The bedrock of Carna consists primarily of metamorphic rock belonging to the Morar group and originally being formed up to 1 billion years ago. These were formed in shallow seas form deposited sediment then altered by low grade metamorphism from volcanic activity.
The taller ridge on the western side of the island is made up of Pelite Schists which are harder than the Psamite and Granofelsic formation that makes up the flatter eastern side of the island.
Around the edges of the island you can find numerous patches of igneous intrusion of different ages and types including Microgabbro, Basalt, Hornblend, Microdiorite and Lamprophyre. In several places the island also has various superficial deposits of gravel, sand, silt and peat formed in the quaternary period.
Left Half of Carna (34) – Mica-schist
Right Half of Carna (33) – Quartzose felspathic schistose flags
Reproduced with the permission of the National Library of Scotland
British Geological Survey Map of Carna
Legend
British Geological Survey Onshore GeoIndex Map