Rock Types

Throughout this series, we have discussed the layers of the cliffs, but what are they actually made of? Understanding these specific minerals and rock types helps us decode the ancient environments—from violent landslides to drying salt lakes—that shaped Sidmouth.

1. Breccia: The "Angular" Rock

While many rocks on the beach are smooth, Breccia is famous for being sharp.

• What it is: A rock composed of large, angular fragments "cemented" together by a fine-grained material called a matrix.

• How it formed: In Sidmouth, these are often Sedimentary Breccias, formed when landslides or mudflows rapidly piled up sharp rock fragments before they had time to be rounded by water.

• Why it matters: Geologists study breccia to understand ancient mountains and the violent events, like flash floods, that shaped them.

2. Gypsum: The Ancient Evaporite

Found within the Sidmouth Mudstone, Gypsum is a soft, white mineral that tells a story of extreme heat.

• Desert Clues: It forms as an "evaporite," meaning it was left behind when ancient salt lakes dried up under the Triassic sun.

• Properties: It is very soft—only a 2 on the Mohs scale—and can be dissolved by water over time.

• Fun Fact: Gypsum is the main ingredient in blackboard chalk and was even used by ancient Egyptians to make mummy cases!

3. Halite: Much More Than Table Salt

Commonly known as Rock Salt, Halite (NaCl) is another evaporite found in our local strata.

• Formation: Like gypsum, halite precipitates out of water in arid regions where intense heat causes seawater or salty lakes to evaporate.

• Modern Uses: While we use it to season food, its most widespread use today is de-icing roads in winter.

• Geological Marker: Finding halite in the cliffs is a "smoking gun" for a past environment that was once a restricted, salty basin.

4. Glauconite: The Green Secret

The Upper Greensand at the top of our hills gets its name from this specific silicate mineral.

• Marine Origins: Unlike the "desert" minerals above, Glauconite forms only in marine environments, typically at moderate depths with low oxygen.

• The Colour: Its characteristic green hue comes from its iron content.

• Scientific Value: Because it forms under very specific conditions, geologists use it to reconstruct ancient ocean temperatures, salinity, and nutrient levels.