Laid down during the Upper Devonian period (377-360 million years ago).
Slate is the metamorphosed equivalent of mudstone and shale
i.e. the result of heat and pressure applied to these mudrocks. Slate is
a hard, splintery mudrock composed of variable proportions of quartz
silt and clay minerals with grain sizes less than 32um (0.032mm). The
fundamental feature of slate is the cleavage, which results from
well-crystallized platy clay minerals arranged along a single set of
micron-spaced parallel planes.
(From "The definition and characteristics of very fine grained sedimentary rocks: clay, mudstone, shale and slate." By RJ Merriman, DE Highley, & DG Cameron of the British Geological Society).
Delabole slate is a dark silvery grey and because of the small grain size allows detailed drawing.