War Memorial #1018 ~ Chelmsford Civic EarthCache

This is an accessible EarthCache that will take you to the War Memorial in Chelmsford.

Here you will undertake a geology lesson as you visit this memorial.

As this is an EarthCache you need to send me your answers via email or the message centre.

Limestone is a sedimentary rock. A common building material especially in ancient times due to the decorative look of the rock and its propensity to being easily carved. This is often a light coloured rock, but other limestones may be variously coloured including darker colours. which suffers from chemical weathering, as a result of acid rain. It may contain bedding planes and many variety of fossils including corals, shells and marine life.

Portland Stone has been used in buildings and monuments throughout the world, examples of this can be found in the UK, where the stone has been used to construct famous buildings like St Paul's Cathedral and Buckingham Palace. The stone has also been used in war memorials around the country and is the stone used in the Chelmsford War Memorial. Portland Stone is a limestone from the Jurassic period and is quarried on the Isle of Portland, Dorset.

Portland Stone is formed in a marine environment, on the floor of sub-tropical seas. When seawater is warmed by the sun, the seawater is unable to hold its dissolved gas. This increase in temperature causes a reaction in the water. As the water is warmed it releases any dissolved carbon dioxide as a gas which in turn allows the calcium and bicarbonate ions, which are in the water, to combine and create calcium carbonate (CaCO₃). Calcium carbonate is the main compound in most limestones and indeed Portland Stone.

After the compound is formed, calcium carbonate crystals are deposited on the seabed forming a mud called micrite. Materials on the seabed such as sand, shell fragments and other organic matter collect layers of calcium carbonate as they move around on the micrite. The calcium carbonate gradually builds up and creates tiny ‘balls’. Over time these balls grow bigger in size and start to merge together and eventually start to form limestone. Portland Stone's formation and make up lends itself to construction. The stone is very resistant to weathering and such will stand up to the elements incredibly well, but crucially the stone is still able to be worked relatively easily by stone masons.

Weathering

The weathering of rocks is quite simply the breaking down of the stone at the Earth's surface, by action of rainwater, extremes of temperature, or biological activity. It doesn't involve the removal of rock material. There are three main types of weathering:

Physical weathering is caused by physical processes such as changes in temperature, freezing and thawing, and the effects of wind, rain and waves. An example of physical weathering is freeze-thaw weathering, where water expands slightly when it freezes to form ice. The formation of ice can break rocks, as if water gets into a crack into the rock, it freezes into ice, expanding the crack and making it bigger. This process then repeats itself with the crack in the rock getting bigger every time.

Chemical weathering occurs when the minerals within the rocks are chemically altered. In the process of carbonation, rainwater and carbon dioxide in the atmosphere combine to form carbonic acid. This means all rainwater is slightly acidic. This carbonic acid dissolves minerals within the rock, weakening the structure and resulting in damage and wear. On statues or monuments this can also result in the appearance of staining on the surface.

Biological weathering is where rock is weakened and disintegrates from the impact of plants, animals and microbes. These can release acid forming chemicals that contribute to the rock breaking down (weathering). Examples of this would include moss or lichen growing through the rock. Growing plant roots can exert stress or pressure on rock, potentially causing it to crack.

Questions

1. Observe the carved faces of the memorial - there is evidence of rock being worn away here. What type of weathering caused this and what can you see that tells you this?

2. Please examine the side of the plinth with text 'Our Glorious Dead'. Please describe for me the composition (e.g. Colour and Texture) of the rock at this specific part.

3. Compare the texture of the memorial base and the main plinth (where the text is). Explain if there there is any difference; do you think the lichen has had a detrimental effect on the stone - what weathering has acted here? 

4. Please add a photo of you or your GPS at the memorial though not giving any answers away :)