Many of the UK's mountain regions contain hills made of slate, but it is really the Northern Welsh slate quarries which have a reputation for their climbing.
Slate is a low-grade (ie not very high pressures or temperatures, corresponding to burial depths of perhaps 10 - 15 km) metamorphic rock produced under anisotropic stress conditions. Any platy clay or mica minerals in the rock will align with their platy cleavage direction (see video on micas to see what that means) at right angles to the direction of maximum compressive stress. This makes it possible to cleave rock which is rich in these platy minerals into thin sheets as all the individual minerals break in the same orientation. That is slaty cleavage, and slates are very rich in clays or micas. They started off a mudstones where the original sediment was very clay rich. Often the starting sediments are deep-sea muds (tiny clay particles can be washed or blown far out to sea where larger particles settle out closer to land) - this is the case for quarried slates in Wales. Alternatively slates can be formed from volcanic ashes which are laid down under water. In this case the fine ash particles react with the water to make clay - this is what Lake District green slate is (eg that mined in Honnister and Langdale).
Cleavage can be at any angle to bedding, and normally it is at quite a high angle. This means that any fossils will not be well exposed when the rock cleaves, so even quite fossil-rich rocks can appear to be barren. Several exceptional preservation localities only reveal their fossils when blocks of rock are analysed by X-ray imaging to show the three-dimensional forms inside.
Climbing-wise, the strong cleavage makes for large slabs with only very thin features, at least in quarried slate. Get strong, smear well and make sure you can edge on match-stick holds.
See also:
Micas: [ Ссылка ]
Rigghead green slate quarry: [ Ссылка ]
Deep sea graphite slate, Ballahuilish: [ Ссылка ]
The hoem of metamorphiosm (2): Index minerals in Glen Esk: [ Ссылка ]
