Orogeny: The earth’s crust is made up of ‘plates’ which are continually in motion relative to each other. As a result some parts of the earth’s crust are under compression while others are under tension. In areas undergoing compression, rocks are deformed by folding and faulting, forming mountains in the process and altering the nature of the constituent rocks by a process known as metamorphism. Although in many cases the resulting mountains have been eroded away, evidence of their existence remains in the deformation and metamorphism of the rocks exposed at the surface. Slate is an example of a mudstone which has been metamorphosed due to heat and compressive stress. The conditions required for this to happen can only be found at a depth of 10-15km, hence wherever slate is found close to the surface, it can be inferred that the overlying rock has been worn away.
There were two principal mountain building events in Britain during which slate was formed. The earlier of these was the Caledonian Orogeny which occurred during the Ordovician, Silurian and Devonian periods, resulting in a mountain belt stretching from Scandinavia in Europe to the Appalachians in North America. In Britain the main activity took place in the Ordovician period resulting in the Highlands of Scotland and the mountains of North Wales. Most of the slate in Britain was metamorphosed from mudstone during this Orogeny.
The second orogeny affecting Britain was the Variscan or Hercynian Orogeny which occurred during the Carboniferous and Permian periods. The folding and faulting associated with this orogeny can be seen in North America and Central Europe. In Britain the main deformation occurred in the south of England and is associated with the formation of Cornish slate.