Quantock Hills - Geology

Geology

The Quantock Hills are largely formed by rocks of the Devonian period, which consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock. In the higher north-western areas older Early Devonian rocks known as Hangman Grits predominate and can be seen in the exposed rock at West Quantoxhead quarry, which was worked for road building. Further south there are newer Middle and Late Devonian rocks, known as Ilfracombe beds and Morte Slates. These include sandstone and limestone, which have been quarried near Aisholt. At Great Holwell, south of Aisholt, is the only limestone cave in the Devonian limestone of North Devon and West Somerset. The lower fringes around the hills are composed of younger New Red Sandstone rocks of the Triassic period. These rocks were laid down in a shallow sea and often contain irregular masses or veins of gypsum, which was mined on the foreshore at Watchet.

Several areas have outcrops of slates. Younger rocks of the Jurassic period can be found between St Audries and Kilve. This area falls within the Blue Anchor to Lilstock Site of Special Scientific Interest (SSSI) and is considered to be of international geological importance.

Kilve has the remains of a red-brick retort built in 1924 after the shale in the cliffs was found to be rich in oil. Along this coast, the cliffs are layered with compressed strata of oil-bearing shale and blue, yellow and brown Lias embedded with fossils. The Shaline Company was founded in 1924 to exploit these strata but was unable to raise sufficient capital. The company's retort house is thought to be the first structure erected here for the conversion of shale to oil and is all that remains of the anticipated Somerset oil boom.

At Blue Anchor the coloured alabaster found in the cliffs gave rise to the name of the colour "Watchet Blue". The village has the only updraught brick kiln known to have survived in Somerset. It was built around 1830 and was supplied by small vessels carrying limestone to the small landing jetty. Now used as a garage, the kiln is thought to have operated until the 1870s, when the large-scale production of bricks in Bridgwater rendered small brickyards uneconomic.

Cockercombe tuff is a greenish-grey, hard sedimentary rock formed by the compression of volcanic ash and is found almost exclusively in the south-eastern end of the Quantock Hills.