Rock types:

Old and new red sandstones, Devonian slates, shales and limestone

Sedimentary rocks of the Devonian system lie under most of Exmoor. The name Devonian' refers to the fact that the rocks are common and accessible in Devon. It is also used to name rocks found elsewhere but formed at the same time i.e. between about 410 and 360 million years ago, the Devonian period.

Most of the rocks were deposited as layer on layer of mud or sand in the shallow waters of seas, lakes or river deltas. These layers gradually hardened into rocks and were later squeezed between two colliding crustal plates, one to the north and the other to the south. The intense pressure folded the rocks across the West Country into an east-west arch-like fold (anticline) and trough (syncline) within which were lots of smaller folds. The crest of the arch lies close to the coast of Exmoor but has been eroded to the west where the Bristol Channel now lies. The oldest rocks can be seen in the Valley of Rocks in the eroded core of the anticline along the northern side of the moor. Progressively younger layers become exposed towards the southern edge.

Small patches of other rocks, once more extensive, have survived erosion. In the Vale of Porlock and down the eastern edge of the exposed Devonian rocks there are some red, stony, sandy and silty sediments with no fossils. The larger fragments in these New Red Sandstone rocks are broken down pieces of Devonian rock. This shows that the New Red Sandstone sequence was formed after, and from the erosion of, the rocks of Devonian times. Their general nature suggests that the erosion took place on land in desert-like conditions.

After this period when the area was dry land, about 290 to 210 million years ago, the sea once again invaded at least part of the area, for rocks of Jurassic age (210 to 145 million years ago), with many marine fossils, are preserved near Selworthy and further east.

Unconsolidated, surface deposits are widespread and of much more recent date. One of these relates to the last Ice Age when ice-sheets were probably pressed against the West Somerset and North Devon coasts. Usually the ground would have been frozen but during times of thaw at least a surface layer melted and large quantities of frost-shattered rock fragments slid down-slope in a muddy matrix to form an unsorted deposit on the lower valley sides.

These deposits are known as 'Head'. River-laid alluvium covers the floors of some valley bottoms while blown sand forms Braunton Burrows and peat is found on some of the high moors.