History

Introduction

Purbeck Limestone History
The United Nations Educational, Scientific and Cultural and Organization (UNESCO) granted the coastline World Heritage status. The internationally protected site is now called the “Jurassic Coast”. A more appropriate name for the region would be the Mesozoic coast, but Jurassic has a popular culture association due to the “Jurassic” Park movies, where fictional dinosaurs have been reintroduced into the present by scientists. This Mesozoic includes three well known periods called the Triassic, Jurassic, and Cretaceous periods and is an interval of geological time from about 252 to 65.5 million years ago, it is also called the Age of Reptiles. A mass-extinction of the Dinosaurs marked the end of the Mesozoic Era.

Leger A. 2006 from the University of Colorado, USA states: ".....The Purbeck limestone spans from the lower Cretaceous 140–125 million years ago down to the upper Jurassic 150–145 million years ago. The Purbeck stone is a large grained limestone that originated in soft lime muds, shells and other fragments of organisms living in the water. Below the Purbeck limestone lies the Portland limestone that consists of smaller grains of calcite clasts. The Portland limestone, commonly called “free stone”, is a high quality working stone for building. However, in the towns and villages of Dorset both the limestones are found. The more important characteristic of the limestone is their strength against erosion. Despite their differences the Purbeck and the Portland limestones share this quality. Compared to surrounding layers these limestones are very resistant and will often form ridges ¹⁴...."

Dr Ian West, a retired geologist from Southampton University states that: ...Durlston Bay, Swanage is the type-section of the Purbeck Formation and the geology has been much studied since it was first described by Thomas Webster in 1816. This classic Purbeck section of lagoonal and lacustrine limestones alternating with shales and marls is the thickest exposed on the coast. Shelly limestones, the Purbeck Stone and Purbeck Marble, have long been quarried. Remains of dinosaurs, mammals, turtles, pterosaurs, crocodiles, fish, isopods and insects have been found here... ¹²

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Cretaceous Period: 145.5 million years ago to the Cretaceous-Paleogene (K-Pg) extinction event dated at 65.5 million years ago

About 65.5 million years ago, nearly all large vertebrates and many tropical invertebrates, no spine, became extinct in what was clearly a geological, climatic and biological event with worldwide consequences. Geologists call it the K-Pg extinction event because it marks the boundary between the Cretaceous and Paleogene periods. The "K" is from the German word for Cretaceous, Kreide
In 1979, a geologist who was studying rock layers between the Cretaceous and Paleogene periods spotted a thin layer of grey clay separating the two eras. Other scientists found this grey layer all over the world, and tests showed that it contained high concentrations of iridium, an element that is rare on Earth, but common in most meteorites. Also within this layer are indications of “shocked quartz” and tiny glass-like globes called tektites that form when rock is suddenly vaporized then immediately cooled, the effect of an extraterrestrial object striking the Earth with great force. The Chicxulub (CHEEK-sheh-loob) crater in the Yucatan, Mexico, dates precisely to this time. The crater site is more than 110 miles (180 kilometers) in diameter and chemical analysis shows that the sedimentary rock of the area was melted and mixed together by temperatures consistent with the blast impact of an asteroid about 6 miles (10 km) across striking the Earth at this point.

When the asteroid collided with Earth, its impact triggered shockwaves, massive tsunamis and sent a large cloud of hot rock and dust into the atmosphere. As the super-heated debris fell back to Earth, they started forest fires and increased temperatures. This rain of hot dust raised global temperatures for hours after the impact and cooked alive animals that were too large to seek shelter. Small animals that could shelter underground, underwater, or perhaps in caves or large tree trunks, may have been able to survive this initial heat blast. Tiny fragments likely stayed in the atmosphere, possibly blocking part of the sun's ray for months or years. For example two months after impact any animal bigger than a crocodile would have been wiped out. Acid rain would have fallen due to the billions of tons of carbon dioxide, methane and carbon monoxide created in the atmosphere, when layers of limestone were vapourised in the impact crator. With less sunlight, plants and the animals dependent on them would have died ¹⁵.

140million years ago, Sauropod Dinosaurs gathered at the shoreline, now an Acton stone quarry, of the Purbeck Lagoon

An Iguanodon was a Sauropod Dinosaur ²⁰

⁹ Map 1886-1903 Stone Location

The cliff quarries were very productive and used in the construction of Ramsgate Quay after an act of Parliament gave approval for construction of a new harbour, following the Great Storm in 1749, the quarries and mines supplied 15,000 tons of stone. It took 52 sailing ships two years four months to complete the shipment.

The cliffs are solid stone, unlike the Purbeck beds where blocks are separated by loose clay, the Portland stone beds are joined by a 60 centimetre capping stone, known as under-picking cap, separating the seam from the upper layers. To separate the 20 ton blocks, the capping stone had to be carefully blasted away with gun powder to ensure the ceiling and payload are not damaged. It was separated horizontally and vertically from its neighbours using wedges, known as Gads. This is a highly skilled task that required the workmen to create a hole, with a hammer and punch, half the length of the wedge and a slightly larger diameter, this could take up to two days to complete. From the image, there are five men hammering in wedges together, to ensure a uniform rock fracture. The Portland stone weighs 12 cubic feet/ton 25% lighter than Purbeck stone at 8 cubic feet/ton.

From the Support Leg image, the leg is not constructed manually but is part of the cliff stone, the reason for this was the height of the mine shaft, some greater than three metres, a height physically impractical when handling the size of the block required, although in a few cases when absolutely essential they were built by hand, but this required excessive man power to be the norm. The cliff quarries closed in the late 1950’s because improved internal combustion engines opened up many new commercial transport opportunities, killing off the sea-borne trade. ⁹

Winspit Portland Stone Mine

At the base of Peveril Point, a member of the Durlston Formation is placed at the base of the massive Broken Shell Limestone Bed - also known as “the Burr”. Above the Burr, are the principal marble beds. In the Middle Purbeck, the Freestone is the best stone for building as it occurs in thick beds that can be cut for ashlar. Ashlar is the finest stone masonry unit, generally a rectangular cuboid.

The manor house at Dunshay was one of the earliest to be built of Freestone in 1642. Freestone was used a great deal during 19th Century for churches in Poole and Bournemouth. The Roach, Thornback and Grub from the Freestone Vein have also been used for paving and kerbs. Thinner slabs of the Downs Vein, which can be up to 2cm thick, have been used for roofing tiles throughout the Isle of Purbeck and the Frome Valley. Open cast quarrying in Worth and Langton Matravers is now providing Middle Purbeck Limestone for new-build projects and repairs across Dorset

Swanworth Quarry Stone Beds ¹²

Limestones of the Portland Group are quarried in south Dorset on the Isle of Portland and the Isle of Purbeck. The group is divided into a lower Portland Sand Formation and an upper Portland Stone Formation. The white, ooidal limestone most commonly used in many cities including London, Dublin and Cardiff. In the 14th Century, the stone was exported to Exeter for the Cathedral, and in the 17th Centry to London for St Paul’s Cathedral. Ooidal limestone is composed mainly of spherical grains ('ooids', from the Greek word for egg)

On the Isle of Purbeck, similar though more thinly bedded ooidal limestones are worked as freestone, but are commonly described as Purbeck-Portland Stone, which is a very much harder stone suitable for naval fortification and harbour construction as opposed to the softer more decorative structure of Portland Stone. The eastern course of the Portland bed from Portland Bill to Durlston head is: at the Bill it runs under the seabed for 25 Kilometre, rises steeply 70 metres to the surface at St Albans Head, at Winspit the seam is approximately 30 metres above the sea, it then slopes gently downwards and appears in the lower structure of Durlston Head as viewed from the sea ¹⁶.