Sea arches are natural openings found primarily along rocky coastlines. These formations are tunnels carved through a headland by the power of the sea. They are ephemeral features, constantly being shaped and eventually destroyed by the same forces that create them.
Initial Erosion of Headlands
The formation of a sea arch begins with a headland, a mass of rock that juts out into the sea, often composed of rock layers with varying resistance to erosion. Waves tend to refract, or bend, around these protruding landforms, concentrating their energy on the sides of the headland. This focused wave energy targets pre-existing weaknesses in the rock structure, such as faults, joints, or bedding planes.
Two primary erosive forces initiate this sculpting process: hydraulic action and abrasion. Hydraulic action occurs when the sheer force of waves crashing against the headland compresses air and water trapped within the rock’s cracks. This rapid compression and decompression weakens the rock structure, causing fragments to break away. Concurrently, abrasion involves the grinding action of rock fragments, pebbles, and sand, which are picked up by the waves and hurled against the cliff face.
This continuous assault, particularly near the high-tide line, enlarges the initial cracks into small indentations known as notches. As these notches deepen and widen, they evolve into sea caves, forming on either side of the narrow headland. The process is accelerated when the rock type, such as limestone or chalk, is susceptible to chemical weathering or is interbedded with softer material. The caves grow progressively deeper.
The Breakthrough: Creating the Arch
The arch forms when two sea caves, eroding inward from opposite sides of the headland, finally meet. Wave energy removes the intervening rock mass, creating a tunnel that extends completely through the promontory. This leaves behind a bridge or span of rock connecting the two sides of the headland, forming the characteristic arch structure.
Once the arch is established, the erosive processes shift their focus. Hydraulic action and abrasion continue to widen the base of the arch’s pillars, increasing the span of the opening. Simultaneously, sub-aerial weathering processes, such as rain, wind, and freeze-thaw cycles, begin to act upon the rock material that forms the arch’s ceiling. This weathering weakens the structural integrity of the rock bridge from above.
The combination of basal erosion by waves and weathering from the top progressively thins and destabilizes the roof of the tunnel. This continued weakening makes the entire structure less stable and more vulnerable to collapse.
The Final Stage: Stack Formation
The life cycle of the sea arch reaches its culmination when the unsupported rock bridge can no longer withstand the forces acting upon it. Eventually, the roof of the arch collapses into the sea. This event marks the end of the arch.
The collapse leaves behind an isolated, vertical pillar of rock, detached from the main headland, which is known as a sea stack. The stack is composed of the more resistant rock that survived the collapse of the arch’s roof. It remains exposed to the relentless pounding of the waves.
The base of the sea stack is continually attacked by hydraulic action and abrasion, causing it to narrow and undercut, forming a wave-cut notch. This erosion and weathering will reduce the stack’s height, eventually transforming it into a much shorter, residual feature called a sea stump. The sea stump is often only visible at low tide.