Thursday, 23 September 2021

A journey through deep time on the Jurassic Coast

We are lucky in Falmouth in having two stretches of coastline nearby which have UNESCO World Heritage status. Thirty miles to the west is the region of Cornwall's submarine mines, while travelling 100 miles east by car or train is the magnificent Jurassic Coast, a 96 mile stretch from Exmouth in Devon, to Studland in Dorset.

Although famous as the Jurassic Coast, it would perhaps have been more appropriately named the Mesozoic Coast, as the exposed cliffs are the sediments of the Triassic, Jurassic and Cretaceous Periods. There are no igneous or metamorphic rocks and what makes this coastline so fascinating is that a slight tilting east during the Mesozoic, and erosion during the Mesozoic and Quaternary eras, has left continuous outcrops representing 185 million years of the earth's history from the Triassic to the Cretaceous. These sediments are regarded as one of the world’s best stratigraphic sequences from the Mesozoic Era, which lasted from 252 million to 66 million years ago and was distinguished by a rapid diversification of life as well as several major extinctions, notably that of the dinosaurs.

In the Triassic, this area was an arid iron-rich Pangean desert, and in our first short venture out of Cornwall during the pandemic Barbara and I last year walked the first 5 miles of this stretch, from Exmouth to Budleigh Salterton, where the cliffs are composed of New Red Sandstones, the sediments from the tropical desert of around 250 million years ago.

"Britain" (red) during the Triassic, 225 million years ago
Source: Open University

Triassic New Red Sandstone cliffs at Budleigh Salterton, Devon

The Triassic sediments were later covered by shallow Jurassic and Cretaceous seas and the basal rocks were overlain by a middle sequence of Jurassic to Lower Cretaceous marine mudstones, sandstones and limestones, that accumulated as the basin deepened in response to the rifting of Pangea, and an uppermost group primarily composed of Upper Cretaceous to Paleogene chalks. In the mid-Cretaceous, tectonic activity gently tilted the Mesozoic sedimentary layers down to the east. Then the sea level rose and Upper Cretaceous chalks, sands and clays accumulated over the inclined layers,the contact between the top of the tilted section and these Upper Cretaceous layers forming a geological unconformity. The time gap is larger to the west, where the underlying rocks are older than those to the east, so that walking from Exmouth is a journey through 185 million years of Earth history.

The modern landscape formed by quaternary erosion

The coast takes its name from the beautiful Jurassic limestones in the Lyme Bay area and a couple of weeks ago we stayed in Lyme Regis, a world famous haven for fossil hunters, including Mary Anning, who was born in Lyme Regis in 1799, and overcame her low social status, lack of formal education and poverty to become one of England’s greatest fossil hunters. 

Lyme Regis

Mary Anning has been credited with the first discovery of ichthyosaur fossils and helped discover the first specimen of Ichthyosaurus to be known by the scientific community of London. This specimen was probably discovered sometime between 1809 and 1811, when Mary was only 10 to 12 years old. Apart from the ichthyosaurus perhaps her most important find, from a scientific point of view, was her discovery of the first plesiosaur in 1823. These early discoveries shook the scientific community, and challenged the religious thinking of the time, as it was obvious that these fossilised remains, buried deep within sediments, were of a bygone world of many thousands of years ago. Many believed the teachings of the Rev. James Usher, whose chronological research of the Bible established that the creation took place around 6pm on the night preceding Sunday 23rd October 4004 BC. Imagine if they had known that the ichthyosaurus died around 195 million years ago!

Mary Annings humble dwelling in the centre of Lyme Regis is now the Lyme Regis Museum, and an essential stop for anyone visiting this bustling seaside town. Mary, one of the great pioneering paleontologists, was only 47 when she died from breast cancer and she is buried with her elder brother Joseph, who helped her unearth the first ichthyosaurus, in the churchyard of St. Michael the Archangel overlooking the Jurassic cliffs where she toiled for much of her life.

Lyme Regis Museum- the site of Mary Anning's home and fossil shop
Kate Winslet as Mary Anning in the 2020 biopic Ammonite
The grave of Mary Anning and her brother Joseph

Lyme Regis hugs the English Channel coast and to the west the harbour is sheltered by the Cobb, a long curling sea wall, and to the east the cliffs tower over the surrounding landscape, dominated by Golden Cap, at 626 feet, the south coast's highest sea cliff (posting of 9th September).

Barbara on the Cobb
The Jurassic cliffs east of Lyme Regis with Golden Cap on the right

East of the Cobb is Monmouth Beach, and between Lyme Regis and Golden Cap is Charmouth Beach, a mecca for fossil hunters, as locked in the layers of Jurassic shale and limestone, known as the blue lias, are the remains of creatures who once inhabited a vast primeval tropical ocean.

The rapidly eroding blue lias cliffs between Lyme Regis and Charmouth

A short bus ride west into east Devon from Lyme Regis took us into the Triassic at Seaton from where after a short walk we encountered striking white cliffs nestling among the red rocks of the coast to either side. These cliffs, in the village of Beer, offer the last glimpse of chalk for anyone travelling west. The Cretaceous chalk is much younger than the Triassic red mudstone and sandstone that dominate the East Devon coast, and would originally have been laid down on top of them. But at Beer the rocks have been folded downwards, bringing the higher layers of chalk into a snug geological hollow called a syncline, where they were preserved from the millions of years of erosion that stripped the rest of the chalk from the surrounding landscape.

The New Red Sandstone Triassic cliffs of Seaton beach
give way to the Cretaceous chalk of the Beer syncline

The furthest east we got on this trip was to the resort of Weymouth, and hopefully in a future visit we will travel further east to the Jurassic-Cretaceous stratigraphic transition near Lulworth. In the meantime I can thoroughly recommend a visit to this fabulous area, and our wonderful base at Lyme Regis. And you will enjoy it even more if you take with you a little knowledge of the incredible geology of this coast.

Looking across the Jurassic beach at Weymouth to the
Cretaceous chalk cliffs of White Nothe in the east


1 comment:

  1. Barry and Barbara,
    So nice to see you walking through "geological history"--being mineral engineers, we should now and then pause and look at the wonders of NATURE in creating these geological wonders and treasures--so much work in these formations--heat,pressure,drift,sedimentation--so much treasure to explore and exploit with due regard to NATURE and SOCIETY.


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