Portland Stone, Jurassic Coast, Dorset
PUBLISHED: 14:47 16 June 2011 | UPDATED: 19:33 20 February 2013
Between Devon's Triassic beds and the Cretaceous cliffs of Ballard Down, much of Dorset's Jurassic Coast consists of Portland and Purbeck limestone.
Between Devons Triassic beds and the Cretaceous cliffs of Ballard Down, much of Dorsets Jurassic Coast consists of Portland and Purbeck limestone. This has been sourced for building as far back as Roman times. Rufus Castle above Church Ope Cove was constructed from Portland stone in the 11th century, Inigo Jones used it for the Banqueting House in London in 1620, and Sir Christopher Wren followed close behind. Wren, who was born at East Knoyle, about five miles from Shaftesbury, continually specified stone quarried on the Isle of Portland for his rebuilding of St Pauls Cathedral, and subsequently many of the finest buildings in London and other major cities have used Portland stone. Here in Dorset, it has been used by local Isle of Portland people for their own homes, churches and public buildings.
What is Portland stone?
Portland is on the southern side of a dome-shaped fold in the limestone rocks called the Weymouth anticline. These sedimentary rocks were formed in Mediterranean conditions, between 155 and 140 million years ago, when minute fragments of dead sea creatures shells and organic debris sank into the sea-bottom mud where calcium carbonate, or calcite, built up around them. Over time, the immense pressure from overlying rocks and earth movements cemented the whole into Portland limestone. Portland stone isnt all the same, I hasten to add. Some layers produce much better building materials than others, and you will see the various types as you follow the Portland walk.
The best building-material beds are the Whitbed and Basebed limestones, each being about 2m-deep layers of fine-grained freestone. Whitbed can be cut easily in any direction, both are excellent for fine-detail carving and both are very durable, although Whitbed is slightly more weather-resistant than Basebed. At the north of Portland, where we will be walking, the Whitbed is covered by deep layers of Roach and Purbeck rubble, which have to be removed before the better stone can be quarried. The Roach is full of holes and fossils of the Portland Screw, so it is only really suitable for rock armour coastal defences. The Purbeck rubble, above the Roach, is only suitable for dry-stone walling.
Between the Whitbed and the lower Basebed, there is a layer about 1m thick of sandy and shelly curf and chert, which also has to be discarded, often piled high for back-filling but historically dumped over the sea-cliffs. Continuing below the Basebed are the 50m deep Cherty Beds (used only for crushed aggregate), 35m-deep Portland Sand and, visible as you walk above West Weares, Kimmeridge Clay just above sea-level.