Southern England. Peter Friend

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      FIG 39. The major bedrock structures of Southwest England and South Wales.

      Another feature of the mountain building is that muddy material – the most abundant sediment in the Southwest Region – was often converted into slates that can typically be split into thin sheets and are said to possess a ‘slatey cleavage’. These rocks are locally referred to as killas, to distinguish them from other rocks with no cleavage, particularly the granites. The conversion into slates took place during the folding and fracturing of the mountain building, when the original muds, rich in clay minerals, were buried deeply below other sediments and then compressed to produce a new layering (or cleavage).

      One large feature visible in the bedrock is the ‘Culm fold belt’ or ‘synclinorium’, a large and complex downfold representing horizontal crustal convergence (Figs 39 and 40). The centre of this feature is a belt of bedrock of Carboniferous age that extends between Bude and Exeter running across the centre and north of the Southwest Region. To the north and south of this, older (Devonian) rocks occur at the bedrock surface, forming the margins of the large downfold or syncline (Fig. 37). Culm is an old term much used by miners and European geologists for Carboniferous sediment, and synclinorium is a name for a downfold (or syncline) which contains numerous smaller folds.

      In the Lizard area, much of the bedrock consists of a distinctive group of igneous rocks (Figs 39 and 40). These rocks cooled and solidified earlier than the main mountain building, and were mostly formed by intrusion of hot molten rock in a way that is typical of the floor of an ocean basin. The Lizard area provides one of the best examples now visible on land in Britain of material formed originally as ocean-floor crust. In Late Devonian times, as a result of early Variscan convergence, this large area of oceanic crust was forced northwards over and against sedimentary rocks lying just to the north. This shows how, in a large mountain belt, an area of crust (tens of kilometres across), with a distinctive history as the floor of an ocean basin, can be uplifted and incorporated into a mountain belt as its margins are squeezed together.

      FIG 40. Schematic cross-section representing major structures of the Variscan mountain belt. The deep structure shown is speculative but shows how crustal shortening seen at the surface may be related to deeper, flat-lying fractures (faults). Located on Figure 39.

      FIG 41. Zigzag folding due to horizontal convergence during the Variscan mountain building is spectacularly exposed at Hartland Quay (Area 3). (Copyright Will Brett/www.lastrefuge.co.uk)

      FIG 42. Zigzag folding due to horizontal convergence during the Variscan mountain building, this time at Bude (Area 2). (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)

      Some of the bedrock of this Lizard Complex is called serpentinite, after the common occurrence of the green mineral serpentine in sinuous cracks and veins. This gives the rocks an attractive colour patterning, and the absence of quartz makes them surprisingly easy to work with steel tools, giving rise to a local industry of carving serpentinite ornaments.

       Granites and valuable minerals

      The granites of Southwest England are an important later feature of the Variscan mountain belt. Granites are igneous rocks with coarse (millimetre across) crystals that have grown, interlocking with each other, as the molten material cooled slowly and solidified at some depth in the Earth’s crust. The minerals of the granite are most commonly quartz (typically about 30 per cent) and feldspar, generally with some other minerals such as mica (Fig. 43). The granite liquid (often called magma) formed as a result of melting deep within the outer layers of the Earth and was then forced upwards, sometimes pushing aside the overlying bedrock and sometimes replacing it by melting. The granites solidified at depths of several hundred metres or more below the surface and are now at, or near, the surface because of landscape erosion. The main granite areas include the Isles of Scilly in the west, followed by Land’s End, Carnmenellis, St Austell, Bodmin Moor and Dartmoor in succession to the east. These distinct granite areas at the surface can be visualised as the tops of fingers extending upwards from a single continuous granite body detectable by gravity surveys at greater depth under the spine of Southwest England (Fig. 44). The deep body extends for some 200 km along the length of the mountain belt.

      FIG 43. Polished slab cut in the Dartmoor granite showing typical granite texture. Crystals of quartz (light grey), feldspar (white) and biotite (black) have interlocked as the magma (molten rock) solidified on cooling. (Copyright Landform Slides – Ken Gardner)

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