‘No vestige of a beginning, no prospect of an end’
You don’t have to go far on the Island to see rocks. That’s the big advantage of islands : cliffs and coastal exposures. Here the rocks are all sedimentary, but formed at various times and in different environments. Many contain fossils; excellent fossils. The oldest are along the south coast west of Atherfield, where a large river flowed across a broad, flat and partly forested floodplain in the Early Cretaceous (Barremian) about 127 million years ago (127 Ma.) These deposits produced the Wessex Formation (Fm.) of the Wealden period. It was the time of the dinosaurs, whose bones may be found in the red muds left behind when the river burst its banks. Trees, ferns and other plants left their fossils too. The climate had long dry seasons when muds cracked to crazy paving, followed by severe thunder storms, lightning-strike forest fires and flash-floods that carried soil and debris overland, forming distinctive pale-grey ‘plant-debris beds’ which are exceptionally rich in bones. According to the Geologists’ Association (G.A. 1998), the very lowest (earliest) Wessex Fm. beds are near Brook Chine, at the centre (‘hinge’) of an updoming called the Brighstone Anticline.
Large quantities of sediment (muds, silts and sands) were carried in to the area from uplands in Devon/Cornwall and Amorica/Brittany. These continued to be deposited over a very long period of time because the land was slowly subsiding : the Island was part of the large Wessex-Channel Basin. Subsidence was not just a gentle sagging of the ground. The Island was on a major earthquake-prone fault zone.
The solid surface or lithosphere of the whole Earth on land and below the oceans consists of the crust plus the top of the underlying mantle (which has a different mineral composition). Picture this surface as being fractured into huge ‘plates’. Some plates have continents perched on them. All move relative to each other over the Earth, which is a very slightly flattened sphere of constant size. At plate margins where two plates move towards each other, one plunges below the other and continues sinking into the deep lower mantle. The forces of compression generated at a collision zone spread outwards, causing movements even on distant faults.
The movement of whole plates is controlled by convection currents in the deep mantle, as a result of radioactive energy release and heating which makes some regions more buoyant and mobile. Mantle rocks rise and then flow horizontally. It is creep rather than current, but ‘flows’ below the plates before cooling and sinking again, imposing a direction of motion on the plate. Usually some section of a plate margin is growing, where part of the upwelling mantle generates basaltic magma which creates new ocean-floor, while other sections of the margin are sinking down into the mantle after colliding with another, over-riding plate. Once these sections start sinking, gravity pulls them deeper because they long remain colder and denser (heavier) than the surrounding mantle as they go down. Heat escapes through the solid (brittle) lithosphere by conduction, but below this the mantle is ductile and heat can travel by convection. Heat below the constructive margin of new ocean-floor elevates this region, and gravity assists the motion of the plate down a very long imperceptible slope, as it cools and contracts, to the destructive margin where it sinks into the mantle. The base of the lithosphere is about 100 km. below continents (but can reach 300 km in places), whereas below oceans it varies from just 4 km at growing (constructive) plate margins, to 100 km below old (over 100 million years) ocean crust. Solid mantle makes up about sixty percent of the depth of lithosphere below continents, but ninety percent below oceans.
At locations within a plate far from the margins, the crust may experience a pull (tension) from different directions, causing the land to sink into a basin as happened over part of the Island during the Cretaceous. The Island is crossed almost centrally, from east to west, by a major fault zone. Movement on these faults, sometimes a pull-apart caused by tension, sometimes the push-together of compression, has affected the topography, the erosion, and the deposition of sediment from long before the Cretaceous through to more recent times. To understand the origin of these faults we have to go further back in time, to events that are not visible here but have been deduced from more ancient rocks elsewhere in Britain.
Tectonics and Faults: Variscan Colliding Continents
Our planet is 4550 Ma. old, so events around between 383 Ma. (Late Devonian) and 305 Ma. (Late Carboniferous) are not too remote to have affected local geography during the Cretaceous. Southern Britain (once part of the Avalonia continent) was on the south east edge of a large ‘Old Red Sandstone Continent’ called Laurussia which extended far to the S.W. (N.America), N. (Greenland) and N.E. (Scandinavia). The Laurussia plate was moving north-east, but closure of the Rheno-Hercynian (or Rheic) Ocean to the south brought it into oblique collision with the relatively small microplate and landmass of Amorica moving south-westwards. This collision was intensified when Amorica was shunted northwards from behind first my the colliding landmass of Iberia and then by the enormous continent of Gondwana (or Gondwanaland) both moving directly northwards. The suture or line of ‘welding’ of Laurussia and Amorica runs east-west below the modern English Channel. In effect, Gondwana collided with Laurussia and compression buckled the collision zone into a line of mountain ranges called Variscan (or Hercynian) extending from the Appalachians in N.America, through S.W. Britain to Central Europe and the Urals. The united landmass, with Britain deep in the interior, is known as Pangaea.
North of a line from Bristol to Dover there was apparently no major faulting or uplift. Everywhere south of that line, and extending across the Channel into France, the Variscan Foldbelt was riven by major faults, some extending deep into the crust. These faults subsequently remained permanent lines of weakness.
Two of the major faults converge on the Island, but diagrams published by the British Geological Survey (BGS) give two different interpretations of their connection: their closest approach or intersection was either near Newport or close to Culver Cliff (Sandown).
The Portland-Wight Thrust Fault extends east-west from Newport to the Needles, to near Swanage and across Dorset to Devon where it terminates at the N.W./S.E. Sticklepath Fault (which goes inland from near Torquay and cuts the Dartmoor Granite (309 Ma), itself of Carboniferous age). Portland-Wight is a ‘normal fault’ angled downwards (‘dipping’) towards the south, and the land moved downwards (‘downthrown’) on the south side of it. It is the most important of a whole series of almost parallel discontinuous east-west faults found across Dorset and Hampshire.
The Wight-Bray Fault, almost vertical, is the second major fault on the Island. It is of a very different kind, known as transcurrent, along which the two sides moved in different horizontal directions (rather than vertically). Wight-Bray Fault and runs
N.W./S.E. from near Culver Cliff (Sandown) to Dieppe. Some BGS diagrams show it continuing beyond Culver by curving W. onwards to near Newport. Wight-Bray is just one of a series of roughly parallel N.W./S.E. transcurrent faults along the English Channel. The others cross the south coast close to Dover, Weymouth, and Torquay (Sticklepath Fault). They are all dextral. meaning that if you stand directly above the fault-line and look along it (in either direction) the land on your right has been displaced towards you (relative to the land on your left). This pattern of transcurrent faults (which are oblique to the normal faults) was caused by the angle of collision of the two land masses being partly sideways rather than head-on.
In the southern and central English Channel area, mountains were raised by massive upfolds of rock becoming detached at their base and being thrust northwards along low-angle thrust faults as ‘nappes’. Three Nappes have been identified and named west of Sticklepath Fault, at the south edge of the Cornubian Massif (Cornwall/Devon). Similar nappes are a large feature of the present Swiss Alps. It is not clear how far north nappes came towards the Island. Over time the Variscan Mountains here were worn down to their roots, and became the ‘basement’ above which later sediments were deposited. But the major faults remained as permanent lines of weakness, dividing the crust into a series of rigid blocks. The faults defined the margins of sedimentary basins when the forces of compression associated with continental collision were gradually replaced by tension (extensional stress).
Basins and Uplands
A whole system of subsiding basins developed, collectively called the Wessex-Channel Basin, extending southwards from Pewsey (near Marlborough) to the central English Channel. They form two main groups: (1) Pewsey and Weald Basins north of the Wardour-Portsdown Faults and (2) the Portland-Wight Basin (sometimes called the Vectian Basin or Wessex Basin and including the southern half of the Island). Between these groups was relatively high ground called the Hampshire-Dieppe High, which covered the northern half of the Island as far south as the Portland-Wight and Wight-Bray Faults. The basins sank slowly, accumulating sediment, from Permian times (290 Ma.) through into the Early Cretaceous.
Upland areas are exposed to the elements and erode faster than lowlands. Eroded sediment moves downhill, by rainwash, rivers or wind scour on land, or by slumping and water currents on the seabed. Sediment accumulates wherever the rate of supply exceeds the rate of removal. Each new layer (bed) of sediment is deposited horizontally, or at a very shallow angle. However steep the angle you may see a sedimentary bed take on the Island today, it was originally horizontal: the angle is an abberation produced by later tectonic movements. There can, however, be steep angles within a single bed: for example thin laminae within a sandstone which formed on a river bed represent the migrating front layer on sand-banks or dunes, where sand grains were carried over the crest to fall in a very thin steep layer on the downstream side. A rapid influx of new sediment can cover and seal-in small-scale features like this.
During Permian and Triassic times the western Wessex-Channel Basin sank so much that it accumulated 3000 metres of sediment. Sinking continued through the Jurassic, and extended further east when, for the first time, vertical downward movement occurred on the west side of the Wight-Bray Fault. The sea swept in and by Kimmeridge Clay times (154 Ma.) very little of the Variscan uplands remained as dry land.
Subsidence continued into the Early Cretaceous, but because global sea-level fell from the end of the Jurassic, the sea retreated from elevated areas including the Hampshire-Dieppe High across the northern half of the Island. On these uplands the Late Jurassic sediments were eroded away, often down to Middle Jurassic rocks. Erosion continued until the end of the Barremian period (121 Ma.) supplying sediment to the nearby Wealden lowlands of Portland-Wight Basin. The thickness of Jurassic plus Lower Cretaceous rocks in the centre of the Portland Wight Basin (south of a line from Brighstone to Sandown) exceeds 2000 metres.
Two factors are thought to have contributed to the rise of the Hampshire-Dieppe High: (1) Isostatic uplift of the buoyant upthrown block (the ‘footwall’ side of the E./W. fault) which had previously been jammed in place by compression and (2) Uptilting from the west, where the continent was being heated by an upwelling flow from the mantle and the first stages of opening the new Atlantic Ocean were taking place. Pronounced cooling deep in the crust nearer to the Island caused contraction and rapid subsidence of the fault-bounded basins in the Early Cretaceous, increasing the rate of erosion of intervening uplands (which were fault-bounded ‘horsts’).
There are no rocks from the base of the Cretaceous (113 Ma.) at the surface on the Island. To see the oyster-rich Cinder Bed, a short-lived marine incursion within the Purbeck Beds which marks the Jurassic-Cretaceous boundary, you need to visit Swanage. Terrestrial conditions were soon restored, and the west end of the Hampshire-Dieppe High, a fault scarp which we can call the Wight Scarp (continuing somewhat further east into the modern English Channel), continued to separate the Portland- Wight Basin from the Weald basin. This upland covered the northern half of the Island and expanded rapidly to the west and N.W. as the Cranborne-Fordingbridge High, part of the continuous high ground which ran north to the Welsh uplands (extending into the English Midlands). To the east of these uplands and N.W. of the Weald Basin in Hampshire, broad lowlands known as the Bedfordshire Straits extended in a loop W. and then E. to the area of The Wash.
This lowland was hemmed-in further east (Essex/Suffolk) by uplands of the London-Brabant Massif. The Boreal Sea on the site of the modern North Sea was the closest marine water to the Isle of Wight, but some barrier across the lowlands near Peterborough prevented it advancing southwards to the Weald. However a large freshwater lake or lagoon did develop over much of the Hampshire Basin and Weald in the Early Cretaceous, extending south east into the Paris Basin. South of the Wight Scarp and Cranborne-Fordingbridge High, the Island had a normal terrestrial environment with sediments being carried in off these adjacent uplands, and from the Cornubian and Amorican uplands.
Although Wealden sediments began to form at 136.5 Ma., the Wealden deposits exposed on the Island all date from the Barremian stage (127 Ma to 121 Ma.) of the Lower Cretaceous, and form two distinct groups: (1) the lower (early) Wessex Formation (Fm.) once called the Wealden Marls, and (2) from about 114.5 Ma. an upper (later) Vectis Formation, known before as Wealden Shales. The Wealden period was much wetter and less seasonal than the hot semi-arid Purbeck period which preceeded it. A doming of the continental margin west of Britain in the Early Cretaceous gave an abundance of detritus from the west including material from the Iberian Peninsula. Wessex Fm. rocks are mainly red mudstones deposited over successive river floodplains during overbank conditions of ponding. There are also sandstones and conglomerates, generally lag deposits left in the beds of broad meandering rivers, but sometimes they represent ‘cravasse-splay’ deposits where a river burst through its natural levees (raised banks). Wessex sediments are rich in kaolinite, feldspar and tourmaline minerals originating in the Cornubian highlands and brought in by rivers flowing eastwards. Other river borne sediments came north from the Amorican massif. A major east-west river flowed just to the south of the fault-bounded northern margin of the basin. The Sudmoor Point Member contains ‘point-bar’ sands deposited on the inner side of river meanders, in a meander zone believed to be over 1.8 km. wide. Typical Wessex Fm. fossils are freshwater animals: Unio mussels, gastropods and Ostracods, tiny crustaceans with shells.
The Vectis Fm. is mainly dark grey siltstones and mudstones indicating a different environment. Shallow brackish lagoons produced the Cowleaze Chine Member, and a river delta building out (prograding) into the lagoon gave the highly distinctive yellow-orange sands of the overlying Barnes High Sandstone Member. As the basin continued to subside, this delta was in turn succeeded by a shallow lagoon into which there were periodic increases in sediment inflow from flooded rivers, giving a series of particular grey beds of fine sand, silt and mudstone called the Shepherd’s Chine Member. Several thin beds of limestone chock full of Filosina bivalve shells occur near the top of this Member. Chamositic pebble deposits, formed under fully marine conditions, sometimes occur in the Vectis Fm. There are abundant Ostracod fossils, and some beds rich in the gastropod Viviparus. BGS estimates that the combined thickness of Purbeck Beds (the lowest of which are Jurassic) plus Wealden beds is 900 metres south of a line from Atherfield to Shanklin, and (closer to the north edge of the basin) 700 metres south of a line from Compton Bay to Sandown.
After the Wealden, but still in the Early Cretaceous, a marine incursion (‘transgression’) from the Boreal (North) Sea occurred due to regional downwarping resulting from the early stages of opening of the Atlantic Ocean. The sea caused some erosion of the Wealden, leaving a ‘disconformity’ above which shallow marine conditions produced the Lower Greensand (121 Ma. to 105 Ma.) This name is rather a misnomer due to early misidentification of rocks at some locations with the true (now Upper) Greensand. In the Portland-Wight Basin it begins with the brown Atherfield Clay, which includes the Perna Bed with its large and strangely shaped eponymous bivalve. This Bed, rich in bivalves, corals and ammonites, also has fragments of Jurassic rocks and fossils eroded off the nearby uplands. Atherfield Clay was deposited on a shallow marine shelf, and severe storm waves created wave-ripples on some beds. Scouring also occurred, making channels which were later infilled by ‘gutter-casts’ often containing a rich lag deposit of tiny bones. Westwards across Dorset the fossils in the upper Atherfield Fm. indicate progressively less saline conditions, and Rawson (1992) suggests they represent the estuary of an eastward-flowing river which briefly sent sand, now known as the Crackers Member, as far at the Isle of Wight. Above the Atherfield clay came marine sand waves, now forming the rusty coloured Ferruginous Sandstone (145 metres thick at Atherfield), which is in fact clearly dark green towards its top at Compton Chine in Compton Bay. These Ferruginous Sands (118Ma.)mark a lowering of sea level and an uplift of the London-Brabant Massif which supplied much of the sand. They were followed by the white or yellow sands of the Sandrock (114 Ma.), and then up to 22 metres of pebbly Carstone (110 Ma.). The Sandrock (57 metres thick) has a succession of upward-coarsening units (associated sequences) each beginning with a low-energy environment of black estuary-muds, and ending with high-energy sandy shoals and channel-fills, suggesting that a nearby shoreline was advancing seawards (prograding) but then repeatedly retreated landwards as the basin subsided further. Eventually the seas retreated (a marine ‘regression’) due to movement on the faults, and the top of the Sandrock was eroded away producing an angular unconformity. After the seas returned, the overlying horizontal Carstone covered an irregular surface at a slight angle to the formerly (now displaced) horizontal bedding of the Sandrock. The Carstone, which forms the top of the Lower Greensand, is the earliest Cretaceous rock to still be found in the area north of the Portland-Wight Fault.
Many sands of the Lower Greensand are thought to have come from the NNE down the Bedfordshire Straits, beginning near The Wash, but one enigmatic feature of this period is the ‘Portsdown Swell’. This raised area, running from Portsdown (near Southampton) to Eastbourne, may have remained as dry land above some of the Lower Greensand seas. Both then, and perhaps at other times, it hindered the flow of sediment between the Weald Basin and Portland- Wight Basin.
Marine influences increased rapidly after the Lower Greensand. The dark bluish-grey Gault Clay (105 Ma. and known colloqually as slipper clay) is up to 30 metres thick and represents deposition of fine particles under very low energy conditions, in water 100 to 200 metres deep, probably far offshore on an outer shelf. Then the shoreline advanced seawards, giving the Upper Greensand which is a shallow water, near-shore deposit of sands brought from the west (Cornubia and Amorica) and containing the special green marine mineral glauconite. It is up to 45 metres thick, and near the top has siliceous nodules known as the ‘Chert Beds’. Ironically the very dark green glauconitic sandstone above this is actually the base of the overlying Chalk Group. Considered together, the Gault and Upper Greensand represent a coarsening-upwards sequence off a prograding (seawards-building) coastline. The marine basin was filling with sediment allowing the shoreline to advance inwards from the margins. Muds which became the Gault Clay had travelled long distances on very gentle currents before settling out, but the Upper Greensand sands required much stronger, nearshore currents to move them. The Upper Greensand has been widely used as a building stone, with quarries particularly around Ventnor and Shanklin.
The general palaeogeography from Lower to Upper Greensand times consists of a broad zone of usually shallow marine conditions extending from The Wash to Lyme Regis in the west, swinging east into the Paris Basin, and returning north through Dover and Tilbury to Great Yarmouth. The London-Brabant Massif became eroded and gradually retreated east, but both the Welsh High (extending into the English Midlands) and the Cornubian Massif were still present to the west, and the Amorican Massif (Channel Islands and Normandy) to the south.
Upper (or Late) Cretaceous
A global (eustatic) rise in sea-level marked the beginning of the Upper Cretaceous (98.9 Ma.) and seas spread over much of Europe, an event termed the Cenomanian Transgression. The top surface of the Upper Greensand was removed by erosion, leaving a time gap of about two million years in the rock record. London-Brabant Massif was submerged and the Amorican uplands much reduced. Continued subsidence of the Portland-Wight Basin produced locally deep water conditions. The Lower Chalk begins with a muddy Glauconitic Marl, followed by the progressively clay-depleted Chalk Marl and then Grey Chalk, with a final thin but very significant muddy bed of Plenus Marls (95 Ma.). The Lower Chalk has abundant fossils of bivalves, sponges. serpulids and ammonites like Schloenbachia and Mantilliceras. Then came the nodular chalks and ‘hardgrounds’ (hardened sea-beds formed at times of low sediment input) of the largely flint-free Middle Chalk (92 Ma.), and above this the very white Upper Chalk (89 Ma.) with seams of flint nodules. Flint comprises tiny silica crystals, recrystallized from biogenic material such as sponge- spicules and microfossils such as radiolarians. It often infilled crustacean burrows in the chalk ooze. Within the Upper Chalk (White Chalk Fm.) are thin bands of grey marl which originated as volcanic ash.
Late Cretaceous sea-level rose to a maximum of about 300 metres above modern sea-level (in the Late Campanian 74 Ma.) as a result of rapid sea-floor spreading (and a consequently elevated ocean-bed) in the Atlantic and S.E. Pacific. Rising seas meant that dry land was far distant from the Island, so terrestrial sediments usually failed to reach the area. Instead, the calcareous skeletons of countless microscopic calcareous ocean plankton (coccoliths) settled to the seabed forming chalk ooze which later hardened into a fine grained micritic limestone. In seas 100 to 600 metres deep, with a surface temperature of 20 to 30 degrees centigrade, chalk accumulated over a period of about 35 million years to a thickness of over 400 metres in parts of the Hampshire-Dieppe Basin. A borehole off Swanage shows that Upper Cretaceous deposits on the north side of the Portland-Wight Basin (which includes the Island outcrops eastwards from the Needles to Culver Down) reached a thickness of 477 metres.
By contrast the former uplands across the northern half of the Island, like large sections of the former Hampshire-Dieppe High, are believed to have accumulated only 300 metres of chalk, and this is now concealed below later sediments, with the base of the Upper Cretaceous now 800 metres below ground.
Tertiary Basin Inversion (66.5 Ma.)
The Cretaceous period ended with a bang in North America because of a major meteorite impact on the Yucatan Peninsula of east Mexico. Fallout from this is found as clays in Denmark and Italy, but not on the Island because the top of the Chalk (the whole Maastrichian period) and the base of the Tertiary have both been lost to erosion. A time gap of about 15 million years separates the youngest surviving Upper Chalk (71.3 Ma.) from the base of the oldest surviving Tertiary rocks (56 Ma.) Some uplift had occurred during the earliest Tertiary (65 Ma.) producing a slight dip towards the south-west during the Palaeocene. Consequently the eastern side of the Island was raised higher, and has lost a greater thickness of Chalk (about 25 meters more) than the west side. Potholes (soon infilled) which were excavated vertically into the chalk at this time, can now be seen as horizontal features in the chalk cliffs at Alum Bay.
By the end of the Cretaceous, crustal extension (tension) and basin subsidence had continued for about 200 million years. This regimen was replaced in the Tertiary by crustal compression. The direction of vertical movement on major faults was reversed, and areas which had been elevated uplands became the new basins in a process termed ‘basin inversion’. The amount of reversed movement was not huge, but was continual and sufficient to maintain this reversed topography while sedimentation progressed. The Hampshire-Dieppe High running east-west across the northern half of the Island slowly changed into the Hampshire-Dieppe Basin (280 kilometers long) which sank sufficiently to preserve over 400 metres of Tertiary sediments. Sandhills borehole on the Island near Porchfeld recorded 652 metres of Tertiary sediment. The Portland-Wight and Wight-Bray Faults became now the northern margin of an uplifted area.
The timing of these changes is controversial, because later tectonic events have obscured the evidence. BGS suggests that small scale reversals on major faults began in the Late Cretaceous, but thermal subsidence remained inportant throughout the area until the mid-Tertiary. Consequently, despite the erosion of the top of the Chalk, later sediment accumulation occurred over the whole area, both uplands and basins. The Tertiary sedimentary beds now seen standing almost vertically on end north of the Chalk at Alum and Whitecliff Bays were originally deposited horizontally over a low lying land surface. Major tectonic changes were delayed until the Miocene (23.8 Ma.) when strong compression was associated with the Helvetic mountain-building (orogenic) event in the Alps and Pyrenees, caused by the collision of the northward-moving African plate into Europe. This event post-dates all the rocks found on the Island. However, Anderton (2000) claims there is good evidence that the Portland-Wight High (formerly Basin) had already risen above sea-level by Selsey Fm. (46 Ma.) times, and continued to rise in Barton Clay (44 Ma.) times.
Early Tertiary compression caused an upward arching of rocks in the southern half of the Island. The Portland-Wight Fault and Wight-Bray Fault became the Portland-Wight and Wight-Bray Monoclines, so named because the steep northward dip along them was easily visible, whereas the very gentle southern dip was not appreciated. They actually mark the northern limb of anticlines (an uparching of the rocks). BGS show the summit (axis or hinge) of the anticline ran somewhat to the south of the Portland-Wight fault, beginning at Sticklepath Fault in Devon and running east to near Brighstone, then just south of Newport, before curving to the south east and crossing the coast. The G.A. prefers a different interpretation, with two separate, unconnected anticlines crossing the coast near the same locations: Brighstone Anticline and Sandown Anticline. Both the Portland-Wight High (formerly Basin) and on the mainland the Weald Anticline were raised over 1000 metres along their central axes due to compression from the south. These former basins, filled with a great thickness of weak, uncompacted and poorly cemented sediment were clearly the areas most affected by compression.
Early Tertiary (Paleocene)
The earliest extant Tertiary sediments on the Island are the bright red and mottled muds of strongly weathered terrestrial soils in the Reading Formation of upper Paleocene age (late Thanetian) about 56 Ma. No record remains of the interval since just before the end of the Cretaceous at 65 Ma. though it is thought that the entire English Channel area was dry land during that time. About the time of the Reading Beds, but further east, the Boreal (North) Sea advanced into the Paris Basin. It entered a semi-enclosed basin, with its western margin from Brighton to Le Havre, and swampy lowlands extending forther west. The Reading Beds generally lack fossils.
A major marine transgression onto the land marks the start of the Eocene period (55 Ma.) The seaway advanced west through the Channel to link up with the warmer waters of the English Channel. On the Island the marine London Clays (Ypresian age) were deposited. The northern coastline of this sea ran from Lyme Regis to The Wash. Cycles of deepening and then shallowing water can be seen in repeated sequences (units) of sediments with upwards reduction to fine grains followed by coarsening to larger grain sizes. They represent offshore silty muds, followed by tidal flats with laminated silty sands and muds, and cross-bedded sands from tidal channels (the ‘Bagshot Sands’). A new marine transgression initiated each cycle. London Clay is mainly brown to blue-grey and contains over 350 species of both cool water and warm water molluscs, mainly gastropods and bivalves (notably Pinna, Arctica and Turritella) but also cephalopods like Nautilus.
The Bracklesham Group represents a time when global sea-level fell (52 Ma. Early Lutetian age). The seaway became less salty (brackish) due to freshwater inflow from rivers, and the relatively shallow (100 metres) waters were turbid and muddy. Again there are rhythmic repeat sequences of coarsening-upwards sediment, including storm-lag shell beds. The water depth repeatedly shallowed from offshore shelf conditions to intertidal and even brackish lagoon conditions, Sediments include a lignite bed derived from coastal marsh vegetation at Whitecliff Bay. In the west at Alum Bay some of the sands represent beaches and even terrestrial aeolian dunes. Bracklesham Group has warm water mollusc fossils, and provides the well known ‘coloured sands’ of Alum Bay.
Next came the Barton Group (41.3 Ma. Bartonian age) with clays and sands. The marine ‘Solent’ embayment at first had normal salinity, but changed to a muddy, low salinity water as river inflow increased. Barton Group records the last prolonged marine environment on the Island (though later brief marine incursions did occur) and it terminated with the Becton formation beach sands.
Headon Hill Formation (39.4 Ma. Priabonian age) represents freshwater and brackish water sediments, probably from coastal lagoons and alkaline lakes (up to 15 metres deep) fringing a marine inlet along the present day eastern Solent. Several freshwater limestones occur: How Ledge, Hatherwood, and Lacy’s Farm Limestone Members. The Barton Group (Middle to Late Eocene) and the overlying base of the Headon Hill Fm. (Totland Bay Member) represent the gradual infilling of a marine basin, originally 100 metres deep and eventually a few metres above sea-level. North east of a line from Brockenhurst (New Forest) to Newport on the Island, the Totland Bay Member is folowed by dark clays with marine fossils (Brockenhurst Beds). On the Island this brief marine incursion produced the Colwell Oyster Bed, probably in an estuary. Thereafter terrestrial conditions returned.
The overlying Bembridge Limestone (about 35 Ma.) has abundant moulds of freshwater molluscs, and locally of terrestrial milluscs. It was once an important ecclesiastical building stone, shipped to the mainland. It varies in thickness across the Island, possibly due to tectonic processes, and contains some muddy marl layers formed in more brackish conditions, and thin lignites. A number of fossil mammals like Anaplotherium have also been found.
Next came the Bouldnor Formation (32 Ma.) which began with a brief marine incursion producing the Bembridge Oyster Beds. These sands and flint pebbles have fossil mulluscs including Mytilus and Ostrea vectensis. The environment was less saline westwards, and the bed is absentwest of Newtown river, suggesting the sea advanced from the east. The following sediment is the Bembridge Marls Member, mainly of grey clays formed in brackish lagoons or on a flood plain and in extensive, shallow, freshwater coastal lakes. Mollusc fossils include both brackish water species such as Corbicula and Potamides, and freshwater forms like Viviparus. The base of the Bembridge Marls contains a very fine- grained limestone, the famous Insect Bed with arthropod fossils.
BGS (1992) placed the base of the Oligocene (33.7 Ma) immediately above the Bembridge Limestone, but the G.A.(1998) places it above the Bembridge Marls. After Bembridge Oyster Bed, the North Sea withdrew eastwards to Belgium, while the Atlantic withdrew south to a line from Dieppe to the central English Channel and westwards. The Weald-Artois uplift cut off the North Sea from the English Channel, while the Portland-Wight High (formerly Basin) had restricted the sea to the northern half of the Island. At the top of the Bouldnor Formation the Hamstead and Cranmore Members are still Early Oligocene (31 Ma.) and there are no later Tertiary beds on the Island. Hamstead Member has freshwater clays with plant seed fossils and some mammals. Cranmore Member fossils suggest first estuarine conditions and later a marine environment. It just preceeded a global fall in sea-level caused by the formation of an ice cap in Antarctica. Rocks from the Miocene and following Pliocene are completely absent from the Island, due to the major uplift which occurred in the Miocene followed by erosion of the resulting uplands.
The Pleistocene (Quaternary) began at 1.67 Ma. and heralded the Ice Ages in Europe. Angular flint gravels on St.Boniface and Bowcombe Downs, eroded out of the chalk. are believed to date from the lower Pleistocene. They may have originated from erosion during the Pliocene (5.2 Ma to 1.67 Ma) when sea-level in Kent is thought to have reached 183 metres above modern sea-level (O.D.)
Uplands running from Surrey to The Boulonnais separated the North Sea from the English Channel. Local sea-level fell in the early Pleistocene as a result of both a global (eustatic) fall and regional tectonic uplift which raised the coastline. The English Channel dried up, possibly first during the early Cromerian (770,000 to 560,000 years Before the Present (BP)) and then again during the Anglian Ice Age (560,000 to 440,000 BP) when the Dover Straits were cut through by the overflow channel from an ice-dammed lake in the southern North Sea. It was also dry during the Wolstonian Ice Age (300,000 to 130,000 BP) and during the late Devensian (120,000 to 13.000 BP). At these times rivers like the Rhine, Meuse and Thames cut deep channels into the bed of the English Channel. One tributary incised the Palaeosolent Valley, at least 46 metres below modern sea-level, extending east from Southampton Water and around the north east coast of the Island before heading south south west. Although there is no evidence that ice sheets ever advanced as far south as the Island, possible glacial erratic ‘dropstones’ carried by icebergs in Cromerian or earlier times from Brittany and the Channel Islands have been found at Portsmouth and Selsey Bill.
St. Catherine’s Deep, off St. Catherine’s Point, descends 60 metres below the surrounding seabed, and may be due to tidal scouring of a pre-existing palaeoriver valley. West of the Island, an eastwards flowing river emerging from Poole Harbour, swung southwards opposite Christchurch during the Pleistocene glaciations, cutting a channel across the line of the Portland-Wight High (formerly Basin). The chalk ridge connecting the Needles to the Isle of Purbeck was eventually completely breached at this point, either by river or marine erosion, either during the late Devensian ice age (13,000 BP) or in the early Flandrian (or Holocene) about 10,000 BP. A seaway opened here as sea-levels rose after the Devensian ice age, making Wight insular and allowing gradual development into the Island we know today.
- Stratigraphy based on BGS (1992): British Geological Survey (R. Hamblin et al) (HMSO 1992) ‘The Geology of the English Channel’
- Dates based on P.Hancock and B. Skinner Eds (Oxford 2000) ‘The Oxford Companion to the Earth’
- Interpretations based on :L.N. Warr (Oxford 2000) ‘The Variscan Orogeny : the welding of Pangaea’ in N. Woodcock and R. Strathan ‘Geological History of Britain and Ireland’
- A. Gale ‘Early Cretaceous: rifting and sedimentation before the flood’ and ‘Late Cretaceous to early Tertiary pelagic deposits: deposition on greenhouse Earth’ in Woodcock and Strathan (2000)
- R.Anderton ‘Tertiary Events: the North Atlantic plume and Alpine Pulses’ in Woodcock and Strathan (2000)
- P.F.Rawson ‘The Cretaceous’ in P.Duff and A, Smith Eds (Geological Society 1992) ‘Geology of England and Wales’
- G.A. (1998): Allan Insole, Brian Daley and Andy Gale ‘The Isle of Wight’ (Geologists Association Guide No.60) (1998)
- Building materials based on H.J. Osborne White (HMSO 1921) ‘A Short Account of the Geology of the Isle of Wight’