Otter Sandstone

While much of the Triassic landscape was a harsh desert, the Otter Sandstone Formation (now formally known as the Helsby Sandstone Formation) tells the story of powerful water systems that once cut through the arid plains. Dated to the Middle Triassic (approximately 247 to 237 million years ago), these rocks are a cornerstone of the Jurassic Coast World Heritage Site.

1. A Landscape of Braided Rivers

The Otter Sandstone was not formed by wind alone, but by braided rivers flowing through a desert environment.

• Powerful Currents: The sandstone features "cross-bedding"—diagonal layers within the rock that act as a physical record of the strong river currents that once flowed here.

• Rock Composition: It is primarily made of fine- to medium-grained sandstone that often weathers into soft sand at the surface.

• Varying Layers: Within the sand, you can find thin units of hard conglomerate (cemented pebbles) and reddish-brown mudstone, reflecting times when the river energy changed or floodwaters pooled and settled.

2. A Sanctuary for Life

The Otter Sandstone is famous among geologists because it preserves a record of non-marine biota (land and freshwater life) at a critical turning point in history. This was the era when animals were rapidly diversifying after the End-Permian mass extinction.

Fossils discovered in these layers include:

• Amphibians: Large, flat-headed creatures known as temnospondyls.

• Early Reptiles: This includes beak-headed rhynchosaurs and small, lizard-like animals.

• Predators: The formidable ancestors of dinosaurs and crocodiles, known as archosaurs.

• Fish: Evidence of the aquatic life that thrived in these ancient freshwater systems.

3. The Chemistry of the Cliffs

The nature of the sandstone also influences the local environment today. While sandstones are generally neutral or slightly acidic, the specific mineral "cement" holding the grains together can change the pH balance of the rock. This unique chemistry, combined with its ability to hold fossils, makes it a prime location for scientific study and fossil hunting today.