Portland Limestone

Portland Limestone

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The Portland Limestone is a valuable building stone that has been used in numerous structures throughout history. It is important to understand its geological formation, its lithological characteristics, and its applications to appreciate its significance. This academic paper aims to explore the geological formation of the Portland Limestone, including its Basebed, Whitbed, Roach, Portland Cap, and Portland Blue variations. The paper will also examine the stone’s usage in various famous buildings and villages.

Geological Formation of the Portland Limestone The Portland Limestone, which is also known as the Portland Stone, is a limestone formation that is made up of calcium carbonate. This stone was formed during the Jurassic period, approximately 150 million years ago (BGS, n.d.). It is a marine limestone that is composed of the skeletal fragments of organisms such as corals, mollusks, and foraminifera that lived in shallow seas.

The Portland Limestone formation is divided into four main categories: the Basebed, Whitbed, Roach, and Portland Cap. The Basebed is the lowest level of the Portland Limestone formation, and it is characterized by the presence of sandy layers, fossil fragments, and clayey materials (BGS, n.d.). The Whitbed, on the other hand, is the most valuable and commonly used stone in the Portland Limestone formation. It has a uniform texture and is distinguished by the absence of sand and clay layers (BGS, n.d.). The Roach is a darker, more fossil-rich limestone that is usually used as a building stone for details or sculptures. The Portland Cap is the uppermost level of the formation, and it is a harder and coarser limestone that is less frequently used as a building stone (BGS, n.d.). Lastly, Portland Blue is a rare and highly prized variety of Portland Stone, distinguished by its dark blue-grey colour, and is sometimes referred to as Roach Bed.

Lithological Description and Geographical Limits The Portland Limestone is a hard, dense, and durable stone that is easy to work with. It has been used extensively in buildings throughout the UK, and its lithological characteristics make it suitable for a wide range of applications, such as exterior cladding, paving, flooring, and walling. The thickness of the Portland Limestone varies depending on the level within the formation, with the Whitbed having a thickness of up to 6m, while the Basebed is up to 12m.

Alternative Names

The Portland Limestone has several alternative names, including Portland Stone, Portland Stone Formation, Portland Roach, and Roach Bed.

Use in Buildings

The Portland Limestone is primarily found on the Isle of Portland, located off the coast of Dorset, England). However, it can also be found in other areas such as the Cotswolds, Bath, and in London’s historic buildings. The stone’s unique characteristics, such as its durability and resistance to weathering, make it ideal for use in structures in coastal areas.

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Further reading:

Portland Group PL BGS Lexicon of Named Rock Units – Result Details

Portland Stone Formation POST BGS Lexicon of Named Rock Units – Result Details

• British Stone Selector
• Portland Bowers Roach
• Portland Bowers Basebed

• Portland Fancy Beach Whitbed
• Portland Grove Whitbed
• Portland Bowers Whitbed
• Portland Bowers Basebed
• Portland Bowers Roach
• Portland Jordans Basebed
• Portland Jordans Whitbed
• Natural Stone Data
• Portland Jordans Roach
• Natural Stone Data
• Albion Stone Plc
• Natural Stone Data
• Natural Stone Data
• Portland Beds [Obsolete Name and Code: Use HADD, PL, PLND]
• Portland Cherty Member [Obsolete Name and Code: Use POCH]
• Portland Limestone Formation [Obsolete Name and Code: Use POST]