Richmond Bridge was built between 1774 and 1777 to a design by James Paine and Kenton Couse. Its construction was overseen by 90 commissioners appointed under the 1772 Richmond Bridge Act. These included the landscape architect Lancelot 'Capability' Brown, the actor David Garrick and Horace Walpole, son of Britain's first prime minister. The bridge was built to a stone arch design, with five elliptical arches of differing heights. This gives it a humpbacked appearance but allows river traffic to pass through its central span, which has a clearance below of up to 7.9m depending on the tide.
The material used throughout is Portland Stone, a hard-wearing pale white oolitic limestone. Portland Stone was formed on the floor of a shallow marine environment known as a carbonate ramp during the Tithonian stage of the very late Jurassic period, when the climate of Southern England was sub-tropical. As dissolved carbon dioxide (CO2) was released through the effect of warming by the sun, positively and negatively charged ions of calcium and bicarbonate in the supersaturated seawater bonded to form a precipitate of calcium carbonate (CaCO3) known as calcite. The calcite crystals coated minute particles of sand and organic material, over time forming tiny balls (oolites). The spherical oolites settled into the sediment, becoming buried and gradually cemented into limestone by the deposition of more calcite. Quarried since Roman times, Portland Stone has been prized by architects as a monumental stone because of its colour, strength and resistance to weathering. It is widely seen around London on important buildings.
Once exposed to the elements, all stone is gradually worn away through weathering. There are three main types of weathering: physical (chiefly freeze-thaw or exfoliation), chemical (typically dissolution, hydrolysis or oxidation) and biological (caused by living organisms, most often plants). Limestone is particularly susceptible to chemical weathering, because the dissolved CO2 in rainwater reacts easily with the CaCO3 in its structure. However, Portland Stone is one of the most durable limestones, with an average retreat rate of only 1mm/50 years in natural conditions. That said, the introduction of acidic pollutants into the atmosphere since the 18th century has accelerated this process. Other chemical factors can also speed up weathering, such as the presence of salt : because limestone is porous, saltwater can penetrate easily but will cause structural decay similar to freeze-thaw as the stone dries and the salt crystallizes.
To log the EarthCache, you now need to answer four questions.
Q1. Walk towards the middle of the bridge on the upstream (SE) side and find converted gas lamp #10. Continue a metre or so beyond the lamp towards to the top of the bridge and look at the side of the balustrade rail. There you will find objects embedded in the stone. Describe the length, texture and shape of the objects that you see.
Q2. What do you think these objects are and how did they get into the stone?
Q3. Go over the top of the bridge and down towards the Twickenham side. Examine the lower section and base of the last whole baluster before lamp #14. How do you think this damage has been caused?
Q4. Return to the Richmond side of the bridge and find the original 18th century milestone in the form of an obelisk. Add London Bridge to Hounslow. What is the total in Roman numerals?
Please email or message me your answers using the link to my profile at the top of the cache page. A photo of you at GZ is welcome, but optional, and please do not include pictures that give clues for any of the questions. You may log the find as soon as you have sent me your answers but if it is obvious that you have not visited the bridge then your log will be deleted.
Congratulations to Team GIM for their joint FTF!