Tick Tock – Weather that Rock! EarthCache

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spooky_luke: Thanks to all who visited.

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Hidden : 8/4/2017
Difficulty:
1.5 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:


This EarthCache is located on publically accessible land on Exmouth seafront and is available 24/7. Parking is available along the Esplanade near GZ although there is a fee payable.
The clock tower that you are looking for


History
The prominent clock tower on the Esplanade was built to commemorate Queen Victoria’s Diamond Jubilee in 1897. The land on which the tower is built and the clock were donated by The Honourable Mark Rolle who was the High Sheriff of Devon in 1864, Deputy Lieutenant of Devon and High Steward of Barnstaple. Due to a sizeable inheritance from his uncle by marriage (John Rolle, 1st Baron Rolle), Mark became the largest private landowner in Devon, owning approximately 55,000 acres. However, he was a prolific philanthropist and builder and restorer of churches, farmhouses and cottages, the latter for his estate workers.

The clock tower recently benefitted from a two-and-a-half-year long refurbishment project and was finally unveiled in March 2017.

Geology
A number of rock types have been used in the construction of the tower and now 120 years later, despite the refurbishment, some are looking decidedly weathered.

Blocks of two different types of local Devonian-age limestone have been used to build the foundation plinth and the bulk of the tower. Formed during Middle to Late Devonian times as thick reefs of limestone around Plymouth, Torquay and Brixham, these rocks were formed in relatively shallow marine conditions created as a result of local fault movements. These faults gave rise to shallower water areas called ‘highs’ on which the reefs grew. Following their formation, major Earth movements during late Carboniferous times (during an event called the Variscan Orogeny), led to some of the limestones undergoing partial metamorphism, which changed their appearance. Some limestones were so altered that they could be polished. This resulted in them being termed ‘marble’, even though they weren’t true marble. Some of the blocks used for the base of the clock tower contain fossil shells as shown below:


Fossil in the limestone foundation blocks


Jurassic-age oolitic limestone has been used where the blocks needed to be shaped/carved, such as around the inscribed panel at the front and the doorway at the back. Oolitic limestone is made up mostly of ooliths (or ooids) which are sand-sized carbonate particles that have concentric rings of calcium carbonate. These rings are formed around grains of sand or shell fragments that were rolled around on a shallow sea floor, gathering layer after layer of limestone. The rock has an even structure a little bit like cod roe and it can be cut or sculpted in any direction.


Oolitic limestone with a fossil shell


2 varieties of an igneous rock (red granite) have been used for the inscribed panel and curved pillars at the front of the tower.

Weathering
Weathering is the breakdown of rocks at the Earth’s surface, by the action of rainwater, extremes of temperature, and biological activity.

The three major types of weathering are physical, chemical and biological:

Physical weathering is caused by physical changes such as changes in temperature and the effects of wind, rain and waves. When a rock is warmed, it expands a little but when cooled it contracts a little. If a rock is heated and cooled many times, cracks form and pieces of rock fall away. Although this type of physical weathering happens mainly in deserts, (due to the hot days and cold nights), it can happen anywhere that experiences significant fluctuations in temperature. Wind and waves can propel tiny grains of sediment against a rock. These wear the rock away and weather it like a natural shot-blasting process. Even pure water can cause damage if it freezes. As water expands slightly when it freezes into ice, this formation of ice can break the rock. If water gets into a crack in a rock and then freezes, the expansion pushes the crack further apart. When the ice melts, water flows down further into the crack. When the rock freezes again, it expands and makes the crack even larger.

When rocks are weathered by chemicals, this is called chemical weathering. Rainwater is naturally slightly acidic because carbon dioxide from the air dissolves in it to form carbonic acid. However, other acidic gases too can dissolve in it such as sulphur dioxide and nitrogen dioxide to further acidify the rainwater. Under these conditions, even unreactive metals such as copper and some minerals in rocks may react with the rainwater, causing the rock to be chemically weathered. Some types of rock are more easily weathered by chemicals than others. Limestones and chalk comprise a mineral called calcite (calcium carbonate). When acidic rainwater falls on limestone or chalk, a chemical reaction happens with the calcite and new soluble substances are created in the reaction. These are then washed away and the rock is weathered. Not all rock types are easily weathered by chemicals - Impervious igneous rocks such as granite and gabbro are weathered only very slowly as most of the minerals that they are formed from are not very reactive with rainwater.

Finally, animals and plants can wear away rocks. This is called biological weathering. Burrowing animals can excavate a crack in a rock, making it bigger and perhaps splitting the rock. Plant roots growing into cracks in rocks can also widen the cracks and eventually break the rock. Humans can also cause biological weathering by walking over a rock surface.

In order to claim this EarthCache, go to the published coordinates - You should be standing in front of the clock tower.

Please message or email me your answers to the following questions through my profile at the top of this page or via this link:-

1) Observe the various rocks used to build the clock tower. Using the information in the description, identify which rock type has been most weathered. (HINT – Look at all the faces of the clock tower to compare the relative rate of weathering).

2) Describe the evidence that you can see that shows the rocks have been weathered over time. (HINT – Look at the third vertical level from the ground and/or the two crosses and flowers at the front AND rear of the clock tower).

3) Explain and justify which types of weathering you think have been responsible for changing the various rocks used to construct the clock tower since it was first built. (HINT – Refer to the types of weathering in the Long Description and relate your answers to the various faces/rocks of the clock tower).

4) Suggest what colour staining is present on some of the rocks of the clock tower along with the element responsible for causing the staining and the most likely source of that element. (HINT – Look up at the roof – What is it made of?)

Feel free to log your find without having to wait for confirmation from me. However, if your answers are not correct or incomplete etc., I will contact you to verify your find.

EarthCache guidelines now make posting a photograph optional, but do upload any non-spoiler pics (i.e., not showing any close-up images of the weathering features) with your log if you wish.

Thanks for taking the time to visit this EarthCache and I hope you enjoy your visit.

Additional Hints (Decrypt)

Ybbx ng gur ebpxf gung znxr hc gur pybpx gbjre!

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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