Sea Caves in Port Quin Bay explained by Geologist Jon King

Along the North Cornwall coast the storm waves of the Atlantic Ocean crash and nibble away at the land, forming cliffs and indented sandy coves. Sea caves are the vanguard of this attack, each representing a line of weakness exploited by the sea.  Erosion concentrates on areas where the slatey country rock is broken and faulted or altered along contacts with igneous rocks, and weakened where solutions rich in antimony, arsenic and gold have passed through. Once a cavity starts to form it provides a focus for attack, as waves fling themselves into the tiniest cracks and loosen rock with hydraulic and pneumatic pressures wedging and splitting to enlarge the cave.

The rocks of this coastline are from the Devonian Period, the Age of Fishes, some 400 million years ago. They formed in the Trevone Basin of the Rheic Ocean on the continental shelf of Avalonia and the Old Red Sandstone continent, Laurasia, to the north.  For over 70 million years mud accumulated up to 6km thick in the basin and as it subsided under the load, fissures in the earth’s crust fed pillow lava domes and dolerite sills that intruded the wet sediments. To the south, the continent of Gondwanna was moving northward towards Laurasia, finally colliding to form the super continent of Pangea and close the Rheic Ocean. The layers of sediment in the Trevone Basin were squashed between the continents, pushed up into great overturned folds and dismembered by gravity slides and thrust faults into slabs riding over each other to build the Variscan Mountains. By the end of the Carboniferous Period, 300 million years ago, the stresses had imprinted a cleavage of closely spaced partings as the muddy sediments became slates. Finally, granites were emplaced in the heart of the mountains, leaking volatile enriched fluids to deposit metallic ores in the surrounding slates and complete the genesis of the rock mass from which North Cornwall will be formed.

Over the next 100 million years the mountains eroded to form the hills of the Cornubian Island, lapped from the east by the warm Jurassic sea. Later, as the Atlantic Ocean started to open, sea inundated low ground, only receding at the end of the Cretaceous some 65 million years ago when the Cornish peninsula started to take its present form. Sub-aerial erosion planed off the slates, cutting the Reskajeage Erosion Surface, an undulating plain that sloped gently from Davidstowe to the Isles of Scilly. The ice ages of the last 2 million years brought sea level changes and periglacial erosion, driving the river valleys to deeply dissect the surface and leave the Killas Plateau of the modern landscape. Finally, in the Flandrian Transgression 15,000 years ago, sea level rose to drown the river valleys while coastal erosion cut the cliff line back to its present location, as sea caves develop and collapse in an ongoing cycle.