The geology of Ramsey is both varied and complex, including examples of sedimentary and volcanic rock as well as intrusive igneous rock from the Paleozoic Era, to the extent that even geologists have not always agreed on certain elements of it. The island is divided by a north/south fault, from Aber Mawr to Porth Lleuog, into two distinctly different rock types which range in age from the Arenig Llanvirn (mid Cambrian Period) through to the Ordovician Period. The majority are igneous rocks from the Ordovician Period, 443-485 million years ago, largely making up the southern and western area of the island. On the northern and eastern area of the island, sandstones and mudstones from the older mid-Cambrian period are to be found. During the Ordovician Period, when much of the island’s rocks were created, there was widespread volcanic activity in the area of their origin and many of the offshore islands and higher mainland peaks are made predominantly of igneous rocks.

Even to the untrained eye it is easy to see, as you move around the island, that the rock type changes dramatically. The igneous rocks on Ramsey tend to be characterised by the more rugged coastline and dramatic scenery, with high cliffs including both Carn Llundain, the highest hill at 130m high, and Carn Ysgubor to the north, which is an igneous intrusion. Most of the volcanic rocks are made up of rhyolite and various tuffs, including turbiditic and ash-flow tuffs, all of which are now believed to have been formed by violent subaqueous or underwater eruptions. There is also a fine example of rhyolitic conglomerate of the Ogof Colymenod Conglomerate Member to be found overlying the sandstones, where rounded pebbles of rhyolite are set in a finer mix of rhyolite and rhyolitic sandstone. The extensive exposures of rhyolitic volcanic rocks provide some of the most important sites in the UK for interpretation of submarine, silicic volcanic processes, and the first record of submarine welding of a silicic ash-flow tuff in the world. Fine examples of columnar rhyolite can be seen in various areas around the south of Ramsey, where rapid cooling of magma has formed distinctive column structures.

The background sediments to this volcanic activity consisted largely of black muds and sand, which probably accumulated in a low-energy outer shelf-like marine environment. They are rich in marine fossils, including trilobites, graptolites and brachiopods, making them of national importance to paleontological research.

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