Sidmouth Mudstone

The Sidmouth Mudstone Formation is a geological layer from the Mid-Triassic period, dating back roughly 247 to 237 million years ago. It serves as the foundation (or "basal formation") for the much larger Mercia Mudstone Group found across England and Wales.

1. Composition and Character

If you look closely at the cliffs, you are seeing fine-grained particles of mud and clay that have compressed over millions of years into rock.

• Structure: The mudstone is largely "structureless," meaning it doesn't always have clear layers, and it breaks away in a blocky weathering habit.

• Acidity: Chemically, this rock tends to be more acidic; unlike limestone, it is not rich in calcium carbonate.

• Thickness: In South Devon, this formation is incredibly deep, reaching over 200 meters in thickness.

2. The Story of the Colours

The striking colours of the Sidmouth Mudstone are actually a record of ancient chemistry.

• The Red: The dominant red-brown color comes from iron oxides. This tells us the mud was deposited in "oxidizing" conditions—where oxygen was present.

• The Green: You will often see grey-green patches or spots. These are reduction patches, where iron sulphides formed because oxygen was absent (reducing conditions).

3. An Ancient Shallow Sea

While earlier parts of our series focused on deserts, the Sidmouth Mudstone was deposited in a shallow sea that once covered much of the UK.

• Fossil Clues: We find evidence of this marine past in fossils of bivalves (shells), gastropods (snails), and ostracods (crustaceans).

• Salinity: These fossils suggest the water wasn't always the same; the salinity (saltiness) of this ancient sea varied over time.

4. Why It Matters Today

Beyond its beauty, the Sidmouth Mudstone has been a practical resource for centuries.

• Industry: It has been quarried for building stone and used extensively in brickmaking.

• Minerals: It is a valuable source of evaporite minerals, including gypsum and halite (salt)