Cliffs of western Stroma. Image by Bill Fernie
Definition: steep to vertical or overhanging rock slope at the coast, free of soil and regolith
Around 20% of the Caithness 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 Caithness 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. The cliff face is commonly stepped, with numerous overhangs, reflecting undermining and removal of joint-bounded blocks by wave water rushing up the cliff face during storms.
Experiments have shown that breaking waves are much more effective than broken waves at notch cutting. When cliffs plunge into deep water, waves are reflected and little other erosion appears to take place - leaving open the question of how the cliff formed. When waves break some distance offshore, owing to a low beach or shore platform angle, erosion also occurs slowly. Cliffs whose base has a relatively narrow steep beach are more likely to experience the maximum erosive forces of breaking waves on the lower cliff. The narrow shore platforms that fringe the cliffs of Caithness 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 Caithness, the shore platforms show a relatively steep slope ending in deep water. This is probably related to the general rising sea level over the past six millennia.
Superimposed on this process of cliff erosion is the subsidiary process creating cliff morphologies. This gives, along the exposed coasts of Caithness, an endless succession of headlands, bays and geos, with caves, gloups or blowholes, arches and sea stacks in every stage of development and destruction. The process starts with wave-induced erosion of the rocks along fracture lines at the base of the cliff and initiates a small opening following the weakness. Cave enlargement proceeds by hydraulic action of the waves enhanced by the pneumatic action of trapped air. When long caves are formed, collapse of the roof at the inward end forms a gloup, while total collapse of the roof forms a geo. Intersection of geos can leave isolated sea stacks and arches.