Limestones

2014-09-16 13.26.51

This group of attractive and commercially valuable sedimentary rocks is responsible for much of the English landscape: the Yorkshire Dales, Derbyshire Dales, Cotswolds, Mendips, Chilterns, north-west Lancashire and Wenlock Edge. A form of silica known as chert is an accessory mineral, occupying the place of the more familiar flint in chalk. Both limestone and chalk consist of the mineral calcite, calcium carbonate. The great chalk deposits of the Wolds, Norfolk and the Sussex Downs are solely from the Cretaceous period, the last great age of dinosaurs. Unlike chalk limestone was formed in many geological eras. There is Cambrian limestone in Scotland and a small amount of Silurian limestone in Shropshire. The limestone of Yorkshire and the north-east is Carboniferous, and in central England there are later linear deposits of Permian and Jurassic material.

Limestones form in quiet seas away from land, a process which continues today. Some were precipitated from evaporating oceans and others consist of calcium salts that were extracted by various living organisms from sea water. Marine organisms use three kinds of calcium carbonate to construct their shells: aragonite, a dense compact form, magnesium rich calcite, and ordinary calcite, which is fully stable. The amount of calcium carbonate soluble in water increases as the pressure is raised and the temperature falls. If the sea is deep enough calcium carbonate in shells will re-dissolve (at 5000m in the Atlantic ocean). Most British limestones formed in relatively shallow water; some consist almost entirely of fossils, in others fossils are hardly visible. Descriptive terms for limestone include: reef (Wenlock Edge and Cracoe, N.Yorks), crinoidal (formed of sea lilies in the Carboniferous), oolitic (typical of the Jurassic Cotswolds and forming in rough sea conditions).

Chalk, as all ready mentioned, formed only in the Cretaceous and originated from soft coccoliths forming external to algae in sea too deep for reefs. Most shelly remains are aragonite or magnesium rich calcite but coccoliths are made from ordinary calcite and the original particles have remained separate. Within chalk there are horizons of the accessory minerals marcasite (iron pyrites) and flint. Flint consists of bulbous masses of finely crystalline quartz which ultimately presumably came from the ‘skeletal’ remains of sponges. Flints shatters along curved conchoidal fracture sites, like glass; where two curved surfaces meet there is formed a very sharp edge. This property made flint extremely attractive for prehistoric tool making.

Strong brines percolating through limestones may replace half the calcium with magnesium making the still stronger rock, dolomite. Named after a range of Italian mountains this rock forms a ridge in North Yorkshire along which the Romans constructed Dere Street linking York and Corbridge. If calcium carbonate is heated up with silicates (as clay) unstable calcium silicates are formed. This is ground to powder to form cement. In the presence of water cement forms a new hydrated silicate mineral form that conveniently bonds with sand and ballast to make the valuable synthetic rock known as concrete.

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