There is widespread evidence around Orkney of the emergence of erosional landforms at the coast from beneath a cover of till. Whilst long stretches of the Atlantic coasts are undergoing active erosion, more sheltered shores give examples of platforms, geos and even cliff lines that are currently being re-occupied by the sea.
Around 20% of the Orkney coastline has cliffs more than 15 m high. Marine action plays a dual role in cliff formation. The obvious role is in cliff erosion, undercutting the slope base and thereby forming debris both directly and indirectly due to mass failure of the over-lying rock. The hidden process is the removal of debris by a variety of marine processes including long shore drift.
These linear clefts in the cliff line reflect marine erosion along the line of weakness. Where joints and other planes weakness are close together, these long, narrow slots develop in response to selective marine erosion between the neighbouring planes or weakness.
Gloups are a reminder that cliffs may become riddled with tunnels which remain largely unseen from the cliff top. The excavation of blowholes is a hidden process but the opening of the shaft must involve the removal of material along intersecting lines of weakness provided by crossing joints and fractures.
Caves form in response to the structural control of local geology on cliff form. The exploitation of joints, faults, cracks and other irregularities all lead to the opening of caves. The caves can be either tunnel or dome shaped, reflecting the type and inclination of the geological structures. Caves are most common where massive layers overlie weaker beds with resistant ribs.
Arches are natural openings eroded out of the cliffs by marine processes. Some arches appear to have developed from surge channels, which are created by wave refraction causing the focussing of wave fronts on the side of a headland.