deposition

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Coasts A coast is the edge of the land where it meets the sea or ocean. The coast, or coastline, around Britain is very varied. It includes beaches, cliffs and sand-dunes as well as mudflats, saltmarshes, shore platforms and estuaries. These features are not all found at the same place, however. Each part of the coast is different. If thousands of rivers are flowing into the sea, why doesn't the sea level rise? The answer lies in The Water Cycle. When the energy of the sea or waves is high (with big waves and rough seas), boulders and large pebbles can be moved. When the energy is low (with small waves), only sand can be moved. Waves have the power to erode, or break up, and wear away rocks over time. Storm, or high-energy, waves breaking at the foot of a cliff dislodge blocks of stone - rocks. Waves fling pebbles at the foot of a cliff and wear the cliff away. This process is abrasion. The broken rocks get moved around on the beach and are rounded and worn down smaller. This way pebbles, and eventually sand, are made. The Yorkshire coast is famous for rapid erosion. Here, cliff erosion has caused homes to fall down the cliff and into the sea. In 1993 a hotel collapsed down the cliff at Scarborough! Where the rocks on the coast are hard they resist erosion. In such places cliffs form that stick out into the sea - called a headland (or head, on a map), peninsula, cape or ness. Cliffs are eroded more rapidly in some places than others. Where there is a softer rock or one with more joints (cracks) in it, the sea will erode much quicker, making a cave. Sometimes the cave is eroded right through a narrow headland and reaches the other side (so you can see straight through - like a tunnel). This is a bit like a bridge with an arch of rock over the top so it is called an arch. As erosion continues, the arch of rock will get thinner and weaker until (usually during a storm) it collapses, leaving a large rock standing in the sea - a stack - and a gap where the arch used to be. So over hundreds and thousands of years cliffs can be eroded by the energy of the sea, into rocks, then into pebbles and eventually sand. Sometimes large quantities of sand are deposited by the sea. Some of this can be blown inland by the wind, to form dunes. On a steeply sloping beach - usually pebbles - high water mark and low water mark are close together. On a gently sloping beach (usually a sandy one) high water mark and low water mark are far apart. This often exposes, or uncovers, a wide area of sand and mud. The sea is 'a long way out' at low tide! The difference between water level at high and low tide can sometimes be seen directly on vertical cliffs. On a beach, high tide is usually marked by a strand line. This is a line of debris - seaweed, plastic waste, litter, dead fish, shells, bits of fishing net, etc. - washed up by the in-coming tide and left there as the tide goes out. There is about 12 hours between each high tide. On a steeply sloping beach - usually pebbles - high water mark and low water mark are close together. On a gently sloping beach (usually a sandy one) high water mark and low water mark are far apart. This often exposes, or uncovers, a wide area of sand and mud. The sea is 'a long way out' at low tide! The difference between water level at high and low tide can sometimes be seen directly on vertical cliffs. On some coasts there is mostly deposition (addition) of material. The material is deposited, making sandy beaches, pebble beaches and some beaches that are part sand and part pebbles. Sand often collects, or is deposited, in bays, forming beaches. Beaches protect the bottom, or foot, of cliffs from erosion by the sea - they absorb the force of the waves. On part of the coast of Devon, a shingle beach was removed at the end of the eighteenth century so that the stones could be used for building. This meant the cliff was no longer protected from storm waves. As a result, the village of Hallsands collapsed into the sea in 1917. People build structures like sea walls to protect the coast from natural changes. Sea walls are very expensive to built so people argue about whether to build a sea wall or to let the land get eroded away or flooded. Coastlines are dynamic! They are constantly changing. Sometimes the changes are sudden and dramatic - perhaps the result of a violent storm or a landslide. Beaches usually change more in winter than in summer. This is because the sea is often rougher in winter. It has more energy so it can do more damage. Strong winter waves can move more material than gentler summer waves. These strong waves may pull or drag sand and pebbles out to sea. The next season, gentler summer waves may push the sand back. Wind blowing from the sea to the land - an on-shore wind - may also move the sand farther up the beach, forming dunes. Usually changes are much more gradual - they happen slowly, over hundreds, thousands or millions of years. A long time ago, during the Ice Age, the sea level was lower and land that later became known as Britain was joined to the rest of Europe. The coastline was very different. As the ice melted at the end of the last Ice Age (it started to melt about 20,000 years ago) the sea level rose and some of the land became covered with water. This stretch of water formed what we know as the English Channel and the North Sea. Scientists believe that the world's climate is changing. They think it is getting warmer. If it continues to change as scientists predict, the ice caps at the North and South Poles will melt. Sea levels will rise and coastlines all over the world will change. Many small, low, islands, especially coral reef islands - the Maldives, for example - would disappear. Much of Bangladesh would be flooded permanently. Low-lying land would be flooded, producing a new coastline. In Britain this would affect the coast of east and south-east England the most.