more objects of geological interest than the Isle of Wight. It presents the characters of the tertiary formations (the Eocenic period of Lyell), consisting of depositions partly derived from the sea, and containing detritus of various kinds, such as shells and plants, the bones of fish and turtles, of saurians, or animals of the lizard tribe, and also of some extinct mammalia. Teeth of the crocodile and shark are also found in this division, as well as wood and the fruit of some former species of the Cycadeae or Pine tribe.

Immediately below the Tertiary Eocene lies the Cretaceous group, which consists of the marine deposits of former oceans, many of them highly fossiliferous. This series is made up of the two beds of Chalk, Upper and Lower, superimposed on Chalk Marl; then follows the Upper Greensand, with its various strata of Chert, Firestone, Limestone, Freestone etc.; and to this succeeds the argillaceous bedsd of Gault, which again rest upon the Lower Greensand, or Ferruginous sand, as it was formerly called; below which we find the Wealden or Hastings sand, which crops out at various parts of the Island, as at Sandown Bay, and along portions of the south-western coast from Atherfield Bay westward, forming as it were a transition-series between the last-named and the Oolitic formation. "
(Martin, 1849).

"The Country of the Iguanodon. .. Whether that country was an Island or a Continent cannot be determined; but that it was diversified by hills and valleys, and irrigated by streams and rivers and enjoyed a climate of a higher temperature than any part of modern Europe, is most evident..."
(Mantell)

INTRODUCTION:

Location Maps and Geological Maps

INTRODUCTION CONTINUED:

General Introduction

The Isle of Wight is often described as being lozenge or diamond shaped with concave sides (White, 1921) , (Bird, 1997) . Its long axis runs approximately east-west and is 23 and a half miles. It is nearly 14 miles at right angles. The island is fairly evenly divided into two parts by the roughly north-south Medina River. A more prominant feature is the range of Chalk downs extending east-west from the Needles (shown above) in the west to Culver Cliff in the east.

The Isle of Wight in its southern half is mostly of Cretaceous strata, whereas north of the Chalk ridge it is of Tertiary sands, clays and some limestones. The oldest Cretaceous strata is Wealden, a continental, fluvial deposit of mudstones and sandstones that is famous for its dinosaur remains. The coast near Brook is of particular interest. The Lower Greensand above has a good assemblage of marine fossils and has ferruginous sands (with chamosite or berthierine oolites partly oxidised to limonite). The Gault and Upper Greensand are fossiliferous and well-seen in coastal exposures such as Compton Bay (see above). The Chalk is fossiliferous and very well-exposed in the west of the island in the region of Freshwater, the Needles and Alum Bay and in the east at Whitecliff Bay and Culver Cliff, as shown below (left photograph).

Above the Chalk come the Tertiary strata which include fossiliferous, marine, Eocene sands and clays, seen in the right-hand photograph above at Whitecliff Bay in the east of the island. This is an excellent reference section but they are also well-seen at Alum Bay in the west. Whereas the Bracklesham Group of the Middle Eocene is most fossiliferous at Whitecliff Bay, it is the overlying Barton Clay which is better exposed and more fossiliferous at Alum Bay. At both the eastern and western ends of the Island the lacustrine and lagoonal Solent Formation is well-exposed north of these marine Eocene cliff sections, particularly at Headon Hill just north of Alum Bay and in the northern part of Whitecliff Bay.

The marls, clays and limestones of the Solent Group are also exposed at various places, mostly coastal sections, around the northern part of the Island. In particular, the Bembridge Limestone of the Solent Group is a prominant freshwater limestone with ancient pondsnails and there are several places like west Cowes and Gurnard where it forms a natural sea-wall. In the Bembridge area it forms extensive ledges. The Bouldnor Formation includes the strata referred to in the older publications as the Bembridge Marls and Hamstead Beds. An insect bed occurs in the Bembridge Marls and there is a variety of mollusc shells to be found and occasional remains of turtles. Salinity conditions varied from fresh to brackish to marine. At Hamstead and Bouldnor Cliff some of the youngest of these Oligocene strata contain abundant remains of the small marine bivalve Corbula .

There are no strata on the island of late Oligocene or of Miocene age. In the Eocene succession of the northern part of the Isle of Wight there is evidence of early folding. This would have intensified in the late Oligocene. By the Miocene Epoch there was uplift and major folding associated with the Alpine Orogeny, and the effects of this are clearly visible. In the photographs above you will notice that the Chalk is steeply dipping towards the north. There is a continuation of this Chalk outcrop eastward under the seafloor to the Isle of Purbeck, and there, as Ballard Cliff, Swanage and at Lulworth Cove, the Chalk is vertical or overturned. This steeply north-dipping or vertical Chalk is within the middle limb of the Purbeck - Isle of Wight Monocline (or "Purbeck - Isle of Wight Disturbance" in some oil-industry and structural publications). The structure is easily understood in the Isle of Wight because the south limb of the fold is seen as gently south-dipping Chalk at St. Catherine's Down. To the north the Chalk descends deeply beneath the Solent and the New Forest, eventually coming to the surface again in Salisbury Plain. The nature of the folding is somewhat more complicated than it first appears. It is due to compression from the south, but not in a simple manner. Deep-seated thrust faults, of Hercynian origin (Hercynian - of the Carboniferous-Permian Hercynian Orogeny or mountain-building movements, about 300 million years ago), in the Palaeozoic basement have been reactivated as an effect of the distant impact of the African Plate on the European Plate (i.e. northward movement of Africa and collision with Europe). Although this was a long way away it still had some effect in southern England. The thrust faults are not seen at the surface here but the visible monoclinal folding is above them and results from uplift and northward movement of the block under the southern half of the Isle of Wight.

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Footnote:

There is an interesting curiosity related to this and it is the occurrence of reworked Jurassic (Kimmeridge Clay) ammonites in the Cretaceous strata of Redcliff, Sandown. Beneath the Isle of Wight, concealed and known only from boreholes and seismic is a thick sequence of the Jurassic strata; in the northern part of the island, however, about a kilometre of this is missing. This surprisingly large gap is the result previous extensional movements on the same faults producing depression to the south and uplift to the north. The northern uplifted area, similar to the structure at Wytch Farm Oilfield, was eroded in Cretaceous times. We will leave the oil-geology of the Isle of Wight for now, though, and I hope to discuss it in a later section.
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If the Miocene folding had happened as a sudden brief event after deposition of the Tertiary strata, then these poorly consolidated sands and clays would have been uplifted as a mountain of more than 600m in height in the southern part of the Isle of Wight. There probably never was much of a "soft mountain of Wight", a topic I discussed briefly in print in the 1960s. The uplift probably started in the Eocene Epoch and the Tertiary strata almost certainly thinned or disappeared because of non-deposition or progressive erosion over the slowly rising anticline. Creechbarrow Hill in Dorset provides evidence of angular unconformity between Tertiary strata and the Chalk in the similar Purbeck Monocline. As in the case of the older (Bartonian) Creechbarrow Limestone, the Bembridge Limestone is probably the result of a lacustrine deposition in proximity to eroded Chalk (on the anticline to the south) providing very carbonate-rich water for the pondsnail limestones (cf. the Holocene "shell marls" of the Test Valley in Hampshire). It is fair to point out, though, that the large quantity of reworked subangular flint that would be expected to be present in the Upper Eocene and Oligocene has not been found. Why is this?

An interesting sideline to the history of folding is origin of the mysterious coloured sands of parts of the Bracklesham Group at Alum Bay and Redend Point, Studland in Dorset. Both of these localities are close to the middle limb of the monocline. At Studland previously vertical, cylindrical pipes of limonitic sandstone in the Redend Sandstone have been tilted from the vertical (Arkell, 1947). This suggest that there, at least, some weathering and oxidation took place quite early, before the folding was completed. Probably there was some limited early uplift on the northern margin of the anticline at Studland and Alum Bay and this resulted in the oxidation that gives the reds and yellows of these sands. It was followed later by the main folding, the effects of which were slight at Studland Bay, but major (vertical) at Alum Bay. The coloured sands are not developed at Whitecliff Bay because the anticlines plunge eastward and there was probably insufficient early uplift in this lower eastern region. The Bracklesham (including "Bagshot Beds") are not brightly coloured elsewhere and are largely grey or green and glauconitic; there is much iron but it is mostly not in ferric condition.

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Footnote: How deep has been the Pre-Tertiary erosion?

Usually, as at Alum Bay, the uppermost zone of the Chalk is that of Belemnitella mucronata. Rowe (1908) considered that in certain places Tertiary strata either rest directly on the next zone down, that of Actinocamax quadratus and in some others only a thin representative of the mucronata Zone separates the quadratus zone from the Tertiary. White (1908) did not agree with this and considered that the mucronata Chalk is continuous from Alum Bay to Whitecliff Bay. He recognised, however, that there has been drastic thinning due to Pre-Tertiary erosion with the mucronata Zone being 145m at Alum Bay and only 46m at Whitecliff Bay. It would be interesting to know whether there is a marked topographic unconformity beneath the Tertiary, rather than just a gradational change due to regional uplift over some distance, and whether there is any truth in the old theory of a rather deep sub-Tertiary valley at Freshwater Gate. This data, however, does not see, to help a theory of the Alum Bay coloured sands being oxidised by fairly early Tertiary uplift because the greater thickness of the last Chalk zone here, seems to oppose uplift at the very beginning of the Tertiary here. Perhaps you would like to think further about this mystery.
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After the Miocene folding there is no record of deposition on the Isle of Wight until Pleistocene times. Obviously much major erosion took place over several million years. It is possible that, at a time of relatively high sea-level, there was some Pliocene marine deposition, as the products of it are known elsewhere in the southern part of Britain (in East Anglia and at St. Erth in Cornwall), but no Isle of Wight Pliocene sediments have been discovered. By comparision with other regions Pliocene deposits, had they existed, would probably have been present on the Chalk downs, such as St. Catherine's Down, perhaps 130m or more above sea-level.

Pleistocene deposits include a raised beach of the Foreland near Bembridge, and this was formed by an Interglacial sea-level that was a little higher than that at present. During the many glacial phases, which severely affected northern Britain, the Isle of Wight area was not glaciated but was in periglacial tundra conditions. During the melting of the snow each Spring the flooded rivers transported much flint downstream from the source areas of the Chalk uplands. As in nearby areas, such as the New Forest, there are, as a result, various subangular flint gravels that originated as braided river deposits of flood origin. Some of these are associated with the Solent River, a major Pleistocene river that once flowed on what is now the bed of the Solent eastward round the Isle of Wight and then southwestward down the English Channel. This was, of course, at a time of lower sea-level during glacial advance. Other gravels have originated on the flood plains of local northward-flowing tributaries to the Solent River.

There has been a suggestion that during the Pleistocene there was an English Channel Glacier approaching the area of the Isle of Wight from the west and blocking river inflow to so as to form a glacial Lake Solent ( Kellaway, 1975 ). This seems unlikely, at least partly because of lack of a sufficiently high ice source area in the west. Erratics at low levels on the Hampshire-Sussex coast can be adequately explained by the presence of pack-ice in the sea at times and needed no glacier for their transport. Elephants, particularly mammoths, and other large mammals lived in the periglacial tundra of the Isle of Wight (not at that time an island). Their remains are found in Pleistocene chalky gravel and loam on both sides of Freshwater Bay, in brickearth near Shide in the Medina Valley, and in old river gravels at Grange Chine near Brighstone and at Brook Chine. See White (1921).

Strata of the Isle of Wight

Quaternary - Holocene (Recent)		Landslips Blown Sand Tufa Alluvium and Peat
Quaternary - Pleistocene		Raised Beach Valley Gravel Plateau Gravel Angular Flint Gravel
Palaeogene    (Tertiary)	Solent Group       Barton Sand Formation Barton Clay Formation   Bracklesham Group           "Bagshot Sands"     Thames Group   Reading Formation	Bouldnor Formation (Bembr. Marls & Hamstead) Bembridge Limestone Fmt. Headon Hill Fmt. (Headon & Osborne)         Elmore Formation Selsey Formation Marsh Farm Formation Earnley Formation Wittering Formation       London Clay Formation (Oldhaven Formation)
Cretaceous	Upper Chalk Middle Chalk Lower Chalk     Upper Greensand Gault     Lower Greensand         Wealden Group     (Purbeck Fm. - in boreholes)	(Chalk with flints)               Carstone Sandrock Ferruginous Sands Atherfield Clay   Vectis Formation ("Wealden Shales") Wessex Formation ("Wealden Marls")
Jurassic	(Jurassic underground only)

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Some Fossils of the Isle of Wight

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Various fossils from the Cretaceous and Tertiary strata of the Isle of Wight (and other regions) are shown here. Note the lagoonal Purbeck Formation is underground and is not present in the cliff, but known from the Arreton Boreholes. The mainly fluvial Wealden consists of mottled marls, yellowish sandstone beds, some shales and thin limestones. It has some very interesting non-marine faunas, including remains of Unio-like molluscs, ostracods ("water-fleas") and pondsnails (Viviparus) It is particularly notable for dinosaur remains and such creatures as Polacanthus, Brachiosaurus, Iguandodon and Hypsilophodon have left their bones in these strata. The main exposures are near Brook in the southwest and around Sandown in the southeast. The Lower Greensand is a notable fossiliferous formation in the Isle of Wight with remains of marine organisms such as lobsters and various molluscs, including the robust Mulletia mulleti, formerly known as Perna. The unit can be seen at Compton Bay, Atherfield area (where it is particularly well-developed) and at Sandown. The Gault Clay is fossiliferous in places, but because this "Blue Slipper" is often associated with landslides it is not usually cleanly exposed. The Upper Greensand forms prominant cliffs and is particularly fossiliferous. Some examples of Upper Greensand fossils are shown here. They are of marine origin. Chalk fossils can be found at Compton Bay, Culver Cliff, the St. Catherine's Down area and at many other places. The Lower Chalk is a unit which shows many obvious fossils. They can be found in other parts of the Chalk with more searching.

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

Fossils of the Solent Group

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. Pondsnails 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. It is also present at Colwell Bay, Totland Bay and elsewhere. There are freshwater lacustrine, brackish water and some marine fossils. The Bembridge Limestone Formation is present around the shores of much of the northern (Solent) coast of the island. It contains various freshwater pondsnails. The Bouldnor Formation contains freshwater to marine molluscs. It also contain fossil insects, not shown here. Remains of crocodile (rare) and turtle (fairly common) are present in the Solent Group.

Younger strata than the Solent Group are Pleistocene and Holocene. These Quaternary deposits contain remains of elephants (as at Freshwater Bay), bison (as at Newtown Estuary), hazelnuts (near Brook) and other fossils.

References and Bibliography

Please go to Isle of Wight Bibliography .

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.

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at his private address, in connection with the School of Ocean and Earth Science, National Oceanography Centre, Southampton (NOCS),and web-hosted by courtesy of iSolutions of Southampton University.

Written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.

Romsey, Hampshire