Summary of site:
The importance of this site lies in the range of different rock types of the Runabay Head Formation that can be seen here. The rocks form part of the ancient Dalradian Supergroup of metamorphic rocks and belong to the Southern Highland Group, the second half of the period. The Runabay Head Formation is the middle of three formations that comprised the group.
The formation is exposed intermittently along the beach and in the cliffs of the bay where quartz schists and quartz and feldspar-rich gritstones predominate. There are also green metamorphic rocks within the sequence. Unusually, both tourmaline (dark needle-like crystals) and larger, well-formed crystals (porphyroblasts i.e. crystals that grew during metamorphism) of the feldspar, albite, occur in most of these rocks. In one part of the site rocks tilt 30 degrees to the east. In the small bay called Portnahorna some of the best green beds can be seen in the form of dark green hornblende and biotite schists.
All the rocks seen here were formed around 600 million years ago when the supercontinent of Pangea that extended from the South Pole to the equator was beginning to break up. The rift, passing very near the South Pole (then ice-free), was due to deep convection currents parting under the continent and thinning the crust. This thinning created deep basins close to the coast of the newly separated fragment, another large continent called Laurentia, and they rapidly filled with mainly sandy sediments washed off the barren land into shallow water to form unstable masses that later detached and plunged into the basin depths. The thinning also allowed molten rock to reach the surface where volcanic activity discharged ash and lavas both of which weathered to form new sediments. The green beds could have formed either way. 140 million years later as Laurentia crept north, it collided with an arc of active volcanic islands causing extensive compression and thickening of the sediments resulting in their metamorphism and regional buckling. At least four phases of deformation resulted, one of which created the gigantic Altmore Anticline that extends west into County Tyrone and beyond and east into Scotland. This fold was effectively pushed over to the south and this site is on the inverted southern limb which means that all these rocks are now upside-down.
The quality of access and the range of rocks exposed make this a site of national importance that should be preserved for posterity. Fortunately it is owned by the National Trust which therefore offers access protection. The site is sufficiently robust for hammering to be allowed.