Natural Heritage

Geological and Geomorphological

The St Kilda archipelago illustrates ongoing geomorphic processes in the coastal and submarine environments. The igneous rocks of the archipelago, exposed by marine and glacial erosion processes, have produced a spectacular assemblage of rock coast landforms unique within the Palaearctic Realm and of global significance. Sea cliffs in excess of 400m high are found on the island of St Kilda itself.

The rocks of the St Kilda archipelago form a tiny but highly significant portion of the North Atlantic Igneous Superprovince recording Paleocene-Eocene events associated with the opening of the North Atlantic some 65-52 million years ago. The rocks exposed in the St Kilda archipelago represent the remnants of a large ring volcano with the islands themselves rising from a seabed plateau lying approximately 40m below sea level.

During the Ice Age of the Quaternary Period (approx. 2.5 million years ago to present) the St Kilda landscape was modified by local mountain glaciers and during at least the last glacial maximum, parts of Hirta remained ice-free. Sediments and pollen within the peat deposits provide a regionally important palaeoenvironmental and palaeoclimatic postglacial record for the NW European margin.

During glaciation, sea level dropped by as much as 120m around St Kilda. Lying at the periphery of the Scottish ice sheet, corresponding glacio-isostatic depression and consequent uplift of the archipelago have been limited. This has resulted in the formation of a series of former coastlines that have been ‘drowned’ following the ‘postglacial’ rise in sea level. These extend from the present coastline down to 120m below present sea level and are represented by two major submerged shore platforms and cliff lines.

The coastline of the archipelago currently experiences some of the most extreme storm levels on the western European coast and these conditions have facilitated the development of superb sheer–faced and composite cliff forms, geos, sea caves, tunnels, arches and stacks at all stages of development.

The combination of marine and glacial erosion imposed upon a hard rock geology, along with sea–level change, has resulted in a complex and spectacular terrestrial landscape that extends uninterrupted into the submarine zone. This combined terrestrial/marine landscape in an island setting is unique within the Palaearctic Realm. The archipelago is globally significant in terms of the the physiographic features of the archipelago and significant ongoing geological and geomorphological processes that have created, and continue to influence, the terrestrial and marine landscape.