Differences between areas
The median grain size of the aeolian material in the coastal dune fields of Finland varies between 0.15 and 0.56 mm. This material is usually well or very well sorted. The coarsest and most poorly sorted aeolian material is found in the hummock dunes surrounded by deflation surfaces, which are covered with coarse grains, and from the transgressive dunes which have accumulated on the distal sides of these surfaces. The earlier literature (e.g. Uusinoka 1984: 19) mentions that the grain-size distribution of dune sand is often skewed towards the small fractions, but this is not a general rule, as the grain-size parameters of aeolian deposits vary from place to place in accordance with the characteristics of the source material (cf. Carter 1988: 308).
On the grounds of the grain-size of the material, the steepness of the coastal profile, the coastal sediment budget and soil moisture, the ecological and geomorphological succession of the Finnish coastal dune fields can be divided into the following types:
1. The material is silt or very fine sand and the coastal profile is very gentle. Successive parallel dune ridges alternate with damp depressions or damp, grazed, flat deflation surfaces. The older dune ridges, i.e. intermediate dunes, have been lowered or broken down into separate hummocks. The dune fields of Hailuoto, the Tauvo Peninsula, Letto at Kalajoki and the Karhuluoto area at Yyteri belong to this type, and the successive beach ridges at Yrjänä south of Tauvo do not differ much from it, other than in the fact that the depressions separating the ridges are filled with water, which limits the aeolian processes. On all these beaches the sediment budget is positive.
Agrostis stolonifera is frequent on damp coastal sand plains, while landward of the foredunes bound by lyme-grass, the most common grasses are Festuca rubra and Deschampsia flexuosa. Festuca ovina grows on the grazed deflation surfaces. In addition to Empetrum, both Juncus balticus and Salix repens are common on the damp surfaces, which are also rapidly covered by mosses.
2. The coastal profile is quite gentle, but the sand surface behind the foredunes is so far above sea level that no extensive damp surfaces can occur. The material of these beaches varies from silt to coarse sand. The beach is bordered by a foredune consisting of only one ridge or several successive or partly overlapping ridges. If the coastal dune meadows were grazed earlier, there is a broad, dry sand flat on the distal side of the modern foredune. Deflation bowls may occur on ungrazed beaches, as at Yyteri.
If the coastal sediment budget is clearly positive and the beach is rapidly prograding seaward, the foredunes will be successive parallel ridges. If the sediment budget is only moderate and the shoreline remains in the same place for a long time, then the beach will be bordered by only one high foredune. If the coastal sediment budget is almost in balance or negative, the windward slope of the foredune will be steepened by marine erosion. With a negative sediment budget the foredune is covered with blowover layers and a flora which indicates that leaching of the soil has been going on for a long time. In Kalajoki the coastal profile has been steepened during this century as a consequence of the rapid rate of land uplift. In accordance with this, coasts on which the grain-size and steepness are most favourable for aeolian processes (not so gentle that the soil would be damp nor too coarse-grained and steep) can be divided into the types (Fig. 1) proposed by Carter (1990), but seaward dipping beds occur usually only on the seaward slopes and landward dipping beds dominate.
On these coasts the Honkenya communities are followed by Leymus dunes and then by dune meadows with mosses and lichens. The most frequent grass is Deschampsia flexuosa, while Festuca ovina grows on grazed areas. As the soil becomes more acid, a meadow can change into an Empetrum heath.
3. The coastal profile is steep and the material is quite coarse. The foredune is often replaced by a beach ridge or berm deposited by the combined action of winds and waves. Landward from this, a dry deflation surface is dotted by hummock dunes. Hyypiänhiekat at Virolahti, Lappohja and Kolaviken on the Hanko Peninsula, Padva and Koppana belong to this type. These beaches have only a limited amount of material suitable for aeolian transport.
The shore face is often covered by a thick carpet of Honkenya. The beach ridge is bound by Leymus arenarius, and also by Elymus repens and other grasses in the south. Festuca ovina predominates on the dry deflation surface, but on the Hanko Peninsula it is replaced by F. polesica. The sand surfaces feature patches of lichens and mosses, Arctostaphylos uva-ursi and Empetrum hummocks.
4. On rocky coasts aeolian accumulation is usually restricted only to the formation of isolated dune hummocks or one foredune. Festuca rubra and Juncus balticus grow on the shores of Pietarsaari. The tombolos of the Hanko Peninsula are covered by flowering, lichen-rich meadows.