the two northerly islet groups are formed from gabbro, a hard-wearing rock of igneous origin whilst those of the daufraich group to their south (and indeed llechau-isaf) are formed from microtonalite, another intrusive igneous rock. the most southerly, emsger is formed from rhyolite. all these are of probable lower palaeozoic origin. llechau-uchaf is formed from the late cambrian 'lingula flags' whilst carreg-trai is formed from ordovician acid tuffs of the llanrian volcanic formation. the submarine geology between the main groups is formed by sandstones and mudstones of cambro-ordovician age, though the bedrock is partly covered by sand and gravel.