The rocks of Cornwall have an amazing story to tell. They have been on a journey of 8,000 miles in just 400 million years. This journey has included tropical seas, deserts, volcanic eruptions and hot granites, mineral vapours rich in tin and copper and ever changing climate and sea levels.

Our Cornish journey starts in an ancient tropical ocean south of the equator where primitive fish swim in the warm waters and deserts stretch north across England and Europe. At this time you can walk to America.

On the horizon the African supercontinent slowly moves towards us as volcanoes erupt ash and lava. The ocean floor is squeezed away and the Lizard emerges. Further north, Coal Measure swamps and deltas form. Continents collide over a period of 80 million years, buckling the strata. If we could stretch the rocks out flat again Cornwall would be 300 miles long. Cornwall becomes part of Pangea. Click to jump to our Wow! page to see an animation of Cornwall's plate movements!

Hot granites from inside the earth rise and cool to form the backbone of Cornwall. On their margins veins rich in tin and copper await the arrival of man 250 million years later. Warm seas lap our Cornubian Caribbean island while dinosaurs roam the landmass further east. The Atlantic Ocean finally opens up as Cornwall continues its journey northwards. Chalk and flint are deposited in the clear waters and a mass extinction event wipes out the dinosaurs and many other species, just 66 million years before the present time.

Sea levels fluctuate again and West Penwith becomes an island. A beach forms at St Erth. Ice sheets then move southwards, they miss Cornwall but there are icebergs offshore. Sea level drops over 300 feet and it would be possible to walk to France. Man appears and is faced with tundra conditions including permafrost, cave dwellings, wolves and bears.

What has changed since?

The ice sheets have melted, river valleys have flooded and the English Channel has cut us off from mainland Europe. Erosion and weathering have shaped the Cornish landscape and formed our magnificent coastline.

Man has discovered the mineral legacy in our rocks. They gave us mines, mineral wealth, building stones and china clay. Cornish inventors, such as Richard Trevithick who pioneered high pressure steam locomotion, pushed back the frontiers of science and discovery. Poets, writers and artists were - and are! - inspired.

Below is a BGS (British Geological Survey) map of the main rock types found in Cornwall .

Now let the rocks tell their story, choose your destination now...

Granites The Lizard Killas Coastline

Granite

Granites form the backbone of Cornwall stretching from Dartmoor in Devon westwards to the Isles of Scilly. With their associated minerals they have underpinned the Cornish economy for hundreds of years. Granites are still used today in the China Clay industry, as building stones and as great climbing pitches for budding mountaineers. They also provide spectacular scenery and the infamous radon gas.

The granites started life around 300 million years ago. The first sign that things were really cooking deep down was the intrusion of lamprophyres, strange rocks derived from the Earth's mantle. The easiest place to see an example of these rocks is on the beach at the south end of Prisk Cove near Mawnan Smith. The rocks that started life in the high pressure and temperature conditions of the mantle are now very weathered and lying on a beach roamed by cows!

Soon afterwards, the main bodies of the granites started to rise up one after the other, like giant hot air balloons, into the overlying rocks. As they rose, they pushed like fingers into the surrounding slates (killas) of which they swallowed great chunks. You can still see these foreign slate bodies ('xenoliths') within them. The best places to see giant tapering fingers of granite in the slates are at Megilligar Rocks near Porthleven, or Porthmeor and Wicca Pool on the north coast of Penwith. It was here that Henry De la Beche, who was to become the first director of the British Geological Survey, sketched the jagged granite boundary more than 150 years ago.

What was happening on the surface above the granites? Most of the overlying strata has been removed by erosion but there is evidence, at Withnoe and Kingsand, that the granites literally blew their top in the form of violent volcanoes like Vesuvius or Mount St Helens in North America in more recent times. Here rhyolites, the fine-grained version of granite, can be seen as a volcanic neck and lava flows, and there are even some volcanic ash deposits.

When the rising granite balloons reached as far upwards as they could go they started to solidify and form a crystalline mass. This initial crystallisation caused the remaining molten rock to change composition and become increasingly volatile. Every so often the pressure got so much that the volatile mineral-rich components escaped through fissures within and beyond the granites. This spectacular geology can be seen at places like Cligga, Roche Rock and the mineral lodes and elvans which criss-cross many parts of the county.

A long period of more than 200 million years separates the formation of the granites and associated mineral lodes from today. During much of this huge span of time, which saw the dinosaurs come and go and flowering plants evolve, the climate was tropical and dominated by monsoons. The resulting downpours caused weathering deep into the ground, rotting the granites. This effect was particularly marked in the granites around St Austell where the feldspar minerals were transformed into kaolin, more popularly known as China Clay. The Eden Project is located in one of these deeply weathered pockets from which most of the better clay had been removed leaving low grade clay in weathered granite and also unaltered granite.

During the Ice Ages the rotten rock was further broken up by the growth of ice crystals repeatedly forming in the freezing conditions. During the thaws in the arctic summers the rotted material became so waterlogged that it slumped downslope leaving the stronger unweathered parts of granite to stand proud as tors. Excellent examples occur throughout the region, like the Cheesewring and other tors on Bodmin Moor and Pulpit Rock on the Isles of Scilly.

Lizard

The Lizard peninsula is Cornwall's and Britain's most southerly point and its geology, landscape and flora are very special too. Nowhere else in Cornwall has such a density of nationally recognised Sites of Special Scientific Interest (SSSIs) or regionally important County Geology Sites (formerly known as RIGS). The main reason for this is that the rocks on the Lizard are totally different from the rest of Cornwall.

The most extensive rock type is the serpentine which, spanning 20 square miles, is the largest outcrop of such rock in mainland Britain. It is found nowhere else in England. When Queen Victoria and Prince Albert visited the area in 1846, they were so struck by serpentine's unusual characteristics that their ensuing royal patronage spawned an industry of architectural and decorative stone working that continues, albeit much diminished, to this day.

Originally this altered peridotite was thought to be the root of a volcano, but it is now recognised to be part of the Earth's mantle, today normally tens of kilometres below your feet, which was bulldozed onto the newly evolving Cornish mainland in front of an advancing continent. The actual junction between the Earth's mantle and the crust is today exposed on the foreshore at Coverack.

This is why you will often see visiting groups of university students on the beach investigating the relations between the serpentinised mantle peridodites and the overlying crustal gabbros. It was a Croatian meteorologist called Mohorovičić who, having constructed his own seismograph to study earthquake shock waves, discovered the distinct boundary that exists between the lighter crustal rocks and the denser mantle rocks, and his name is given to this junction. Today we have shortened it to the Moho.

Between Coverack and Kennack Sands there is a marked narrow promontory known as Carrick Luz. This is the remains of a great fissure in the serpentine that acted as the feeder for the overlying gabbro. In a few places, such as around The Manacles, Leggan Cove and the West of England Quarry at Porthoustock, instead of gabbro we can see dolerite and basalt dyke swarms that injected up through the gabbro onto the ancient sea floor. The actual lavas have since been removed by erosion or metamorphosed almost out of recognition.

Some of the oldest known rocks in the whole of Cornwall occur around The Most Southerly Point. These rocks, the Man of War Gneiss and the Old Lizard Head mica schists, are over 500 million years old. The gneiss, with its corrugated texture, can be seen only as boulders on the beaches around The Most Southerly Point. They have been brought in by the waves from the offshore reefs, such as Shag and Labham Rocks. Another rock unique to the Lizard is the enigmatic Kennack Gneiss. No prizes for guessing where it is best exposed.

There is a huge time gap of 350 million years between the great events that emplaced the rocks of the Lizard on Cornwall, and today. It is becoming clear that many of the faults that control the development of the current shape of the coast and the location of the inland valleys had formed in the first 75 million years after the Lizard was thrust up. It is within these and associated cracks that you find spectacular zeolite minerals, very different in nature from the minerals associated with the granites. Particularly fine collections can be seen in the Museum of Cornish Life in Helston and in the Rashleigh Gallery at the Royal Cornwall Museum in Truro.

It is probable that much of Cornwall and the Lizard were covered by the high sea levels that deposited the chalk elsewhere in Britain 65 million years ago. Inland the evidence has been eroded away but flint (weathered out of chalk) is regularly washed up along the south coast, perhaps from the still existing bed of Cretaceous rocks under the sea off Black Head and The Lizard Point. Nearby Loe Bar (SW 643243) is composed of 86% flint.

Just before the turn off for Coverack on the main Helston to St Keverne road there is a small exposure of quartz-rich pebbles and cobbles of Tertiary age, probably deposited 25 or more million years ago and derived from the killas and granites north of the peninsula. There is small picnic area here which should be treated with respect for this really is ancient ground. You can see the quartz Crousa Gravels on the banks of the little pool, known as Dolly's Pool, here. This area is a County Geology Site.

Since these gravels were laid down there has been a tremendous amount of erosion. Much of the peninsula, being composed of hard, dense serpentine gabbro and schists has remained relatively high and treeless, whereas the softer Kennack Gneiss, fault zones, and killas country (The Meneage) that arcs around the peninsula have been eroded into deep wooded valleys.

Each district of the peninsula has been uniquely shaped by its underlying rocks and nowhere is it more evident than near Lizard Town itself. Here you can see a vast difference between the fields to the south of the village and those a little further north and west. The very tip of the Lizard is composed mostly of schists and these are quite fertile and grow cauliflowers and potatoes. Whereas, close by (to the north and west) you will see heathland that is quite barren in appearance and this overlies the ultrabasic serpentine with its poor soils. You may also be lucky enough to see the emblem of Cornwall itself: when a pair of choughs recolonised Cornwall from Ireland, in 2001, they bred in a cave by The Most Southerly Point. Twenty years on, after the original pair were joined by others widening the gene pool, choughs can be seen all around the Lizard, and as far as the Roseland, as well as around West Penwith and up to Newquay and Port Isaac.

Serpentine and gabbro produce magnesium or calcium rich soils and it is the resulting alkalinity of the soils that has enabled a large number of quite rare plants to thrive on these parts of The Lizard. These include dropwort, salad burnet, bloody cranesbill and the rare Cornish heath which is only found on The Lizard in Britain. On the flattest parts of the peninsula underlain by serpentine, is a fine quartz-rich windblown dust, known as loess, that was derived from the area now occupied by the Celtic Sea during the last cold phase of the Ice Age. The quarry by Goonhilly Earth Station, which is a County Geology Site, is the reference section for loess in Cornwall.

There is a tremendous amount of variation among the farms and villages of individual Lizard parishes. Every village and farm is unique because, throughout history, people have always used the local rocks to build with. For example, on the areas underlain by gabbro, the land had to be cleared of large boulders (core stones) to make fields. These ended up in the stone-faced Cornish hedges or in the walls of farmhouses. In places like Main Dale and Crousa Downs you can still see what the landscape would have looked like in the distant past. The churches on The Lizard characteristically have beautifully shaped and polished serpentine fonts, lecterns and bible stands from Victorian times. The medieval Landewednack church, east of Lizard Town, is notable for its checkerboard tower, composed of alternating blocks of granite and serpentine.

Killas

Killas is a wonderful Cornish miners' term used to describe the sediments metamorphosed by the pressures of subsequent mountain-building episodes and heat emanating from intrusion of the granites. They are a very important sequence of rocks as they underlie two thirds of the county. They are mostly of Devonian age. Strata of the overlying Carboniferous are confined to the north of the county.

The killas sediments were deposited in many different environmental settings. We know this from the rocks themselves and their fossil content, which help us to piece together the type of ancient environments existing in Cornwall at this time. Trilobites, crinoids, corals and bivalves (including the famous "Delabole butterfly") occupied the shallow shelf areas; brackish water fish the intertidal areas; and primitive fresh water fish the pools, rivers and lakes of the lowland fringes. In the deeper basins, fetid anaerobic conditions prevented sustained life due to the lack of oxygen. However, plant debris and a few free floating creatures that died and sank into these hostile conditions, have been preserved.

The intrinsic geodiversity of the killas was further enhanced shortly after deposition when the sediments were bulldozed into great folds and slices, and literally dismembered. Subsequently beneath the resulting mountainous pile, molten granite started to form and rise up into the sediments above, heating and baking them. By this time Cornwall was part of a new supercontinent called Pangaea of which we remained a part until the late Jurassic period but that's another story.

Devonian 419 to 359 Ma (million years ago)

Britain lay just south of the equator and the site we now know as Cornwall was covered by an ancient tropical sea, the Rheic ocean. Its coastlines lay to the north across north Devon and southern Wales, and to the south in southern France and North Africa. The coastal and marine Devonian strata of Cornwall and Devon differ from those seen elsewhere in Wales and northern Britain, which were part of an arid continent with alluvial fans, debris choked rivers, desert lakes and dunes. The Devonian is divided into lower, middle and upper; the lower being the oldest. Very few people realise that there is a major dislocation in the Earth's crust which divides the county in half, known as the 'Start-Perranporth Line'. It crosses from Holywell Bay to Pentewan and was probably in existence before Devonian sedimentation took place and certainly during the mountain building which took place at the end of the Carboniferous. Rocks of proven Lower Devonian age only occur north of the line and can be seen at Watergate Bay on the north coast and around Polperro/Fowey area in the south. They are the purple and green Dartmouth Slates and were deposited in shallow water, but no marine fossils are found, only primitive freshwater fish.

These non-marine Dartmouth Slates represent the closest conditions in Cornwall to those that persisted further north in Britain throughout the Devonian. They are the oldest sedimentary rocks in Cornwall. Later the water deepened slightly and sea invaded Cornwall, so on top of these rocks are the Meadfoot Beds, which are slates, siltstones and sandstones, with occasional beds of limestone indicating shallow waters. Volcanic activity occurred at this time and various agglomerates and tuffs can be seen around St Austell Bay and Carlyon Bay, where squished lava blobs and ash, together with some small intrusions in the form of sills, are exposed.

At the end of Meadfoot times the sea again became shallower and the sandstones and muds of the Staddon Grit were deposited in front of a delta. They can be seen on the shore east of Kingsand. An example of the overlying Middle Devonian rocks are the Trevose Slates of north Cornwall and the Padstow area. They consist of muds, now slates, containing sections of crinoid stems (ossicles), fish, goniatites, trilobites, brachiopods and corals, mostly replaced by pyrite but still distinguishable. The sediments indicate deposition in a moderately deep basin, perhaps sited to the outer side of the continental shelf.

In Upper Devonian times, mainly muds were deposited and are now found in north Cornwall, for example, Upper Delabole Slates, Tredorn Slates and Polzeath Slates. The deeper water here allowed only the finest grained muds to develop which have since been heated and squeezed producing high quality Cornish slates, especially in the Delabole and Tintagel areas. Volcanic activity also took place in the north of the county at this time, especially at Pentire Head, where lava oozed out on to the sea bed and cooled and consolidated as pillow lavas. Sediments south of the Start-Perranporth Line were laid down in a deep water basin; the Gramscatho Basin. It was here that the first effects of the northward advancing Gondwana continent were felt, as it shed larger and larger debris in front of it the closer it came.

As a result this basin contains Middle to Upper Devonian sediments. These range upwards from deepwater radiolarian cherts and muds, through sandstones to coarse breccias (angular coarse fragments) and even huge blocks of pillow lavas, pre-Devonian limestones and quartzites. These can be seen in the Roseland Breccia near Carne, from Mevagissey to Pentewan beach, and north of Porthallow on the Lizard Peninsula. To the northern side of this basin Porthtowan Formation basinal slates and outer shelf deposits of Mylor Slates and Porthleven Slates were deposited in Upper Devonian times.

Carboniferous 359 to 299 Ma

At the end of the Devonian and continuing into the early lower Carboniferous the sea deepened slightly and black muds, siltstones and some thin limestone bands (in shallower areas) were deposited. An example of this can be seen at the Barras Nose Formation at Tintagel, which consists of siltstones and shales interbedded with thin crinoidal limestones. Lower Carboniferous rocks are amply demonstrated south west of Rusey between Trebarwith Strand and Boscastle.

The geological interest of these 300 million year old sediments is highlighted in Thomas Hardy's third novel 'A Pair of Blue Eyes', published in 1873. At one point in the story Henry Knight, an essayist and geologist, is clinging to the high cliffs just to the north of Boscastle when ... “opposite [his] eyes was an embedded fossil, standing forth in low relief from the rock. It was a creature with eyes. The eyes, dead and turned to stone, were even now regarding him. It was one of the early crustaceans called Trilobites. separated by millions of years in their lives, Knight and this underling seemed to have met in their death”.

Volcanic activity prevailed in Lower Carboniferous times too and superb squashed lava bombs can be seen below the Port William pub at Trebarwith Strand, along with volcanic ash. This volcanic activity continued as far as Launceston where some volcanic rocks were used to build Launceston Castle.

Sediments of Upper Carboniferous age are confined to the north east of the county. These strata are of deltaic origin and represent southward extension of the delta flats on which tropical swamp forests were established further north in Wales and the north of England.

Coastline

Cornwall's spectacular coastline stretches for over 400 miles and is the longest of any county. Take the challenge of 731 feet high cliffs between Bude and Boscastle. Go across the sandy beaches and golden dunes of Penhale, Perranporth and Gwithian. Walk around the granite peninsula of Land's End and past Lizard Point, the most southerly place on mainland Britain. Meander along the beautiful south coast estuaries and headlands from the Fal to the Tamar.

Every mile is controlled by the rocks under your feet and the continual action of the sea, wind, time and tide.

The coast is probably our most dramatic and dynamic environment and there have been some amazing changes since the end of the last Ice Age only 12,000 years ago. Sea level has fluctuated like a seesaw. Imagine being able to walk to France when the English Channel was an enormous river valley. You could have done this as recently as 8,000 years ago because much of the Earth's sea water was still locked up in the polar ice sheets. At the height of the last Ice Age sea level was about 400 feet lower than today and all of Cornwall's rivers cut long deep valleys. As the ice sheets melted sea level rose again submerging woodlands and forests that became fossilised. Our estuaries were flooded to form the deep water rias that now provide the sheltered harbours and anchorages along the south Cornwall coast.

Raised beaches provide evidence of even higher sea levels, some dating back to pre-glacial times. We even have curious glacial debris or erratics left behind from the Ice Age. Then Cornwall had a climate similar to present day Siberia or northern Canada. Rocks were shattered by the freezing conditions and icebergs floated offshore. On land the frost shattered debris, known as Head, accumulated on coastal and valley slopes and massive dust storms deposited layers of fine sand or loess across much of the county.

Experts tell us that sea level is still rising, the climate is changing and Cornwall is still sinking. Parts of Cornwall even bounce by about 4 inches every time the tide comes in and out twice a day, making Cornwall one of the bounciest places in Britain!

So where should you go to see the best geological features that the Cornish coastline has to offer? View our stunning Sites page to find out.

Tufa and speleothems around Cornwall

Local and national geologists and ecologists have been surprised by the discovery of tufa and its underground equivalent, speleothems, on the coasts of Cornwall.

These formations are found where rainwater flowing through lime-bearing deposits dissolves calcium carbonate. Carbon dioxide is lost from the water into the air, and calcium carbonate is deposited as tufa near water sources such as springs, or as speleothems in caves. These conditions usually occur in areas underlain by thick limestone, but Cornwall is so poor in lime-rich rocks, that limestone was imported from the 18th century onwards.

Tufa is a white to straw-coloured deposit formed on cliffs open to the atmosphere. While it is forming it is soft, spongy and has been largely ignored as ‘yucky’. As it gets older it hardens and becomes a rock. On cliffs it is commonly associated with green, filamentous algae close to sea-level, but higher up mosses take over. The plant material, as it decays, provides a framework for further carbonate deposition, leading to a naturally porous fabric. There are also higher plants characteristic of tufa deposits: the calcium-loving maidenhair fern for example.

The flowstone, stalactites and stalagmites of speleothems, are far more spectacular. The best example in Cornwall is the often photographed cave at Holywell Bay, but other examples have been coming to light in many places.

Where the calcium carbonate comes from is still a mystery. On the north Cornwall coast the beaches and dunes are known to have an abnormally high calcareous content due to all the shell fragments. However, it is becomingly increasingly clear that there must be other sources and that inland occurrences of tufa and speleothems in stream beds and on quarry faces, railway cuttings and mine shafts and adits need to be mapped to help solve the mystery.