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Geological map of Devon

Geological map of Devon showing the location of different types of rocks

Rocks

RAMM holds a comprehensive collection of local rocks that reflect the geological diversity of Devon. They draw us a picture of the county as we have never seen it before: a terrain covered by ancient tropical reefs or vast desert dunes, an area disturbed by sudden rock avalanches or erupting volcanoes.

Devon’s rolling hills, multi-coloured cliffs, narrow gorges and steep valleys tell us a story of an eventful past. Red sandstones, hard granites or grey slates are the main characters in a tale that started about 400 million years ago on the southern hemisphere of the Earth.

Granite

Granite is best known in Devon as the rock that forms tors and hill tops on Dartmoor. It was once molten rock that rose from inside the earth. Once it had reached near- surface levels it cooled down very slowly and turned into a vast body of solid rock. This body extends all under Dartmoor and further West into Cornwall and the Isles of Scilly.

Over millions of years erosion and weathering transformed the Dartmoor granite. Rain, humidity, freeze and sun weakened the usually resistant rock. Cracks and holes, huge blocks and boulders broke off. The process continues today and shapes bizarre landmarks such as Hay Tor or Bowerman’s Nose.

People build pavements, houses and bridges out of granite because it splits easily along fractures into regular blocks. Prehistoric standing stones and stone circles on Dartmoor are made of the gritty rock as well.

Read more: Dartmoor National Park Discover Dartmoor

Volcanic ‘trap’

Who would have thought that Exeter was once a place of rumbling volcanoes spitting lava and fire? Take a closer look at what is underneath your feet or in surrounding buildings on your next walk through Exeter and you will find clues to an explosive past that started during the Permian period.

And then?

Since the Permian large parts of the volcanic deposits have been eroded. Fragments of volcanic rock occur as angular components in much younger rocks which were laid down by rivers and flash floods during the Triassic period.

Not only natural forces had a go at the ancient volcanic rocks. Humans have quarried the durable stone since Roman times to build churches, walls and arcades.

Why ‘Trap’ rock?

The term ‘trap’ rock has been used by quarrymen, builders and architects for centuries. It derives from a Scandinavian word meaning steps or stairs. Volcanic rock tends to appear in orderly structures resembling piles of blocks, sometimes reminiscent of stairs.

Read more: Exeter City WallExeter Trap Stone

New red sandstone

Who has not been to the beach in Exmouth or Teignmouth admiring the red cliffs in the glowing evening sun? Today an attraction for locals and tourists, the thick piles of sand, gravel and rock debris once formed in a dusty desert. During the Permian and Triassic period Devon was positioned near the equator. The climate was dry and the county resembled a modern savannah. Wind and water shaped the landscape.

Dawlish and Lympstone

Ancient winds piled vast amounts of sand up into big dunes. The red cliffs at Dawlish are an impressive exposure of what were once, in effect, dynamic desert dunes as they exist in the Sahara today. The angle of the torrential rains wore down the uplands that then existed to the west and washed vast amounts of gravel, sand and mud into surrounding gorges and plains. Layers of ancient rubble with sharp-edged stony fragments, the so called breccia, filled the valleys. The cliffs near Lympstone are an excellent area to study the breccia which can be up to 400m thick in places.

Exmouth and Budleigh Salterton

The cliffs between Exmouth and Budleigh Salterton are much more muddy. Not torrential flash floods but slow moving rivers deposited the fine red mud. Huge and powerful rivers shifted tons of gravel from Northern France to Devon. The up to 30m thick Budleigh Salterton Pebble Beds tell a fascinating story that goes back to the early days of this planet.

Ladram Bay

The reddish brown and orange cliffs near Ladram Bay have a different story to tell. Great rivers formed stacks of sandstone with peculiar angular patterns. This so called ‘cross-bedding’ is characteristic for sediments laid down by rivers. About 240 million years ago, long before the English Channel formed, these rivers flowed from further south to the north.

Bones of early reptiles such as the rhynchosaur are hidden within these sandy cliffs. Remains of fish and amphibians have been discovered in the finer muddy deposits. Plants that probably grew profusely on sandbars and in abandoned river channels occur in the sand but are rare finds.

Pebble beds

About 245 million years ago, long before the English Channel developed, huge powerful rivers eroded blocks of Ordovician quartzite in Northern France and shifted them northwards into Devon. The rivers deposited the big heavy pebbles in thick layers across the hot desert that covered Devon at the time.

Wind sculptures

Once exposed to the air powerful desert winds started to shape the pebbles. Small sand grains driven by the wind abraded and polished the pebbles and turned them into sculptured stones, called ventifacts.

Taken by the sea

Today exposed in the cliff sections along the beach at Budleigh Salterton the pebbles have still not come to a rest. Wind and water wash away the soft mud that holds the pebbles in place in the cliffs. Thousands of pebbles drop out of the cliff every year and add yet another stone to the beach. Sea waves and tides swallow some of the beach pebbles each day and push them out into sea. One wonders how this story is going to continue.

Hidden creatures

Ever seen someone with a hammer on the beach being busy splitting pebbles? Some of the rounded stones hide remains of prehistoric life from the Ordovician period. One can find fossilised shells of ancient mussles, lamp-shells and trilobites that once lifed in a sea covering the North of France.

Read more: East Devon Pebble Beds Project

Chalk

Once up on a time, when dinosaurs ruled the land and early birds conquered the air, Devon was covered by a shallow warm sea. The turquoise water was teaming with life including sea urchins, shells, sharks and sponges.

The sea floor was not covered by golden sand but by a white muddy substrate that consisted of incalculable numbers of tiny organisms called coccolithophores. They are single-celled algae that float through the ocean waters while alive. After death, their delicate white outer shells sank to the bottom of the sea, bit like snow, and piled up in thin layers. Over millions of years the layered mud hardened and turned into a solid stone that we call chalk today.

Powerful movements in the top layer of the earth lifted the chalk out of the sea, tilted and bend it. It rose high above the sea forming gigantic cliffs.

Geologists believe that large areas of Devon were once covered by chalk but weathering and erosion washed away most the soft sediment. A most spectacular exposure can be admired at Beer Head in East Devon. The piles of chalk stretch from there all the way into Dorset and Sussex.

Hidden in the chalk

The chalk contains a precious raw material that humans have used for thousands of years. One can find lumps of a dark grey rock called flint scattered through the white sediment. Flint is a very hard rock that is easy to split. It was an essential resource for early humans to make stone tools like hand axes, scrapers and arrow heads. In modern times people use it to built or decorate walls.

In some flints one can find amazingly preserved fossils. The very fine-grained to glassy stone has the ability to immaculately preserve many and very small details of a once living creature.

Some of the irregularly shaped lumps of flints which are also known as ‘nodules’ are aligned in layers that one can trace along the coast of Devon or Dorset for miles.

Greensand

The Blackdown Hills, the East Devon plateau and parts of the Haldon Hills contain seams of a hard and weather resistant sandy stone called Greensand. The now prominent ridges and high grounds were once sandy patches at the bottom of a shallow sea. Waves and currents moved tons of sand across the sea floor and piled it up thick layers.

The name Greensand derives from the typical olive-green tint of the rock. Amongst the millions of quartz grains one can spot small dark green specks of the mineral glauconite which generally only forms in marine conditions.

Tool making

In the 18th and 19th century a specialised tool-making industry grew in East Devon. Using local, very porous and abrasive sandstone, people started making sharpening tools, called whetstones or Devonshire ‘Batts’. The whetstones were used for sharpening edged tools, like the blades of scythes and sickles.

Pieces of sandstone the size of a horse’s head were obtained from horizontal shafts dug into the hillsides of the Blackdown Hills. They were then roughly shaped on the spot just outside the mines. People used a unique double-headed tool called a ‘basing axe’ to work the stone. The axes were made locally in Kentisbeare.

The stones were then brought down to the local villages to be hewn to their final dimensions of about 30 centimetres in length. Afterwards, women rubbed them down in hot water on a large stone of the same material. Shaping the stones was relatively easy since the rock was very moist and soft when it came out of the mines. Once tried, the whetstones became very hard and were fit for sale. They were taken to the ports of Topsham and Bridgewater to be shipped to London.

For 200 years the industry flourished, providing whetstones of a very high quality to a huge market. By 1900 there were only three mines left. By 1910 the invention of Carborundum stones spelt the end of whetstone mining. The last solitary miner left in 1929.

Coral reefs

Most people are busy watching the anglers trying to catch yet another mackerel or pollack at the bottom of the cliffs when visiting Hope’s Nose near Torquay. On your next visit have a look at what is underneath your feet and you will find intriguing white structures in pale grey rock.

About 390 million years ago, during the Devonian Period, what we call Devon today was an area at the bottom of a tropical sea on the southern hemisphere. Sunlight flooded the waters that were teeming with life. Corals and sponge-like stromatoporoids grew in turquoise waters and formed large reefs. Trilobites and snails lived inbetween the reefs. Once hardened into what is called a limestone shells and external skeletons of some of the organisms became preserved as fossils.

RAMM holds an extensive collection of accessioned and unaccessioned polished specimens that came from various locations. Good exposures of Devonian limestone can be admired at Hope’s Nose Torbay and Chudleigh.

Tektites

The idea of rocks falling from outer space can get people very excited. These fascinating messengers from the sky can give us information about the age of the solar system, and the formation of planets and the asteroid belt from which they came. Most small meteorites burn up in the Earth’s atmosphere and we see them as shooting stars, but the larger ones will hit the Earth at tremendous speed. When they impact the Earth they can leave large craters.

A less well known result of meteor impacts are tektites. Tektites, from the Greek ‘tektos’ meaning molten are usually dark glassy objects that are believed to have been melted from the Earth’s crust when major meteorites impacted the Earth. This molten rock was then thrown up into a sub-orbital trajectory before falling back to Earth.

We know of four ‘strewn fields’ of tektites. Three of these fields are associated with known meteorite craters. However, the most extensive, which covers parts of Indonesia and Australia, does not have a known source.

RAMM has just acquired six Australite Tektites for its collections. They were collected by an army engineer in the Nullarbor region of South Australia in 1962 (hence the local name ‘Australites’). They are probably around 700,000 years old.

The tektite on the left in the photo above shows its natural shape which was caused mainly by its re-entry through the Earth’s atmosphere. Aborigines flaked the one on the right to form a tiny tool. Both are about a centimetre in diameter.

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