Quantocks is a varied county, which has the rocks and sediments spanning the 440 million years of time. The landscape of this country is varied, with contrasts being proffered by the Quantock Hills, Mendip, Brendon and Polden. In the east and the west of the County, there is a complicated landscape of hills and vales which represents the southern end of the Cotswold escarpment and plateau. This landscape diversity is largely because of the variation in the underlying geology and the history of climatic environmental change which has affected its development.
Mendips are made in blocks of folded Carboniferous Limestone, and show the remaining of a much higher range of hills which existed millions years ago. The Quantocks and the Brendon Hills lying at the eastern end of Exmoor are made by thick sequences of slates and sandstones of Devonian age which were deposited by enormous deltas which built out into a shallow sea. All of these hill ranges are encircled by mudstones and sandstones of Triassic age that represent the deposits of big river systems which crossed a desert plain. In the south and east of Somerset where the marine lower Jurassic clays form undulating and rich pasture between Castle Cary and Staple Fitzpaine before the succeeding sands and limestones give rise to a more complicated landscape of scarp slopes, hills and small steep valleys which rises to the south and east. In the far south of the County, the healthy windswept plateaux and ridges of the Blackdown Hills are covered by the marine sands of the Cretaceous Upper Greensand, which in turn is overlain by the purer white limestone of the Chalk in the Chard and Crewkerne area.
During the last two million years the area was not impacted directly by the repeated advances and retreats of the great ice sheets of the Ice Age. Nevertheless, the arctic climate which prevailed during glacial periods propelled to the development of tundra-like conditions. In the warmer periods warmer periods between glacial, sea-level rose to invade and cover the Somerset Levels on many occasions.
The most ancient rocks occurring in Somerset are of Silurian age and represent the most southerly known outcrop of rocks of this age in Britain. These comprise a sequence of mudstones, tuffs shales and lavas, which form a narrow outcrop to the northeast of Shepton Mallet in the eastern Mendip Hills. Such sediments have produced a fairly rich fossil fauna of trilobites and brachiopods indicating that they were deposited in a shallow marine sea into which the lavas were extruded.