Slate Islands Heritage Trust

The geology of the Slate Islands

Geology

The bedrock of the district consists of some of the oldest sedimentary rock exposed in the British Isles. 650 million years ago during the Dalradian Period there were two major land masses covering the area now known as Scotland. The mountains and plains of ancient Caledonia were bounded on their southern borders by a shallow sea occupying the area which lies between the Great Glen Fault and the Highland Boundary Fault. This area of sea included Argyll and the Inner Hebridean Islands of Islay, Jura, Scarba and the islands in the Sound of Lorn.

In the absence of vegetation and with great extremes of temperature the rock was rapidly broken down into boulders and stones small enough to be carried away by the fast flowing waters shed from the hills. Particulate material, breaking continuously into finer and finer particles as it travelled, was eventually deposited as silt and mud on the bed of the shallow sea.

Approximately 440 million years ago during the Ordovician period, extensive land movements took place during which great folding of the shallow sea deposits occurred. The concertina effect of this folding created enormous pressure in the layers of mud compressing and heating the deposits until they were converted into slate rock.

Easdale slate is blue-black in colour, bears a ripple on its surface which distinguishes it from other smoother slates from Wales and Cornwall and is identified by the large quantity of iron pyrites it contains. This occurs as cubic crystals of what is often referred to as Fool's Gold. The crystals may be anything up to 15mm across and can be easily seen glistening on the wet slate roofs. They rarely rust and add an extra dimension to the attractive appearance of the slate.

During the folding of the slate beds they were tipped over to an angle of forty five degrees. The dip of the rocks is clearly seen where slate is exposed in the now abandoned quarries and along the shore.

55 million years ago during the Tertiary Period, extensive volcanic activity centred on Mull resulted in the intrusion of dykes and sills of molten lava which forced their way upwards through the existing strata. The Easdale slate beds were at this time crossed in a NW to SE direction by one large dyke and several smaller dykes and sills of Dolerite, a hard basaltic granite known locally as Whinstone. Where the intrusions occur the slate is deformed and there are often veins of quartz and greater concentrations of iron pyrites crystals at the junction of slate and Dolerite. At these sites the slate was not worth cutting out and was left undisturbed.This accounts for the often jagged outlines of rock surfaces produced by weathering and for the uneven cutting of the quarries themselves where buttresses and platforms can be observed to great depths through the water on a calm day. The Whinstone was particularly used as a building material on the island forming many of the cornerstones of the cottages and the industrial buildings.

The early masons were particularly skilful in cutting and shaping this rock. Even using modern cutting and drilling equipment it is a difficult material to work. The builder's worst nightmare is to encounter a whin dyke while digging out the foundations of a house or excavating for a septic tank!

Roofing the World History Index Slate Industry