swuklink: Limestone

Making up some ten per cent of all sedimentary rocks, limestone is composed of the mineral calcite (calcium carbonate) which has been laid down by of the shells made by marine organisms from calcium carbonate dissolved in the seawater. After they die, their shells sink to form the pelagic ooze of the ocean floor.

Limestone is porous and also comparatively soluble in water and rivers crossing the rock frequently become subterranean, carving large caverns. Dripping water in limestone caves becomes supersaturated as it evaporates and produces stalagmites and stalactites.

Limestones may be crystalline, clastic, granular, or dense, depending on the method of formation and white or almost white when pure. Many limestones, however, such as the grey Portland Stone or "golden" Ham Hill Stone or simply Ham Stone exhibit various colours, particularly when weathered, because of impurities such as clay, sand, iron oxide, organic remains or other materials. The limestone frequently contains fossils, particularly of the shells or their fragments from which it was formed. Sometimes these may be individual fossils and sometimes may be so numerous as to give the limestone a marble-like appearance such as that of the "Purbeck marble used as a decorative stone. Small cavities in the rock may be lined with calcite, quartz, dolomite or barite crystals and limestone layers frequently contain nodules of cherts or flint.

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BRITISH ISLES
ENGLAND
GEOLOGICAL TIME SCALE
GEOLOGY OF THE BRITISH ISLES
PORTLAND STONE
PURBECK MARBLE
ROCKS

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Wikipedia
Limestone

DORSET

The Jurassic limestone ridge which starts on the North York Moors runs the length of England through the Cotswolds into the South-West and ends in Dorset on the Isle of Purbeck.

Isle of Portland

The "Isle", attached to mainland Dorset by Chesil Beach, is famous for the Portland Stone quarried there.

Isle of Purbeck

Winspit

A ledge on the cliff allowed the limestone to be quarried by the villagers of Worth Matravers about a mile inland. The "caves" which they carved from the rock are now a popular visiting place for locals and visitors alike.

Worth Matravers

The picturesque village, about a mile inland, is built of the local limestone quarried at the cliff ledge at Winspit. The village pub is, appropriately enough, named the "Square and Compass" and has a room used as a small museum.

The Square and Compass public house, meritious of a visit in its own right, is the venue for a festival of stone carving during the last week of July and the first week of August every year. Amateur and professional alike descend on the village from throughout the country to practice their craft in full view.

WILTSHIRE

Limestone used to cover most of England but much has been eroded leaving only a few outliers in the county as at Old Town (Swindon) and Kington St Micheal, the Vale of Pewsey and the Vale of Wardour.

Swindon

Old Town

The hill on which Old Town evolved stands about 120 feet above the surrounding clays of the Thames valley on an outlier of the Portland beds.