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.