Category Archives: Highland Boundary

Highland Boundary Slate

Aberfoyle Quarry

Southern Highland Group is the youngest member of the Dalradian Supergroup. The rocks of this Group are located  to the north of the Highland Boundary Fault, which extends across Scotland from the Mull of Kintyre in the west to Stonehaven in the east. Associated with this fault zone is a rampart of hills which makes a striking topographical feature marking the bounday betweenthe low rolling countryside of the Midland Valley and the rugged Highlands.  The Highland Boundary Slate quarries of the Group are not from a continuous belt but from different formations within the Group, located at intervals  between Arran in the west and Dunkeld in the east, just to the north of the Highland Boundary Fault.

The original sediments of the Southern Highland Group were deposited by turbidity currents on a subsiding continental shelf, forming major submarine fans of terrigenous sediments. The slate, formed from fine-grained mud, represents the  more distal parts of these fans. Due to the oxidising conditions during deposition of the original mud, Highland Boundary slates do not contain graphite or sulphide minerals. The typical iron ore mineral  present in these slates is haematite and the usual carbonate mineral is calcite. Colour is variable with blue-grey, green and purple often found in the same quarry. Different bands of colour are indicative of primary bedding freatures.  The largest quarries of Southern Highland Group are Aberfoyle, Birnam and  Dunkeld. Other smaller quarries are located in Arran, Bute, Luss, Comrie and Logiealmond some of which no longer appear on the OS maps.  For more on the information on the individual quarries read Scottish Slate Quarries Technical Advice Note 21 published by Historic Scotland in 2000.