The Holy Island Group is a sequence of metasedimentary rocks of Cambro-Ordovician (Furongian to Tremadocian) age found in northern and western Anglesey and the adjacent Holy Island in North Wales.

The South Stack Formation outcrops on Holy Island, between Holyhead and South Stack and at Rhoscolyn, and also inland on Anglesey itself between Mynydd Mechell and Carreglefn.

It extends across much of northern and western Anglesey and Holy Island.

(2014) Footsteps through Time: the rocks and landscape of Anglesey explained.

GeoMôn, Isle of Anglesey County Council