#GeologyoftheIsleofWight how the Isle of Wight was formed/shaped

On Bank holiday Monday - a fine spring day - I headed down to the annual village May Fair to buy some plants for the garden. Typically I got side tracked by a secondhand book stall rather than plants!

I ended up buying (among others) a copy of "A Geographer's Look at the Isle of Wight" as told by Roy Hollis - first published in 1995. It is an easy read - suitable for someone like me - a generalist - albeit with an A level GCE in Geography (acquired 45 years ago - ha!) and a healthy interest in the Isle of Wight where I have lived and loved for coming up to 30 years. (On the author -  I first came to the Isle of Wight in 1982 as a Building Society Manager. Commander Roy Hollis (RN retired) was a regular customer. I remember him well and fondly and his book reflects how I remember him - energetic - precise and no nonsense.)

While Commander Hollis's book does cover human geography as well, the opening paragraphs are focused on the physical geography of the Isle of Wight - how it was formed - how it has taken shape. That is what I want to focus on. It won't take long to share it with you - so I will do so now. For some reason it makes me smile - perhaps because of the simplicity of the explanation - perhaps because millions of years of geological activity - often extreme sometimes violent can be summarised in just a few paragraphs! This is my précis based on Roy Hollis's book, some Wikipedia extract (in italic) and other bits and pieces I have in my head garnered over the years - ha! :

• The Isle of Wight is made up ENTIRELY of  SEDIMENTARY rocks laid down gradually over 140 million years - almost entirely underwater - probably in a shallow lagoon in the main.  (Definition Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils.)
• Geologists have been able to determine by observation of rock layers that the land which now forms the Isle of Wight has been covered by sea water, huge rivers or lakes and risen up again on no fewer than 7 occasions.
• On the point of rising up and sinking Commander Hollis makes short reference to the Isle of Wight currently thought to be sinking again. I wish to add this because I think it is interesting and graphic. Imagine a see-saw. Scotland at the top and at one end of the plank - Isle of Wight at the bottom and the other end of the plank. Put the weight of up to 3 kilometres depth of ice (during the Ice Age) on the Scotland end and the Isle of Wight rises! The ice melts - the weight on the Scotland end of the see saw consequently reduces and gradually the Isle of Wight sinks (lowers) again by a process referred to and defined as POST GLACIAL REBOUND. During the last glacial period, much of northern Europe, Asia, North America, Greenland and Antarctica was covered by ice sheets. The ice was as thick as three kilometres during the last glacial maximum about 20,000 years ago. The enormous weight of this ice caused the surface of the Earth's crust to deform and warp downward, forcing the viscoelastic mantle material to flow away from the loaded region. At the end of each glacial period when the glaciers retreated, the removal of the weight from the depressed land led to slow (and still ongoing) uplift or rebound of the land and the return flow of mantle material back under the deglaciated area. Due to the extreme viscosity of the mantle, it will take many thousands of years for the land to reach an equilibrium level.
• Back to the Isle of Wight sedimentary rock. They were laid in bands - the oldest of which were in the Cretaceous Period - 65 to 140 million years ago. These rocks form the southern half of the Island where chalk is the most prominent. Chalk is a soft white limestone (calcium carbonate) formed from the skeletal remains of sea creatures. This chalk layer was laid over an immense time period - at a rate of 1" (inch) every 2500 years!
• The northern half of the Isle of Wight is formed by sedimentary rocks from a later period - the Palaeogene - 35 to 54 million years ago.
• The rocks that make up the Island are, in oldest to youngest - Wealden Clays, Lower Greensand, Gault Clay (Blue Slipper), chalk, and then clays, limestone, sands and gravels.

two maps showing the geology of the Isle of Wight.

• Because sedimentary rocks are almost entirely laid by water they are laid in flat horizontal bands - the oldest at the bottom the youngest on the top. But of course the Isle of Wight is anything but flat and some of the older rocks - like the chalk are exposed at the highest points - so how can that be?
• I will take the hilly nature of the Island first - particularly the chalk "downs" that run from Culver in the east - right through to Tennyson Down and The Needles in the west. (the route of the annual "Walk the Wight" organised by the local Hospice to raise money - a great walk - 26 miles - with up to 4000 participants.).  
• In order to be able to explain why the Islands high ground is high I have to ensure you know the basics of "continental drift". The earths continents are not in a fixed position on the earths surface but "float" over millions of years on 7 major and some smaller tectonic plates. (Please do some background reading if you want to understand more.) These tectonic plates can drift apart but they can bang into each other. When this happens massive and destructive forces occur.
• Now a graphic explanation using a table cloth laying over your dining room table to simulate crashing continents. Go to one end - put both hands flat on the table cloth and push firmly towards the other end. You are simulating what happened millions of years ago when the African tectonic plate crashed into the European tectonic plate through continental drift. Your hands on the table cloth have pushed up one major fold and a number of smaller ones by ripple effect. In the case of the Europe/Africa crash the large fold was the formation of the Alps - and amazingly the chalk downs - the hills on the Isle of Wight are one of the small ripple folds from the same event! So this is why flat sedimentary rocks are no longer always flat - in fact in some instances they are now vertical.

the table cloth effect on sedimentary rock 

while this picture is NOT of the Isle of Wight it does graphically demonstrate how rock strata can be "folded"

• Next - to understand the effect of subsequent erosion of the land (rocks) by the elements - by the weather ie by water in the form of ice, rain, streams, rivers and sea (waves) and by wind of course. Obviously the elements have been acting on the landscape for millions of years and much rock has been worn away, moved elsewhere, broken down or otherwise exposed. Erosion has shaped our landscape.
• All sedimentary rocks are relatively soft and quite easily eroded. However some are harder - more durable than others. In the case of the Isle of Wight the hardest rock is chalk. Although as the chalk was formed on the sea bed millions of years ago it is now the rock evident through the "backbone" of the Isle of Wight. It was lifted up there as an anticline. The softer layers of rocks that covered it have been worn away so we now have the exposed chalk down. It is for this reason we see the massive chalk promontory at Culver Down and at the famous Needles at the west end. They have resisted the weather - and the pounding of the sea because they are harder. Bays have been formed because the soft rock has been scoured out by wave action and washed out by rivers and streams. Chines (sharp short deep valleys - mini canyons - for which the Isle of Wight is well known - ie Black Gang Chine) evident on the south coast of the Island (back of the Wight) have been formed by water running off the Downs and cutting through the soft sedimentary rock - very easily - in the short run to the sea.
• The entire back of the Wight - the coast facing south - is subject to serious erosion and landslip. Soft sedimentary rock is no match for the pounding from the sea. The Isle of Wight is getting smaller. The upside is ancient sedimentary rocks contain fossils - which is why the island is famous for Jurassic finds - Dinosaur Isle!

I have included this diagram to show how low layers of folded sedimentary rock can be exposed on the surface by millions of years of erosion.

• There are 2 other geological phenomenon that shape the Isle of Wight I want to quickly mention. The first is the effect of Gault clay - known on the Isle of Wight as "Blue Slipper" clay. Unlike most sedimentary rocks - certainly chalk - which is very porous Blue Slipper clay is not. Water cannot pass through it. So water soaks through the chalk downs and other porous rocks before it hits the clay barrier. The Blue Slipper acts as a lubricant - and the rocks on the top "slip" to create landslides and what is referred to on the Island as "undercliffs". This can be very destructive and continues to blight the Island (the Ventnor Niton road is still closed through landslip.) This phenomenon and the effect of Blue Slipper clay has in the same way damaged many Isle of Wight properties - by undermining foundations and causing movement. (be careful where you buy!)
• The final thing to mention is of course the Isle of Wight is an island - cut off from the mainland by a body of sea called the Solent. The Solent is generally not very deep - on average between 16 and 33 metres. Roy Hollis illustrates you could place St Paul's Cathedral in the middle of the Solent at high tide and still see more than half of it! There are long periods in history when the Island was not an island. Geologist can demonstrate the Solent is in fact the valley of the River Solent - and possibly a freshwater lagoon fed by its tributaries the Rivers Frome, Avon, Itchen and Test. As sea levels rose at the end of the Ice Age and the land started to sink (see post glacial rebound described above) the sea broke through the chalk barrier (by undermining softer rocks) at either end of the Island - and the River Solent valley was swamped to become a sea and create the island of the Isle of Wight! 

So there is my potted explanation of the geology of the Isle of Wight - perhaps a bit longer than I intended but I have covered 140 million years of activity - ha!. I hope you find it interesting. It might give you something to think about when you are plodding those hard (but beautiful) 26 miles of charity walk which is "The Walk the Wight!"