{ { Magnesian Limestone and } { { Sandstone } Red Sandstones and { Permian { Marl Slate } Magnesian Limestone { { Lower Permian Sandstone } { { { Coal Measures } Sandstones, Shales and { { Millstone Grit } Coals at top { Carboniferous { Mountain Limestone } Sandstones in middle { { Basal Carboniferous } Limestone and Shales { { Rocks } below { { { Upper } Devonian } Red Sandstones, { Devonian { Mid } and Old Red } Shales, Slates and P { { Lower } Sandstone } Limestones R { I { { Ludlow Beds } Sandstones, Shales and M { Silurian { Wenlock Beds } Thin Limestones A { { Llandovery Beds } R { Y { { Caradoc Beds } Shales, Slates, { Ordovician { Llandeilo Beds } Sandstones and { { Arenig Beds } Thin Limestones { { { Tremadoc Slates } { { Lingula Flags } Slates and { Cambrian { Menevian Beds } Sandstones { { Harlech Grits and } { { Llanberis Slates } { { { No definite } Sandstones, { Pre-Cambrian { classification } Slates and { { yet made } Volcanic Rocks Again, after the formation of the beds many changes may occur in them. They may become hardened, pebble-beds being changed into conglomerates, sands into sandstones, muds and clays into mudstones and shales, soft deposits of lime into limestone, and loose volcanic ashes into exceedingly hard rocks. They may also become cracked, and the cracks are often very regular, running in two directions at right angles one to the other. Such cracks are known as _joints_, and the joints are very important in affecting the physical geography of a district. Then, as the result of great pressure applied sideways, the rocks may be so changed that they can be split into thin slabs, which usually, though not necessarily, split along planes standing at high angles to the horizontal. Rocks affected in this way are known as _slates_.