The Quantock Hills AONB comprises a narrow plateau of Devonian rocks, running north westwards for approximately 19 km from just north of Taunton to the Bristol Channel.

The physical landform of the Quantock ridge is an important backdrop and skyline feature over a wide area. To the west, the hills rise dramatically from the wide valley which separates the Quantocks from the Brendon Hills and Exmoor. The eastern side of the ridge rises in longer, gentler slopes from the River Parrett and the low-lying land of the Somerset Levels and Moors.

Streams have cut deep, steep-sided valleys into the Devonian rocks, creating the distinctive ‘folded’ landscape of the Quantock Hills. The Quantock landform often appears more imposing than its actual height, which ranges from approximately sea level to a general summit height of about 300 m at the northern end of the ridge.

The AONB boundary also encompasses a green mantle of farmland surrounding the main ridge. This undulating countryside, with its patchwork of hedged fields, copses and settlement, provides further diversity in landscape character.

The geology of the Quantock Hills is formed by rocks mainly of the Devonian Period. In simple terms, the higher north west part of the hills are formed by older, Lower Devonian rocks, while the south east is composed of rocks of the Middle and Upper Devonian, with lower fringes around the hills composed of younger rocks of the Triassic Period.

The Devonian rocks consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock. This rock was subsequently folded into an upstanding ridge that was then subject to further complex sequences of erosion, inundation and deposition to create the landform present today.

The oldest rocks in the AONB are the Lynton Slates, which outcrop in a small area on the western edge of the hills. The northern plateau of the hills, is underlain by the Hangman Grits consisting of sandstones with shales and quartz-pebble and mud-pellet conglomerates.

The central region of the hills is a transitional zone where Ilfracombe Slates overlie the Hangman Grits. The Ilfracombe Beds consist of more varied rocks and include a mix of slates, siltstones, sandstone and bands of limestone.

The third main geological division of the hills lies roughly south of a line between Cothelstone and Enmore and is composed of the Morte Slates, which overlie the Ilfracombe Beds. These softer slates have been weathered to create the lower, more rounded hills of the southern Quantocks. They support a rich soil that is suitable for both pasture and arable. The fertile farmland on the flanks of the upland plateau skirting along the western edge of the hills, and around the coastal fringe to Holford on the east, is underlain by the younger Permo-Triassic rocks.

During the Triassic, New Red Sandstone was deposited on the old land surface surrounding the hills, followed by the Mercia Mudstone Group. Both consist of materials eroded from the hills.

Geologically, the most interesting part of the AONB is the coastal edge, between St Audries and Kilve, where younger rocks of the Jurassic Period can be found. This area falls within the Blue Anchor to Lilstock Site of Special Scientific Interest (SSSI) and is considered to be of international geological importance. The marine deposits, known as Lias, consist of alternating limestones and shales.

The landform was further shaped during the Ice Age. The hills were probably never covered by extensive ice sheets, although the ground would have been permanently frozen with seasonal snowfields. Products of ice age erosion form a deposit known as ‘Head’, a flinty soil mix, which can be found covering extensive areas on the hills. The frozen ground did not permit percolation of water, and the resultant surface drainage created many of the valleys and combes that are characteristic of the Quantock landscape today.

Countryside Agency, Quantock Hills Landscape, 2003