Geology: Weald Clay Formation
Rock unit: Freshwater limestone
Age: Lower Cretaceous
Provenance: West Kent
Where to see examples
These cathedral floors and steps contain numerous paving slabs of limestone extracted from the Weald Clay Formation. Bethersden Marble is the name given to a limestone found in the eastern Weald in small isolated outcrops within the beds of the Weald Clay. The stone has been extracted as a building stone, a paving stone and as an ornamental stone from numerous shallow pits or quarries many of which were in the vicinity of the village of Bethersden.
The stone is composed largely of the shells of freshwater snails (gastropods) set in a fine matrix of lime-rich silty clay. The snails are of the species Viviparous fluviorum. The Viviparous snail was formerly known as Paludina, hence the reference in the older literature to the stone being the larger Paludina limestone. These shelly beds, representing snail-rich ponds and lakes, occur throughout the Weald Clay outcrop in Kent and Sussex. The stone can be known by other local names. As a rough rule of thumb, stones with snail shells predominantly greater than 1cm are usually from the Weald (Kent and Sussex) and stones with snail shells less than 1cm are usually from Dorset (Purbeck Marble).
The calcite shell outlines of the snails are usually very clear. The shell interior can be the same as the dark and cloudy matrix, it can be a crystalline white or a bit of each. When the snail dies the soft organic parts rot away leaving a void. The void can be filled by the surrounding lime-rich mud (micrite) or, if remaining a void long after burial, carbonate-rich groundwaters moving through the beds may precipitate white crystalline calcite (sparite) within the shell. Sometimes the lower section of the shell is first filled with mud and later the upper void is filled with a calcite. When the piece of stone is removed from the ground the position of the mud/calcite divide can act as a ‘way-up feature known to geologists as a geopetal indicating which way up the limestone was originally deposited.
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© Geoff Downer 2019