Both marine and sub-aerial processes have a part to play in the formation of cliffs but the emphasis on each depends on the type of cliff that is being eroded.
On a cliff made from hard rock, such as chalk or Granite in Land's End, it is likely that marine processes will contribute most towards erosion, whereby a wave-cut platform will be formed. The wave cut platform is created by many forms of coastal erosion, such as hydraulic action and abrasion, from destructive waves. Initially undercutting at the base of the rock, where wave energy is at its greatest, causes a notch to be formed. The processes of undercutting from abrasion, where sand and pebbles collide against the rock breaking of peaces of rock, are continuously repeated increasing the stress and tension in the cliff until it eventually collapses. Abrasion is the most prominent and rapid form of erosion experienced by the cliff but other process also occur, with corrosion, the chemical action on rocks, attrition, the process of particles colliding with the rock face and being broken into smaller sizes, also contributing the retreat of the cliff. Other important forms of coastal erosion also occur, including hydraulic power, where air is trapped and compressed in a crack and eventually pressure builds up causing rocks to break away, and wave pounding, where the sheer force of the waves on the cliff face causing the cliff to erode. Therefore, on hard rocks where a wave cut platform is formed and the hard lithology remains its steepness coastal erosion from destructive waves will be the greater process on the cliff as it is the attack of the waves at the base of the cliff, not at the top, which will be the main contributing factor to its retreat.
However, on soft rocks such as sand and clay, a stepped profile is seen, which is mainly caused by sub aerial processes rather than marine erosion. The act of coastal erosion should not be underestimated in this example as it still plays a part in weakening the rock but the greater proportion of erosion is done from above.
