Читать книгу Flowers of the Coast - Ian Hepburn - Страница 13

Salt-marshes and their growth are described in detail in Chapter 5. They show better than any other feature the intimate relation between ecology and physiography in coastal evolution. They are built up of the mud, silt, and fine sand carried by the tidal (and other) currents, and deposited in quiet places. Their nature will depend much on the material of which they are composed; there is a great difference (see here) between the firm mud marshes of Norfolk and the sandy wastes of Morecambe Bay. Marsh growth begins in certain favoured places on the sea floor, especially on that part of it which is bared at low tide. Ideal conditions obtain on the coast of North Norfolk where formations like Blakeney Point, Wells Headland, Scolt Head Island, and the small high-tide island at Thornham provide shelter. Marsh growth takes place outwards from the original coast and inwards from the protecting ridge. Moreover, where structures like Blakeney Point throw off a number of laterals, or recurved ends, the best possible conditions for marsh formation often exist between neighbouring laterals, especially if they are so built as to make a narrow entrance to a relatively broad expanse inside (e.g. the Marram marshes at Blakeney Point). Into these quiet backwaters tidal water pours, and stands quietly for a period before the ebb (Pl. XIa). Sedimentation takes place readily, and since the water has to drain out through the narrow mouths, material deposited round the margins is undisturbed. Hence, a considerable quantity of mud soon gathers. The same thing happens in less enclosed places, but the speed of accumulation is generally less.¹ Between Wells and Blakeney there are extensive marshes, only here and there fringed on their seaward side by a ridge. The larger waves break well out, so that in the quiet inshore water sedimentation can proceed.

The same quiet conditions often prevail in parts of estuaries and other embayments. There is considerable marsh growth in Hamford Water, around Canvey Island and Sheppey in the Thames Estuary, in Chichester Harbour, in Southampton Water and Poole Harbour, on the upper parts of Plymouth Sound and Milford Haven, and in the Bristol Channel. Many other places round our coasts show similar growth. The principles of accumulation are similar everywhere and need not be analysed in further detail.

Reference has been made to beach-drifting, to the movement of dunes, and to tidal and other currents. Let us look generally at Great Britain in relation to wind-systems. The prevalent winds (i.e. those blowing most frequently) in any part of this country are from a westerly direction, usually somewhat south-westerly in England and Wales. On our western shores the dominant winds (i.e. those having greatest power or effect) also blow from the same general direction. On the east coast, however, the dominant winds come from the quarter between north and east. How any wind will affect a particular stretch of beach must depend greatly on local conditions. To take an example: south-westerly winds will have great effect in Mount’s Bay, but not just east of the Lizard Peninsula. Allowing, however, for detail of this kind, the westerlies are responsible in the main for eastward directed beach-drift along the Channel, for that up the Bristol Channel, and for that along the coasts of Cardigan Bay. Another factor is also important—the relation of wind-direction to the amount of open water off a particular coast. On the Cumberland coast, the direction of beach-drift is north and south from approximately St. Bee’s Head. This is in general conformity with the amount of open water off these two parts of the coast. However, the relationship is better seen on the east side of England. Along the Norfolk coast, excluding minor exceptions, the travel of beach material is on the whole westwards from Sheringham along the north coast and south-east and south from that same place along the east coast of the county. The dominant winds and waves approaching the Cromer-Sheringham coast are from the quarter between north and east: these, working in with the extent of open water offshore and with the general trend of the coast, are mainly responsible for the outward drift from that locality. The southward drift of beach material continues, apart from a few minor interruptions, as far as the Thames.

On the whole (except in the inner parts of the Firths of Tay and Forth) beach material travels southwards from north-eastern Aberdeenshire all down the east coast of Great Britain. It is, however, along the more open coast south of Flamborough Head that this is most noticeable. Along the south shore of the Moray Firth and the coast as far as Banff and even Rosehearty the general movement of beach material is to the west, and southwards from Wick it is also directed towards Dornoch Firth and Inverness.

On an indented coast of hard rocks it is difficult to generalise. Each separate bay usually has its own beach, and whatever solid stuff travels round the enclosing headlands does so below water level and cannot easily be traced. The individual coves of Cornwall, Devon, Pembrokeshire, the north coast of Scotland, and elsewhere may have their beaches temporarily removed by storms, but they will gather again in normal times. It is probably true to say that each bay has its own shingle and sand economy. On relatively deep water coasts, such as that of the west of Scotland, it is impossible to generalise about the travel of sand and silt.

The main contrasts we have made between the different parts of the coasts of Great Britain may perhaps be related to an even more general factor. Apart from the Lancashire coast, and excluding local occurrences of boulder clay, it is approximately true to say that a line joining the mouth of the Exe to that of the Tees separates a region of softer rocks and simpler structure to the south and east from a more complicated region of harder rocks to the west and north. The former is associated with long lines of open beach and sweeping curves along which beach and long-shore drifting are well exemplified. The latter is often a coast broken by inlets and hard and rocky lines of cliff, along which lateral movement is irregular.