Lesson:
This EarthCache hopes to teach you about the characteristics of Portland stone and the weathering risks it is prone to
Questions:
- Describe the colour and texture of the stone plinth
- Where on the memorial are the ooids most visible, is it the horizontal or the vertical edges? Why do you think that is?
- Can you see any signs of weathering on the soldier on top of the statue, if so please describe them.
- What signs of chemical erosion can you see on the plinth, what do you think has caused this? You made need to think about what the plaques are made from
Send your answers by email or via the Message Centre.
History of the location:
The Monnmouth War Memorial stands in the centre of town in a small garden on St James' Square, it has three bronze plaques dedicated to the names of those who lost their lives in both world wars as well as the Korean war. The memorial plinth has been created from portland limestone with a marble statue atop, the statue depicts an infantry soldier carrying full kit. Behind the memorial is an Indian Bean Tree that was planted in 1900, it has a controversial history in the town after being condemned in 2005 after being deemed a danger. But the actions of the Monmouth Action Group saved the tree by consulting with experts who recommended treatments which have lead to the tree flourishing today.
Background information:
Portland stone is a type of limestone formed in a marine environment when calcium and bicarbonate ions combine to form calcite, which accumulates with other particles on the ocean floor. These particles gather together to form slightly larger balls called "ooids" which then go on to cement together to form the limestone. Ooids can be identified as they appear as small sand like particles. I have added an image below as well as to the gallery of a cross section of the portland stone and of some ooids before they have cemented together to help identify them.
Weathering is a variety of processes that cause damage to rocks over time, it makes rocks more prone to erosion. Weathering processes can be classed into three types:
Physical weathering - Caused by the effects of changing temperature on rocks. As the temperature rises the rock will expand and then as the temperature cools it contracts again. This also occurs to any water which fills any gaps in the rock, as ice has a greater volume than water as it freezes it causes any gaps to enlarge.
Chemical weathering - Rain reacts with the minerals inside the rocks, the rain absorbs CO2 and chemicals from other fossil fuels. These chemicals ake the rain acidic which react with the alkali rock.
Biological weathering - Living organisms grow into gaps of the rock and cause these to expand. Algae and lichens can also impact rocks by releasing chemicals and extracting minerals causing the rock to weaken.
Bronze is an alloy consisting of 88-90% copper and the remainder is tin, bronze is subject to natural patination and over time it will change colour. Once the patination has formed the metal needs maintained and waxed in order for the colour to not be absorbed by the surrounding materials.
