The rocks found in the main ridge of the Hills were formed in a geological period called the Pre-Cambrian which includes all the very long period of time before there were many forms of life preserved as fossils. The age of the Malvern rocks is thought to be between 600 and 800 million years. The rock types are varied and many have been altered since their formation. The oldest of them are probably sediments laid down in a sea but these have had large volumes of molten rock squeezed into them under pressure. This has changed the sediments into rocks called schists and gneisses whilst much of the molten matter has solidified into coarse-grained rocks like granite.
In the Warren House area east of the Herefordshire Beacon, there are lavas possibly of similar age which were erupted onto the sea floor. One of the best places to see them is around Clutter's Cave. After their formation the ancient rocks were planed down to a flatish surface partly by the action of waves on an ancient shore and were covered by later sediments now hardened into rocks. These can be seen at the Gullet quarry. They were formed 400 million years ago in the Silurian age. The contact between these and the pre-Cambrian rocks is visible at the north-west corner of the quarry where the oldest local Silurian beds are seen. Now tilted at a very steep angle they originated as near horizontal layers of silt, mud and some thin limestone beds all now consolidated into hard rocks. In the area between the Malverns and Ledbury to the west of the Hills beds of clay/muds, hardened into shale, alternate with limestones. The limestones form the wooded ridges whilst the shales have been worn away to form lower ground. The limestones contain fossil corals and other shelly creatures indicative of their origin in warm, shallow tropical seas.
Great geological upheavals affected this area just under 300 million years ago as a result of which earlier rocks were vigorously folded and broken by faults. Some faults affecting the pre-Cambrian rocks correspond with the gaps at the Wyche Cutting, the British Camp, the Gullet and Hollybush, while a long and very spectacular fault system running north-south marks the abrupt eastern side of the Hills.