Geology

Compton Cliffs © Martin Williams
www.island-photography.co.uk

Geologists believe that the Earth's crust is composed of separate plates which move in relation to one-another. The movement of these plates means that the positions of the continents has changed through time, and that oceans have come and gone. To understand the geology of the Isle of Wight we have to look at its rocks and fossils in relation to such movements.

The rocks which form the Isle of Wight date from Early Cretaceous times (127 million years ago) to the middle of the Palaeogene some 30 million years ago. All of the Island's rocks are sedimentary (one of the three groups of rocks - igneous, metamorphic and sedimentary), and are composed of mineral grains derived from pre-existing rocks.

After these grains were deposited, they consolidated to form the rocks we see today such as mudstone (from clay), sandstone (from sand) and limestone (from lime - calcium carbonate). The Island's rocks are rich in fossils, and by studying them, we gain an understanding of past environments, how environments change, and how different plant and animal groups have evolved.

The Cretaceous 127 - 65 million years ago

The Cretaceous was a time of progressive rise in global sea levels, probably linked to the melting of polar ice caps, and coincided with the opening of the Atlantic Ocean as the European and North American plates separated. At the time the area of southern Britain was about where North Africa is today. On the Island, the rocks called the Wealden Group record those times. The Wealden Group comprises mudstones and sandstones, and were deposited by, and in a mosaic of rivers and backwater ponds. These predominantly red coloured rocks tell us that the climate was hot and dry. The landscape was home to the dinosaurs, the bones and foot prints of which can be found at Yaverland and along the Island's south-west coast.

About 110 million years ago, rising sea levels flooded southern England, and a shallow lagoon formed. With continuing rises, the sea drowned the lagoon, and the marine rocks of the Lower Greensand were deposited. A warm sub-tropical shallow sea now covered the area, and was home to many different types of shellfish, including ammonites and oysters, other creatures included corals, lobsters and lethyosaurs (dolphin-like marine reptile). Sea levels continued to rise, and deep water clays, known as the Gault Clay (known locally as blue-slipper) were deposited. At this point sea levels remained stable, this caused the sea to fill up with sediment, and shallow layers of sand, called the Upper Greensand were deposited, again a rich source of fossils.

Approximately 99 million years ago, sea levels again began to rise, and the Chalk was deposited. Initially the seas were rich in ammonites, but as the sea deepened, the waters cooled, and ammonites became scarce. The sea floor became home to sea urchins and big oyster-like inoceramids. Cold ocean currents from the North Atlantic brought in shoals of the squid-like belemnites, which characterise the youngest parts of the Chalk.

The Cretaceous ended 65 million years ago, but the youngest parts of the Chalk, remaining on the Island date to about 75 million years ago. Cretaceous rocks can be seen in Sandown Bay, St Catherine's Point and along the south-west coast. Locally, the late Cretaceous was marked by small-scale earth movements, which gently tilted the rocks, but with extensive erosion. Globally there was wide spread extinction of groups such as ammonites and dinosaurs.

The Palaeogene 65 - 23 million years ago

In the early Palaeogene, southern England was a low lying land area, on about the same latitude as Portugal, today. However further sea level rises led to a warm shallow sea spreading over southern England into northern France and Germany. Many new types of gastropods (snails) and bivalves (clams) evolved, and began to dominate the sea floor. Over 600 different types of gastropods and bivalves are known from the marine sands and clays of the Palaeogene. The waters were home to many forms of sharks and rays. Most of the life forms of the Palaeogene have living relatives today. On the Island, sands and clays deposited in that sea can be seen at Whitecliff Bay, and include the famous coloured sands at Alum Bay.

About 37 million years ago the area of the Isle of Wight had become a shallow embayment, this rapidly silted-up, and a complex of ponds and swamps formed, within this complex, the muds and limestones of the Solent Group were deposited. These are a rich source of fossil shellfish, turtles, crocodiles and mammal bones. The youngest part of this sequence dates to about 30 million years ago, these rocks can be seen along the Island's north coast.

The Neogene 23 million years ago - today

At some point after 30 million years ago the rock sequence was folded into its present form, comprising a central vertical axis with near horizontal limbs (a monocline). These are the outer ripples of the Alps of central Europe, produced as the African plate collided with the European plate.

About 2 million years ago polar temperatures dropped markedly, and extensive ice sheets formed, and periodically spread south during the Ice Ages (glacials) and retreating during the warm inter-glacials. On the Island gravel terraces and clay and peat deposits record parts of the sequence of events. During these times the Solent was formed as a river valley, carrying melt water from Hampshire, Dorset and the Isle of Wight.

About 10,000 years ago the last glacial ended, and as the ice caps retreated, the sea level began to rise. Low lying areas were flooded, eventually, probably as recent as 7000 years ago the island was formed.