Shell gravel deposits on the Åland Islands, southwestern Finland, with special reference to the molluscan assemblages

Twenty-six shell gravel sites at elevations between 3 and 35 m a.s.l. were found on the Aland Islands. The faunal composition was determined on 14 of these locations and Mytilus edulis and Macoma balthica were found everywhere. Another common species was Hydrobia ulvae , while H. ventrosa , Littorina saxatilis and Cerastoderma glaucum were more sporadic. This species assemblage implies that the salinity of the area was about 10‰ when the animals lived there. The deposits on the N-NE hilltops generally have southerly to westerly aspects, while those of the south-central hilltops are easterly. On the basis of a 14 C-dated stratified deposit it is suggested that each deposit was formed during a relatively short period of time. As a hilltop emerged from the sea, glacial debris was flushed down. Shell-bearing molluscs colonized the bare rock surface and were flushed down and formed the shell gravel deposit as the shoreline displacement continued. At a later stage, as the more concave part of the hilltop was subject to wave action, erosion was weaker and the shell gravel deposit was covered by beach sand. Since the hills may be of different elevation, it is concluded that trying to date ancient shorelines from shell gravel deposits is not justified, as each deposit originates from the hill with which it is associated.