Killas is a wonderful Cornish miners' term used to describe the sediments, metamorphosed by the heat emanating from 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.

See the Killas Geological Timeline

The killas were deposited in many different environmental settings. We know this from the rocks themselves and their fossil content which helps us piece together the type of ancient environments existing in Cornwall at this time. Trilobites, crinoids and various other bivalves, including the famous Delabole butterfly, along with corals 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 (no oxygen!) but a few free floating creatures and plant debris have been preserved after they died and sank into these hostile conditions.

The intrinsic geodiversity of the killas was further enhanced shortly after deposition when the killas 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 member until the late Jurassic period - but that's another story!

DEVONIAN 408-360 Ma (million years ago)

Britain lay just south of the equator, and the site we now know as Cornwall was covered by an ancient tropical sea, the Rheic Ocean, whose coastlines lay to the north across north Devon and southern Wales, and to the south in southern France and N. 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 (see geological map). 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 ever came in Cornwall to those that persisted further north in Britain throughout the Devonian and are the oldest sedimentary rocks in Cornwall.Later the water deepened slightly and sea invaded Cornwall, so on top of these are the Meadfoot Beds, which are again of slates, siltstones and sandstones with the 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 and 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 e,g, Upper Delabole Slates, Tredorn Slates, Polzeath Slates. The deeper water here allowed only the finest muds to develop which have since been heated and squeezed producing fine 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 (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. As a result this basin contains Middle to Upper Devonian sediments ranging 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, and from Mevagissy 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 360-290Ma

At 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 e.g. 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.

Volcanic 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 northeast 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.

'Save our Stones’ an award winning leaflet (written by Jane Anderson) helping to conserve Crackington Haven’s beach and cliffs.

However in Devon and Cornwall whilst some coal seams are present, e.g. 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 (315Ma) 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 310-300 Ma. when the Rheic Ocean closed and the basin was finally squeezed out of existence thus heralding 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 the ocean basin was slowly but surely replaced by a mountain range. The southwest of Britain became part of a bigger mountain belt which included Brittany

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Granites | The Lizard | Killas | Coastline