125 million years ago

Long before man appeared on the planet, the south east of England was a low-lying landscape covered by a large shallow freshwater lake with several rivers flowing into it. These rivers carried vast amounts of clay or mud, which started to build up in layers on the bed of the lake. Eventually the clays reached a thickness of nearly 200 metres, forming the first layer known as the Weald Clay.

110 million years ago – the sea breaks through

The land continued to sink until eventually the ocean broke in laying down massive layers of sand known as Lower Greensand. The sea gradually deepened and the waters became still. Under these conditions, a thick dark mud collected known as Gault Clay. After this period, there were strong underwater currents in the sea and the sandier sediments of the Upper Greensand were deposited.

Flint is often used in walls and buildings

97 million years ago – chalk and flint

It was during this time that the sea began to lay down the chalk of the South Downs as we know it today. Chalk is a white soft limestone, which has been formed from the skeletons of marine creatures deposited, squeezed and eventually fossilised on the sea-bed. This process continued for 20 million years and to a thickness of more than 300 metres . The white, pure limestone of the Chalk is superbly exposed at the great chalk cliffs of the Seven Sisters between the River Cuckmere and Beachy Head near Eastbourne.

The chalk contains flint which is the only hard rock to be found on the Downs. If you look at the great chalk cliffs of the Seven Sisters near Eastbourne you will be able to see narrow, dark grey bands of flint at regular intervals of a metre or so, up the cliff face. Flint was formed from the skeletons of minute animals, such as radiolarians, that floated around in those ancient seas.

After a field has been freshly ploughed, it looks like a stony wasteland as you can see how the flints have been unearthed and risen to the surface. Early people on the Downs found that they could use the razor-sharp edges of flaked flint as a cutting tool. Some of the best preserved flint mines can be found on the South Downs, especially at Cissbury, above Worthing. You can also see displays on flint working at Worthing Museum.

65 million years ago

It was at the beginning of this period that the ocean floor began to rise, the sea became shallower and the formation of chalk stopped. Deposits of clays, pebbles and sands were laid down.

20 million years – The Alpine Storm

During this time the South Downs were raised from the seabed, through the movement of the earth’s crust. The land masses or ‘tectonic plates’ of Africa and Europe moved towards each other and collided. The rocks were pushed up and created mountain ranges, including the Himalayas and the Alps. The south east of England was caught up in this ‘Alpine Storm’ and the ripples pushed the layers of rock upwards forming a vast extended dome of chalk (See Figure 3). The neat layers of sands, clays and chalk, laid down over millions of years in fresh and salt water gradually hardened into rock.

Over millions of years, the landscape has gradually changed shape to form the South Downs as we know it today. The centre of the dome has been eroded. The soft chalk at the top of the dome gradually cracked and crumbled and the falling rain carried off these shattered pieces of chalk. This left an outer upstanding rim of chalk surrounding a lowland plain formed from older layers of clay and sandstone. The outer rim of chalk forms the uplands of the North and South Downs and the central plain is known as the Weald.

2 Million years ago - The Ice ages

Although the South East of England was not covered in ice, an intensely cold climate dominated this area. This meant that the rock and soil was frozen for most of the year. Summer rain and melt-water could not soak into the frozen chalk. So this water formed streams which carved out valleys on the Downs. The rapidly melting snow during the last ice age also carried rock and soil from the hillsides on to the floors of the valleys.

The dry valley at Devil's Dyke near Brighton

When the climate became warmer, the frozen ground eventually thawed and the water soaked into the little holes in the chalk, leaving the valleys dry. These dry valleys, known in Sussex as coombes, are V-shaped with steep sides. The Devil’s Dyke, to the north of Brighton, is the most famous of all the chalk dry valleys. Patches of Clay-with-flints can be found in places on top of the Downs. This is the remains of some clay that was once on top of the chalk that got mixed up with flints from the chalk.