The underlying geology of the islands is Dalradianslate, which was quarried widely until the mid-twentieth century. Quarry working began in 1630 and at the turn of the 20th century, the quarries were yielding some eight million slates every year. Eilean-a-beithich ("island of the birches") was once one of the Slate Islands, located in Easdale Sound. However, it was quarried to a depth of 76 metres (249 ft) below sea level leaving only the outer rim of the island. This was eventually swept away by the sea and little visible sign of the island now remains.
The Garvellachs lie immediately southwest of the islands.