The Isle of Wight is made up of layers of sedimentary rocks. These rocks are formed from sand and mud laid down by rivers and in the sea, between 126 and 30 million years ago. You can tell how old these rocks are from the fossils found in them.
The Island’s chines cut through these rocks revealing the layers of history written through the Island. This helps us to explore and understand the Island’s geological heritage. The Island’s geology is on show for anyone to see in the chines and along the cliffs.
The Cretaceous period: age of dinosaurs and tropical seas
The Isle of Wight has some of the most complete series of Cretaceous rocks in Britain, these range from about 126 to 72 million years ago. They form the southern part of the Island – south of a line between Alum Bay and Whitecliff Bay. From about 126 to 120 million years ago dinosaurs like iguanadons and sauropods roamed the land, and their footprints can still be seen today around the chines. Towards the latter part of the Cretaceous the Island was flooded and its tropical seas were home to ammonites, sharks and large marine reptiles. The Island has become known as one of the best sites in Europe for the wealth of fossils of these creatures that are found here.
Different rock layers or strata are grouped into named units. The muds and shales of the Wealden group were laid down on the land and in rivers. When the sea invaded the land the sandstones and muds of the Lower Greensand and Gault were deposited in a progressively deepening sea. The Upper Greensand and Grey Chalk were deposited later as the sea deepened again. The sea then became very deep, perhaps as much as 350 metres in places, and the White Chalk was laid down from the microscopic remains of marine plants.
The Palaeogene period: silty lagoons and shifting seas
The Island’s geological record for the end of the Cretaceous Period was eroded away in the early part of the following period, the Paleogene. Many groups of plants and animals became extinct around this time, including ammonites and dinosaurs.
The oldest Palaeogene rocks on the Island are red muds, thought to have been laid down on land around 55 million years ago. As time passed, warm seas invaded again, but they remained shallow. At this time the area now known as the Isle of Wight was believed to be a shallow bay. This bay began to silt up, becoming narrower and shallower until it consisted of low lying lagoons in which the Solent Group muds, sands and limestones were deposited. The sea briefly invaded the area several times but by about 30 million years ago the bay had become land. No rocks have been found on the Island dating from between thirty million and one million years ago.
The rocks of the Palaeogene era form the northern part of the Island. To explore the Island’s Palaeogene era you could visit much of the northern coast, good access points are at Alum Bay Chine and Colwell Bay Chine.
The Quaternary period: The Island is formed
The Quaternary is the name given to the last 2.6 million years of time. It has been characterised by frequent, and sometimes rapid, changes in global temperature and hence sea levels. During these times straight-tusked elephant, woolly rhino, cave bears,wolves, bison and hippo roamed the land where we live today.We know about the Quaternary not from consolidated rocks but from scattered layers of gravel, peat and mud found around the Island. Some contain the tools of our ancient human ancestors. Plant remains in the form of tough seeds and logs are common.If you visit the chines of the south west of the Island, particularly Shippards Chine or Churchill Chine you will see that the cliffs are capped by gravel deposits. These once formed the beds of ancient rivers which flowed to the sea further away and lower than today.Around 6,000 BC the Solent was flooded by rising seas following the last ice age.This separated the Isle of Wight from the mainland, and also created the conditions necessary for the current system of chines to form.