The geology of Llŷn is complex : the majority is formed from volcanic rocks of the Ordovician period . Rocks of Cambrian origin occur south of Abersoch . Numerous granite intrusions and outcrops of rhyolite form prominent hills such as Yr Eifl , whilst gabbro is found at the west end of Porth Neigwl . The western part of the peninsula ( northwest of a line drawn from Nefyn to Aberdaron ) is formed from Precambrian rocks , the majority of which are considered to form a part of the Monian Complex and thus to be closely related to the rocks of Anglesey . Numerous faults cut the area and a major shear zone - the Llyn Shear Zone - runs northeast to southwest through the Monian rocks . In 1984 there was an earthquake beneath the peninsula , which measured 5.4 on the Richter Scale and was felt in many parts of Ireland and western Britain
earthquake beneath the peninsula
1984 Llŷn Peninsula earthquake
Llŷn Peninsula