TYPES OF COASTS

Coastal regions may be either submerged or uplifted by changes in land or sea levels. Thus, coasts are either submerged or emerged types. When a highland coast is submerged the lower parts of its river valleys become flooded. These submerged parts of the valley are called rias. Rias are common in S.W. England, S.W. Ireland, and N.W. Spain. Due to submergence the coast becomes in­dented and the tips of the headlands may be turned into islands. Longitudinal coasts are formed when a highland coast whose valleys are parallel to the coast is submerged. Some of the valleys are flooded and the separating moun­tain ranges become chains of islands. These valleys are sometimes called sounds. This type of coast occurs in Yugoslavia and along parts of the Pacific coasts of North and South America.

When glaciated highland coasts become submerged, the flooded lower parts of the valleys are called fiords. During glaciation the river valleys become widened and deepened. After the glaciers have disappeared and the sea has risen, the steep-sided valleys are drowned. The water inside the fiord is much deeper than it is at the entrance of the fiord. Fiords have steeper sides and deeper water than rias. All the fiord coasts lie in the belt of prevailing westerly winds and are on the western sides of land masses. It was in these regions that vast amounts of ice accumulated in the Ice Age. Some of the best examples of fiord coasts lie in Chile, South Island of New Zealand, Greenland, Norway and British Columbia. Both rias and fiords often provide good natural harbours.

A rise in a sea level along a lowland coast causes the sea to penetrate inland along the river valleys. The flooded parts of the valleys are called estuaries. When a part of the continental shelf emerges from the sea it forms a coastal plain. Such emerged lowland coasts have no bays or headlands and deposition takes place in the shallow water offshore, producing off-shore bars, lagoons, spits and beaches. The development of ports is difficult in such areas.

There are some coasts marked by coral formation. Coral is a limestone rock made up of the skeletons of tiny marine organisms called coral polyps. Polyps form below' the level of low tide as they cannot grow outside water. They
. thrive in sunlit, clear salt water down to a depth of about 55 metres in sea temperatures of about 21°C. Extensive coral formations develop between 300N and 300S, especially on the eastern sides of land masses where warm currents flow near to the coasts. Coral masses are often called reefs, of which there are three kinds. A fringing reef is a narrow coral platform separated from the coast by a lagoon which may disappear at low water. The surface of the platform is usually flat or slightly concave and its outer edge drops away steeply to the surrounding sea floor. A barrier reef is a wide coral platform separated from the coast by a wide, deep lagoon. The Great Barrier Reef off the east coast of Australia is famous. Barrier reefs also occur around islands forming a continuous ring of coral. An atoll is a circular or horseshoe shaped coral reef, enclosing a lagoon within it.