These are narrow, deep indentations carved out through vertical planes of weakness in the rocks by wave action. With time, further headward erosion is hindered by the chasm mouth, which itself keeps widening till a bay is formed.
When the sea waves strike against a cliff, the cliff gets eroded gradually and retreats. The waves level out the shore region to carve out a horizontal plane or a wave-cut platform. The bottom of the cliff suffers the maximum intensive erosion by waves and, as a result, a notch appears at this position.
It is the seaward limit of coast which is marked by a steep scarp.
Differential erosion by sea waves through a rock with varying resistance across its structure produces arched pockets in rocks. These are called sea caves.
When the waves attack a rock- form from two opposite sides, the differential erosion produces bridge-like structures or sea arches.
Stacks/Skarries/ Chimney Rock:
When a portion of the sea arch collapses, the remaining column-like structure is called a stack, skarry or chimney rock.
If the fluvial erosion by streams flowing down the coast is not able to keep pace with the retreat of the cliff, the rivers appear to be hanging over the sea. These river valleys are called hanging valleys.
Blow Holes or Spouting Horns:
A narrow fissure through the roof of a sea arch is called a blow hole or a spouting horn because the wave action compresses and squeezes out the air from the sea caves through blow holes making a peculiar noise.
Plane of Marine Erosion/Peneplain:
The eroded plain left behind by marine action is called a plain of marine erosion, and if the level difference between this plain and the sea level is not much, the agents of weathering convert it into a peneplain.