Upper Greensand

8: Sidmouth’s Green Crown: The Upper Greensand

Discover the Upper Greensand Formation in Sidmouth. Learn about the green mineral glauconite, marine fossils, and how this rock built the great Cathedrals of England.

While the base of Sidmouth’s cliffs is famous for its deep red mudstone, the very top of hills like Peak Hill and Salcombe Hill is capped by a different story entirely: the Upper Greensand Formation. Dating from the Cretaceous Period (roughly 113 to 93.9 million years ago), this rock was formed in a shallow sea that once covered southern England.

1. Why is it Green?

The formation gets its name from glauconite, a distinct green mineral found within the fine-grained sandstone.

The Process: Glauconite grains formed through the chemical weathering of iron-rich minerals sitting on the ancient seafloor.
The Look: This mineral gives the rock its characteristic greenish-yellow hue, which contrasts sharply with the red rocks below.

2. A Marine Record

The Upper Greensand is a treasure trove for paleontologists, acting as a biological record of the marine life that inhabited the Wessex Basin.

Ocean Dwellers: It frequently contains fossils of bivalves, gastropods, and ammonites.
Surprise Finds: Occasionally, scientists even find dinosaur remains that were washed out to sea and preserved in these sandy layers.

3. Chemistry and Composition

The Upper Greensand is unique because its chemical makeup can vary depending on where you look.

Not Quite Limestone: While it contains some calcium carbonate (chalky material), it is primarily a sandstone.
A Geological Mix: Some sections are more calcareous (alkaline), while others are more acidic due to the presence of clay minerals.

4. Building the West Country

Because it is durable and easy to work with, the Upper Greensand has been a vital resource since Roman times.

Cathedrals: It was used to build some of England's most magnificent structures, including Exeter Cathedral, Winchester Cathedral, and Salisbury Cathedral.
Hidden Uses: Beyond building, it has served as a source of iron ore and continues to be an important water aquifer (a rock layer that holds groundwater).

Before you go

The "Hidden Sponge"

Goal: To understand how the Upper Greensand acts as an "aquifer" and why this causes cliff falls.

The Setup: Place a dry sponge (representing the porous Upper Greensand) on top of a flat piece of plastic or clay (representing the impermeable Sidmouth Mudstone).
The Rain: Slowly pour water onto the sponge. Watch how it disappears into the "rock."
The Barrier: Notice what happens when the water reaches the bottom of the sponge. It can't go through the plastic/clay, so it begins to pool and leak out of the sides.
The Discussion: This "leaking" is exactly what happens in Sidmouth's cliffs. The Upper Greensand stores water like a sponge. When that water hits the mudstone, it creates a slippery "slide" that can lead to landslides.