Coastal Landforms

The archipelago provides representative examples of vertical, composite and reversed cliff profiles, geos, sea caves, arches, tunnels, stacks and blowholes. Some particular examples are highlighted rather than an exhaustive survey presented.

The south-west coast of Dun is characterised by vertical lower cliff faces which plunge directly into deep water, and a complex of stacks, caves, arches, blowholes and narrow inlets at all stages of formation. It is extremely irregular in plan with several deep geos passing into caves that penetrate through this narrow (in places less than 100m wide) island. These are partially controlled by north-east – south-west fault lines. A spectacular natural arch occurs near Gob an Duin, which is almost 50m long and 24m high. The caves and geos have generally been eroded along the line of dykes or thin, inclined sheet intrusions, where they crop out within wave reach. Hence, marine erosion is in the process of dissecting the island into a series of sea stacks. The stack of Levenish to the south-east of Dun was probably formerly attached to the island.

The coastline of Hirta is characterised by deep-set embayments and spectacularly high clifflines. Cliff top scour processes are active along part of the cliffline, and the vegetation has been stripped to altitudes of over 60m by wave spray. The lowest point of Village Bay is fronted by a cobble and boulder beach complex, and is defined by the presence of two north-west – south-east orientated fault lines. Cobbles and boulders form extremely steep storm beach ridges above high water mark, whilst the lower intertidal at the head of the bay is characterised by an ephemeral sand bar. The south-west and west coast of the island displays cliff forms that have a strong geological control. The Mullach Sgar Complex is characterised by dykes and sheets of microgranite and granite, producing an irregular, notched and stepped cliff profile with remarkable colour contrasts. Triangular cliff forms occur along the basal part of the Mullach Sgar ridge, where joints or possibly intrusions run obliquely up the cliff face. Free faces have developed in the high cliffs with massive scree deposits or block fields beneath, on the lower cliff faces, and boulder slopes forming close to the angle of repose. The gabbro and dolerite breccias that comprise the northern slopes have led to the development of an irregular, stepped profile to the cliffs. The eastern cliff face is incised by deep vertical fluting. It is unusual in the context of Hirta, in that it occurs as a continuous, very high angle cliff.

The sound between Hirta and Soay contains several impressive stacks including the knife-like ridge of Stac Shodhaig (Soay Stac) which is dissected by a natural arch, the dog-tooth pinnacle of Stac Biorach and the triangular An Stac Dhona (Stac Dona). These stacks represent successive stages of coastal development, with Stac Biorach and Stac Shodhaigh probably having been separated by the cutting of an arch between the two features. Hence Sodhaigh could be seen as a sea stack which has become detached from its host cliffline at An Cambir on Hirta. An Cambir itself will, in geological time, become a sea stack once the narrow connecting neck of land is breached by the sea. Superb steeply sloping rock ramps have developed at the foot of the lower cliff face on the south eastern side of Soay. These platforms have facilitated rapid wave run-up over the lower cliff face, and therefore enhanced erosion and undercutting of the cliff, to form overhanging or vertical bedrock cliffs along the inner margin of the ramp.

The spectacular plunging cliffed coastline around Boreray reaches altitudes of 384m on its eastern side, and is cut in dolerite and gabbro breccias. The coastline is characterised by lower overhanging cliff profiles, and is honeycombed with caves and geos. Two of the most impressive features associated with Boreray are Stac an Armin and Stac Lee both of which plunge into deep water. A north-west – south-east fault line cuts across the eastern side of Stac an Armin to produce its steep triangular cliff form.