A Journey Through Time

Sidmouth serves as the western gateway to the Jurassic Coast, a UNESCO World Heritage Site. While the name suggests a world of dinosaurs, the geology beneath your feet here tells an even older story.

1. Why are the Sidmouth Cliffs Red?

The most iconic feature of Sidmouth is the deep red colour of its towering cliffs. This striking hue comes from iron oxide (rust).

This chemical signature tells us that 250 to 200 million years ago, during the Triassic Period, this part of Britain was located much closer to the equator and consisted of a hot, arid desert environment.

2. The Key Rock Formations

The landscape is primarily composed of three distinct geological layers. Understanding these layers helps explain why the coast looks the way it does today.

The Otter Sandstone (OS)

• Location: Found in the lower sections of the cliffs and to the west toward Budleigh Salterton.

• Origin: Formed by ancient braided river systems.

• Significance: It is world-renowned for containing rare fossils of Triassic reptiles and amphibians.

The Sidmouth Mudstone (SM)

• Location: These are the crumbly, red-brown layers often seen in the upper parts of the cliffs.

• Origin: Formed in ancient desert lakes and floodplains.

• Nature: Because mudstone is softer than sandstone, these sections are highly prone to erosion and frequent rockfalls.

The Upper Greensand (UGS)

• Location: A yellowish-green layer visible at the very top of Peak Hill and Salcombe Hill.

• Origin: Deposited during the Cretaceous period (approx. 100 million years ago) when rising sea levels submerged the former desert.

3. The "Missing" History: The Great Unconformity

Sidmouth is one of the best places in the UK to witness a geological unconformity—a significant gap in the Earth's rock record.

• The Gap: Cretaceous rocks (100 million years old) sit directly on top of Triassic rocks (240 million years old).

• The Missing Link: Roughly 140 million years of history—including the entire Jurassic period—is "missing".

• The Cause: These layers were eroded away by natural forces before the Cretaceous sands were ever deposited.

4. How Geology Shapes Sidmouth Today

The relationship between these rock layers directly impacts the modern coastline and public safety.

The combination of porous Greensand sitting on top of impermeable Mudstone creates a "slip plane". When it rains, the water soaks through the sand but becomes trapped when it hits the mudstone. This lubricates the layers, causing the cliffs to become unstable and leading to the frequent landslides and rockfalls Sidmouth is known for today.