The Lleyn protrudes about 25 miles, and separates Cardigan Bay from Caernarvon Bay. On the southern side, under shallow water mostly less than 20 metres deep the Llyn joins the rock of the Cambrian Harlech Dome, and small exposures of Cambrian appear on the far south of the Llyn. The structure of the peninsula itself continues into Snowdonia. The main spine of Llyn, like most of Snowdonia, is Ordovician and synclinal, the younger Lleyn Ashgill-Caradoc is exposed at the centre, with older Arenig-Llandeilo around it. Between them are exposed the acid lavas and tuffs associated with the early Ordovician subduction volcanics. Along the northern edge of Llyn are the prominent hills of the Northern Granodiorites; the line of these intrusions extending nearly to Conwy. Granodiorites also appear in other places on Llyn, and there are many diorite intrusions as well. The far west and south west is much older Precambrian Mona Complex, with mélange, pillows and schists, metamorphosed in the Cadomian Orogony and then accreted to the rest of Wales.

On Day 1 in lovely weather we looked at the Precambrian of the most SW part of Lleyn, regarded as one of the finest excursions in North Wales, parking at 142255 and traversing SE through the geologically separate Precambrian Gwna Group, from the chloritic tuffs to the underlying mélange, and then to a small area of overlying Ordovician Gwyddel tuffs, until Porth Felen (lunch) where the Ordovician is faulted against the greenschists of Gwna Group. This shear zone (near a major fault) becomes more distorted and schistose moving south-west, until around 153245 the schisty rocks are faulted against Ordovician Arenig which, some 300 metres further, at Pen y Cil is intruded by a large dolerite dyke.

The morning of Day 2 was spent near Llanbedrog and Mynytho, starting with a walk to the top of an igneous boss hill near our B&B at Llanbedrog. Here, at 329309, in driving rain and with zero visibility of the 360 degree panoramic degree view, we looked at a fascinating leucocratic microgranite, locally filled with what appear to be pyroxene crystals! (and not described in the GA guide). Then we went to 302311, a plug of pink microgranite in a small quarry at Mynytho, and onto the higher part of Mynytho Common, where the microgranite is both flow-banded and contains riebeckite (acicular fibrous crystal in high alumina, high alkali granite). In appalling weather we just managed to find examples of both. Later we visited the surface works of the manganese mine 212266 near Rhiw, with high grade ore lying in blocks from the last days of working in World War II. On the nearby beach at Porth Ysgo we looked at the Ordovician Arenig sandstones, rich in trace fossils; but our main interest was the large dolerite sill within which appeared confusingly similar areas of indurated sandstones.

The morning of Day 3 was spent at Gimblet Rock 387343, ducking sea spray on a small exposure of Ordovician dolerite on the tip of a rather wild headland at Pwllheli. This displays remarkable small pods of pegmatitic structures where late-stage accumulation of exsolving volatiles lowered the melting point of the dolerite, allowing large feldpars and beautiful pyroxene crystals to grow. In the afternoon we went to Criccieth, parking around 520380 and walking along the railway line to look at part of the Upper Cambrian in a forlorn attempt to find the Dictyonema beds (dendritic graptolites). We were defeated by the fact that they seemed to lie within a fenced area. We did finally and definitely identify the slate beds which lay to one side of them, and then we wandered through more superb countryside looking at small exposures, big igneous erratics and rocks in stone walls.

Day 4 (Monday) was very lucky with the weather. A small remaining group which didn’t have to return to work spent all day traversing round the spectacular headland of Mynydd Cilan from 290251 above Hell’s Mouth, to 302234 at Trwyn Llechydoll. This few kilometres of coastal Cambrian of Llyn is much thinner than that of Harlech but exposes the same succession. The traverse started on the overlying Ordovician Arenig sandstones and progressed onto a generally east dipping Cambrian, which therefore younged through the Cambrian around the headland from the Hell’s Mouth Grits to the Caered Mudstones, and onto the Ordovician. Right on the beach at Trwyn Llechydoll beneath towering vertical cliffs of Arenig sandstone was a splendid angular unconformity with the underlying Cambrian. However, perhaps the day’s best find was well-shaped rich manganese ore nodules in an old working on the clifftop Cambrian Mulfran Formation.