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The Mendip Hills are situated 30 km south of Bristol. There is a wide variety of landscapes formed by the underlying rocks, ranging from Silurian to Jurassic in age. The oldest rocks are the igneous pyroxene andesites, found at Stoke St.Michael. This rock was created by a series of active volcanoes 425 million years ago. It is now quarried by John Wainwright and Co Ltd at Moon's Hill Quarry. Sandstones of Devonian age can also be found on the Mendips. These were deposited when the region was a hot, arid desert about 380 million years ago. The rock is exposed at Stoke St.Michael and Beacon Hill Wood.
The larges deposits of rock forming the Mendips came from the Carboniferous Period 350 - 300 million years ago. This area was covered in a shallow tropical sea, creating Limestone full of corals, crinoids and bivalve shells. Later, as the sea receded, swamp conditions led to the deposits of sandstones, shales and coal, now found around Radstock in the North East Mendips.
The Triassic period (268 million years ago), brought a massive folding event that created the Mendip Mountains, forming an east-west ridge, up to 1500m high. These mountains then became eroded, and the sea rose, so the Mendips became a series of island. These became further eroded into a flat sea bed, covered in a shallow sea. This brings us to the Jurassic Period (150 million years ago), when a cream coloured limestone was deposited on top of the eroded limestone. This is called an Unconformity because the sequence of rocks does not conform to the expected geological sequence (i.e. Carboniferous rock-Permian Rock-Triassic Rock-Jurassic Rock). This Unconformity is exposed in Vallis Vale, Nr Frome, and was described by one of the founders of modern geology, called Henry De la Beche in 1846. These important rock exposures are still visible and maintained so people can visit them today.