Shales and mudstones which are subject to very low grade regional metamorphism become slates. Slates are very fine grained, and often do not have any significant sheen because the phyllosilicate minerals (chlorite, mica) are not large enough. Slate has an extremely well developed cleavage induced by compressive tectonic deformation. This slatey cleavage is the characteristic which makes these rocks excellent for use as building materials.

On Arran slates are Dalradian in age. They form the core of a large scale regional metamorphic structure called the Lochranza Anticline. The slate belongs to the Lochranza Slate Group, the name being from the area where it was once mined.