Geological Influences

After completing this 10-part series, you have transitioned from a beginner to someone with a basic understanding of how hundreds of millions of years of history created the modern Sidmouth landscape.

1. The "Jurassic Coast" branding in Sidmouth is a bit of a historical misnomer.
   While the coast is famous under this name, the vast majority of the rock making up Sidmouth's dramatic cliffs actually belongs to the older Triassic Period (dating back roughly 250 to 200 million years), meaning they were deposited long before the first Stegosaurus ever existed.

2. The stunning deep red colour of the cliffs is actually ancient rust. 

During the Triassic, this land was part of the supercontinent Pangaea near the equator. Under a relentless sun, iron-rich minerals in a massive, scorching continental desert weathered and oxidised, leaving behind a chemical signature of iron oxide.

3. There is a staggering "silence" of 140 million missing years in the stone. 

At the summits of Peak Hill and Salcombe Hill, yellowish Cretaceous rock sits directly on top of red Triassic rock. Because the entire Jurassic period was completely eroded away, millions of years of planetary history and evolutionary change were simply deleted from the local geological record.

4. A natural geological "sandwich" is the secret engine behind Sidmouth's frequent landslides. 

Highly porous Cretaceous Upper Greensand sits directly on top of impermeable Sidmouth Mudstone. Rainwater filters easily through the sandy top layer but gets trapped when it hits the clay-like mudstone below. This traps water, builds immense hydrostatic pressure, and creates a highly lubricated "slip plane" that causes the cliff tops to slide.

5. Devon was once a parched land carved by high-energy "braided rivers." 

The lower western sections of the cliffs consist of the Otter Sandstone Formation. This sandstone was not laid down by a calm sea, but by powerful networks of shallow, fast-flowing river channels. Geologists prove this by identifying "cross-bedding"—slanted stone layers that preserve the direction of ancient water currents.

6. These cliffs served as a post-apocalyptic "recovery room" for life on Earth. 

The Otter Sandstone is of international paleontological significance because it formed in the Middle Triassic, just after the end-Permian mass extinction (the most devastating mass extinction in Earth's history). It offers a rare archive of how land-based and freshwater ecosystems rebuilt themselves, preserving rare fossils of ancient fish, heavy-bodied amphibians, and early relatives of dinosaurs like predatory archosaurs.

7. The Sidmouth Mudstone is a globally recognised, 200-metre-thick scientific reference point. 

Formed in stagnant desert lakes and floodplains, this formation is exceptionally thick and beautifully preserved in East Devon. Because of its immense scale and clarity, geologists around the world use it as a primary reference to reconstruct and understand Triassic sedimentary environments.

8. The red cliffs feature scattered grey-green spots representing "chemical rebellions." 

While oxygen-rich environments turned the vast majority of the mudstone red, tiny scattered grey-green patches show localised zones where oxygen was completely absent. In these pockets, chemical reduction occurred, creating iron sulphides and locking a miniature chemical battle into the stone.

9. The green tint at the top of the hills is formed by Cretaceous "feacal recycling." 

The Upper Greensand layer gets its colour from glauconite, an iron-rich mineral. Because the Cretaceous sea was exceptionally calm, sedimentation occurred at a glacial pace, allowing iron to slowly weather on the sea floor. Interestingly, this mineral often formed when seawater chemically altered the feacal pellets of bottom-dwelling marine organisms over deep time.

10. The pebbles on Sidmouth's shingle beaches are ancient travellers from the Alps. 

Unlike sand beaches, shingle beaches act as sophisticated, self-restoring natural shock absorbers that dissipate wave energy by rolling against one another. Many of these pebbles are not local cliff debris; they were carried thousands of miles from the mountains of Central Europe by massive glaciers during the last ice age before being rounded by the sea