Notes on the Geology of Sula Sgeir and the Flannan Islands

Abstract

Sula Sgeir consists of hornblende-gneiss with occasional biotite and augite. The whole is intersected by small pegmatite veins which merge along the borders into the gneiss. Though evidence is small, it seems probable that the island, together with North Rona, came under the influences of the Quaternary ice.The Flannan Islands consist also of hornblende gneiss, biotite being a fairly common accessory. Numerous and varied pegmatite Veins occur throughout the islands, and in certain cases have affected the foliation of the gneiss. Owing to the proximity of the islands to the Lewis and the Harris, and the presence of adeposit of cemented sand on Eilean Ghobha, it seems probable that the Quaternary ice also reached these islands.