The Rose Window

Rambling Rose: Travel Blog
 Katie’s Fantastic Jurassic Dorset Adventure
 I: A Story of Rocks

The Magic of Wandering…

“Jude continued his walk homeward alone, pondering so deeply that he forgot to feel timid. He suddenly grew older. It had been the yearning of his heart to find something to anchor on, to cling to—for some place which he could call admirable." - Thomas Hardy, Jude the Obscure
 
Walking, for me, is a magical movement medicine, a way to ponder, wander and reinvigorate inspiration. During Covid, I learned to be content walking within the circumference of where my feet could take me from my door.  The time had come to widen the circle and wander a part of the world I have always loved, the Dorset Coast.  To walk these cliffs is to track the motions of Gaia over centuries and to follow in the footsteps of giants - dinosaurs, pioneering palaeontologists, kings on horseback, feisty saints and literary geniuses.   Come wander with me… we start with the story of the cliffs themselves….

Jurassic Coast, Dorset

I: A Story of Rocks

Once upon a time, a very long time ago, about 252 million years ago, (Ma) the great mothership continent Pangaea, surrounded by one ocean Panthalassa, started a vast process of breaking into what would become the continents we know today.   Scientists call this period between 250-66Ma the Mesozoic (Middle) Era, formed of three phases - the Triassic, Jurassic and Cretaceous (TJ&C).

In this area we now call the Jurassic Coast of Dorset, these three phases (TJ&C) are captured in a rocky freeze frame. As the earth's crust expanded and contracted, layers of sinking sediments formed rock basins including the Wessex Basin. The hot, arid atmosphere of the Triassic era formed baking desert sediments - basal ‘red beds’  from the core of Pangaea.  The high concentration of iron oxide formed the crimson-coloured cliffs, we now see on beaches including Sidmouth. Dinosaurs stalked the earth from the middle of the Triassic period until a mysterious Mass Extinction event wiped them from the face of the earth 66 Ma.

Diagram via Wikipedia/ USGS

During the Jurassic Era, rising sea levels transformed deserts into tropical seas, lagoons and swamps. The sea was full of great marine reptiles, including the ichthyosaurus and plesiosaurus first found by Lyme Regis fossil hunter Mary Anning (1799-1847) and the skies were full of great flying birds and pterosaurs. Land-dwelling dinosaurs roamed further inland.  As Pangea cracked, the Wessex Basin deepened, filling with marine clays, sandstone and limestone.  As sea levels dropped in the Late Jurassic a forest rose, sank and was submerged in watery landscapes. The resulting soft grey cliffs of the beaches around Charmouth and Lyme Regis are filled with Jurassic fossils - ammonites, belemnites (an early squid ancestor).  These soft rocks are also prone to landslides - Mary narrowly escaped one alive.

Mary Anning's Statue Lyme Regis

During the Early Cretaceous, low sea levels formed mudstones in coastal areas.  In the Upper Cretaceous, movement and collisions of tectonic plates (continents) of the earth's crust tipped the sedimentary layered rocks to the east.  The top layers of rock that were pushed upwards in the west were eroded. In the Late Cretaceous, sea levels rose again, submerging the area in a sea full of algae whose calcium carbonate shells formed a layer of white chalk sediments.  These are now seen in spectacular chalk cliffs and rock formations.

Chalk Cliffs on the South West Coastal Path to Durdle Door

During the Late Cretaceous, the African and European tectonic plates collided creating a massive folding and buckling of rock which we now know as the Alps and sending shockwaves across the planet.  Lulworth Cove was sculpted by a river of glacial meltwater loosened by the collision, which cut a gap in the strong Portland limestone, allowing the sea to rush in and hollow the bay out of the softer stone behind it.  Looking over the bay, you can see the outer formations of Portland Rock, followed by layers of Purbeck Beds, Wealdon Beds and Greensand rocks.  The sea is constantly eroding the chalk cliffs, causing rock falls.

Diagram Credit: K. Cantner, AGI.

Lulworth Cove

Similarly at Durdle Door, (Durdle meaning thirl/ drill/ pierce and door from dure in Old English) the tipping of rocks forced the strong Portland stone to stand almost vertically, and over time the sea hollowed out the arch from softer stone behind it.  These geological strata, mapping timelines of 185 million years of history, were exposed as natural erosion carved out the coastline we see today, which is still very much on the move.

This massive geological story forms the bedrock of the Jurassic Story, written into the lines of the cliffs, compressed in the millions of creatures buried in the rocks and the paths along which I am to wander in my Fantastic Jurassic adventure.

Next Up: II: Weymouth

Durdle Door and Man O'War Bay from the South West Coastal Path

Sources:

https://www.jurassiccoast.org/what-is-the-jurassic-coast

https://www.earthmagazine.org/article/travels-geology-mesozoic-masterpiece-englands-jurassic-coast/