Portland stone is a Limestone from the Jurassic period quarried on the Isle of Portland. The quarries consist of beds of white-grey limestone separated by Chert beds. It has been used extensively as a building stone throughout the British Isles, notably in major public buildings in London such as St. Paul’s Cathedral and Buckingham Palace. It is also exported to many countries—Portland stone is used in the UN Headquarters building in New York City, for example.
Portland Stone formed in a marine environment, on the floor of a shallow, warm, sub-tropical sea probably near land (as evidenced by fossilized driftwood, which is not uncommon). As seawater was warmed by the sun, its capacity to hold dissolved gas is reduced, consequently dissolved carbon Dioxide (CO2) is released into the atmosphere as a gas. Calcium and bicarbonate ions within the water are then able to combine, to form calcium carbonate(CaCO3) as a precipitate. The process of lime scale build up in a kettle in hard water areas is similar. Calcium carbonate is the principal constituent of most limestones. Billions of minute crystals of precipitated calcium carbonate (called calcite) accumulated forming lime mud (called micrite) which covered the sea floor. Small particles of sand or organic detritus, such as shell fragments, formed a nucleus, which became coated with layers of calcite as they were rolled around in the muddy micrite. The calcite gradually accumulated (by accretion) around the fragments of shell in concentric layers, forming small balls. This process is similar to the way in which a snowball grows in size as it is rolled around in the snow. Over time, countless billions of these balls, known as "ooids" or "ooliths" (from the Greek for "egg-shaped" or "egg-stone"), became partially cemented together (or lithified) by more calcite, to form the oolitic limestone we now call Portland Stone. Fortunately, the degree of cementation in Portland Stone is such that the stone is sufficiently well cemented to allow it to resist weathering but not so well cemented that it can't be readily worked (cut and carved) by masons. This is one of the reasons why Portland Stone is so favoured as a monumental and architectural stone. Dr Geoff Townson conducted three years doctoral research on the Portlandian, being the first to describe the patch-reef facies and Dorset-wide sedimentation details . Dr Ian West of the School of Ocean and Earth Sciences at Southampton University completed a detailed geological survey of Withies Croft Quarry before the Portland Beds were quarried by Albion Stone plc.
Stone has been quarried on Portland since Roman times and was being shipped to London in the 14th century. Extraction as an industry began in the early 17th century, with shipments to London for Inigo Jones’ Banqueting House. Wren’s choice of Portland for the new St Paul's Cathedral was a great boost for the quarries and established Portland as London's choice of building stone. The island was connected by railway to the rest of the country from 1865 and new quarry firms, steam-powered stone sawmills and masonry works soon followed. The Bath & Portland Stone Firms Co. Ltd. was formed in 1911. The Easton Railway closed in 1965 and since then stone has left by lorry. After a period of decline, the 1980s saw a revival in the use of natural stone and in 2000 there were two quarries and a large masonry firm on the island.
The Coordinates take you to the “Eastbourne Youth Council Millennium Sundial”, as this has been roughly hewn it is a great example of the Portland Stone in it’s raw form.
To claim this Earthcache please answer the following questions
- At what stage in the Jurassic period is the limestone from?
- What is Chert ?
- What size must the Ooids be to be classed as Ooids?
- Can you describe the naturally occurring characteristics that you can see on the stone and what made them ?
- How many hand prints around the base are there?
- Finally an optional picture of yourself at GZ
Please email/ message answers to CO before logging, thanks