COASTAL ENVIRONMENTS 4 coastal geology sand dunes and salt marshes sea level changes

1. COASTAL ENVIRONMENTS 4 • coastal geology • sand dunes and salt marshes • sea level changes • coral reefs

2. Discordant coastline occurs where bands of differing rock type run perpendicular to the coast. The differing resistance to erosion leads to the formation of headlands and bays. A hard rock type is resistant to erosion and creates a promontory whilst a softer rock type is easily eroded creating a bay. Part of the Dorset coastline running north from the Portland limestone of Durlston Head is a clear example of a discordant coastline. The Portland limestone is resistant to erosion; then to the north there is Swanage Bay where the rock type is a softer greensand. North of Swanage, the chalk outcrop creates the headland which includes Old Harry Rocks.

3. A concordant coastline occurs where the bands of differing rock types run parallel to the coast. The outer hard provides a protective barrier to erosion of the softer rocks further inland. Sometimes the outer hard rock is punctured allowing the sea to erode the softer rocks behind. This creates a cove which is a circular area of water with a relatively narrow entrance way from the sea. Lulworth Cove has occurred on a concordant coastline. The outer hard rock is Portland limestone. The sea has broken through this barrier and easily eroded the clays behind it. A chalk cliff face is at the back of the cove.

4. Many coasts have developed sand dune formations. The source of the sand may be ancient offshore deposits or material eroded from local cliffs. In order for sand dunes to develop, you need an abundant supply of fine sand, a prevailing onshore wind and then something to trap and retain the sand. Seaweed and beach debris may initially trigger a small embryo dune. Once the pioneer plant species such as lyme grass take root, more sand accumulates and the dune grows. Marram grass is the main coloniser and stabiliser of sand more mature yellow and grey dunes. Eventually other plants, which are less salt tolerant will colonise the dunes, leading ultimately to a climatic climax woodland ecosystem. In low-lying areas or slacks, marsh plants such as juncus will colonise. If the dune vegetation cover is broken by human activity or rabbit burrowing, blow-outs may result with large amounts of sand being removed from the damaged dune.

5. In quiet coastal environments such as behind coastal spits, in estuaries, etc sedimentation will be an active process and lead to the formation of salt-marsh. As alluvium is deposited, salt-tolerant plants such as Salicornia begin to colonise the muddy patches which show at low tide. The plants slow the water currents leading to increased deposition until the salt marsh begins to be covered only by the highest spring tides. Plants such as Spartina begin to dominate the vegetation cover. Seawater may be trapped in hollows that are not colonised by plants and as water evaporates, it leaves behind a strong concentration of salt which reduces the chances of plants colonising. Eventually these saltpans become a semi-permanent feature of the salt-marsh. Water drains from the salt marsh along distinctive creeks.

6. Apart from daily and seasonal changes in sea level due to the tides, the average sea level changes long term due to a variety of factors. • During the last ice-age, water was held back from re-entering the oceans as it remained frozen on the land. This eustatic fall in sea level resulted in a drop of between 100- 150 metres compared to present sea level. • At the same time, the great weight of ice that accumulated (many kilometres thick in places) was sufficient to depress parts of the earths crust leading to an isostatic change in sea level. • After the ice age, there was a relatively rapid eustatic rise in sea level as the ice melted and water returned to the oceans. It has, however, already taken at least 10,000 years for the land to very slowly rise again as the weight of the ice was removed. Thus the isostatic readjustment is still going on in areas such as Norway and Scotland (4-20mm per year). • Other causes of sea level change include: • tectonic uplift of new mountain ranges • tectonic tilting of the land • local earthquake and volcanic activity

7. Changes in sea level lead to distinctive coastal scenery and landforms. Submergence of the coastline leads to the drowning of many low-lying coastal areas. Many river estuaries have been formed or enlarged by submergence. Rias are the drowned lower portions of river valleys. Many have only small rivers or streams flowing into large, deep estuaries. These form excellent natural harbours and are a common landform in south-west England. Fjords are the drowned lower portions of glaciated valleys. They are broader and deeper than rias and have very steep sides. They are a common landform along the coast of Norway. A dalmation coastline is formed where the geology creates valleys parallel to the coast so that when sea level rises, a series of elongated islands remain offshore.

8. Coral reefs are among the most diverse and productive communities on Earth. They are found in the warm, clear, shallow waters of tropical oceans worldwide. Reefs are formed from calcium carbonate produced by tiny coral polyps. Reefs have functions ranging from providing food and shelter to fish and invertebrates to protecting the shore from erosion. Revenue from tourists attracted to the beauty of coral reefs can be a significant source of income for human communities in these areas.

9. Corals and coral reefs are extremely sensitive. Slight changes in the reef environment may have detrimental effects on the health of entire coral colonies. These changes may be due to a variety of factors, but they generally fall within two categories: natural disturbances and anthropogenic disturbances. Although natural disturbances may cause severe changes in coral communities, anthropogenic disturbances have been linked to the vast majority of decreases in coral cover and general colony health when coral reefs and humans occur together. Discharge from power plants and sewage works alter the sea temperature and the amount of nutrients in the water. Agriculture, particularly where soil erosion is a problem, decreases the amount of light reaching the coral due to increased sedimentation. Overfishing by humans has altered the food chain. The collection of coral for tourist souvenirs has damaged many reefs.

10. Summary of key points: • discordant coastline occurs where bands of differing rock type run perpendicular to the coast producing headlands and bays. • concordant coastlines occur where band of differing rocks run parallel to the coast producing straight coasts or coves and cliffs. • sand dune coasts occur where there is abundant sand exposed at low tide and a strong onshore prevailing wind. Specialised plants such as marram grass colonise and anchor the dunes • in quiet coastal environments deposition of fine mud leads to the formation of salt marsh colonised by salt tolerant plants. • sea levels change long term due to glaciation (isostatic and eustatic changes) and tectonic activity • coral reefs produce unique maritime habitats which are increasingly in danger from human activity.