Scottish Geology Website

The Highland Boundary Fault (HBF) which traverses Scotland from Stonehaven to Arran, is in most places only identifiable by the change in topography - from highlands to the northwest and lowlands to the southeast. However, the coast to the north of Stonehaven is one of the sites where the fault can be distinguished more readily. It is also a site where the Highland Border Complex, a thin sliver of unmetamorphosed sediments that lies between the Dalradian highlands and Devonian lowlands, is exposed well.

The rocks to the northwest of the HBF are Dalradian metasediments of the Southern Highland Group. These are deep marine deposits metamorphosed to schists, phyllites and slates during the Caledonian Orogeny. The HBF came about as the Highland and Midland Valley crustal blocks came together during the orogeny. The last major movements at the fault took place in the Silurian and Lower Devonian times. Overall, the Dalradian Supergroup underwent four main phases of deformation during this orogenic activity.

At Stonehaven, to the southwest of the HBF lie the steeply dipping Lower Old Red Sandstone sandstones and shales of the Stonehaven Group and the sandstones and conglomerates of the younger Dunottar Group. These sediments unconformably overlie the Highland Border Complex.

The Highland Border Complex is a suite of rocks exposed in a series of lenses along the HBF. Exposed here between Ruthery Head and Garron Point are pillow lavas, shales and jasper (other lithologies exist at other sites). The suite represents an ophiolite and associated marine sediments, obducted during the Caledonian Orogeny and containing no Dalradian source material. The Complex is believed to have been deposited in early/pre-Arenig to Caradoc and partly deformed by the overthrusting of the Dalradian block by the end of the Silurian, i.e. prior to deposition of the Lower Old Red Sandstone sequences.

The Highland Boundary Fault at Craigeven Bay Stonehaven

The Highland Boundary Fault at Craigeven Bay Stonehaven. The sea stacks and central point of the image mark the position of the fault. On the right there are the Dalradian rocks of the Grampian Highlands and on the left there is the Highland Border Complex. To the far left, seaward of the headland known as ‘Slug Head’, there is the Cowie Formation of Silurian age.
Image provided by Diane Mitchell.

Further reading:

Stephenson, D. & Gould, D. 1995. British Regional Geology. The Grampian Highlands. 4th edn. British Geological Survey (Her Majesty's Stationery Office, London).

Trewin, N.H., Kneller, B.C. & Gillen, C. 1987. Excursion Guide to the Geology of the Aberdeen Area. Scottish Academic Press, Edinburgh.