The county of Cornwall, nearly 500km west of London, is a holiday playground with a stunning coastline, sandy beaches, picturesque villages, wild high moors and hundreds of great walks. For the geologist, however, south-west Cornwall, in particular Land’s End and the Lizard Peninsula, holds particular fascination.
The greatest influence on both the geology and landscape of Cornwall was the Late Palaeozoic Variscan orogeny, when a granite batholith, which forms the deep core of the Cornish peninsula, was emplaced, crucially folding and metamorphosing the surrounding sediments. Almost simultaneously, a tectonic front advancing from the south produced the intriguingly named Lizard Peninsula.
The Lizard ophiolite Complex
Attractive though the idea is, the Lizard Peninsula is not named after some mythical giant reptile, but is probably derived from a Cornish word for a high area or headland. Bounded to the west, south and east by the sea and about 15 km wide and 10 km deep, to the north the peninsula is almost separated from the rest of Cornwall by the Helford River.
The Lizard is geologically different from the rest of Cornwall because it is an ophiolite complex. A section of oceanic crust was thrust over continental crust after a collision between two continents, overlying metamorphosed mostly sedimentary rocks, originally slates, sandstones and volcanics, now changed into mica-schists and quartzite. These original rocks, known as the Old Lizard Head Series, are rarely exposed, one of the only places where they can be clearly seen being at Lizard Point. The oldest rock in Cornwall is the 500 million year old Man of War gneiss, which forms small rock islets off Lizard Point.
A classic ophiolite has at the base ultramafic rocks (those with little silica), representing the mantle, overlain by layered intrusive gabbros and volcanic dykes, with at the top pillow lavas and sedimentary deposits formed on the ancient seafloor. The junction between the gabbros and peridotite represents the boundary between the mantle and oceanic crust, and is marked by a fundamental change in seismic velocities due to the density contrast. Subsequent movement and faulting means that these rocks are not always found in an ophiolite complex, but most do feature somewhere on the Lizard.
Much of the peninsula is dominated by originally deeply buried mantle material such as peridotite. This contains large amounts of minerals like olivine and pyroxene, which have been altered to create the rock serpeninite – a rare example of metamorphism resulting from a decrease, rather than an increase in temperature and pressure. This rock, so named because it resembles snake skin, is banded and streaked with veins, and polishes to a very attractive red, green and black rock. It was extremely popular for ornaments in Victorian times and is still quarried in small quantities for tourist souvenirs.
One of the best places to appreciate serpentinite is Kynance Cove on the south-west coast of the Lizard. A famous beauty spot for centuries, it is approached down a steep winding path which suddenly opens onto the small bay, where the wet cliffs and small shoreline rock islets shine red and green where they have been polished by the sea.
One of the many ways in which geology is responsible for the distinctive landscape of the Lizard is the characteristic flora of slow growing, low-lying heathland plants resulting from the toxic nature of serpentinite to vegetation.
Picturesque Mullion Cove
The Lizard actually comprises several fault bounded slices of ophiolite. One large section in the south-east, separated from the rest by a major thrust fault, has the peridotite, gabbro and igneous sections, although the transition into pillow lavas and oceanic sediments is missing. The contact between the mantle and the oceanic crust is clearly seen over a 200m stretch of beach at the village of Coverack, where steeply dipping gabbro sheets, cut through by basalt dykes, give way to ultramafic rocks, having passed through a transition zone where the gabbro and mantle rocks intermix.
Much of the rest of the peninsula to the west of this thrust is dominated by peridotite and serpentinite, but there are a number of exposures of metamorphosed igneous basalt or gabbro, called amphibolite or hornblende schist. A faulted contact between hornblende schist and serpentinite is visible at Mullion Cove on the west coast of the Peninsula. This pretty harbour, still used by local fishermen, has an old lifeboat station – vital on a wild rocky coastline that has claimed thousands of lives over the centuries.
A short distance off Mullion Cove is Mullion Island, where there are excellent exposures of pillow lavas, formed when hot magma was extruded onto the ocean floor, as well as cherts and limestones, representing the top of the ophiolite sequence.
The contact between the Lizard ophiolite complex and the surrounding material is a fault zone running across the peninsula. It is seen at Polurrian on the south-west coast, where hornblende schists of the ophiolite complex have been thrust against metamorphosed Devonian mudstones, and at Porthallow on the east coast, where serpentinised peridotite and a small section of the Old Lizard Head schists are faulted over the Devonian Meneage melange.
Granite at Land’s End
Away from the Lizard Peninsula, the geology of south-west Cornwall is dominated by marine clastic rocks of the Middle Devonian Gramscatho group, predominantly siltstones and mudstones. The exact age of these rocks has been the subject of much debate, primarily because they have undergone changes both from tectonic pressure which induced folding and faulting, and as a result of massive granite intrusions near the end of the Variscan orogeny, 300 Million years ago.
The granite was emplaced at great depth, but subsequent erosion has exposed it to form large areas of relatively bleak uplands, seen in Cornwall at Bodmin Moor, Land’s End, and the Isles of Scilly, as well as smaller exposures such as St. Michael’s Mount. The granite is predominantly a coarse crystalline rock, although there are occasional intrusions of finer grained material, and there are also dykes and sills of igneous material running into the surrounding rock. As the hot magma intruded into this sedimentary rock, the heat and pressure altered them significantly, creating a ring of altered rocks, the ‘metamorphic aureole’, around the granite.
There are some excellent exposures of the contact between the granite and the metamorphosed sediments on the Land’s End Peninsula. Cape Cornwall, 7km north of Land’s End, is composed of Devonian Mylor Slates, but in a cove just to the south of the headland these have been metamorphosed into hornfels by contact with the solidifying magma, and the transitional contact between the two can be clearly seen on the rocks of the small beach.
Another intriguing exposure of the granite contact is at Porthmeor Cove, 15 km north-east of Land’s End, which like Cape Cornwall has been designated an SSSI (Site of Special Scientific Interest). On the northern side of this beautiful remote bay the roof of the granite intrusion is exposed at the surface, with veins of fine and coarse grained igneous rocks intruding into the surrounding rocks, which are extensively veined and altered.
Walking West Cornwall
If large quantities of Mesozoic and younger sediments ever overlaid the Cornish Peninsula, there is little evidence of them now. The greatest changes made to the landscape in more recent times have been as a result of changes in sea level, with erosion platforms and raised beaches visible at several levels. This has also created another major feature of the Cornish landscape, in the form of drowned river valleys or rias, such as the Fal estuary and the picturesque, tree-lined Helford River.
By far the best way to appreciate the geology of Cornwall is to walk the coast, and it is possible to do just that using the South West Coast Path National Trail, which stretches 1,000 km from Somerset to Dorset and includes the entire coast of Cornwall. Much of it passes along the top of the cliffs and it includes all the sites discussed here, plus many more, with equally gorgeous scenery and fascinating geology.