Morte Point, (Death Point), is a peninsula on the North West coast of Devon, England belonging to the National Trust. Many ships were shipwrecked on these trecherous rocks, often lured by wreckers wanting to plunder their cargo, before the building of a lighthouse in 1879 at Bull Point. In one year alone, 1852, five ships went down here. One shipwreck carrying a cargo of live pigs gave a small cove to the south of the point the name of Grunta Beach, (most of the pigs survived).
The rocks in this area are from the Devonian age, (419.2 - 358.9 million years ago), and are predominantly Morte Slate, which is greenish grey to purple in colour, with some thin sandstone bands and siltstone.
These rocks began as a mud in a Sedimentary Basin which was compacted and over time formed mudstone and shale, sedimentary rocks. These were then folded and faulted in a time of mountain range creating known as the Variscan Orogeny. These processes exposed the mudstone and shale to low level heat and pressure which resulted in the clay minerals (clay sized particles) within them rechrystalising to become thicker chrystals of white mica and chlorite. These new minerals are arranged along a new “Slatey Clevage” plane which is parallel and runs at a perpendicular angle to the original bedding planes. At this point it has become slate, a metamorphic rock. It is Slatey Cleavage that controls the splitting properties and thickness of slate tiles and flagstones and unlike shale, slate can only be split with a hammer and chisel.
Exposed to higher temperatures the rock transforms to become phyllite, at higher temperatures still schist and at even higher temperatures it becomes gneiss, (pronounced nice). During this process the numerous fossils usually present in shale are often damaged, flattened and distorted or destroyed.
Rocks can also contain inclusions - quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals which can often be seen, although with slate, as it is the finest grained foliated metamorphic rock, these minerals are not visible to the naked eye. Inclusions can cause several problems - staining around the inclusion or a mineral coloured ‘run;’ as the inclusion decomposes it can leave anything from a pit in the surface to a hole, or it can cause the rock to split or disintegrate completely.
Rocks can also be affected by weathering, which is when they get broken down due to contact with the Earth's atmosphere, waters and biological organisms. Weathering happens where the rock is sited rather than during any movement it has undergone. Two important classifications of weathering processes exist – physical and chemical; physical, (sometimes known as mechanical), weathering involves the breakdown of rocks through direct contact with atmospheric conditions such as heat, water, ice and pressure. Chemical weathering happens when chemicals present in the atmosphere act directly on the rocks to break them down. These two processes happen together, e.g. a crack in a rock subjected to physical weathering increases the surface area that is then exposed to chemical weathering. Individually these two processes are sped up in very different environments; physical weathering when it is very cold or very dry and chemical weathering when the climate is wet and hot. The rate at which rocks weather depends on their composition.
Rocks are also subject to erosion. This occurs when surface processes like water flow or wind move the broken down rock products from one location to another. Again this is divided into physical and chemical processes, physical being when the rock is broken down into sediment that is then moved and chemical being when the rock particles are disolved or suspended in a solution, (usually water), which then flows away. This transportation can be anything from just a fraction of an inch to thousands of miles. According to Wikipedia, whilst erosion is a natural process, human activities have increased the rate at which erosion is occurring globally by 10-40 times.
Logging requirements:-
Message me via the message centre the answers to the following questions BEFORE logging as found. Please go ahead and log as found after sending the answers and I will contact you to confirm as soon as I can, which may be several days if I am away on holiday. If a "found it" is logged without the answers to the questions being sent to me your log will be deleted. If someone else is sending the answers on your behalf please state this in your log.
GZ is beside the path on the landward side.
1. Observe the rocks at GZ. Describe them - what colour/texture are they and are they composed of crystals or grains?
2. What rock do you think this is?
3. What is the elevation above sea level at the co-ordinates?
4. In the rock at the co-ordinates is a hole like shape where the rock is missing. Describe the colour and internal shape of this "hole" and state its width and depth.
5. What do you think happened to result in this hole like shape forming in the rock?
6. Why do you think that the back of it is the shape that it is?
7. A photo with your log (NOT OF THE SHAPE) would be appreciated, but is not a requirement.
With thanks to the National Trust for permission to site this earthcache.