This cliff section of Upper, Middle and Lower Chalk is part of the “Great Unconformity”. The sea has eroded a natural arch in the cliff headland, “Bat’s Hole”. The Chalk here has been twisted by 90 degrees to the vertical.
Coastal Features: The Great Unconformity and The Cretaceous Blanket, Chalk Cliff Erosion, Bat's Hole
Chalk Cliff Erosion
Erosion of coastal Chalk cliffs is slow. Chalk is relatively hard, and only becomes unstable when the sea has carved a cave into its base, destabilising the weight of cliff above. The sea then has to wash away the cliff fall before the process can start all over again. The milky coloured seawater found at the base of Chalk cliffs, particularly around fresh falls, is evidence of coastal cliff erosion in progress.
The Great Unconformity and The Cretaceous Blanket
The sediments which make up the Jurassic Coastline were deposited in the following order, Triassic, Jurassic and Cretaceous, with the oldest rock, the Triassic sitting at the at the bottom of the pile.
Part way through the Early Cretaceous Period the sea levels dropped and the layers of rock were tilted to the East by earth movements. Overtime the exposed rocks were eroded before sea levels rose again, depositing more Cretaceous sediments.
The combined tilting and erosion explains why the Jurassic Period is completely missing in East Devon and why areas of cliff all along the coastline are capped with Cretaceous rocks which are millions of years younger than the sediments laying directly beneath them.
A wave cut arch in Chalk can be seen at Bat’s Head at the western end of Durdle Door Beach. The Arch, called “Bat’s Hole”, is the start of an erosion process which may eventually produce a whole series of Chalk sea stacks similar to those of the Old Harry Rocks near Swanage. Alongside the arch is a sea stack, the upright remains of a previous sea arch called “Butter Rock”. The Chalk here has been twisted vertical, the result of an impact ripple that was sent out when the continents of Africa and Europe collided around 30 million years ago. The collision was immense and threw up the mountain range of the Alps.