Shale is any type of fissile rock consisting of lithified silt or clay grade sediment. It is deposited in a variety of environments, often representative of low or waning energy conditions, such as in lakes, floodplains and in deep marine settings.

 The photograph shows shale of Ordovician age. Exposed in North Glen Sannox, these rocks were deposited as deep marine muds and are associated with pillow lavas and chert. Interpreted as part of the Highland Border Complex, these rocks represent fragments of an ancient ocean floor caught between two colliding continents.

Deltaic sequences of the Carboniferous, lacustrine deposits of the Triassic and deep marine rocks from the Jurassic on Arran also contain shale.