Geological diversity on Llŷn

The geology of Llŷn has had a tremendous impact on the landscape which we know today.

A major fault line runs through the peninsula and marks the boundary between the older rocks to be found in the west, from those much younger in the east. This fault is a continuation of the line which formed the Menai Strait and divided the Isle of Anglesey from mainland Gwynedd.

A striking landscape

Around 450 million years ago there was great volcanic activity in North Wales, and it was during this period that the spine of Llŷn, so obvious today, was formed as igneous rock was forced upwards.

Subsequently the surrounding and overlaying softer rock has been eroded by glaciers leaving the chain of peaks from Yr Eifl in the North East, through to Mynydd Nefyn and Garn Fadryn, and finally to Mynydd Rhiw in the west.

The most interesting geological spectacles are to be found at Pen y Cil where a major dolerite dyke is visible, Porth Ysgo where many fossils are visible in the Ordovician Arenig sandstone, and near Ffynnon Fair on the cliffs below Mynydd Mawr where folded strata give evidence of the extraordinary power of volcanic activity.

Local extraction

The granite formed by this volcanic activity has been exploited by man with quarries on all the peaks. Each one has its own distinctive colour due to small changes in the chemical composition of the rock, so keep an eye out for these subtle changing in the walls and buildings around the peninsula.

One of the most important mineral resources of Llŷn has been the Manganese which is to be found in the gabbro rocks at Nant Mine near Porth Ysgo. The mine was in its heyday in the Victorian period, but the mine was reopened during the Second World War due to the importance of the mineral for the war effort. During this period 150,000 tonnes was quarried from this valley.