A third of a kilometre beneath the Essex landscape lies a worn-down, buried land of hard rock more than 400 million years old. The mud that was compressed to form this hard rock was laid down as fine sediment around a volcanic island. The island was on a sliver of continent drifting away from its large continental parent near the south pole. Continental drift has brought that rock a long way north.
Deserts to Dinosaurs
For a very long time (and before the age of the dinosaurs) the ancient rocks, of Silurian and Devonian age, formed the surface of the land that was eventually to become Essex. The continental sliver upon which they rode drifted northwards. During Carboniferous times, when Essex was on the equator, tropical forests grew and the trees that fell into the swamps around Essex became coal; but no coal is preserved in Essex itself. During Permian and Triassic times, between 300 million to 200 million years ago, Essex was just a small part of a desert upland in the middle of a vast continent known as Pangea. Around 200 million years ago, at the start of Jurassic times, warm seas were spreading around this land, a dinosaur-infested island with pine trees and palms.
Buried Island
Now, if you could dig down 1000 feet (300 metres) under Essex you would reach the hard rocks of that dinosaur island. All trace of forests and animals from this time have been swept away by the sea from the eroded surface of the island, so there are no dinosaur fossils in Essex.
By 100 million years ago, in Cretaceous times, the sea flooded right across the island to spread dark mud and sand called Gault Clay and Greensand. The sea then deepened to deposit hundreds of metres of Chalk, a white limestone. Chalk contains lumps of hard mineral called flint which formed in its seabed. The soft white chalk covered over the island as well as much of what is now the area of Britain and Europe.
Pebbles and Clay
The North Atlantic Ocean, which did not previously exist, was starting to open out to the west of Britain around 60 million years ago. As this happened, the continental edges on either side of the new ocean rift lifted up. So the land including Essex gradually tilted up from the west. The layers of chalk were worn down and countless flints were eroded out of it. Billions of these flints were tumbled on beaches to form beautifully-rounded pebbles and layers of sand across our area. During a spell of extreme global warming, some of these sands and gravels were cemented by hard silica to form very tough rocks called puddingstone and sarsen.
Around 50 million years ago, in Eocene times, muddy rivers spread thick London Clay across the sea floor of southern England. London Clay contains evidence of a tropical climate, with fossil remains of many plants such as magnolia and cinnamon, and animals including tiny horses, birds, sharks, turtles, crocs and sharks. Where the Atlantic Ocean was opening out to the west huge volcanoes erupted. They poured their ash over the seas and lands including Essex.
The Alps and the Thames
During this time – and ever since – the continent of Africa has been moving north towards Europe and pushing up the Alps. This movement even affects south and mid-Essex. The Earth’s crust has become compressed from the south. Together with a slow lifting of Essex from the north west, the push from the south left Essex in a broad vee-shaped sag, known as the London Basin. Sand was spread offshore in tidal shell banks now seen as beds of shelly Red Crag, best viewed at Walton on the Naze.
Global cooling led to the Pleistocene ice age. Over the past 2.5 million years there have been many alternating warm and very cold periods; right now we are in a warm period known as the Holocene. Through this ice age, with its swings in climate and the slow tilting of the land from the north west, the course of the ancient Thames has repeatedly changed. A series of ancient riverbeds has been left behind as the river was shifted across the land, leaving a ‘staircase’ of flinty gravel and sand terraces. The river eventually flowed across where Harlow, Chelmsford and Colchester now lie, and out across Doggerland to join the Rhine where the North Sea is now.
Ice and people cover Essex
During an exceptionally icy stage 450,000 years ago a gigantic ice sheet covered most of Britain and Essex as far south as Hornchurch. The moving ice diverted the Thames towards its present-day course and dumped its load of glacial till (‘boulder clay’), on top of the old Thames gravels. Rivers now drain downslope across Essex from the north west and along the axis of the basin from the west.
Through the numerous cold and warm stages of this Ice Age, humans have migrated to and from Essex together with the animals they hunted. They have left thousands of flint tools and tool-making debris on the banks of the ever-changing Thames and its tributaries. In south Essex we have the best environmental and archaeological record in Europe of the last half a million years of cold and warm episodes. Such a record in the rocks provides clues to the prospects of our future climate.