Learn about the Sidmouth Mudstone Formation. Discover why the cliffs are red and green, what fossils lie within, and how this 200m thick layer formed 240 million years ago.
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)
Goal: To understand how oxidation (rusting) creates the red colour in Sidmouth’s cliffs.
The Comparison: Find a piece of rusted iron and compare it to a photo of the Sidmouth cliffs. Notice how the shades of burnt orange and red are nearly identical.
The Science: Discuss why we see red in the cliffs (oxidation/oxygen present) but green spots in other areas (reduction/oxygen absent).
Field Observation: Walk along the seafront and look for the "blocky" way the mudstone falls away. Can you find a grey-green "reduction spot" on a fallen fragment?.