The story of slate

Slate is basically squashed mud that was formed four to five
hundred million years ago. Mud contains large amounts of
clay minerals. Old mud which lay on the seabed, now forms
hillsides where the slate quarries are.
Great earth movements involving the collision of whole continents,
{continental plates} produced pressure that forced the old seabed
upwards. With low heat and pressure the mud which had already
been turned into mudstone, was now turned into slate.
Such a process starting with one kind of rock is called Metamorphism.
As a result slate is a fine grained metamorphic rock derived from an
original shale type sedimentary rock that consists of clay, quartz or
volcanic ash.