Bunter (geology)

Bunter beds are sandstone deposits containing rounded pebbles. They can be found in Warwickshire, Cheshire, Staffordshire, Nottinghamshire, Yorkshire, Devon and Dorset in England. They are thought to be alluvial deposits and, judging from the rounding of the mainly quartzite pebbles, to have resulted from prolonged transportation in a large and turbulent river, resulting in powerful abrasion. The deposits in the English Midlands are thought to have been transported in this way northwards from Brittany, France. This supposed river has been called the "Budleighensis", after the Devon village of Budleigh Salterton, a site where such deposits were discovered. The depositions took place in the lower Triassic period. Some newer conglomerates, e.g. near Ryton in Warwickshire, are thought to have arisen during the Ice Age by reworking and southward transportation of older deposits by ice flows.

The pebbles, also called cobbles, which can be used as gravel, as ballast or as cobblestones, are mainly milky-white quartzite but can vary in colour and composition, including some that are hard, reddish-coloured sandstone. The sandstone in which these pebbles are deposited can be used for building or as an aggregate for cement or concrete.

The name "Bunter" derives from the German term "Buntsandstein", "bunt" meaning "variegated" or "colourful", referring to the colour of the sandstone deposit, which varies from reddish to greenish. This sandstone is widespread across central Europe, notably in the Black Forest and Odenwald region of Germany, as well as the Vosges Mountains in northeastern France.

The sandstone can be hard enough for building, yet easy enough to "work", resulting in bridges, castles, cathedrals and churches constructed of reddish sandstone, throughout the relevant areas of Europe (e.g. Germany, Luxembourg, Switzerland, Alsace in France, Denmark, Poland). The land under which these beds lie is generally very well drained, creating heathlike conditions. Because of the drainage, the soil tends to be of low fertility. A notable area in Britain that has these characteristics is Cannock Chase, a designated Area of Outstanding Natural Beauty (AONB).

The long shingle tombolo of Chesil Beach in Dorset and the raised beach of Portland, Dorset are partly composed of Bunter pebbles. As yet, no fully satisfactory and universally accepted geological explanation has been formulated to explain their precise origin and mode of transport, as each proposed theory has its difficulties.

Further reading