Jersey's coastline is blessed with many caves, particularly in the north of the island and nowhere more so than here at Plemont. Once on the beach you will discover that there are several different caves that one can explore however the subject of this earthcache (and therefore the one that you'll be most interested in) is the cave that is situated directly underneath the cafe.
The educational bit:
The coastline can be weathered by mechanical, chemical, or biological means. Rocks can be broken down by physical forces such as heat, water and ice. Rainwater is usually slightly acidic and can dissolve certain types of rock. And plant roots will sometimes help to dislodge rocks as they work their way further and further into the ground.
Listed below are a number of different ways in which coastal erosion can occur. It is important to note that not all of these things have happened here! The climate and the type of rock for instance will play a significant part. But perhaps through your own careful observations and by using your powers of deduction you can work out what’s going on in this corner of Jersey.
Hydraulic power - as waves smash into the cliffs trapped air is blasted into holes and cracks in the rock causing it to break apart. The explosive force of trapped air in cracks is known as cavitation.
Corrasion - fragments of rock are picked up by the waves and slammed into the cliffs. Each time this happens the rock face is gouged and scraped some more.
Abrasion - the effect of pebbles constantly grinding over a rocky platform causing it to flatten and become smooth.
Solution - some rock types are particularly susceptible to being dissolved by sea water.
Attrition - rock fragments in the sea knock against one another causing them to become smaller and more rounded.
Freeze-thaw weathering - Sometimes referred to as frost shattering is a process that will take place wherever there is plenty of water and where the temperature fluctuates repeatedly above and below freezing. It begins when water collects in cracks and holes in the rock during the day (when the temperature is above zero degrees celsius) and then freezes overnight (when the temperature drops below zero degrees celsius). When water freezes and ice is formed it expands and this in turn causes fractures in the rock to enlarge. After repeated freezing and thawing the rock eventually breaks off and drops to the foot of the cliff.
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In Jersey, coastal erosion has resulted in a number of distinctive landforms. The bays of St Ouen to the west and St Aubin in the the south have been formed as a result of the softer shale being worn down by the sea at a more rapid rate than the harder granite situated at both ends of these bays. And in the north of the island there are many gullies, inlets, and caves that punctuate the coastline. Caves are usually formed when lines of weakness in the cliff such as joints or faults are perpetually battered by the waves causing them to break apart and grow wider and/or deeper. After a very long period of time caves that are situated at a headland may eventually form an arch which in turn could become a stack (an isolated outcrop of rock) when the roof of the arch becomes too heavy to be supported and collapses into the sea.
In order to log a find you MUST answer the following questions/satisfy the following criteria:
1. A nice easy starter - What is the name of the predominant rock found here at Plemont in the north-west region of Jersey?
2. Study the rock immediately above the cave entrance and that which is found at the back of the cave. Do you notice anything? If so, how might this explain how the cave was formed?
3. When you first approached the cave entrance you will have spotted two arches on your right hand side. Measure (or estimate) the height and width of either of the two arches. Please specify which of the two arches you are providing measurements for in your answer (the big one or the small one).
4. Stand in the cave entrance and look out to sea and one cannot fail to notice 'Pinnacle Rock' (see gallery). Estimate the height of this coastline feature and explain how it has been formed.
5. OPTIONAL - Take a photograph of yourself in either: a) the cave entrance, b) either of the two archways, c) in front of 'Pinnacle Rock';.
Please email your [OPTIONAL] photograph(s) and your answers to me before logging a find. If any of your answers are incorrect I will let you know in my reply. Logs that are submitted without the criteria being met may be deleted.