The geology of Cornwall is dominated by its granite backbone, part of the Cornubian batholith, which was formed during the Variscan orogeny. The Cornubian batholith takes its name from Cornubia, the Medieval Latin name for Cornwall. The main exposed masses of the batholith are seen at Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Lands End and the Isles of Scilly. This is part of the Lands End mass and has views over St Ives Bay to the north and St Michaels Mount to the south. Granite can be predominantly white, pink, or gray in color depending on the mineralogy. It is an igneous rock with at least 20% quartz and up to 65% alkali feldspar by volume. It is the potassium feldspar that gives many granites a distinctive pink color. The intrusion of this granite into the surrounding sedimentary rocks is associated with significant mineralization, particularly tin, for which the area has been famous since about 2000 BC. and which led to Cornwall being one of the most important mining areas in Europe until the early 20th century.
On Trencrom there are many rock basins. These are a product of localised weathering and are formed when weak feldspar crystals in the granite are split by the frost along their cleavages. This loosens small fragments which are later blown away by the wind leaving a small hollow. The process continues and gradually the hollow expands outwards and downwards. Water fills the hollow and then freezes, upon thawing more fragments are eroded which once again are blown away by the wind. Over time the hollow widens and deepens and eventually a rock basin is formed.
Logging requirements:-
Message me via the message centre the answers to the following questions BEFORE logging as found. Please go ahead after sending me the answers and log as found and I will contact you to confirm as soon as I can, which may be several days if I am away on holiday. Please be aware that if a "found it" is logged without the answers to the questions being sent to me your log will be deleted.
1. How many rock basins are on the rock at the co-ordinates?
2. What is the length, depth and the width at the narrowest point of the largest rock basin?
3. Describe the difference between the largest and the other basins?
4. What do you think is the reason for the bottom of the largest basin being the colour it is?
5. A photo of you/your GPS with Trencrom and views uploaded to your log would be appreciated, but is not compulsory.
Published with the Permission of the National Trust.