Let me take a moment to introduce myself: my name is Graeme and I am an amateur paleontologist and geologist. Over the past ten years I have developed a deep love of the Jurassic period. As a result of many visits to the Jurassic coastline and the surrounding area of west Dorset, I decided to create this website as a means of sharing this passion with you and with many others. I hope you find it both interesting and useful. In order to increase the educational content of these webpages, I have added hyperlinks to www.dictionary.com for some of the more unusual words that appear.
Graeme
P.S. a Paleontologist [pa-le-on-tol-o-gist] is someone who is involved with a science dealing with the study of the forms of life existing in prehistoric or geologic times, as represented by the fossils of plants, animals, and other organisms, and a Geologist [ge-ol-o-gist] is someone who is involved with a science dealing with the scientific study of the origin, history, and structure of the earth.
CharmouthOne of the most studied and scientifically rewarding areas in England, and indeed the rest of the world, for studying Jurassic paleontology, 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). |
It is fairly near to Lyme Regis, which is another well known area for fossil
collecting. It is also better known through its association with the name Mary Anning.
It was also the backdrop used for the classic novel and film, 'The French Lieutenant's Woman". As you can see from the
sketch map 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 only 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. 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 shallowing rhythms.
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'.
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 the 'Jurassic System'.
The coastline at Charmouth 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. The next feature, which is to the east of 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 topped hill of Golden Cap. This is currently 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.
The Jurassic period, which can be found exposed in the Charmouth / Lyme Regis / 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). Where possible I have tried to adhere to the naming procedures adopted by the Geological Society of London, for the naming of formal lithostratigraphic successions. The following is an example of how the Belemnite Shales Bed fits into the scheme of things:
EON
Fanerozoic 
ERA
Mesozoic
PERIOD
Lower Jurassic
EPOCH
Lower Lias
STAGE
Pliensbachian
ZONE
Tragophylloceras ibex
SUB-ZONE
Acanthopleuroceras valdani
DIVISION
Belemnite Marls
BED 119
Belemnite Shales
Starting at the top level of classification, the Jurassic parts of this website fits in as follows:
| Eon | Era | Period | Time span (Ma) |
|---|---|---|---|
Fanerozoic |
Mesozoic |
Cretaceous |
65 - 144 |
Jurassic |
144 - 208 |
||
Triassic |
208 - 248 |
The Jurassic period itself can be further broken down into three epochs, which can be further divided into a number of stages. The area around Charmouth is primarily of the Lower Jurassic epoch, consisting of the Middle and Lower Lias stages:
Period |
Epoch |
Stage |
Span | |
Jurassic |
Upper |
Tithonian |
146-152 Ma | |
Kimmeridgian |
152-156 Ma | |||
Oxfordian |
156-163 Ma | |||
Middle |
Callovian |
163-169 Ma | ||
Bathonian |
169-176 Ma | |||
Bajocian |
176-183 Ma | |||
Aalenian |
183-187Ma | |||
Lower |
Toarcian |
Upper Lias |
187-193 Ma | |
Pliensbachian |
Middle Lias |
193-198 Ma | ||
Lower Lias |
||||
Sinemurian |
198-204 Ma | |||
Hettangian |
204-208 Ma | |||
Once more we can break down the stages further, this time into beds. The following table contains details of the division formations and their relative thickness as seen at the coast:
| Era | Division Formation | Thickness (at coast) |
| Upper Lias | Bridport Sands [Yellow micaceous sands with bands of blue-centered calcareous sandstone] |
43m |
| Down Cliff Clay [Blue sandy clays with occasional more resistant bands] |
21m | |
| Junction Bed [Fine grained pink-white limestone in this upper layer] |
4m | |
| Middle Lias | Marlstone Rock Bed [Red-brown, oolitic, conglomeritic limestone in this lower layer] |
0.6m |
| Thorncombe Sands [Yellow-brown sands, often indurated leading to formation of massive sandstone blocks or blue-centered doggers. Grey sandy marls in upper parts] |
27m | |
| Down Cliff Sands [Grey silty sands in lower parts, becoming browner and more sandy towards the top. Occasional nodules and sandstone bands] |
26m | |
| Eype Clay [Blue micaceous marls with impersistent bands of calcareous sandstone] |
68m | |
| Three Tiers [Three 0.6m thick bands of calcareous sandstone separated by marls] |
10m | |
| Lower Lias | Green Ammonite Beds [Silty fissured clays with thin, irregular limestones and increasingly sandy towards top] |
34m |
| Belemnite Marls [Hard, jointed, pale grey mudstones and marls] |
23m | |
| Black Ven Marls [Firm dark grey marls and paper shales with nodules and thin limestones] |
46m | |
| Shales with Beef [Upper part is brownish paper shales with numerous seams of beef. Lower part is blue conchoidal marls with bands of impure limestone and infrequent seams of beef] |
25m | |
| Blue Lias [Marls and shales with frequent limestone bands] |
32m |
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.
Mesozoic
The age of reptiles (especially dinosaurs) and of flowering
plants (especially Cycads). The time span and duration of these Periods is open to
discussion. There are some disagreements as to the time frame for many of the periods. I
have seen texts that show the Jurassic starting at between 205-213Ma and ending at between
140-146Ma.
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.
The Jurassic cliff faces in Dorset have played a big part in the creation of a biostratigraphic scale for the Jurassic rock succession. The scale forms the basis for the international correlation of standard zones. These stages represent larger groups of zones and their sub-zones, which indicate levels represented by particular fossils which are usually found only within the zones. For the Jurassic zones, ammonites are used for the scale.
The following is an example of the ammonites used for the Zone/Sub-zone scale that is applied to the Belemnite Marls division.
Zone |
Subzone |
Beds |
Tragophylloceras ibex |
Beaniceras centaurus |
121 |
Acanthopleuroceras valdani |
118d-120 |
|
Uptonia jamesoni |
Tropidoceras masseanum |
118c |
Uptonia bronni |
115-118b |
|
Platypleuroceras brevispina |
110-115 |
|
Phricodoceras taylori |
105-109 |
The best way to date sedimentary rocks is by use of fossils. They provide the a reliable way of understanding the environmental conditions, or paleoecology, at the time the fossilised organisms died and of the way in which evolution of organisms in the past may be studied.
In order to give names to fossils it is necessary to recognise the group of animals to which they belong. For this purpose, the Natural History Museum of London's book 'British Mesozoic Fossils' is highly recommended.
It is useful to visit museums in order to get help with fossil identification. As well as the Natural History Museum of London, the Dorset County Museum is well worth a visit.
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.
The above drawing shows the currently believed paleogeography of the north Atlantic Region at some 200 Million years ago. The arrow shows the approximate position of southern England, and Charmouth, at this time. As you can just see, the Charmouth area of south west England is at the edge of the 'ocean'. This drawing is from Dr. Ron Blakey's Geology website at Northern Arizona University2, where other available drawings show the same area at 170Ma and 150Ma. The question therefore is how did the pattern of land and sea change between the earliest times, through the Jurassic period some 200Ma, to the present day?
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 supercontinent 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.
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.
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.
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.
For an animated history of the world from Big Bang to now, see the bbc.co.uk website. You'll need Shockwave Flash to view it properly.
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].
Some of the images used are copyright by other people and companies with a presence on the WWW. The fossil line-drawings are taken from the 'British Mesozoic Fossils' - 6th Edn. Natural History Museum. The cliff line drawings were taken from 'Finding Fossils in Lyme Bay' by Robert Coram BA Hons and then modified as required. The world maps come from the BBC webpage related to being a Jurassic Journalist.
This is me
and I'm a member of
Open University Geological Society
and also a member of the
with an affiliation to the Dorset branch.
The London branch of OUGS also has its own website at http://www.lougs.freeserve.co.uk
| ||||||||||||||||||||||||||||||||||||||||||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||
