Devonian

The Devonian period lasted from 400 to 350 million years ago. As the Caledonian Mountains, formed at the end of the Silurian period, gradually eroded sediments and gravels ran off them to eventually produce the Devonian rocks of today. The land, a huge continent, was desert and Wales was still located south of the Equator.

Devonian rocks are all sedimentary in origin. Some are red in colour from the presence of iron oxide in them. This gives them their name, the Old Red Sandstone. This type of Devonian rock occurs in a swathe across Pembrokeshire from Skokholm Island via either side if the Milford Haven waterway to Manorbier and Caldey Island. There is also an area of Old Red Sandstone to the south of Narberth.

The sandstone contains many fossils including shellfish and brachiopiods. Devonian rocks also occur around Cosheston, near Pembroke. Here there are greenish sandstones, marls, breccias and conglomerates, which together form the Cosheston Beds.

The sandy soils overlying Devonian rocks support many plants such as Viper’s Bugloss, Dog’s Mercury and Dog Rose. Beech and Oak are common in Canaston Woods, an ideal habitat for many birds and mammals. The soil enabled the development of the early potato industry in south Pembrokeshire, in the 1930’s. The main growing areas are on south facing slopes near the sea where the weather is mildest.