Here in the Copper Coast Geopark, there are hundreds of coastal erosion features. Sea stacks, arches, blowholes, and caves. Natural features carved by Atlantic waves and torrential rains over timescales ranging from hundreds to thousands of years.
But how did they form? And why does the coast look so dramatic in the Copper Coast Geopark compared to other places nearby?
The answer is how erosion impacts the underlying geology.
The dramatic headlands and isolated sea stacks exist because of Ordovician volcanic rocks formed roughly 460 million years old. These formed when volcanoes erupted through the ocean floor near the South Pole, creating rhyolites, andesites, basalts, and volcanic tuffs with an interlocking crystalline structure that resists erosion.
The younger Devonian Old Red Sandstone (370-360 million years old) came from semi-arid river systems about 90 million years after the volcanic activity stopped. Its reddish-brown colour comes from iron oxide, and because its particles are cemented rather than crystallized, it wears away faster. Where these rock types meet, you get the Copper Coast’s signature pattern which is tough volcanic headlands jutting into bays carved from softer sedimentary or meta sedimentary layers.
Glacial deposits from the Ice Age (ending about 12,000 years ago) top the cliffs. These unconsolidated tills (a type of sub-soil) erode quickly, creating unstable sections that contrast with the ancient volcanic rock below.
Where soft tills are in direct contact with the sea erosion happens very fast – sometimes several metres of land are lost to the sea in a single year. When hard volcanic rock faces the sea, erosion is much slower, usually less than 5 cm per year.
Four processes work together to change the coastline in the Copper Coast and elsewhere in Ireland/the world.
Hydraulic action forces water into cracks during wave impact, compressing trapped air until rock fragments from within.
Abrasion grinds rock surfaces as waves hurl sand, pebbles, and boulders against cliffs.
Corrosion dissolves susceptible minerals.
Attrition breaks transported rocks into smaller pieces through continuous collision.
Learn more about coastal erosion processes https://www.gsi.ie/en-ie/geoscience-topics/natural-hazards/Pages/Coastal-Erosion.aspx
Wave refraction concentrates these forces on headlands. As waves approach the irregular coastline, varying water depths bend them, focusing energy on protruding volcanic rock while sparing sheltered bays.
Sea caves develop when hydraulic action exploits weaknesses like joints and faults.
When caves form on opposite sides of a narrow headland, continued erosion eventually breaks through to create a sea arch. The roof faces attack from waves below and freeze-thaw weathering above. When it collapses, it leaves a sea stack that continues eroding until only a stump remains.
Blowholes form when sea caves grow inland and upward along weak joints until the roof fails, creating a vertical shaft.
Waves compressing air within the cave chamber force water upward through this opening. On the exposed sections of the Copper Coast, winter storms can create waves that reach over 8 metres in height with wind speeds over 70 km/hr.
The coastline here is not only eroding due to the action of the sea. Heavy rains also impact erosion rate by washing away soil and subsoil and weakening the rocks below.
Kilfarrasy Strand
At Kilfarrasy Strand a mixture of hard and soft rock erosion is ongoing. On the west side of the beach (right as you look out to sea) you can see how the waves are undercutting the harder volcanic rocks.
Kilmurrin Cove
Looked at from the air, Kilmurrin Cove is a conch shell-shaped opening in the cliff line between Annestown and Bunmahon. Cove is itself the product of the gradual erosion of what was once a small fracture in the cliff here but was worked through tens of thousands of years into a semi-circular strand.
Annestown Beach
Annestown beach has some dramatic examples of eroding glacial till. Warning, care is needed here. Do not try to access eroded walkways.
COPPER COAST GEOSITES
Learn about the connected geological, historical and cultural heritage of our area.
