The fresh exposure of yellow Cretaceous Greensand overlying the grey Jurassic sediments is proof of the instability of the cliffs in this area. Rain water seeps through the sand and puts pressure on the underlying Jurassic Clays and Marls. Eventually the cliff gives way and the process starts all over again.
Hotspots: The Jurassic Sea and Fossils, Charmouth Dinosaur
The Charmouth Dinosaur
The cliffs around Charmouth hold a mystery. A local fossil collector, David Sole, has found the remains of a dinosaur, Scelidosaurus, but just how did a land dwelling reptile end up at the bottom of this deep Jurassic sea?
The Jurassic Sea and Fossils
The Jurassic clays around this area formed in a deep tropical sea, the soft muddy bottom of which was often stagnant, creating a sulphurous and anoxic environment, the best conditions for preserving the shells, bones and even soft tissue of dead prehistoric creatures.
Coastal Features: The Great Unconformity and The Cretaceous Blanket, The Cretaceous Blanket and Cliff Erosion
The Cretaceous Blanket and Cliff Erosion
From the air, the instability of the cliffs along this stretch of the Jurassic Coastline is clearly visibly and they are eroded in several different ways.
From above, and where the cliff is capped by Upper Greensand, rainwater seeps through to the underlying impermeable clays. As the water builds up during prolonged periods of rainfall the cliffs below fail to support the extra weight and pressure from above, creating a landslide.
From below, the sea erodes away the cliff base. As more and more sediment is removed, the deposits of clay and shale above become less stable, resulting in cliff falls onto the beach, where the process then starts all over again.
This is why the cliffs here are so unstable and why it is a good idea to keep away. Mary Anning’s faithful dog was killed when a section of cliff fell on him.
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.