Transcript Slide 1
COASTAL ENVIRONMENTS 2 • erosion landforms • depositional landforms In the UK, most coastlines are being eroded. The rate of erosion is very slow in areas such as the granite cliffs of Cornwall but is very rapid in areas such as the glacial till of Holderness on the east coast. Fortunately, the highest energy waves from the Atlantic Ocean usually strike the more resistant igneous and metamorphic rocks of western and northern Britain. The less resistant sedimentary rocks of southern and eastern Britain receive more low energy waves. High rates of coastal erosion can result in cliff recession of 10 – 20 metres per year. Headlands and bays are characteristic of areas where the geology consists of alternating bands of hard and soft rock. The least resistant rocks are eroded into bays and the more resistant rocks form headlands. Due to wave refraction, irregular coastlines are a temporary feature and there is a cycle which is repeated over thousands of years from straight coastline to irregular coastline and back again. Image adapted from: http://www.geosci.unc.edu/faculty/glazner/Images/Coastlines/BeachProcesses/Headland.jpg Many coastlines are backed by cliffs. The cliffs vary in height, steepness and shape according to the geology of the area. In general, cliffs increase in height over time as the coast is eroded back. Chalk rocks tend to form vertical cliffs sometimes of great height (up to 50 metres). The lower part of the chalk cliffs is relatively easily eroded and regular collapse of the well-jointed rocks keeps the steep profile. Some well-jointed but more resistant limestone rocks develop overhangs. If the rocks occur in layers, the dip of the rock also helps control steepness and form. Beds dipping seaward can produce overhangs whilst beds dipping landward tend to produce more sloping cliffs. As cliffs are eroded at the base, a wave-cut notch forms, eventually the cliff collapses and the cliff retreats. It leaves behind at its base, a wave-cut platform. This may be bare rock or more usually, it is masked by layers of sand and shingle. In places, the platform may have depressions which form rock pools at low tide. The wave-cut platforms have gently sloping angles of around 4 degrees and continue to be eroded (if exposed) by abrasion and corrosion processes. The width of the platform is determined by: • the rate of retreat of the cliffs • the length of time that the sea has been at its present level • the tidal range Since an increase in platform width means that more wave energy is absorbed as the wave approaches the cliff, there is a theoretical width limit of about 0.5 kilometres. Photo source: http://content.answers.com/main/content/wp/en-commons/thumb/a/a9/500px-Wavecut_platform_southerndown_pano.jpg Along irregular coastlines, joints and other weaknesses in headlands tend to be exploited by the waves to form caves of various shapes and sizes. These caves may eventually be eroded back through the headland to form an arch. Continued erosion and weathering will eventually weaken the arch so that it collapses to leave behind a pillar of rock or stack. Ultimately the stack will be reduced to a stump and eventually disappear. Sometimes the sea cuts inland along a joint or fault line to form a narrow, steep-sided inlet called a geo. In well jointed rocks, the sea may erode vertically at the back of caves to form a blowhole. Photo source: http://www.habs-monmouth.org/userImages/hmsg/H0511_OldHarry01.jpg Deposition along a coast occurs in areas of low-energy waves where the input of material exceeds the output. Beaches and other depositional landforms represent a store within the marine system. Beaches tend to be composed of either sand or shingle but sometimes both occur at different places along the beach profile. Shingle beaches tend to be steeper than sandy beaches. At the upper end of the beach, storm ridges occur. As these are often beyond the reach of waves except in the highest tides, they may become colonised by salt-tolerant plants. Lower down the beach, there are ridges or berms which correspond to successively lower tides. The profile of a beach is constantly changing but will show the greatest contrast between spring and neap tides and between summer and winter seasons. Photo source: http://www.soton.ac.uk/~imw/jpg-Worbarrow/3WB-south-cusp.jpg When longshore drift transports material along the coast, it sometimes comes across an estuary or a change in the direction of the coastline. In either case, the transport process tends to carry on moving the material in the same direction. Over time, a ridge of material will build up into the deeper water. This will form a spit. Eventually the spit may form a substantial feature, many miles long with sand dunes and plants, possibly even buildings on it. Many spits show a series of recurved ends which relate to material moved by winds creating waves from a different direction. In the low-energy environment behind the spit, deposition of fine silt and mud will occur and salt marsh forms. This helps to stabilise the landform. Over time, spits may move inland and many are regularly broken by storms and reform in subsequent years. Image adapted from: http://piru.alexandria.ucsb.edu/collections/geography3b/p-s/ps17-17.jpg If longshore drift carries material across a bay, it may form a bay barrier or bar. Such bars rarely form across estuaries as the power of the river erodes material from the ends of the spit if the gap becomes very narrow. If longshore drift transports material across to a nearby island, then it forms a tombolo linking the island to the mainland. Barrier islands are common depositional landforms along many coast such as the east coast of the USA. They are created from a series of sandy islands totally detached from, but running parallel to, the mainland. Summary of key points: • in Britain rapid rates of erosion occur in the less resistant sedimentary rocks of the south and east • headlands and bays are characteristic of areas where the geology consists of alternating bands of hard and soft rock • many coastlines are backed by cliffs. The cliffs vary in height, steepness and shape according to the geology of the area. • as cliffs are eroded at the base, a wave-cut notch forms, eventually the cliff collapses and retreats. It leaves behind at its base, a wave-cut platform. • in headlands, lines of weakness are exploited to form caves, arches, stacks and stumps. Geos and blowholes may also form • deposition creates beaches with varying profiles, spits jutting out into estuaries, tombolos joining islands to the mainland, bars and barrier islands.