The Jurassic Coastline – From Devon to Dorset

Another holiday away from the lecture theaters and practical labs – another “extra” week of learning in the form of a geological field trip.  This time, we headed south to Devon and Dorset to study the Jurassic Coast – a world heritage site.

Firstly a map!

Dorset Coastline

It was with the aid of Google Earth, and it’s many contributing photographers, that I was able to piece together the trip as we made it.  The photos are in sequence from west to east, although the chronological order of the geology is otherwise.  This is not meant to be a detailed geological field guide.  I no longer have my field trip notes, and if I had, they would be complete gibberish!  If you wish to know more about the geological record of this coastline then click here.

However, it might help to briefly remind you of what period was when:

  • Triassic Period is from 250-200 million years ago,
  • Jurassic Period is 200-140 million years ago
  • Cretaceous Period is 140-65 million years ago – ending in the mass extinctions of dinosaurs etc.

Looking west along the beach at Budleigh Salterton

Budleigh Salterton: These red cliffs are made up of quartzite pebbles that are derived from 440 million-year-old rock formations identical to that found in Brittany, France.  They would have been transported by a massive river that flowed through the Triassic desert 240 million years ago.

Budleigh Salterton Pebble Bed Cliffs - horizontal bedding, faulting & honeycomb weathering textures

Sidmouth:  The Triassic cliffs are capped with a yellow sandstone of the Cretaceous Period.  The entire Jurassic period is missing, having been eroded away. This gap in the records is referred to as an unconformity.

Looking east towards Sidmouth from Jacob's Ladder - red cliffs = Triassic, yellow cliffs = Cretaceous

Waves crashing at Jacob's Ladder

Pinhay Bay: Westwards from Lyme Regis is a fossil collectors dream paradise! Ammonites, belemnites and fossilised bone fragments of the icthyosaur can be found in the rocks that form the cliffs along this coastline.  I was obviously too busy trying to prize out some of these fossils that I didn’t take any fossil photos!

White (Triassic) and Blue (Jurassic) Lias at Pinhay Bay

View eastwards along Eype Beach towards West Bay

West Bay: The Bridport Sands that form the East Cliff are alternating bands of sandstone.  Soft, friable sandstone is layered with harder, carbonate-cemented sandstone indicating cyclical periods of deposition during the Jurassic Period.

Bridport Sands - The East Cliff of West Bay

Chesil Beach: Protecting The Fleet, a natural lagoon, this natural beach barrier is 28km (17 miles) long.  The size of pebbles range from “pea-sized” in the west increasing gradually to “potato-sized” in the east.

Looking east along Chesil Beach from Portland Heights

Portland Heights:  On the Isle of Portland, near the car park for the local hotel, is an uprighted fossil tree.  However, Portland is best known for its quality of building stone, a fine white limestone, which many a building in London, including St Paul’s Cathedral, has been constructed.

Geologists lurking behind fossil tree, Portland Heights

Pulpit Rock at Portland Bill

Osmington Mills: More Jurassic fossils can be found here – only they are trace fossils, which include burrowing marks left by ancient marine animals.  In addition, natural oil seepage occurs from the limestone rocks.

Osmington Mills with the Isle of Portland on the horizon

Wreck of the Minx - a coal barge that lost its moorings off Portland in 1927

Purbeck Fossil Forest:  West of Lulworth Cove is the best preserved Jurassic forest in the world – but it lies within a militarized zone, so access is restricted, but obviously not entirely prohibited.  Rather than fossil trees remaining, it is the mould of the tree that exists, and the algae that formed around the base of cypress/juniper-type trees as sea-levels rose.  These raised moulds are referred to as “thrombolites”.

Mould of fossil tree

Purbeck Fossil Forest

Mupe Bay: Is also within the Army firing range… access may be restricted.

Mupe Bay

Lulworth Cove: The limestone of the Purbeck formation forms a hard-rock barrier against the sea.  Where the sea has eroded through this barrier, the softer chalk beds overlying the limestone have been more readily eroded forming beautiful coves.

Stair Hole with crumpled zones of the Purbeck Formation

Lulworth Cove

Kimmeridge Bay: The ledges seen at Kimmeridge bay are dolomitic (limestone) and bituminous shales.  Wytch Farm Oilfield is located nearby, pumping oil and gas from the layer of rock called the Cornbrash Limestone – also found at Osmington Mills.

Kimmeridge Bay with Clavell Tower in the distance

Devon and Dorset Coastline, England – April, 1993

Info: Pentax P30T, ISO 400, Scanned negatives with CanoScan 8800F & cleaned up in Photoshop (version 5).


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