Slate deposits of Cornwall & South Devon [UK]

The southwestern peninsula of Great Britain lies at the outer edge of the Rhenohercynian and consist mainly of marine Devonian units. Compared to the other Variscan zones the Caledonian orogeny only weakly influenced Cornwall.
The complete Devonian succession of SW-England shows intercalations of Old Red sandstone and its stratigraphy is hardly dividable due to the quick change of the facies and the multiple deformation (Anderton et al., 1979).

Cornwall itself is situated within the synclinal structure of SW-England whose Carboniferous rocks are southernly and northernly flanked by Devonian units, containing the Upper-Devonian roofing slate deposits ('Tredorn Slates'). The sedimentation of the Devonian units is characterized by several fault controlled subsidence rates and thus in Cornwall different fault-related sedimentation basins are distinguished (Selwood et al., 1998).

In the Upper-Frasnian the marine transgression started towards Wales in Central England. During this time, the Armorican Massif probably represented the southern rim of the basin and supplied coarse grained material into the area of South Cornwall. To north, in South Devon, cephalopod limestones and ostracode shale deposited and further in N-Devon and Somerset shallow-marine and fluviatile rocks sedimented (Edmonds et al., 1975). The original material for the roofing slates was deposited within an outer shelf facies.

During the Carboniferous the flysch sedimentation of the Armorican Massif moved towards north and so moved the folding and the rejuvenation of the age of metamorphism (Schönenberg & Neugebauer, 1987).

The Armorican orogeny (Upper-Carboniferous - Early Permian) is characterized by North-South directed compression with a general E-W strike of the tectonic elements which were mainly formed by folding. There can be distinguished three stages of deformation or folding, respectively (Edmonds et al., 1975).

The first stage D1 is observable in the whole of Cornwall and gave rise to west-vergent and NE-SW to ENE-WSE striking folds as well as large scale thrusts which mostly create lithostratigraphical borders. The folding style varies between closed and tight and the axial planes are horizontal or dip moderately to SE. With this folding the formation of an axial planar cleavage S1 was related. In the vicinity of the thrusts the development of the cleavage is more intense and lead to a god splitability of the 'Tredorn Slates' that means to the origin of roofing slates.

For this main stage of deformation a low-grade and intermediate regional metamorphism is typical. During this regional metamorphism the 'Tredorn Slate' were modified by a contact

The second stage D2 is locally developed and again lead to NW-vergent and ENE-WSW striking folds with a moderate to steep dip of the fold axis planes to SSE. The deformation was accompanied by the origin of a regionally occurring crenulation foliation S2. This ductile deformation regime was completed by dextral lateral displacement (Selwood et al., 1998).

The last stage D3 was related with the origin of E-W striking folds and a second crenulation foliation S3.

The Upper-Devonian 'Tredorn Slates' have a fine-grained sedimentary pattern and a well developed, homogeneous and smooth fracture cleavage. The colour varies from dark grey to green or bluish to grey, respectively.
Slate mining was carried out since the 16th century in a slate quarry between Delabole and Tintagel. Yet the peak of the mining of roofing slates was in the 19th century when slate replaced the straw on the roofs.
The area at Tintagel is a zone of intense deformation with recumbent folds which are overprinted by kink folds as well as extensive thrusts. Other quarries are within the region of Wadebridge where slate was mined for the typical English walls and other architectural purpose. Numerous small slate quarries are distributed throughout Cornwall which have neither mined roofing slate nor have a more then locally restricted economic importance.

Except for the Launceston area in which Carboniferous slates are mined, most of the slate quarries mine their slates within the Upper-Devonian (Selwood et al., 1998).
Today, roofing slate is only mined in the Delabole quarry.