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A LEVEL GEOGRAPHY ESSAY: DUNES and MARSHESEXAMINE THE ROLE OF VEGETATION IN COASTAL DEVELOPMENT

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Introduction

A LEVEL GEOGRAPHY ESSAY: DUNES and MARSHES EXAMINE THE ROLE OF VEGETATION IN COASTAL DEVELOPMENT IN THEFOLLOWING HABITATS. (a) SAND DUNES (b) SALT-MARSHES (a) Sand Dunes Before considering the role of vegetation In the development of coastal sand dunes, it may be useful to summarise the general conditions which are conducive to dune formation In order to place the biological processes involved in a broader context. Factors which encourage dune formation include prevailing onshore winds blowing across a wide inter tidal zone which allows the top sand to dry out between tides and to be set in motion by the wind. There should also be some feature at the head of the beach to trap the sand being driven onshore by the wind. This might be patches of coarse shingle or drifted debris or existing vegetation just beyond the high water mark. Ideally, there should be an absence of vegetation on those parts of the beach from which sand is being moved, and active colonisation by plants at the head of the beach where dunes are accumulating. Obviously, not all of these conditions win be met in every area of dune formation, but they indicate the optimum conditions for sand dune accumulation and growth. The initial accumulation of sand at the head of a beach is often around a clump of coarse grass, such as sea couch grass (Agropyrum junceum). ...read more.

Middle

The sand thus eroded accumulates down-wind from the dune, where, being fresh, it is usually quickly re-colonised by marram grass and becomes stabilized. Thus, It is common to see large spreads of this grass in thriving condition at the back of old, grey dunes. Good examples are to be seen on the landward side of the main dunes fronting Scolt Head Island on the North Norfolk coast. Dunes may build up to considerable heights. Many contain 15-20m of sand. Dunes higher than this are nearly always found to be piled up on a foundation of solid rock. For example, dunes at Pennard Burrows in the Gower Peninsula reach a height of 60m, but have been built up on sloping ground at the head of the beach. However, whatever their size and complexity, vegetation inevitably plays an important role in the initial formation, subsequent growth and stabilization of coastal dune systems. (b) Salt marshes Salt marshes consist of broad, flat, low-lying expanses of silt and mud deposited by the sea in the sheltered waters of bays and estuaries or on the landward side of spits and bars. Although areas of salt marsh normally support a complex, dendritic system of tidal creeks, and also have small patches of open water on th surface (salt pans), most of their surface is normally covered by a distinctive assemblage of salt-loving plants. Indeed, these plants play a crucial role in both the initial formation and subsequent development of salt marsh areas. ...read more.

Conclusion

At higher levels, sea aster (Aster tripolium) and common marsh grass (Pucinellia maritima ) begin to appear. Higher still, one finds sea pink (Armeria maritima), sand spurrey (Spergularia media) and sea lavender ( limonium humile). At the head of the marsh, around its landward margin, clumps of shrubby sea blite (Suaeda fruticosa) are typical. However, the sequence of plant colonies varies in detail from one area of salt marsh to another. Steers, for example, recorded striking differences between the coastal marshes of the east and west coasts of England and Wales. Two relatively minor features of salt marsh areas should also be mentioned; namely, tidal creeks and salt pans. During the early stages of marsh formation, areas of non- deposition (the original foreshore depressions) are converted into tidal creeks. In this way, the ebb and flow of the tide becomes concentrated along these lines and colonization by plants is prevented by tidal scour. Salt pans are small, roughly-circular pools on the surface of the marsh. The suggestion has been made that the very high salinity of the water contained in these pools inhibits plant colonization and resultant infilling. Finally, mention should be made of the human influence on salt marsh habitats. Once a marsh has built up to such a level that it is seldom covered by tides, It is relatively simple to enclose it with a bank or wall, drain it, and replace the natural vegetation with cultivated pasture. Obviously at that ultimate stage in salt marsh development, the plant cover is almost wholly a result of human interference rather than a response to natural conditions. ...read more.

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