Читать книгу Flowers of the Coast - Ian Hepburn - Страница 12

Shingle ridges of various kinds have been discussed at some length because they form the “skeleton” combining the “flesh” of dunes and salt-marsh. By no means all dunes are built on shingle ridges, and many ridges have no dunes. But if a shingle ridge is being formed in a locality where an expanse of sand is bared at low tide, the wind blowing over the sand will carry much of it on to the ridge and deposit some of it in its interior and some on its surface. This may cause dune growth. But (see Chapter 7) the real dune-builders are the sea couch-grass (Agropyron junceiforme), marram-grass (Ammophila arenaria), and sea lyme-grass (Elymus arenarius) that take root in the shingle, send up shoots, and so begin to trap the sand. Marram-grass is remarkable in this way, thriving best where the sand supply is most prolific (Pl. III). If another shingle ridge is built in front of the old one, the same process will begin on that, and the dunes on the inner ridge will be, at any rate partially, deprived of their sand supply. The dunes may remain, but are often partly blown away, whereas those on the newer ridge increase. Under favourable circumstances they grow and become permanent features. In England dunes seldom exceed fifty or sixty feet in height if built up from sea-level.

The evolution of coastal dunes depends largely on the available sand supply, the vegetation, the prevalent winds, and the nature of the foundation on which they rest. With a constant wind and sand supply they may attain large sizes, but the vegetation rarely forms a close covering, and there is always the chance of minor hollows being enlarged. This is especially the case in older dunes, somewhat cut off from their sand supply. Often these are riddled with blow-outs (Pl. XXI) so that mere fragments of the original sand-dune chain remain. On some dunes the wind may enlarge a hollow, but new sand may sometimes replace that blown away.

Groups of coastal dunes are often arranged in lines, despite their superficial irregularity, and the lines correspond with the trends of old shingle ridges. This is shown at Blakeney Point, Scolt Head Island, and Morfa Harlech. But on Morfa Harlech and even more on Morfa Dyffryn this relatively simple arrangement is often confused by later movement. In both places there are numerous blow-outs, and the inner dunes have locally been set in motion again, so that they are advancing over the low ground on their landward side. On the other hand, at Culbin the major dunes covered groups of ridges and showed no relationship to their trends. The same may be true at Forvie, but unfortunately no detail is known of the surface upon which the dunes rest. Sometimes a line of coastal dunes advances downwind more quickly in its middle and higher parts, than near its lower ends. This is because it is easier for the plants to hold the ends than it is for them to restrain the sand on the higher parts once it has begun to blow again. A dune-line that has advanced in this particular way, when well-developed, is called a parabolic dune. The best examples in Great Britain are at Maviston (really a part of the Culbin Sands) where the dunes have advanced over and buried well-grown forest trees (Pl. VI).

Every possible transition can be found around our dune shores from the tiny heap of sand just beginning to gather round a tuft of grass, or a small obstruction on a beach, through the elementary stage of a foredune based on a shingle ridge, to the more evolved forms on the older ridges which have not been deprived of their sand supply. If the sand supply is great and it blows along or oblique to the shingle ridges, rather than at right angles to them, there is no particular reason for the dunes to have any correlation in trend with the ridges, and they may grow to considerable heights. When the dunes have no relation to the ridges it is possible that there has been a change of direction of the prevalent winds. The old shingle ridges at Culbin are part of a raised beach system; the dunes are the result of conditions extending back only two or three centuries.

When dunes are deprived of a sand supply they take on a dead appearance. The wind may regain mastery, and most of the sand may be blown away, leaving hummocks behind, known as remanié dunes. If the sand blown away begins to accumulate, as well it may, quite near at hand, the observer may be confused by the contiguity of old and new dunes; the new ones in this case may sometimes be to landward of the old. What is more, the new dunes will almost certainly have a covering of fresh-looking marram-grass, although the sand is derived from the old ones! It is essential to keep in mind this liability to rapid change in dunes; a return to a familiar place after an interval of five or ten years may easily result in the visitor being temporarily lost!

Sometimes the sand blown by strong winds is spread out as a layer over considerable areas. To some extent this happens in any dunes; the sandy pasture inside Morfa Harlech is an instance. In the Western Isles of Scotland there is a special development known as the machair (see here). True, it is often associated with prominent lines of dune, but the machair proper usually lies landward of dunes and shingle bars, and forms, as in South Uist and Tiree where it is particularly well developed, a low sandy plain between sea and peat and hills. It is primarily shell-sand and provides a most fertile sward. In high summer the numberless plants and flowers, the strong scent, the vivid colours, the blue sea and white beach, and the wide open views and great expanse of sky make the machair the centre of a most beautiful landscape.