The Jurassic Coast is famous, and famously misunderstood. The "Jurassic" in the name is partly accurate — the middle section of the coast does indeed expose Lower Jurassic rocks. But the coast as a whole tells a much bigger story: 185 million years of Earth's history, from a parched red desert to a tropical sea to a prehistoric river delta, all preserved in 95 miles of cliffs you can walk along in three days. This is the geology, in plain English.
What the Jurassic Coast actually is
The Jurassic Coast is the 95-mile (155-kilometre) stretch of coastline from Exmouth in East Devon to Old Harry Rocks near Studland in Dorset. It was designated a UNESCO World Heritage Site in 2001 — England's first natural World Heritage Site — because it preserves a continuous, exposed sequence of sedimentary rocks covering three geological periods:
- The Triassic (252 to 201 million years ago)
- The Jurassic (201 to 145 million years ago)
- The Cretaceous (145 to 66 million years ago)
That's 185 million years of Earth history, all visible from the South West Coast Path.
The cliffs run roughly east in age — the oldest rocks are at the western end (Exmouth), and the rocks get progressively younger as you walk east. By the time you reach Old Harry Rocks at the eastern end, you are looking at chalk laid down 65 million years ago, while the dinosaurs were going extinct.
The story, in three chapters
Chapter 1: The Triassic — a red desert (252-201 million years ago)
Two hundred million years ago, what is now Devon and Dorset sat 30 degrees north of the equator in the middle of a vast supercontinent called Pangaea. The climate was hot and arid — think modern Saudi Arabia. Enormous rivers occasionally flooded huge dust-bowl plains, depositing red iron-rich sediments that would later compress into the Triassic red sandstones you can see today.
Where to see it: the cliffs at Exmouth, Budleigh Salterton, Sidmouth, and Branscombe. The deep red colour of the cliffs at Sidmouth and Branscombe is iron oxide — literally rusted desert sand. The same colour as the sandstones of the American southwest, for the same reason: iron-rich sediment laid down in an arid climate.
Look for: the colour of the cliffs (rust red), and the pebble bed at Budleigh Salterton — fist-sized rounded pebbles that were once part of a Triassic river that flowed across what is now the English Channel from a mountain range in Brittany.
Chapter 2: The Jurassic — a tropical sea (201-145 million years ago)
At the start of the Jurassic, sea levels rose dramatically and what had been a desert became a warm, shallow tropical sea. This is the period the coast is named for, and the period that produced the famous fossils. Ammonites, belemnites, plesiosaurs and ichthyosaurs lived in these warm waters; their remains accumulated on the seafloor and were eventually preserved as fossils in the dark grey shales and limestones we now call Lias (Lower Jurassic) and Oolite (Middle Jurassic).
Where to see it: the cliffs at Charmouth and Lyme Regis are the heart of the Jurassic Coast. The cliffs here are dark grey shale, full of ammonites and other marine fossils.
Look for: ammonites loose on the foreshore, the dark grey shale of the cliffs, and the spectacular Black Ven landslip between Charmouth and Lyme Regis where Jurassic fossils continually wash out of the cliff. See our fossil hunting guide for what to look for.
Chapter 3: The Cretaceous — a chalk seafloor (145-66 million years ago)
Late in the Jurassic and through the Cretaceous, sea levels rose further and the climate stabilised. A vast warm sea covered most of southern Britain, deep enough to be undisturbed by waves. Tiny plankton called coccolithophores lived and died in unimaginable numbers; their calcium carbonate shells settled to the seabed and slowly accumulated into the soft white rock we call chalk.
The chalk you see at Beer Head, Old Harry Rocks, and the famous white cliffs of southern England is essentially nothing but the compressed shells of trillions of microscopic plankton.
Where to see it: the chalk cliffs at Beer Head (the most westerly chalk on the south coast of England, just east of Hole Mill); the chalk pinnacles of the Hooken Landslip; and at the eastern end of the Jurassic Coast, the dramatic chalk arches of Old Harry Rocks near Studland.
Look for: the bright white of the chalk against the green clifftop turf; the geological surprise of the Beer Stone — a unique cream-coloured limestone within the chalk sequence that was quarried for centuries and used to build Exeter Cathedral and Westminster Abbey. See our Beer Quarry Caves guide for the human history of this stone.
The unconformity at Branscombe
One of the most fascinating geological features on the entire Jurassic Coast is right on Hole Mill's doorstep. At Branscombe Mouth, you can see two completely different layers of rock pressed against each other: the deep red Triassic sandstone of the lower cliff, and the pale Cretaceous chalk and greensand of the upper cliff.
In between — the entire Jurassic period, all 56 million years of it — is missing. The rocks that should be there were either eroded away before the Cretaceous or never deposited at all. Geologists call this an unconformity: a gap in the rock record.
Standing on Branscombe Mouth beach and looking up at the cliff, you are looking at two periods of Earth history meeting along a single line, with 56 million years missing in between. That is, genuinely, one of the more extraordinary things you can see on a holiday.
Why the Jurassic Coast keeps producing fossils
The cliffs along this coast are unusually fossil-rich for two reasons:
- The original sediments were ideal for preservation. Soft mud and shale on a calm seabed buried marine animals quickly enough to preserve them.
- The cliffs are actively eroding. Soft shale and mudstone are easily attacked by winter storms. Every year tens of thousands of tonnes of rock fall onto the foreshore, releasing fresh fossils. This is why Charmouth Beach has been productive for over 200 years and shows no sign of running out.
This is also why fossil hunting on the Jurassic Coast is regulated by a code of conduct — the geological record is constantly renewing itself, but only because the cliffs are constantly collapsing. Walking close to the cliff base is genuinely dangerous, and every winter there are rockfalls large enough to bury cars.
A geological day from Hole Mill
A satisfying day exploring the geology, all within easy reach of the property:
- Morning: walk to Branscombe Mouth and look at the unconformity in the cliff (10 minutes from Hole Mill).
- Mid-morning: walk east along the coast path to Hooken Cliffs and Beer Head — the Cretaceous chalk and the Hooken Landslip.
- Lunch: in Beer.
- Afternoon: drive to Charmouth (25 minutes) for the Jurassic shale and a fossil walk at low tide.
- Late afternoon: drive back via Lyme Regis to see the Cobb and the Ammonite Pavement.
You will have walked through the Triassic, the Jurassic and the Cretaceous in a single day.
Further reading
The Jurassic Coast Trust publishes excellent free guides at jurassiccoast.org. For something deeper, _The Jurassic Coast_ by Denys Brunsden is the definitive popular geological history. The Lyme Regis Museum and the Charmouth Heritage Coast Centre both have small but excellent geological displays.
The Jurassic Coast is, genuinely, one of the most extraordinary stretches of coastline in the world. From Hole Mill it begins on your doorstep — Branscombe Mouth is the unconformity itself — and the next 95 miles unfold to the east with all the Earth history you could want. Check our availability for your dates.