36. Auchinoon Quarry on the A70 "Lang Whang", near Harperigg Reservoir
This quarry is an example of a dolerite sill and the best example of contact metamorphism in West Lothian. At the top are interbedded siltstones and mudstones which are about 335 million years old.
With the dolerite sill, a mass of molten magma has pushed its way between sedimentary rocks, but did not reach the surface. Where the magma came into contact with the sedimentary rocks, the heat changed and hardened them into metamorphic rocks. These are found directly above the sill. This is the best example of "contact metamorphism" in West Lothian.
Subsequent erosion has removed other sedimentary rocks from above the dolerite and metamorphic rocks. Those that remain consist of interbedded grey and brown siltstones and mudstones, which were deposited along with sandstones and coals, laid down in the mid-Carboniferous, about 335 million years ago. The beds of rock in the quarry are dipping south-east towards the reservoir, and overlie lavas of the same age as Arthur's Seat in Edinburgh. The southern top of Auchinoon Hill is a good viewpoint for the Pentland Hills.
The quarry is situated beside the A70 north of Harperring Reservoir. Take care when parking in the informal lay-by on the north side of the road beside the quarry because of the speed of the traffic along this section of the road, known as the "Lang Whang". Nearby geodiversity sites are Baad Park Burn (1), West Cairn Hill (2) and Linhouse Water above Edinburgh - Carstairs Rail line Viaduct (3).
A more detailed leaflet is available on the Council's website: