Jurassic cliffs

One of the most studied and scientifically rewarding areas in England, and indeed the rest of the world, for studying palaeontology, stratigraphy and geology, and for collecting Jurassic and Cretaceous fossils, is along the continually collapsing cliff faces of the west Dorset coastline, in and around the Charmouth area, (the Latitude is approx 50 degrees 44 minutes north and the Longitude is approx 2 degrees 54 minutes west).

Charmouth is fairly near to Lyme Regis, which is another well known area for fossil collecting, especially through its association with the name of Mary Anning. The area was also used the backdrop to the classic novel and film adaptation, 'The French Lieutenant's Woman".

As you can see from the sketch map just below, the Jurassic cliffs of Dorset are located on the southern coast of Great Britain at the western end of the English Channel. It is one of two places where the Jurassic geology of Great Britain is exposed to any great extent, the other place currently being at Whitby in North Yorkshire.

Winter storms along the channel, combined with spring tides and offshore low pressure systems, are responsible for major collapses of the cliff faces, and for uncovering new exposures of fossil beds.

It was after a period of heavy rain during Easter 2000, there was a large landslip of cliff face just east of the River Char. A local collector, Tony Gill, from the Charmouth Fossil Shop, discovered the fossilised remains of a 5m long Ichthyosaur , though to be of the species Temnodontosaurus platyodeon. The fossil has since been named 'Mary' .

The Dorset cliffs form part of the Lower Jurassic (or Lias) which comprise predominantly of clays , thin limestone's and siltstones . The sequence appears to have been deposited during an initial deepening of the sea, followed by two upward rhythms of shallowing sea.

By the way, it was Alexander von Humboldt (1769-1859) who described the massive limestone formations of the Jura Mountains in Switzerland as 'Jura-Kalkstein'. It was later on in 1839 that Leopold von Buch (1774-1853) formally named these limestone rocks as belonging to the 'Jurassic System'.

The coastline of West Dorset is dominated by several 'high level' features. Starting from a position to the west of Charmouth, is the feature called Black Ven (referred to as Black Venn by The National Trust ). The name ven is the local dialect expression for fen, a name which alludes to the very boggy nature of the Black Ven terraces that result from the largest landslip complex in Europe.

The British Geological Society conduct regular surveys of the landslip terraces to check the movement of material down towards the sea.

The next feature, which is to the east of the Charmouth, is Stonebarrow Hill. It is between these two hills that the valley has been formed that creates the mouth of the River Char (hence the local village name of Charmouth).

Next to the east is the very distinctive Cretaceous sand topped hill of Golden Cap. This hill currently holds the record for the highest point on the south coast of England.

The last feature to the east that is included in this website is that of Doghouse Hill to the west of Eype Mouth.

The Lower Jurassic period, all of which can be found exposed in the Lyme Regis-Charmouth-Seatown-West Bay areas of the West Dorset coastline, forms the middle part of the Mesozoic era and spans a time period of between 208 million and 144 million years ago (Ma). Below is an example of the naming sequence used for the Belemnite Shales bed, which form part of the Belemnite Marls, as it might be referred to in Book on this subject:

Several of these terms can be traced back to specific areas, people and events:

Mesozoic The age of the reptiles (especially dinosaurs), and of flowering plants (especially Cycads). The time span and duration of this period is open to discussion. I have seen texts that show the Jurassic starting at between 205-213 Ma and ending at between 140-146 Ma.

Jurassic Named after the Jura Mountains in Switzerland.

Toarcian Stage Named after Thouars, in the Deux-Sevres, France, where this stage is particularly well developed.

Pliensbachian Stage At one time this was known as the Charmouthian Stage, named after the Charmouth area where it was first studied.

Lias An expression that is said to come from the corruption of the word 'layers', used by limestone Quarrymen who worked in the area during the 1800's. An expression that is said to come from the Gaelic word 'leac', meaning a flat stone

With so much geology to cover, this website concentrates only on the divisions and beds starting at the bottom of Black Ven and its Lower Lias Shales with Beef, through to the younger, and more easterly, the  divisions and beds at the boundary of the Middle and Upper Lias, containing the Marlstone Rock Bed/Junction Bed.

If you want to look at more recent divisions and beds, i.e. in the Middle Jurassic, take a look at Ian West's website based upon the geology and stratigraphy found at West Bay/Bridport Harbour . Each of the above divisions has its own dedicated webpage, which details the zones and sub-zones that they can be further divided into. Each sub-zone is then further divided into beds, with details as to their geology and fossil content.

Collecting larger iron pyrite ammonites (1/2 to 1 inch diameter), is best
carried out on the beach under Stonebarrow Hill, as the tide goes out.

Why do the cliff faces of Charmouth have fossils in the that must have formed several hundred metres under the sea? The reason is that Charmouth, or for that matter most of Great Britain, was under the sea millions of years ago.

Following a continental collision, some 420 million years ago, and the subsequent disappearance of the Iapetus Ocean, England and Scotland were united, forming Britain, which was then sited around the latitude of the Tropic of Capricorn. Most of the country was dry land, with the exception of an ocean that lay over Devon and Cornwall. It was at this time that life moved onto the land. The very oldest fossil land plants are to be found in the Welsh border country. Rocks of this age (the Devonian period) are often a deep reddish colour and are often referred to as "the Old Red Sandstone". They outcrop all over Scotland, in the Brecon Beacons and in Devon.

Britain was drifting northwards and by 360 million years ago, the start of the Carboniferous period, was straddling the equator. Scotland was mainly land, with rivers flowing from the Highlands into the Lowlands laying down layers of sandstone around Edinburgh and Glasgow in which fish and amphibian fossils have been found. Cornwall and Devon were still hidden beneath an ocean, but a clear warm sea over the rest of England and Wales saw the deposition of thick layers of gray limestone, in which fossil corals are common. Overlying the Carboniferous limestone are thick layers of sandstone and then the Coal Measures. the Scottish river deltas had extended southwards filling most of England and Wales with sandbanks and swamps in which the coal forests grew. The rocks are like a huge complicated sandwich with layers of sandstone, limestone, mudstone, fossil soils and occasional bands of coal. The whole area was unstable and forests were frequently covered with water charged with sediment which blanketed and preserved the rotting vegetation, which over time turned to coal.

Around 320 million years ago, the ocean covering Cornwall and Devon was shrinking in the same manner as the Iapetus Ocean 100 million years earlier. France collided with England. A great slab of ocean floor was thrust upwards to become the Lizard Peninsular and sediments from the sea where squashed and heated (metamorphosed) into slate. This collision finished the basic building of Britain, and made us part of the great super continent of Pangea. We were land-locked and across the Tropic of Cancer, in much the same position as the present-day Sahara. The whole of Britain was covered by hot, hostile desert with orange sand dunes, salt pans, bare mountains and plateaux, with deep canyons and wadi's. The rocks of this time are preserved as orange sandstones from Devon to the Hebrides, with thick layers of salt in Cheshire and North Yorkshire.

250 million years ago, for reasons that are poorly understood, the biggest mass extinction of all time hit the globe. Ninety-five per cent of all species, in land and on sea died out. Yet despite the devastation, the planet remained intact and life recovered.

Triassic 248-206ma

Around 210 million years ago, fully marine conditions prevailed leading to Blue Lias, Black Ven Marls and Belemnite Marls being slowly deposited on the sea floor over a period of about 12 million years. Throughout the Mesozoic era, Dorset (and the Great Britain), was positioned approximately 35degN of the equator. The Charmouth area was part of a subsiding basin that eventually became a shallow sea. This basin is referred to as the Wessex basin. A flourishing marine life in this shallow warm seas provided a continual supply of dead organisms to the sea bed, where many became fossilised. These sea floor sediments (loose particulate matter consisting of clay, sand, gravel, etc) hardened with age (through a process called lithification that involves compaction, cementation and recrystallisation), and were then buried before being uplifted and tilted eastwards by earth movements and subsequent erosion.

Jurassic 206-144ma

The result is a belt of rocks of Jurassic age that stretch across England from Dorset in the southwest to Yorkshire in the northeast. Isolated outcrops of Jurassic rocks also occur in northern Scotland as well. There are two main rock types; mudrocks, such as those which outcrop at Lyme Regis, and limestone's, which make up the Cotswold escarpment.

At 149 million years ago, Africa was still joined to South America. Antarctica and Australia were joined together off the coast of south Africa.

Cretaceous 144-65ma

Around 131 million years ago, the area was again subjected to marine conditions and the Cretaceous seas re-submerged the earlier deposited sediments. The deposition of the Upper Greensand and then (around 97 million years ago), chalk. Because of the easterly dip of the earlier Jurassic rocks, the Cretaceous sediments now deposited on the sea bed lie on progressively older Jurassic and then Triassic rocks as you move westwards. The boundary between the Cretaceous strata and the underlying Jurassic and Triassic rocks represents a huge gap in time. This is called an unconformity.

In the last few thousand years the advancement of the English Channel has exposed the local geology in a series of fine cliff sections. This coastal erosion is aided by the massive landslips that can occur at any time. These are caused by groundwater flowing along the unconformity surface. This acts as a lubricant between the permeable Cretaceous sandstones and the impermeable lower Jurassic mudstones.

Several of the Jurassic cliffs along the Dorset coastline are capped by a Cretaceous unconformity. These cappings start with a lower level of Gault [consisting of loam and loamy sand on a pebble base (local term), approximately 40ft thick]. This is covered by the Upper Greensand , consisting of a layer of Cowstones [the lower part of the Upper Greensand containing hard sandstone concretions called Cowstones, lying in three bands, inside 20ft of grey sand], above these are the Foxmould Sands [the middle part of the Upper Greensand containing 70ft of Grey, yellow or brown sand (local term)] and finally the Chert Beds [the upper part of the Upper Greensand containing chert beds made from silica formed from the spicules of siliceous sponges].

The Cretaceous 'golden sands' capping of Golden Cap