Geology, Landscapes, and Land Use of
Dorset and East Devon

 

By: Angus Leger, University of Colorado

 

 

Introduction to the Jurassic Coast Word Heritage Site

The south coast of Dorset, England is a geologic wonderland.  Virtually the entire Mesozoic era is represented within the cliffs and outcrops along the coast between the the towns of Swanage and Exmouth. It is possible to view 185 million years of sedimentary rock deposition in 95 miles of coastline. A unique situation where geology, paleontology, natural beauty, cultural and scientific history meet. The United Nations Educational, Scientific and Cultural and Organization (UNESCO) granted the coastline World Heritage status. The internationally protected site is now called the “Jurassic Coast”.  A more appropriate name for the region would be the Mesozoic coast, but Jurassic has a popular culture association due to the “Jurassic” Park movies.

Figure 1.  A general map (http://www.jurassiccoast.com 2006).

The Jurassic Coast (see fig. 1) is a great place to study sedimentalogy and paleontology.  The continual erosion of the coastline exposes more and more fossils to be discovered.  Lyme Regis is the birthplace and life-long home to one of the earliest fossil gathers, Mary Anning.  She was an uneducated lower class woman that lived in the early 1800’s that collected fossils as a child to sell them to travelers then continued here quest the rest of her life.  Some of her discovers were instrumental to the development of the science of paleontology which has a huge role in correlating geologic layers around the world and in affect developing the tectonic history of earth. 

Around the same time Mary Anning was making her discoveries on the Jurassic Coast an unbelievably intuitive man named William Smith was developing some ideas of his own.  Smith is the father of geology because of his discovery of a constant succession in the rock strata all around England. He dedicated most of his life to his masterpiece, the first geologic map.  Smith made a geologic map of England and Wales complete with a cross section which was published in 1819 and was accurate for today standards. 

Smith and Anning are just two of the many people involved in an explosion of interest in the rock layers and in the fossils.  There were many clerics, scholars, naturalists, and unique common people in England that were starting to put the puzzle of earth together. 

The common thought that dominated the early nineteenth century in western civilization was very Christian-centric.  Simon Winchester wrote a book about this era called, “The Map That Changed the World: William Smith and the Birth of Modern Geology”.  The book is a great story of the life of the father of Geology.  In his book Winchester goes into detail about the power of theology during the nineteenth century in England.  At one point in his explanation Winchester writes about a bishop who had a strong influence of the popular beliefs for hundreds of years.

“James Ussher (1581-1656), Archbishop of Armagh, established that the first day of creation was Sunday 23 October 4004 BC based on an intricate correlation of Middle Eastern and Mediterranean histories and Holy writ (Winchester 2002).”

Ussher’s findings were incorporated into an authorized version of the Bible that printed in 1701.  That same version was still common in homes a hundred years later.  Furthermore, the church’s belief was that all the creatures that God created still existed on the planet.  So when the evidence that the earth was quite a bit older than 6000 years and that there were extinct creatures preserved as fossils started to accumulate the western world began a massive transition.  

There has been an explosion of new science in the past two hundred years. However, it is arguable that many of the basic concepts of geology, paleontology and earth history are yet to be accepted by the general public.  Plate tectonics was not taught in school until relatively recently.  The “theory” of evolution is still under suspicion in many parts of the United States.  The conflict between Earth Science and the Christian religion rages on today.  For this reason the Jurassic Coast and southern England is very important to me as a geologist.  It is the Mecca of earth science.   

            The Jurassic Coast has been studied extensive by Geologists and Paleontogists.  Countless papers have been written on various subjects like the Kimmeridge Clays, Chisel Beach, or the marine fossils at Lyme Regis.  To write a paper that contained some original theory or a novel idea regarding the geology of this coastline would take some extensive field work.  The idea behind this paper is simply to make some observations about the coast and apply those observations to basic geologic processes.  The goal was to make a documentary and modest interpretation of the Jurassic Coast.   

 

Geology of the Dorset Coast

            The Geology, landscapes, and human land use of the Jurassic Coast are all directly linked to each other.  The rock beneath our feet plays the biggest role in determining where a ridge of valley will develop or what type of industry will there be.   Furthermore, the type of rock directly underneath the surface will dramatically affect the fertility of the soil.  Farmland, stone quarries, and locations for towns all depend directly to the landscapes and geology of the land. 

            Before we can examine the focus area in detail there are some concepts and facts that need to be clear.  Depositional environments of the sedimentary rocks of the Jurassic Coast are important to review. A description of the strata and their unique characteristic will help in understanding the relative strength of the rocks. A brief tectonic history of area will aid in our understanding of some landscape formations. Finally, a general review of coastal and terrestrial erosion processes that are influencing the landscape will be helpful as well. 

Mesozoic Depositional Environments  

            In the beginning of the Triassic period earth’s continents were group together in a huge supercontinent called Pangea.  This massive landmass was located near the equator.  The majority of Pangea, including what we now call England was far from any coastline.  The climate was dry and arid and the landscape was barren.  It is hard to picture England as a desert, but that is exactly what it was 250 million years ago.  The depositional environments that dominated the Triassic period were terrestrial alluvial fan, river, floodplain and sand dune environments (House, 1989). 

Figure 2: Tectonic history / Continental Drift (http://www.jurassiccoast.com 2006).

            During the start of the Jurassic time period Pangea began to split apart (see fig 2).  This was a very climatically significant geologic event that created a new look for England.  What is now called the mid-Atlantic rift was forming right under what is now the British Isles.  The rift continued to break the northern part of Pangea, often called Laurasia into two separating continental plates.  England was now on the boundary of a continental. Furthermore it was an island or more like a system of islands due to high sea level during most of the Jurassic and even higher sea level in the Cretaceous (Sloan 1993).  England now had many coastlines and all of the diverse depositional environments that come with the coast.  England was also still near the equator. Thus, the landscape looked much like the present day Caribbean (Davies-Vollum, 2006).

There are many different depositional environments that are associated with coastlines.  The amazing thing is that they all exist relatively close to each other.  For example; marine depositional environments that create different kinds of limestone depending on exactly how far the environment is from the coast or shoreline depositional environments with deltaic or tidal flat deposits or swampy near shore lagoon depositional environments.  The presence of a shoreline, that was created during the early Jurassic, helped diversify the depositional environments that existed in England and more specifically, Dorset.   

Relative Erosive Character of the Strata

Figure 3 is a Stratigraphic column courtesy of Michael House who wrote The Geology of the Dorset Coast in 1989. Note that the thickness of the beds vary throughout the Jurassic Coast.

The Cretaceous Chalk is the youngest and most resistant of all the sedimentary beds that are present on the Jurassic Coast.  This chalk is a very fine grained homogenous limestone formed from the off-shore accumulation of fallen Nanoplankton. 

 

Figure 3 (please ignore the “Figure 2” above) (House 1989).

The Chalk is very resistant to weathering.  Flint nodules are very common in the Chalk and are even more resistant than the chalk itself.   

            Directly beneath the Chalk is a relatively thin layer of cretaceous marine marl known as the Greensand.  This layer lives up to its name; it really is green due to the percentage of the mineral Glauconite.  While the Lower Greensand is much less resistant than the Cretaceous Chalk the Upper Greensand and Gualt is only slightly less resistant than the Cretaceous Chalk.

The Lower Greensand is completely missing in the eastern parts of the Jurassic Coast.  This is due to a large scale unconformity that spans more and more of the Jurassic to the east.  In figure 3 it is easy to see how the unconformity gets larger to the east (House, 1989).  The Gualt and upper Greensand were the first layers to be deposited after the unconformity.  This means that on the east Devon coast Triassic rock is directly beneath Cretaceous rock!  

            The Wealden beds are below the Greensand and are also very susceptible to erosion.  These layers are made of a mostly non-marine river bed and floodplain deposits.  The Wealden marks the apex of a transgression during the early Cretaceous.  Mostly sandstone and mudstone, these layers are not very resistant to physical weathering and erosion.  Good soil is able to form on the areas where the Wealden is at the surface and grazing and farmland is a common (Davies-Vollum 2006).

            The Purbeck limestone spans from the lower Cretaceous down to the upper Jurassic.  The Purbeck is a large grained limestone full of shells and fossils.  Below the Purbeck lies the Portland limestone that consists of smaller grains of calcite clasts.  The Portland limestone, commonly called “free stone”, is a high quality working stone for building.  However, in the towns and villages of Dorset both the limestones are found.           The more important characteristic of the limestone is their strength against erosion.  Despite their differences the Purbeck and the Portland limestones share this quality.  Compared to surrounding layers these limestones are very resistant and will often form ridges.

            The Kimmeridge Clays are below the limestone layers.  These are a complex series of clays and marls that contain a very interesting organic rich layer pattern.  There is enough oil in these beds that the Romans actually used to burn the oil shales that were contained within the Kimmeridge Clays (Davies-Vollum 2006).  Like the Wealden these layers are easily weathered and eroded and farmland is found on the clay rich valleys that form where the Kimmeridge is at the surface.

            Through the middle and lower Jurassic there are cycles between thin layers of limestone, shale, and sandstone.  This pattern represents minor transgression/regression periods.  The most famous of these layers would be the Upper, Middle, and Lower Lias.  These layers are part of the lower Jurassic.  It is common to find fossils, organic matter, and calcite in these beds.  Small local difference in erosion rates appear and give the landscape a choppy look. 

            In the Triassic there are mostly terrestrial sedimentary deposits.  The upper Triassic has the Penarth group that does have a thin layer of limestone and the Mercia group below that is a mudstone.  However, the Otter sandstone is below the Merica and its all non-marine sandstones and conglomerates below the Otter.  The high amount of iron in these beds creates the beautiful red in the cliffs of east Devon. 

In general the Triassic layers are easy to erode.  The cliffs form in part by the cretaceous rock, Golden Cap, which lies directly on top of the Triassic due to the unconformity. 

Tectonic History

There have been three major period of deformation in that have affected the Dorset/Devon region.  In the Jurassic Coast website a history of the tectonic past reveals that in the exposures in the Triassic,

“…record evidence of the gradual destruction and denudation of mountains formed in the Variscan orogeny of 330-280 million years ago, and the establishment of a widespread marine environment within a Jurassic basin, formed during the opening of the Atlantic Ocean (http://www.jurassiccoast.com 2006)”.  

The continual break up of Pangea during the early Jurassic and the rifting (tension force) that came with that created many normal faults throughout southern England.  The last period of uplift occurred during the Tertiary period (~10 million years ago) and was caused by compressional forces.  The Alps were forming in neighboring Europe and the compression affected the rocks east to Durdle Door and the Lulworth Crumple.  Thrust faults and folds associated with the formation of the Alps are common on the Jurassic Coast.  Some of the older normal faults were reactivated in the compressional period and became reverse faults. 

There are many local tectonic complications on the Jurassic Coast, but on a large scale there is one major feature that is important.  Throughout East Devon and Dorset the beds all dip to the east.  This is crucial to the succession of outcrops along the coast.  The Triassic all the way to the Cretaceous is exposed along the Coast.

 

Erosion Processes

            The shoreline processes along the Jurassic Coast are spectacular in many ways.  Huge vertical cliffs, sea stacks (see fig 4), headlands, and even arches are common sights.  The differences in strength of the rocks, wave diffraction and longshore currents are the factors that determine what type of shoreline landscape will form.  The harder the rock the more resistant to the wave action it will be.  Once a headland is created wave diffraction may weather it back or isolate it, thus creating a sea stack (Greengrove, 2006).  

 

Figure 4: Old Harry Rocks (West 2006).         

River systems are the other major erosion force in the Dorset/Devon area.  The landscapes that the rivers will form are extremely dependent on the geology below the surface. The rivers will travel the path of least resistance.  Surface water will simply flow over resistant rock and leave ridges or highlands while eroding less resistant rock to form valleys or lowlands.   

General Geology

The current day geology of the Jurassic Coast is quite complex.  The periods of uplift and subsidence of the area have left the sedimentary beds faulted and folded.  It is beyond the scope of this paper to attempt to explain the history behind each piece.  In general the layers dip slightly to east so that there are younger rocks exposed to the east and progressive older rocks exposed to the west.  This is a unique situation.  If the beds were all horizontal the Jurassic Coast would not be well know.  The slight dip of the rocks and the fact that they occur on the coastline makes them ideal for the study of sedimentology.  Figure 5 shows a simplified cross section of the Jurassic Coast.

 

Figure 5: Cross Section (West 2006).

 

Landscapes and Land Use of the Jurassic coast

River Mouth Towns

Most of the towns and villages that are on the Jurassic Coast were built on the less resistant layers at the mouths of rivers.  A perfect example of this type of valley-coastline-town would be Swanage.  Located right on the coast, Swanage sits in a valley between the Cretaceous Chalk and the Purbeck/Portland Limestone ridges.  The valley was craved out by water because the less resistant Wealden lies directly between the two more resistant layers.  Creating a beautiful and effective little fishing/port village with cliffs to both the north and the south, but easy flat access to the ocean.  Many of the towns along the Jurassic Coast exist in very similar situations as Swanage.  Exmouth, Budliegh Salterton, Seaton, Lyme Regis, Charmouth, Bruton Bradstock, and West Bay are all towns located at the mouths of rivers that have cut into easily eroded rock (see Figure 6). 

Figure 6: An example of a river mouth town (Charmouth) and ridge formed by the Cretaceous Golden Cap (West 2006).

 

 

Natural Harbors

Weymouth has a complex and fortified landscape that makes it a very good harbor town.  The isle of Portland is connected to Chisel Beach to the north (see fig 1).  Together they protect Weymouth Bay by creating an eastern sea block.  Breakwaters have been added to the west to further protect Weymouth. 

The Isle of Portland is the southern limb of a large dome structure that extends to the north (Jurassiccoast.com 2006).  The peak of the dome has been eroded away and what we see at the isle of Portland is the resistant limestone that slopes gently to the southwest.  Portland has great access to the sea and an abundance of great building stone.  The Isle of Portland has been quarried for hundreds of years.

Lulworth Cove is a superb example of how the geology can affect the landscape. Locally there is dramatic folding of the rock layers.  Right on the coast the layers are tilted nearly 90˚ vertical. The Purbeck/Portland limestone had put up a mighty effort to resist the sea.  However, a geologically recent breach was made as the limestone gave way to the wave action.  Ever since the breach the less resistant layers behind the Limestone, the Wealden and Greensand, have been eroded more quickly than the limestone as well as the Cretaceous Chalk that acts as a backstop.  Lulworth Cove is a very symmetric little bay.  It continues to grow laterally as time goes on.  It is interesting to contemplate what the future holds for Lulworth Cove.

 

 

 

Figure 7:  Rough Sketch (GUS 2006).

 

Figure 8: Lulworth Cove (West 2006).

In Figure 7 and 8 it is easily see where the limestone has been breached and the less resistant beds have been eroded more quickly than the Chalk.

Quarrying and Smuggling in Beer

Beer is located in a very picturesque and secluded setting. There is a normal fault to the east of beer that takes the Cretaceous down to the Triassic.  This is deceiving however because at this locality there exists an unconformity of about 100 million years.  So, the offset is not quite as significant as you might think, but still impressive.  Figure 9 shows a cross sectional view of the geology around Beer.

Figure 9: Geology at Beer (West 2006).

 Even though the hanging wall of the fault is made of hard Chalk a river has eroded through it and down to the sea.  The Beer syncline surrounds the town with steep sharp cliffs on either side of the town.  The Chalk creates large cliffs and headlands that protrude out into the sea from either side of town. 

Beer is well known for its world class building limestone, the Beer Stone.  The Beer stone is a thin layer that lies beneath the massive Cretaceous Chalk layer and is accessible at the base of a local hill where the layer dips down below the ground level.  At this locality there is a network of man-made caves for quarrying this pure-white limestone.  The caves are quite extensive and contain a lot of history. 

John Scott is a very knowledgeable tour guide at the Beer Caves.  He grew up in Beer as did his family for generations.  Scott has a great passion for the stories about the workers in the caves and of the town of Beer itself.  The Romans were the first to use the cave quarry.  According to Scott, the Romans followed the Beer stone layer as it dipped from a nearby cliff to the base of a the hill (Scott 2006). The Romans must have known that the stone would be there if they dug into the ground.  From a geologic perspective, the fact that they were able to understand that stratrum can be continuous over large areas is breathtakingly ahead of their time.

 Many of the other stories that came from the Beer Caves were related to smugglers.  The seclusion of the town of Beer made it an ideal place for smugglers to avoid the harsh customs of any imported good coming into England in the eighteenth and nineteenth centuries.  The quarrymen, with their hands-on knowledge of the caves used to store the smuggled goods in the Beer Caves (Scott, 2006).

Fossils and Landslides

Near the towns of Lyme Regis and Charmouth the exposures along the coast are mostly Jurassic sediments.  World famous landslides are occurring in this area.  The Black Ven, Shales with Beef, and other Lower Lias members are capped by the Cretaceous Greensand.  This creates a situation where the cap rock produces a cliff, but the underlying rock is too fragile to support a steep cliff.  What happens instead is a very dynamic terraced landscape forms and is continually eroded by the sea.  The significance of this is that there are always new exposures and thus new fossils to be found amongst the landslides and the marine-reptile fossil rich beds.  Mary Anning was known for always venturing out to the landslides when there was a storm in hopes for a new landslide face.  In the picture below (fig 10) the terraced landslide can be seen clearly.

Figure 10: Black Ven Landslides (West 2006).

The Isle of Purbeck and Corfe Castle

One of the most significant landscapes in Dorset for humankind would have to be the Isle of Purbeck.  This small broad peninsula is not technically an island (see fig 11).

Figure 11: Geologic Map of the Isle of Purbeck (West 2006).

There is a large ridge made out of the Cretaceous Chalk that separate a small broad peninsula from the rest of England.  The structural geology of the isle of Purbeck is basically that all the beds dip to the north.  The whole area is part of what is known as the Purbeck monocline.  The affect of this geology is that the resistant beds, the Chalk and the Cretaceous/Jurassic Limestone, form two ridges and the Wealden is eroded between them to form a valley. 

During medieval times the chalk ridge was effectively a barrier from the rest of England.  The Isle of Purbeck was literally in isolation. Virtually the only way to get to and from the isle, without using a boat, was to go through the town of Corfe.  Corfe is located at one of the only breaches in the chalk ridge.  For some reason the chalk was weakened here and a small river was able to erode through creating a gateway into the Isle of Purbeck.  Corfe was obviously a very important town for the trade industry in Dorset. 

The river that cut through the chalk split on the north side of the chalk ridge from one river (evidence that the Isle of Purbeck is an island after all).  Due to the two separate rivers cutting through the chalk nearly parallel to each other an impressive hill has been created.  This chalk hill stands alone with steep sides in every direction (see fig 12).  It’s the perfect location for a castle; in a major gateway town and in an ideal defendable position.  In fact Corfe Castle which stands in ruins yet proudly at the top of this peak and was a stronghold for royalty during the English Civil War.

Figure 12: Corfe Castle and Chalk Hill (West 2006).

Land Use

            The most obvious use of the land in Dorset is the use of stone for a building material.  Rock fences, rock homes, entire villages and towns built out of mostly limestone.  In London you’ll find stone from the Dorset Coast in such buildings as St. Paul’s Cathedral or in Swanage the post office or the Lyme Regis Museum. 

            Tourism has as been a major use of the land in the past and will continuing to be in the future at the Jurassic Coast.  The stunning beauty of the cliffs, sea stacks, coves, beaches, and quaint ocean side towns is certainly worth the trip. Fossil exploration is a scientific endeavor, yet the towns of Lyme Regis and Charmouth have definitely been able to turn it into a leisure activity as well. 

            Farming and grazing are other major uses of the land in the Dorset and east Devon area.  As mentioned earlier the agricultural areas are found in the clay-rich river basins that drain off the more resistant rock layers.  A good example of a fertile river valley would be the valley between the chalk and the limestone on the Isle of Purbeck.  Let me tell you, they are doing something right because I’ve seen the pigs they grow out there and they are enormous!

Conclusion

I hope it has become obvious that there is a huge connection between geology, landscapes, and human societal infrastructures.  The Jurassic Coast is not the only place where this connection exists.  Most people don’t realize what type of rock is under their feet and how that affects the lives of everyone around them.  The fact is that it matters a great deal and understanding geology will provide knowledge which will help us be smarter, make better use of the landscapes, and become more effective when building new infrastructures. 

On a personal note, I would like to say that until recently I had no clue the Jurassic Coast World Heritage Site even existed.  For that matter I had no idea the UNESCO existed.  I’ve learned that “World Heritage Site” is just another way of saying “International Park”.  

I was fortunate enough to take a course through the University of Washington at Tacoma where we spent two weeks in Dorset.  I see now why this area is so well protected.  The Jurassic Coast is a wonderful place in many different ways.  I encourage anyone who has read this paper to check out some of the references of this paper to learn more.  I especially recommend the website done by Ian West. Most.  Most of the figures from this paper come for this unbelievable website.

References

 

Boggs, Sam Jr. (2001) Principles of Sedimentology and Stratigraphy, third ed., Upper 

Saddle River, New Jersey, Prentice Hall, 726 pp.

 

Davies-Vollum, Sian , Phd, Professor of Sedimentalogy,  University of Washington of 

Tacoma,  2006.

 

Greengrove, Cheryl, Phd, Professor of Oceanography, University of Washington of

Tacoma,  2006. 

 

House, M. R., Lister, C.J. (1989) Geology of the Dorset Coast, Geologist Association

Guide, 43 pp.

 

http://www.jurassiccoast.com (2006) Jurassic Coast Dorset and East Devon: World

Heritage Site, UNESCO. 

 

Scott, John (8/3/2006) Live Tour of the Beer Caves, Jurassic Coast World Heritage Site

 

Solan, V.F.,  Birkeland, P.W. (1993) Introduction to Physical Geology: Lab Manual,

third ed, University of Colorado, Upper Saddle River, New Jersey, Prentice Hall,

287 pp.

 

West, Ian (2006) http://www.soton.ac.uk/~imw/index.htm,Geology of the Wessex Coast.

 

Winchester, S. (2002) The Map That Changed the World: William Smith and the Birth of

           Modern Geology, New York, HaperCollins Publishers Inc. 352 pp.

 

 

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