Process of formation:
1. Erosion:
Coastal stacks begin their formation when waves, tides, and other erosional forces like salt spray, wind, and rain start to break down the coastline. The waves constantly batter the exposed sections of the cliffs, creating notches and caves in the weaker parts of the rock.
2. Undercutting:
Over time, the waves continue to erode these weaknesses in the cliffs, causing the caves and notches to deepen and expand. This process of undercutting creates an overhang, leaving behind a separated section of the cliff.
3. Fracturing and Collapse:
As the overhang becomes heavier and less stable, it eventually fractures due to gravitational forces and collapses under its weight. This collapse leaves a vertical column of rock standing alone in the water, creating a coastal stack.
4. Ongoing Erosion:
Even after formation, coastal stacks continue to be sculpted by the ongoing erosive forces. The base of the stack experiences direct wave action, leading to further undercutting and erosion, which causes it to become gradually thinner and taller over time.
Coastal stacks are found along coastlines worldwide and are particularly common in areas with cliffs made of relatively soft or fractured sedimentary rocks like sandstone or limestone. They are often formed in conjunction with other coastal landforms such as arches and caves. Coastal stacks can vary greatly in size, from a few meters to over hundreds of meters in height.
Due to their unique and striking appearance, coastal stacks are popular landmarks and tourist attractions, admired for their scenic beauty and geological significance. Some notable examples of coastal stacks include:
- Old Man of Storr, Scotland
- The Needles, Isle of Wight, UK
- Three Sisters, Port Campbell National Park, Australia
- Giant's Causeway, Northern Ireland
- Haystack Rock, Cannon Beach, Oregon, USA
- Shiprock, New Zealand
