Geology of Somerset - Northern Uplands

Northern Uplands

This is the area between the River Avon to the north and the Axe valley. The north of Somerset is dominated by the tableland of the Mendip Hills, an Area of Outstanding Natural Beauty, stretching from Frome in the east to Crook Peak in the west, with outliers of Bleadon Hill and Brean Down as well as Steep Holm in the Bristol Channel. The highest point is Black down at 324 metres (1,063 ft). There is an escarpment facing south to the Levels and Moors while the dip slope to the north is broken up.

To the north of Bath are Landsdown, Langridge and Solsbury hills. These are outliers of the Cotswolds. Bath is noted for its thermal waters (48 °C) that are rich in calcium and sodium sulphates.

The Old Red Sandstone is a series of red Sandstone, marls and conglomerates. It rises as an anticline in the Mendips and appears in the Avon Gorge and at Portishead. Carboniferous limestone, of maritime origin, covers the sandstone and appears in the Avon Gorge and at Weston-super-Mare where it contains volcanic rocks. There are outlying hills at Worlebury, Middle Hope, the Failand Ridge, Broadfields Down, Portishead Down and Wrington Hill.

The main geological component of the Mendips is carboniferous limestone and represent the remnants of a much higher range of hills that existed hundreds of millions years ago. This has allowed the formation of features such as Cheddar Gorge, Ebbor Gorge and Burrington Combe. There are a wide variety of Caves of the Mendip Hills and swallett holes caused by dissolution of the rock by water. Further east there are silurian volcanos, Carboniferous limestone outcrops, variscan thrust tectonics, Permo-Triassic conglomerates, sediment-filled fissures, a classic unconformity, Jurassic clays and limestones, Cretaceous Greensand and chalk topped with Tertiary remnants including Sarsen Stones. These sediments have yielded a fairly rich fossil fauna of brachiopods and trilobites indicating that they were deposited in a shallow marine sea into which the lavas were extruded. The rocks are quarried at Moons Hill near Stoke St Michael for aggregate.

Coal measures appear in the Radstock district, and surrounding Somerset coalfield (largely concealed by Triassic and newer rocks). There are two series of coal-bearing sandstones and shales separated by Pennant sandstone. Locally the beds are folded and faulted. There were mines in the Radstock and Nailsea areas but these have closed. This was one of the first areas in the world to undergo systematic geological study and mapping by John Strachey and William Smith in the 18th century. They observed the rock layers, or strata, which led Smith to the creation of a testable hypothesis, which he termed The Principle of Faunal Succession.

The Mendips were mined for lead, silver, coal, ochre, Fuller's earth and zinc but this has finished. They were also quarried for stone, notably at Bath and Doulting. Today the Mendips are a major source of aggregates.