Geology of Somerset - Southern Uplands

Southern Uplands

To the south of Somerset there is an upland with a series of rolling valleys and scarps, from Penselwood in the east to the Blackdown Hills, another designated Area of Outstanding Natural Beauty, in the west. This is a geologically complex area of clays, limstone and marl. The honey-coloured limestone at Ham Hill (also known as Hamdon Hill) is particularly important to geologists because of the assemblages of fossils which it contains, the sedimentary features which it displays and the way it relates to other rocks of equivalent age in the close vicinity. It has been quarried since Roman times at least.

The Blackdowns are on the south-west border of Somerset, extending into Devon. They are composed of Upper Greensand. The scarp faces north and is steep and wooded, with a south facing dip slope. There is an open plateau, which is not as high as the Mendips.

The Quantock Hills are a 20 kilometres (12 mi) long broad ridge from the coast near Watchet in the north to near Taunton in the south. They reach 384 metres (1,260 ft) high at Wills Neck and are separated from Exmoor and the Brendons by a rift valley. The Quantocks and the Brendon Hills at the eastern end of Exmoor are formed by thick sequences of slates and sandstones of Devonian age that were deposited by large deltas that built out into a shallow sea.

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 were 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, there is a limestone cave, which is the only one in the Devonian limestone of North Devon and West Somerset. The lower fringes around the hills are composed of younger rocks of the Triassic period, these are known as New Red Sandstone rocks which represent the deposits of large river systems that crossed a desert plain, and often contain irregular masses or veins of gypsum, which was worked on the foreshore at Watchet. The scarp is to the west with a dip slope to the east. The west side is cut by combes with broad valleys on the east. The hill tops are open heathland with woods on the slopes.

Several areas have outcrops of slates and between St Audries and Kilve, 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. At Kilve are the remains of a red brick retort, built in 1924, when it was discovered that the shale found in the cliffs was rich in oil. At Blue Anchor the coloured alabaster found in the cliffs gave rise to the name of the colour "Watchet Blue".