Bishops and Clerks
There are four distinct groups of islets. From the north these are:
- North Bishop (44 metres, 144 feet)
- Carreg Rhoson (44 metres, 144 feet)
- Carreg Rhoson E Island (30 metres, 98 feet)
- Maen Rhoson
- Maen Daufraich
- Emsger or South Bishop (37 metres, 121 feet)
In addition, the three small wave-washed rocks of Carreg-trai, Llechau-isaf and Llechau-uchaf sit in the waters between the Bishops and Clerks and Ramsey Island and the Welsh mainland.
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.
- "Pembrokeshire Coast National Park planning document" (PDF).
- British Geological Survey 1992 1:50,000 scale provisional geological map sheet 209 (England and Wales) St David's