Our aim with this particular Earth Cache is to take you to an interesting spot and teach you some basics, we have presented this earth science lesson on a basic level to accommodate those who avoid EC’s ‘cos they are too hard’ and also to accommodate those travelling with Children (TAWKers) who incorporate Earth caches into their curriculum.
Firstly, some quick interesting & relevant history:
The Red Bridge crosses the Elizabeth River, it was constructed using penal labour, it is the oldest surviving brick arch bridge in Australia, as well as the oldest bridge anywhere on the National Highway. The foundation stone for this three-arch bridge was laid by Lieutenant Governor Arthur on 21 October 1836. It took convicts fifteen months between 1836-38 to build it. The bricks were all made from clay taken from the south-eastern side of the bridge. It has been estimated that 1.5 million bricks were used in its construction. It was created at a time when only a few horse drawn vehicles crossed it. Today over 2 million vehicles cross it each year.
There were 220 men employed at the peak of construction. In a departure from previous practice, brick was chosen as the main construction material to utilise the locally available clay and avoid transporting large quantities of stone. The brickworks were located adjacent to the bridge, was a notable achievement inclusive of five teams of brickmakers and a stone cutter. The use of brickwork also provides a note of modernity in a colony more used to stone. The use of brick can also be linked to several of Campbell Town's most significant buildings, especially The Grange, the former Wesleyan Chapel and St Luke’s Sunday School.
Test holes drilled in the road surface of the bridge in 1988 revealed the original infill and running surface to be river gravel with water worn cobbles and a commonly 15-20% sandy clay binder, over a convex brick surface. Generally speaking, to make a brick like the bricks that were made here, natural clay minerals, including kaolin and shale, make up the main body of brick. Small amounts of manganese, barium, and other additives are blended with the clay to produce different shades, and barium carbonate is used to improve brick's chemical resistance to the elements.
So, what is clay?
Clay is a mineral 'stew' that is the result of the erosion of the earth's crust over vast spans of time. What was originally the mineral feldspar in igneous rocks, primarily granite, breaks down over time and becomes the microscopically fine-particle clay.
Clay minerals most commonly form by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity. Chemical weathering takes place largely by acid hydrolysis due to low concentrations of carbonic acid, dissolved in rainwater or released by plant roots. The acid breaks bonds between aluminium and oxygen, releasing other metal ions and silica.
The clay minerals formed depend on the composition of the source rock and the climate. Acid weathering of feldspar-rich rock, such as granite, in warm climates tends to produce kaolin. Weathering of the same kind of rock under alkaline conditions produces illite. Smectite forms by weathering of igneous rock under alkaline conditions, while gibbsite forms by intense weathering of other clay minerals. Iron oxide often stains clay red, an often desirable colour when clay is used for aesthetic purposes.
There are two types of clay deposits: primary and secondary. Primary clays form as residual deposits in soil and remain at the site of formation. Secondary clays are clays that have been transported from their original location by water erosion and deposited in a new sedimentary deposit. Secondary clay deposits are typically associated with very low energy depositional environments such as large lakes and marine basins.
To log this Earth Cache we require you to visit GZ and the listed waypoint to make some observations, consider the information given, then message us with the following answers to the best of your ability;
1. Look around GZ, can you see evidence that Clay has been used in the construction of the bridge? Describe the colour and texture, do you think the colour and texture and density have changed over time?
2. Head down to the riverbank closest to GZ, this point may fluctuate with the height of the river and vegetation growth, just the general area close to the riverbank will do, this is where some of the clay for the bricks made was excavated. Can or feel or see evidence of clay left over from the bricks that were made nearby? Explain your answer.
3. A photo of your team or GPS nearby without giving the answers away.
You are welcome to log your find straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil this Earth Cache requirement will have their logs deleted without notice.
Source: portal.engineersaustralia.org.au, howmade.com, wikipedia, maggies science connection.