The Quantocks and the Brendon Hills are formed of slates and sandstones, deposited in a shallow sea in the Devonian period (417-354 million years ago). At this time Britain was part of a large landmass which included Europe and North America and lay about 10 degrees south of the Equator. Geologists term this the Old Red Sandstone Continent, because the deposited sediments of which it is formed were rich in iron oxides, colouring them red. These rocks weather to a free-draining red soil which supports heathland and acid grassland.
The oldest part of the Devonian rocks are on the Exmoor coastline, where Lynton slate beds give way to the younger Hangman sandstone that forms the distinctive 'hogsback' hills on this part of the moor. Successively younger layers of rock spreading eastwards from here display features showing that they were laid down in a shallow sea in an intertidal area, where rivers washed sediments down from the Old Red Sandstone, building them up in the sea in fans or deltas.
At the end of the Carboniferous period and the beginning of the Permian period (about 290 million years ago), these rocks were subjected to enormous heat and compression during a mountain-building period known as the Variscan orogeny, caused by the collision of continental plates. Several mountainous areas were formed in what later became South West England, including Exmoor and the Quantocks.
During the Permian and Triassic periods (290 to around 200 million years ago), as these mountains eroded, further large fans of sediment were washed down from them by flash flooding in desert conditions, forming outcrops of sandstones, conglomerates and breccias in the area between the Quantocks and Exmoor. (Conglomerates have rounded pebbles embedded in the sandstone, while breccias have angular fragments of rock which were embedded without having been rolled around in water first).
Over the last two million years, during the Ice Age, the Earth froze in a series of 'glacials', with warmer periods between them when ice sheets melted and sea levels rose. Although the ice sheets did not reach Somerset, in the cold periods this area resembled today's tundra zones, where bitter winds howl through a barren landscape and the ground is locked in a layer of permafrost that never melts. Although little rain or snow falls, the tundra zone is a soggy place even in summer because of the poor drainage and slow evaporation.
Along the Somerset shoreline, sediments were found to contain the bones of reindeer, bison, arctic fox and lemming from this period, and river valley deposits at Doniford have yielded the bones and tusks of mammoth. Remains of elephants, woolly rhinoceros and wolves have also been found in caves in the Mendip Hills (which are visible on the horizon from the field below Walford's Gibbet). During the warmer periods in the Ice Age, sea levels rose in the Bristol Channel and the Somerset Levels were drowned on several occasions.