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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.

Mylor slate, photo by Pat Sargeant

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, A 'Delabole Butterfly' on display in Bodmin Museum, photo by Simon Cammcorals 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.

Folded killas at Jangye Ryn, photo by Cornwall RIGS groupThe 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 408 to 360 Ma (million years ago)

Rheic map by Bill ScoldingBritain 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.

Headland at Porthallow formed of huge lava blocks, photo by Cornwall RIGS groupAs 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 Pendine 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 360 to 290 Ma

Barras Nose Formation with volcanics above, photo by Cornwall RIGS groupAt 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”.

Squashed lava bombs Trebarwith Strand, photo by Cornwall RIGS groupVolcanic 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.

However in Devon and Cornwall whilst some coal seams are present, for Turbidites in Bude Cliffs, photo by Cornwall RIGS.example near Bideford, they are not true seams in that they possess no 'seat earth' or 'tree root seam' and so were given the name Culm. This Culm vegetation was probably rafted in on rivers from the swamps to the north and accumulated in quiet patches on the delta. In Upper Carboniferous times (315 Ma) the ocean basin was silting up rapidly and thick beds of sandstone with minor shales were deposited in the Bude area. These sediments are characterised in part by underwater avalanches on the delta slopes giving rise to a sequence of rocks knows as turbidites.

Volcanic activity came to an end and sedimentation ceased around 310 to 300 Ma when the Rheic Ocean closed and the basin was finally squeezed out of existence. This heralded the end of the Carboniferous in north Devon and north Cornwall. Uplift caused by the northward progression of the African plate into the European plate became so great that the ocean basin was slowly but surely replaced by a mountain range. The south west of Britain became part of a bigger mountain belt which included Brittany.