FOLLOWING on from Thurrock Local History Society's AGM last month, Ian Mercer, a retired geologist and teacher gave members a fascinating talk on Essex Rocks.

His powerpoint presentation showed various maps and charts and his display of artefacts told of the journey Essex has made from being in the South Pole, arriving to where we are today over millions of years.

Studying geology gives us clues as to how the landscape evolved, such as why a hill or valley is there and the type and relationship of rocks. Seabeds also give us clues about the past, including fossils. Our Essex landscape now looks like a series of steps, called heaths. In the past volcanoes happened, deserts and jungles evolved, also ice age rivers.

500 million years ago Essex was a shallow sea with sediments, near the South Pole. Continental drift shifts land a long way around the planet; pieces broke off land masses, forming Australia etc. We were part of Avolovia. Fifty million years later we are now mid ocean, with no plant life on land. Avolovia slowly drifted north, nearing the equator. Then we crunched into the Laurentia continent. Mountain chains formed, sands and river mud in Essex being squeezed to form hard rocks, e.g. slate – there is no coal, gas or oil under Essex.

The hard rock gradually wore down, but we still headed north and crossed the equator over 300 million years ago. Tropical jungle was flanked by a coal forest swamp. Essex became part of a desert, with still no plants. Storms spread rocks and sediment across plains, the ancient slate later stayed. After another 100 million years warm seas spread and Essex was part of a large tropical island infested with dinosaurs; by then there were pine trees and palms. The sea came and the remains were swept away, so Essex has no dinosaur fossils. Coming forward another 100 million years and the sea flooded and hardened to powdery limescale (chalk), containing fossils. This provided silica, later quartz, and Purfleet still shows bands of silica and flint. The land tilted and millions of flints were eroded out, forming gravel layers across Essex. Fifty-six million years ago there was extreme global warming, affected by ground water, and flint pebbles crystalised into pebbles.

After a few million years muddy rivers deposited clay, when we were still tropical. There is London clay in most of Essex and you can find things like sharks' teeth and remains of plants. New volcanoes spewed out vast amounts of volcanic ash. Later still fine sand (bagshot) was spread across Essex and lands continued to rise. A 38km thick crust protects us and we even have sarsen and pudding stones in Essex.

We are in the middle of a long ice age, when climate oscillates, caused by continental drift, with 100,000 years per oscillation, between warm and very cold, ocean currents cooling the planet. Glacial rock debris was brought by the ice sheet. Mammoths roamed and there were layers of dust deposits. The glaciers brought mashed up rocks etc., called glacial till, including topsoil for growing plants.

When warming took place floods caused gravels to spread, forming a 'staircase' across Essex. At that time the Thames ran through north Essex. There are still remnants of riverbeds at the top of hills, Langdon Hills being an example. Essex people left tools etc. behind. Coombe rocks can be found in the Tesco warehouse pit at Purfleet with flint fragments from 300,000 years ago.

Geology is still going on, for example the 1894 and 1931 earthquakes, with land slipping into the sea. We continue to alter the landscape, using clay, cement and gravel for building. Thanks to HS1 we have a complete record of Essex geology: slate, sandstone, gault clay, chalk, Thanet sand and London Clay. Ian Mercer's lecture opened our eyes to what is around us, shaped over millions of years.

The Society's next meeting (the last until September) is at 8pm on Friday, 17 May at St John's Church Hall, Victoria Avenue, Grays, when Georgina Phillips' talk will be on 'Victorian Escapades in Southend'. Visitors are welcome.