Geology of Somerset - Rock Ages

Rock Ages

The oldest rocks are of Silurian age (443–417 million years ago), the most southerly known outcrop of rocks of this age in Britain. They make up a sequence of lavas, tuffs (volcanic ash), shales and mudstones in a narrow outcrop to the northeast of Shepton Mallet, in the eastern Mendip Hills.

Rocks from the Devonian (417–354 million years ago) are found in much of Exmoor, the Quantocks (including Hangman Sandstone and Cockercombe tuff), and in the cores to the folded masses of the Mendip Hills.

Carboniferous Period (354–290 million years ago) rocks are represented by the Carboniferous Limestone that forms the Mendip Hills, rising abruptly out of the flat landscape of the Somerset Levels and Moors. The limestones are very fossiliferous, and contain evidence of the abundant marine life that existed at the time of their creation, including fossil crinoids (sea-lilies), corals and brachiopods.

At the end of the Permian (290–248 million years ago) and Triassic periods, the Variscan orogeny resulted in the formation of several mountainous areas including Dartmoor in the south, Exmoor and the Quantocks, and the Mendips.

In the Taunton area Permian (295–250 million years ago) red sandstones and breccia outcrop, although rocks of Triassic age (248–204 million years ago) underlie much of Somerset and form the solid geology of the Somerset Moors and Levels. There are no glacial deposits.

The Triassic rocks consist of red marls, sandstones, breccias and conglomerates which spread over the older rocks. The Dolometic Conglomerate is an old shingle beach of Keuper Marl age. The Rhaetic Beds are full of fossils due to invasion of the Jurassic Sea. The Lias consists of clays and limestones, the latter being quarried and are famous for their fossils. Blue Lias was used locally as a source of lime for making Lime mortar. It is still used as a decorative building stone. Blue Lias is believed to have been quarried on the Polden Hills as early as the 15th century and was quarried in Puriton from the early 19th century until the 1973, when the local cement works closed.

Above the Lias is the Lower Oolite Series which are chiefly clays and oolile limestone. The famous Bath Stone is obtained from the Great Oolite bed. Oxford Clay is the chief member of the Middle Oolite Series; and above this are the Upper Cretaceous rocks with Gault, Upper Greensand and Chalk. Alluvial flats and peat bogs occupy much of the centre of Somerset.