The Orientation of Dunes in Britain and Denmark in Relation to Wind

There are two main types. First, those which develop singly, or in parallel rows, parallel also to the coastline, as for instance, at Southport (Fig. 6) or in Aberdeen shire (Fig. 4b, c). They are usually not more than a few feet high and several yards apart, and it is to these that botanical workers have related primary vegetation series. Their orientation does not result primarily from the action of and their disposition is consequently not further discussed here. Dunes of the second type develop as masses whose orientation cannot usually be correlated with the coastline, though occasionally, as at Braunton Burrows, it may coincide. In some regions the long axis of the major sand masses appears to be parallel to the prevailing wind, for instance at the Sands of Barry (Fig. 5), and Burghead Bay (Fig. 3); in others apparently at right-angles as at Forvie (Fig. 4a). In many areas a confused pattern may develop?the aufgelosten Dunenlandschaft of continental workers?where no markedly elongated axis is obvious, e.g. Southport or Hansted dunes (Fig. 10a). All these dunes, for our purpose, can be considered as variations of the U-dune type, on scales ranging from several yards to several hundred yards, and which may occur singly, or in laterally connected bands, or may overlap neighbouring dunes. They may also pass through various stages in a repeated cycle of mobility, stability and erosion. In the first phase (Fig. 1, phase 1) the U shape may be hardly visible in the field, the dune having smooth curves, very short or absent upwind arms, and no vegeta? tion, or only sparse marram. When moving relatively fast, the leeward slope may be a slip face 1 and in this respect resembles the desert barchan. Under desert conditions, however, the wind along the base of the dune is unimpeded. The diversion of the oncoming sand round the lower flanks concentrates its stream and tends to make the flanks advance faster than the central high mass. In a wetter climate on the other hand, vegetation is more easily established at the base of the dune near the water table; this impedes the and causes a decrease in sand movement round the flanks, which therefore tend to lag behind the central mass. The dampness of the sand at base level will also impede its movement. This phase is the most mobile, and such a dune can move several miles, leaving trailing parallel flanks, sometimes linked by small crossbars (see Fig. 1), behind it, and showing its path of movement, as deduced by Paul2 for the Kurische Nehrung. Dunes in this phase are to be found at Forvie (Fig. 4a, waves 4-6), Raabjerg Mile near Skagen, and before afforestation, at Culbin

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