Geography : Sea Arch and Stack

• Differential rates of erosion along a coast with alternate bands of hard and soft rocks results in an irregular coastline --- Headland and Bay.

• As waves enter the shallow waters of the headland, the retarding influence of the shallow waters causes the wave fronts to bent and get refracted. Due to the wave refraction, wave energy is more concentrated towards headlands and dissipated towards bays. Marine processes --- hydraulic action and corrosion erode along lines of weakness (e.g. joints, faults and bedding planes). Two back-to-back caves first form at a headland on each side.

• Continuous erosion causes these back-to-back caves to extend backward till the sea caves meet and a natural tunnel is produced. The top portion of the sea caves becomes an arch, linking the tip of the headland with the mainland.

• When the arch collapses, the sea ward pillar is left standing and becomes a stack. Continual wave erosion eventually reduces the stack into a stump.