Carboniferous

Between 350 and 290 million years ago Britain was situated at the Equator. This period is called the Carboniferous.

During it the land gradually sank and warm shallow sea encouraged coastal swamps where forests of giant club mosses grew. Over millions of years the remains of these plants were converted into coal, oil and natural gas. Aquatic animals lived in the water, the harder parts of which later fossilised and became carboniferous limestones and shales.

The Pembrokeshire coalfield, and extension of the South Wales Coalfield, lies across the centre of the county from Nolton in the West to Saundersfoot in the East. The coal measures, along with associated millstone grit and limestone, are the youngest rocks in Pembrokeshire. A wide variety of fossils occur in Carboniferous rocks. The mudstones contain ammonites and bivalves while the limestone contains corals, with brachiopods in the marine shales. The coal measures contain fossils of giant club mosses, ferns and molluscs.

Millstone grit is a hard, coarse-grained rock and was ideal for making querns and millstones for grinding corn. Good quality anthracite was mined in a number of places including Nolton, Broad Haven, Kilgetty, Hook and Saundersfoot. Limestone has been a popular building stone in south Pembrokeshire, it was also used to produce lime, which was used to neutralise agricultural land. Many linekilns can still be seen along the coast. The limestone soils of Castlemartin peninsula were famous for their barley crops in the middle ages.