Sand on a beach, mud in an estuary and pebbles on a rocky shoreline are all examples of modern sediments being deposited in different coastal environments, but did you know that ancient sediments, sometimes hundreds of millions of years old, are also often evident on modern coastlines, excellently preserved in the cliffs?
Next time you visit the coast, take a look at the cliffs to see what they might reveal about past environments (take care not to get too close to unstable overhanging rocks, and NEVER climb up the cliffs to take a closer look).
When sediments are first deposited they are laid down in flat layers. As they become buried beneath further layers, the sediments become cemented to form hard rocks. If you look in the cliffs you may be able to pick out these layers – hard layers will stick out of the cliff whilst soft layers will form notches because they are less resistant to the erosive forces of the wind and waves. Sometimes these layers may be tilted, or even crumpled and folded – the result of intense pressure during later earth movements.
If you can take a closer look at the rocks that make up these different layers, you should be able to notice that they are not all the same. What colour are they? What do they contain?
Some rocks are grey and muddy, possibly with the remains of ancient life preserved as shelly fossils. These might have been formed in an ancient sea, or coastal estuary, far away from the disturbing influence of waves and tides.
Along the South Coast, white chalky rocks, sometimes containing layers of flint, the material fashioned as arrow heads and axes by prehistoric people, are actually formed from millions of minute skeletons of marine plankton which, upon death, sank to the bottom of an ancient ocean.
Grey limestone rocks may preserve corals and other shelly fossils – the remains of creatures found today on reefs in a far more tropical climate. Other rocks may have much coarser grains that resemble sand or even pebbles. These may have formed along ancient coastlines, on beaches, sand dunes and in rivers, in much the same way as they are today.
If they are red in colour, such as along the Devon coastline, they may have been deposited in an ancient desert. The red colour is due to the presence of iron that has gone ‘rusty’ as a result of lengthy exposure to oxygen in the atmosphere.
Some cliffs, such as those along the Norfolk coast, may be soft and crumbly, spilling large amounts of mud and debris onto the beach. These sediments were formed by the grinding action of rocks at the base of large glaciers and were deposited at the end of the last Ice Age as the glaciers melted.
If you look carefully amongst the muddy debris, you may even be able to find small pieces of broken bones and teeth, the remains of woolly rhinos, mammoths and other mammals that roamed the frozen edges of this ancient icy landscape.
This article was originally published Summer 2005 as part of the Coast Postcards pack