Isle of Wight Geology and Forms: A Personal View

Isle of Wight Geology and Land Form

The Isle of Wight has, to a degree, the typical geology of south central and south east England.

The surface strata run from the late Jurassic rocks near Brook Bay to the silts still being laid down and eroded back today in the vicinity of Newtown Bay.

The South Western rocks are of the Wealden Sequences, like the Hasting Beds of the Central Wealden Ridge, which runs from Horsham to Hastings via Tunbridge Wells, on the mainland. These were laid down as sea beds, and are mainly sand based with some other mudstones. In them the fossil record would be marine, unless sea-level fall had exposed them and terrestrial creatures roamed the old sea floor periodically. These would later have been covered by the Lower Cretaceous series. As Iguanodon are evidenced here- there must indeed have been terrestrial or estuarine interludes in this Wealden period.

In the late Jurassic, sand and mud stones,sometimes with iron, can be found.

The Cretaceous period is divided into Upper and Lower: that is, younger and older. The Lower Cretaceous usually appears in English geology as a boundary stratum between the chalk and the Jurassic with, usually, later clay deposits blurring the boundary itself. In the Lower Cretaceous, one finds, typically, Gault Clays and Greensands; the latter being classified as upper and lower. Gault Clays are typically blue and are hard, friable when dry but easily eroded. If they lie on the coast and underlie later strata they are cut away by the sea and undermine the strata above. This is the typical pattern on the south central coast of the island: best seen at Blackgang Chine.

On the Isle of Wight, gault is generally called “Blue Slipper” and the coast from St Catherine’s Point west has this characteristic habit of being destabilised. The same stratum is seen near Folkestone, in Holmesdale (Surrey), and particularly at Equihen in Pas de Calais, where the beach becomes strewn with large blocks of sandstone after the gault has been cut away. Examining a lump of gault and comparing it with the shell rich sandstones above, it seems to be a dead sequence- almost devoid of life- an extinction zone in the Early Cretaceous period.

The Greensands are green due to a mineral glauconite and are typically hard then soft in closely sandwiched strata. Where quarried for building stone these might be termed Rag and Hassock: with the Ragstone being the building rock which is dug out in convenient flattish pieces- almost brick like, and the Hassock being the friable rotten stone between.

A product of these Lower Cretaceous rocks is, in places, Fuller’s Earth: a soft rock with some detergent properties, often anhydrised for absorbance or used in cleaning- traditionally of hats. This has been dug since Roman times.

Between the Greensands and the Chalk of the Upper Cretaceous one finds flat thin strata of an iron rich sandstone, often used for paths – very often seen in churchyards. It is durable and is commonly seen laid end on. Tools can also be found of this material which were generally more deliberately worked that Flint tools. Looking at a bed of this iron rich stone, one sees the ripples of a shallow sea, with the sand patterns of a beach at low tide.

Again there is a tendency for the meeting of Lower and Upper Cretaceous to be hidden by later sedimentary deposits. The Upper Cretaceous might be simply stated as Chalk.

Chalk

Chalk often is found in three strata. The top one is often flint rich and the origin of the silicone which forms this flint is much debated.

The middle stratum is typically pure without flint and must have been laid down in deeper seas.

The Lower stratum is harder and is sometimes called Clunch. This can be used as a building material and was also dug as “hearth stone” and sold as a branded cleaner.

On top of the Chalk one finds gravels, clays and sometimes sarsens which must be Post Cretaceous: that is after the era of the dinosaurs; for the Chalk ends 66 million years ago at the same time as the alleged meteorite strike which finished those creatures off.

Sarsen, a sandstone accreted with silicates, can be found on the Chalk and was extensively used for megaliths and stone structure such as Stonehenge, Avebury, the Cauldron Stones and similar edifices on the Isle of Wight.

Sarsen must indicate a sandy bottomed sea or estuary after that which laid down the chalk.

Chines

A chine is a short valley running to the sea and these occur on the south and east coasts of the island. They cut deeply into the soft rocks down to the Blue Slipper and form a front for erosion. These can erode back very quickly and loose hundreds of feet in a generation. They are often wooded or filled with sea buckthorn and have interesting micro climates due to their sheltered form and southern aspect. Glow Worms are common in some, particularly in the vicinity of Brightstone.

The word is not unique to the island. Chines are also found in Hampshire and Dorset. Bournemouth is formed about 5 chines- some of which are still named.

In the east of the island the word is sometimes applied to a deeper rocky gorges quite unlike the clay chines of the south west coast.

Silicates and Flint

There is a debate about the formation of flints and the source of their silicone. It seems probable that the source was organic life forms such as sponges. As flint is found in the upper of the three typical chalk strata it seems possible that the silicone permeated the chalk from above after that chalk was laid down. It may well have been the same source of silicone which accreted the later sarsen stratum.

The second question must be: what forms the particular shapes seen in the flints? They appear organic and some suggest they are biota and sponges which left cavities in the chalk, later filled by silicone from above. Although organic looking, they seem not to conform to any likely organism and perhaps the more probable cause was gas (itself likely to have been of organic origin) forming the exotic shaped chambers in the Upper Chalk, followed by an infill of silicone from an exterior organic source. Flints appear in clear bands. Conditions for their creation obviously fluxed over millennia- sometimes rich, sometimes non-existent. One can imagine that these coincided with periods when the sea floor was exposed or inundated. It is instructive that the chalk of deeper ocean deposition is, generally flint free.

All the sea floor strata: sandstone, ironstones, gaults, greensands, chalks and Sarsens, must have been lifted up into hills. This happened circa 50 million years ago as a peripheral “shock wave” from the Alpine Orogeny. It is difficult to see the hills of Southern England as foothills of the Alps but simplistically they are, and their alignment, being a “shock wave”, points one, at ninety degrees to the range, towards the epicentre of that force.

The North Coast of the Isle of Wight

It is apparent that the valleys of the northern island rivers are being inundated by the sea causing the fjord-like or ria-like estuaries of the Yar, Newtown Haven, Medina and Wooton Creek. This phenomenon does not occur on the south coast because the rock strata lie in a syncline (declining to the north and the southern coast is generally a cliff.>

This inundation of the north coast is echoed on the mainland with the Beaulieu River and Solent. Southampton Water and the Solent was a river into which the ancient Test and Itchen flowed. It emptied east past Spithead. The severing of the island from the mainland in the Western Solent is much later . The Beaulieu River would have been a stream meeting this east flowing Solent. The Medina would also have been a tributary of it.

Under the Western Solent is an inundated forest which is visible at Low Water on a neap tide. Prior to the severing of the island, the backbone of the Isle of Wight was an eastward extension of the Purbeck Hills of Dorset.

The reason for the inundation of the North Coast and Solent is the corollary of Isostatic Rebound. The North of Britain rises following the removal of the weight of the ice sheets. The south sinks just as the removal of a central weight from a blanket would raise the centre and depress the edges.

Sea Level rise has a complex effect on coast. It is not necessarily true that water level increase leads to land loss. In a salt marsh environment, glass worts and other plants inhabit the inter tidal zone and attract sediment. They adjust to the sea level and will rise as the sea rises. Burrows or Sand dune systems also adjust. A higher tide allows the deposition of sands at a higher level. It is, therefore, quite possible for sea level rise to lead to deposition, not erosion, on a coast. The factors which effect a coast experiencing sea level rise are: the nature of the tides, whether the rivers are sediment rich or poor, and the nature of those sediments.

Human agency is also important for this can trump any natural process. At one time Freshwater was an island. The Yar became a north flowing creek only by the building of the Prom at Freshwater. The Yar rises only yards from that southern beach. Another human intervention is now seen at Cowes. On old maps there were some sand banks or low islands at the mouth of the Medina (these were the original pastures of the eponymous “Cows”. These were eroded away. These have now been restored by an artificial structure.

The shape of the island must be being sculpted now by the absence or presence of reinforced sea fronts such as at Yarmouth, Ryde or Cowes. These will remain while erosion and deposition will continue to the sides of them. One wonders if the inhabitants of this island will ever bite the bullet and build a sea wall from the Needles to St Catherine’s Point: without such a structure, this quarter of the island will be lost in remarkably short order.

Chalk Erosion 

Exposed chalk on a coast is eroded by waves and by frost. Chalk could also be eroded if it sat on an unstable stratum such as gault and was undermined.

When the chalk is eroded to the tide line, it is no longer subject to wave or frost erosion and so that process switches off. The result is a table or plateau of chalk at the rough level of the medium tide. Also the chalk at the base of the Cretaceous sequences, which is likely to be that closest to the tide line, is likely also to be of the lower harder clunch-like form which again- delays erosion.

A chalk stack will erode mainly due to frosting; its overall durability is due to the nature of the stratum in the zone between Low Water and High Water. A stack might last a considerable period of time if the base stratum is of a more durable sequence- such as a Lower Cretaceous ironstone.

Chalk is spread on farmers’ fields to sweeten clay soils. Here it is applied in the late autumn where it becomes sodden, frozen and literally explodes to small fragments. The water which inundates a sea chalk cliff is salt rich if from the sea but fresh if from rain. The brine is less prone to frosting than the fresh water. Also the moderating influence of the sea makes frosting less likely. Thus a chalk stack is most likely to be eroded by waves at its base but by water at its top – which, being rain, is neither brine rich nor temperature moderated. An eroded cliff does not disappear, but must be deposited elsewhere as either flint pebble, chalk nuggets or clay-like sediments.

C. Michael Sargeaunt 2017