Definition: steep to vertical or overhanging rock slope at the coast, free of soil and regolith
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.
Cliff formation and erosion traditionally been related to the formation of a notch at the base of the cliff. The development of the cliff notch through time is important as it controls both the rate of cliff recession and the form of the shore platform left behind. Water-line notches and caves occur at numerous localities on Orkney but also at higher levels on the cliff, reflecting the considerable reach of wave action above sea level on these high energy coasts. Long stretches of rock coast also lack notches, indicating that cliff retreat and shore platform development is not controlled primarily by notch excavation and cliff collapse.
Experiments have shown that breaking waves are much more effective than broken waves at notch cutting. When cliffs plunge into deep water, as on western Hoy, waves are reflected and very little other erosion takes place. When waves break some distance offshore, owing to a low beach or shore platform angle, erosion occurs slowly. Cliffs whose base has a relatively narrow steep beach are more likely to experience the maximum erosive forces of breaking waves. The narrow shore platforms that fringe the cliffs of west Hoy may thus promote rapid erosion.
The continued erosion of the coastal slope and the subsequent removal of the debris by near shore currents cause the shoreline to retreat to form a sub-horizontal wave cut or shore platform. In Orkney, the coastal wave cut shore platforms show a relatively steep aspect ending in deep water. This is related to the general rising sea level over the past six millennia.