The Cliff Coast of Gotland and the Rate of Cliff Retreat

Abstract

Cliff coasts are unusual features in the Fennoscandian landscape of the Swedish mainland but play an important rôle on Gotland. They are connected with the Silurian marls, layered limestones and reef limestones of the island, and especially well developed along the north-western coast, where the layers are dipping slightly towards SE. “Cliff coast” is taken in a wide sense and includes ordinary straight cliff coasts, double cliff's in a step-like arrangement, cliffs dissected into stacks, and cliffs transformed into scree slopes. The height of the cliffs is normally 20–30 m, at most 40–50 m, and the width of the wave-cut platform is 100–200 m. As marine forms of this size are lacking in other parts of Sweden, and as Gotland has taken part in the postglacial uplift, it is natural to ask, whether this cliff coast is created in postglacial time, whether it is of preglacial origin or whether it is to be regarded as a combination of the two alternatives. To test these alternatives the processes at work on the present cliff coast are studied.

The wave-cut platform shows to a great part bare rock, which says that it is still being abraded. The processes in the cliff wall are rock falls, falls of individual particles, and mudflows, probably also some water erosion and limestone solution. The rock waste normally accumulates in talus deposits with the base on the higher beach terraces, and these deposits are more or less removed during great storms. The most important of these latter occurred in 1931, 1948, and probably in 1954. The beach terraces built during the two latter storms are used as reference surfaces for evaluation of gained debris during fixed periods, and the volumes are transformed in wall retreat. Also a snow cover and artificial surfaces are used for reference.

The annual cliff retreat varies, but the most reliable values gather around 0.4–0.6 cm/year—that are to regard as minimum values. This means that the cliff coast cannot mainly have been inherited from preglacial time. The values are on the other hand small as compared with the time available in the postglacial period, and with the width of the wave-cut platform. It is suggested that the rate of cliff retreat might have decreased successively with the widening of the platform. The postglacial cliff formation might also have started from some old cliff-like forms, situated more close to the open sea.