Praa Sands Beach EarthCache

Praa Sands

Praa Sands (formerly Prah Sands) is a white-sand beach and coastal village in Cornwall, and a great destination if you have kids to entertain, they’ll love the beach which is great for sandcastles, the shoreline is great for splashing about in and you won’t have to walk miles from the car park.

Praa Sands is situated in a sheltered indentation within Mount's Bay; it has a long, wide, sandy, south-facing beach that is lapped by large waves for the area, which makes it popular with surfers and walkers.

Praa Sands lies within the Cornwall Area of Outstanding Natural Beauty (AONB) and it was approved as a Regionally Important Geological / Geomorphical Site (RIGS) in 1995.

The Earth Lesson - The Rock Cycle

The rock cycle helps us link together all the processes that make the variety of rocks found on our planet and what we find on Praa Sands Beach.

The rock cycle has 5 main parts: -

1. Magma
2. Sediments
3. Metamorphic Rocks
4. Igneous Rocks
5. Sedimentary Rocks

Planet Earth is made up of 4 x layers – the Inner Core forms the very centre of our planet, which is surrounded by the Outer Core, which in turn is surrounded by the Mantle, where the Magma is found and then there is the outer most layer of planet earth called the Crust, as can be seen on this diagram: -

The Mantle is made of a very hot, almost liquid plastic rock called Magma, which can leak out of the earths crust via Volcanoes, and when that happens, we call the Magma – Lava.

Lava is very hot, almost 1300^OC and when the Lava leaves the earth’s crust via an erupting Volcano, it crystalises which forms Igneous Rocks. Due to the speed of this crystallisation, these Igneous Rocks will have small crystals and form rocks such as Basalt. Sometimes the magma cools underground, very slowly – over millions of years and this forms Granite – an Igneous Rock made up of large crystals due to the slow rate of cooling and crystallisation.

Examples of Igneous Rocks

The earth’s surfaces are exposed to weathering and erosion as weathering is a process whereby rocks are broken down by the actions of ice, wind, and water – known as physical weathering, or by roots of plants – known as biological weathering, or by the acids found in rainwater or rivers – known as chemical weathering.

Erosion is the process of wearing away rocks, such as by moving water or wind or ice. These often carry rock fragments that help further scour away the rocks. Eventually, the water or wind will drop or deposit these pieces of rock, resulting in the fragments piling up, forming sediment.

This sediment action continues with more layers building up on older, lower layers, until the weight of the overlying squashes the lower layers, compacting them together. This compaction, together with minerals in the ground water, cement the grains together, forming sedimentary rocks.

Sandstone is an example of a sedimentary rock formed by layers composed of billions of sand grains, all being cemented together. Sedimentary rocks can themselves be subjected to weathering and erosion, so making more sediments.

Examples of Sedimentary Rocks

Igneous or sedimentary rocks can over a long period of time can be heated and squashed due to the immense pressure inside the earth’s crust, which can deform these rocks and that forms metamorphic rocks.

The term metamorphic is made up of 2 x terms: -

1. Morph – which means to “change”.
2. Meta – which means “heat and pressure”.

Slate is a metamorphic rock, formed by the metamorphism of mudstone, a sedimentary rock. Gneiss is a metamorphic rock formed by the metamorphism of Granite. Metamorphic rocks themselves can also be metamorphosed.

Examples of Metamorphic Rocks

Igneous; sedimentary and metamorphic rocks can all be subjected to weathering and erosion, so making more sediments and continuing the rock cycle, but also Igneous and Metamorphic rocks can over a very long-time be subjected to a lot of heat, causing them to melt and form Magma and thus completing the rock cycle, as can be seen from this diagram: -

The Creation of Praa Sands

Praa Sands story begins 370 million years ago in a long since vanished sea. The mud deposited on the seabed was subjected to great pressure and heat, forming the metamorphosed sedimentary rocks known as Mylor Slates.

The Mylor Slate Formation rocks are well developed in the Cornish area, and they are dark grey, crosscut by many white quartz veins. Features such as bedding (the arrangement of sedimentary rocks in beds or layers of varying thickness and character), cleavage (a type of rock feature that develops because of deformation and metamorphism), folding (a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation) and micro-faulting (a symptom caused by rocks under strain) can be found. The bedding gives the rocks a striped appearance.

About 290 million years ago these were in turn invaded by Cornwall’s most characteristic rock, granite.

Twenty million years later, molten magma from the Earth’s core again forced its way into vertical cracks in the existing rocks. Folly Rock, a fine-grained Elvin Dyke to the West of the beach, was formed at this time and is now listed as a Site of Special Scientific Interest.

Much more recently, only 50,000 years ago, Cornwall was in the grip of an Ice Age. Conditions then were like the modern Arctic Tundra.

Below the ground was permanently frozen, but during short lived summer thaws, the waterlogged surface known as “head” was deposited onto the existing rocks as Solifluction Terraces. Solifluction is a collective name for gradual processes in which a mass moves down a slope ("mass wasting") related to the freeze-thaw activity.

Your Tasks

This EarthCache is designed to take you along the length of Praa Sands to see the change in geology and rock types. Start at the headline coordinates and then make your way East, via the 2^nd way point – answering the questions as you go. The questions are meant to be straight forward observations from the beach aligned with what you have read on the cache page.

1) At the headline coordinates (stage 1), observe the large rocks found at this end of the beach and describe the surface and the texture of those rocks, what colour are they and observe the size of the the crystals within the rocks, are they fine or coarse. Using the pictures above, explain what type of rocks that would make them.

2) At stage 2, observe the cliff face and due to the erosion, you will see some distinctive larger white rocks contained within the layers of peat. Describe their appearance and explain what type of rock that would make them.

3) As you walk along the beach from stage 2 towards stage 3, you will see the site is also prone to severe coastal erosion (up to 10 metres in 100 years). There are two items that have found their way onto the beach as a consequence of this erosion. Name one of them?

4) At stage 3, observe the rocks at this location on the beach, compared to those at stage 1? Describe their texture and colour and using the pictures above, explain what type of rocks that would make them.

5) Optional Task: Please take a photo anywhere along the beach with either yourself or something with your geocaching name showing your visit to Praa Sands Beach.

All Done

Reminder on "EarthCaches": There is no container or logbook on the given coordinates. Just visit the site and answer the questions by e-mail or by the Message Center. There is no need to wait for my reply, feel free to log immediately after. If the answers are way off the mark, I will let you know. Happy caching!

Thanks to the Cormac Beach Ranger and Breage Parish Council for their permission to place this EarthCache.