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Some geological information about the area ... |
West Cornwall has many interesting locations which are both scenic and have interesting geology. The scenery is at least in part due to the geology as well as the climate over the past tens of thousands of years and present weather conditions. Cornwall's geology is largely related to events in the Devonian and Carboniferous periods 420 – 285 million years ago. At that time what we now call the British Isles was south of the Equator and moving north across the Equator (through plate tectonic processes). We were on the northern edge of what geologists call the Rheic Ocean (an ocean which was present but was later destroyed by plate tectonic processes). During the Devonian and much of the Carboniferous periods sediments were deposited on the continental shelf on the northern edge of the Rheic Ocean. These now form the rocks you see well exposed along the coast of Cornwall e.g. Godrevy Point, Hayle, and around Bude. These included turbidite sediments, flows of sediments across the sea floor, triggered by earthquakes linked to subduction of the ocean floor associated with the closure of the Rheic Ocean. Similar processes occur now around the Pacific Ocean margin. The turbidity flow sediments now form greywacke sandstone (which translates as grey rocks!).
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| The picture of Lamorna Cove above shows the jointed granite with the castellated coastline so typical of Cornish granite coastal landscapes. | |
As the Rheic Ocean closed a small fragment of ocean crust was caught up in the collision process and forms what is now known as the Lizard. The rocks were subsequently much altered but geologists have identified rocks that were formerly part of the ocean floor (basalt lavas etc.). The closure of the ocean resulted in a mountain building episode (the Variscan Orogeny) and this caused the sedimentary rocks to be intensely folded as seen at Godrevy Point, Gunwalloe Church Cove and between Boscastle and Hartland Quay. This folding caused some of the sedimentary rocks to be metamorphosed into slate (regional metamorphism involving pressure and heat). Some of the sedimentary rocks such as the sandstones were little altered.
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| The continental collision caused the crust to be shortened laterally but thickened vertically. This caused heating at depth and magma (molten rock) was formed. The melting of the continental crust formed granitic magma that was intruded high into the crust perhaps to within a km or two of the surface. The magma cooled at depth and forms the granite intrusions that we associate with S.W. England from Dartmoor to the Scilly Isles. Each individual intrusion is known as a stock or boss and they are part of a larger batholith. The magma was at least 700 o C and caused thermal metamorphism of the surrounding rock. This formed spotted rocks well illustrated at Mousehole, Cape Cornwall and Porthmeor Cove, and hornfels, a completely recrystallised splintery rock formed closest to the contact with the intrusion. | |
| The granite intrusions are generally coarsely crystalline as the magma cooled slowly; sometime individual crystals are very large (phenocrysts), especially of feldspar. These crystals are sometimes well formed with flat crystal faces (euhedral) as seen at Lamorna Cove. They also show flow textures as the crystals were aligned in the liquid magma as it was intruded. Sometimes the magma is very coarsely crystalline (pegmatite) as seen at Porthmeor Cove and Megiliggar Rocks. Other magma is very finely crystalline (aplite). This variation is thought to be due to the presence or absence of ‘water' in the magma. As the magma was intruded pieces of country rock were incorporated in the magma, the earlier ones were melted, the later ones can be seen as xenoliths in the granite – foreign rocks e.g. at Lamorna cove. |  |
| The image above shows granite with porphyritic texture and flow texture with large feldspare crystals and a xenolith - a piece of country rock that had not been fully assimilated by the granite. | |
| Minor intrusions (sills and dykes) were also formed. Sills (horizontal sheets) follow the bedding – approximately! One of these can be seen at Megiliggar Rocks. Vertical sheets (dykes) can be seen at Mousehole, Porthmeor Cove and Praa Sands. Inclined sheets, neither vertical nor horizontal also occur and are well exposed north of Cape Cornwall. | |
| Following the intrusion of the granite metalliferous minerals were formed from residual liquids left over from the magma. This produced the tin and copper ores that were so important in Cornwall in historical times and help to produce the current landscape. The tin ores are often associated with veins of quartz and tourmaline. | |
The uplifted mountains were eroded over millions of years to expose the granite intrusions. The eroded material forms the red rocks seen in east and south Devon formed in desert conditions of the late Carboniferous, Permian and Triassic. At this time Britain was in the middle of the Pangaea supercontinent and we were at the latitude of the Sahara Desert.
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| During the Pleistocene (the last 2 million years) Cornwall experienced periods of arctic tundra like conditions similar to N. Canada and Siberia. Freeze thaw action created large amounts of angular debris which then was transported by solifluction processes forming periglacial head which can be seen in many coastal section e.g. at Lamorna Cove | |
| The Lamorna Society is indebted to Alan Holiday for his very interesting talk, and guided walk, on this subject, and for permission to include on the website the information shown above. | |