{ { Chalk } { Cretaceous { Upper Greensand and Gault } Chalk at top { { Lower Greensand } Sandstones, Mud and { { Weald Clay } Clays below { { Hastings Sands } { { { Purbeck Beds } S { { Portland Beds } E { { Kimmeridge Clay } C { { Corallian Beds } O { Jurassic { Oxford Clay and Kellaways Rock} Shales, Sandstones N { { Cornbrash } and Oolitic D { { Forest Marble } Limestones A { { Great Oolite with Stonesfield } R { { Slate } Y { { Inferior Oolite } { { Lias--Upper, Middle, and Lower} { { { Rhaetic } { { Keuper Marls } { Triassic { Keuper Sandstone } Red Sandstones and { { Upper Bunter Sandstone } Marls, Gypsum and { { Bunter Pebble Beds } Salt { { Lower Bunter Sandstone } { { Magnesian Limestone and } { Permian { Sandstone } Red Sandstones and { { Marl Slate } Magnesian Limestone { { Lower Permian Sandstone } { { { Coal Measures } Sandstones, Shales { Carboniferous { Millstone Grit } and Coals at top { { Mountain Limestone } Sandstones in middle { { Basal Carboniferous Rocks } Limestone and Shales { { } below { { { Upper } } Red Sandstones, { Devonian { Mid } Devonian and Old } Shales, Slates and P { { Lower } Red Sandstone } Limestones R { I { { Ludlow Beds } Sandstones, Shales M { Silurian { Wenlock Beds } and Thin Limestones A { { Llandovery Beds } R { Y { { Caradoc Beds } Shales, Slates, { Ordovician { Llandeilo Beds } Sandstones and { { Arenig Beds } Thin Limestones { { { Tremadoc Slates } { Cambrian { Lingula Flags } Slates and { { Menevian Beds } Sandstones { { Harlech Grits and Llanberis } { { Slates { { { Sandstones, { Pre-Cambrian No definite classification { Slates and