Coastal Geomorphology

The St Kilda archipelago lies 64km west of the Outer Hebrides. The largest islands and stacks within the archipelago are Hirta, Dun, Soay, Levenish, Boreray, Stac Lee and Stac an Armin. The entire archipelago comprises a coastline of over 35km in length that is notable for its dramatic assemblage of rock coast landforms. The coastline is almost entirely cliffed, with Conachair (430m) on the north coast of Hirta forming the highest cliffline in Great Britain and Ireland, and Stac an Armin (191m) (off Boreray), the highest sea stack. Major variations in the hardness and structure of the different types of igneous rocks that compose the islands within the Hirta group have been differentially exploited by wave erosion to produce an exceptionally varied cliffline. Superb examples of sea caves, arches, tunnels, stacks and stumps are also present along this geomorphologically spectacular coastline. However, shore platforms are largely absent.

Over time, the igneous rocks of Hirta have been affected by numerous processes: glaciation and associated sea-level change; marine erosion and subaerial erosion, all of which have played a part in the evolution of the coastal landform assemblage. The effect of these processes in this high energy environment has produced the distinctive and spectacular assemblage of coastal rock landforms, at both present sea level and lower sea levels, comprising a geomorphological interest that is unparalleled internationally.