Slate was produced from four different areas in Scotland: (1) Ballachulish slate from a group of quarries located in Ballachulish near Fort William in Argyll, (2) Easdale slate from a group of islands, including Easdale from which it gets its name, south of Oban also in Argyll, (3) Highland Boundary slate from a series of quarries just north of the Highland Boundary line, stretching from Arran in the west to Dunkeld in the east. These are grouped together by their common geology rather than location and finally (4) Macduff slate from a range of hills, sometimes referred to as the Slate Hills, just east of Huntly in Aberdeenshire. The name Macduff refers to the geological formation from which they are extracted. Although a different type of slate was produced from each of these groups, they are all metamorphic rocks of the Dalradian Supergroup, located between the Highland Boundary Fault and the Great Glen Fault.
The Dalradian Supergroup consists of sediments laid down in the Precambrian Era between 770 and 560 million years ago and metamorphosed during the Caledonian Orogeny over 450 million year ago. Most of Dalradian consist of rocks which have been intensely metamorphosed and hence too course-grained to yield slate. Instead slate is found in areas of low-grade metamorphism, known as the greenschist facies.
The Dalradian Supergroup is divided into four groups; the Grampian, Appin, Argyll and Southern Highland Groups. No slate was produced from the Grampian Group. Ballachulish slate is part of the Appin Group and Easdale slate is part of the Argyll Group, while the remaining two types, Highland Boundary and Macduff, are located in the Southern Highland Group. The characteristics of slate from each group depend on the environment of deposition of the original sediments and on the degree of deformation during the Caledonian Orogeny (Richey & Anderson. 1944, Walsh 2000, 2002).