Cornish Granite - Jointing at Carn Grey Rock EarthCache
Cornish Granite - Jointing at Carn Grey Rock
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Carn Grey Rock is a very small tor that sits above the now disused Carn Grey Quarry. It is freely accessible to the public over good paths, has parking close by and offers fine views over St Austell Bay (on a nice day!)
In the past, Carn Grey Rock was used as a natural pulpit for public gatherings ‘where a goodly company would assemble’. However, following the destruction of the Tolmen at Constantine on Tuesday 9th March 1869, there were concerns that Carn Grey Rock would suffer a similar fate to the hands of ruthless quarrymen. Thankfully, this never happened and we still have the rock to enjoy today.
The very reason why the rock and quarry are here at all is due to geological processes that have caused jointing. Joints form in rocks that are stretched beyond their elastic limit. Unlike faults, joints are fractures in rock that break the rock into smaller pieces but don’t really move the pieces very far from their original position. The joints are often found as regularly-spaced sets arranged roughly parallel to each other. The spacing and orientation of these sets is controlled by both the physical properties of the rock that they occur in and the conditions of stress and strain that create them. If conditions change over time, generations of joint sets form and intersect at various angles, dissecting the rock into smaller pieces.
The jointing around Carn Grey has reduced the mass of granite to smaller, square-edged blocks suitable for a variety of building applications and for making kerbstones. It also produced the larger tor (Carn Grey Rock) that sits above the now disused quarry. When the quarry was worked, the quarrymen exploited the natural joints in order to work the granite more easily, so the quarry faces now mark the alignment of the joint sets.
Although quarrying has since ceased, the intermediate granite exposed around Carn Grey is still important to geologists studying the St Austell granite mass as a whole, and the quarry is a SSSI for this reason.
If you’d like to find out more about the evolution of the St Austell granite, you may wish to visit my other EarthCaches Cornish China Clay – Wheal Martyn and Tresayes Quarry - Roche Glass Mine
In order to claim this EarthCache, please email me (through my profile at the top of this page or via this link with the answers to the following questions:
1) Describe the colour and texture of the granite that forms the tor and try to identify any of the minerals that it contains.
2) Using yourself or a geocaching companion for scale, estimate the height of the tor.
3) Measure the angle (in degrees) that the two obvious vertical joint sets visible at the tor are aligned. Hint - Although three joint sets actually intersect here, one is horizontal, and is aligned with the top and bottom surfaces of the tor. Therefore, you only need to find the intersection of the two vertical faces of the tor and estimate the compass bearings of these two planes. If you are unsure which of the boundaries of the tor were formed by jointing, look at the quarry faces as they are aligned at the same angle!
4) Suggest what may have caused the joints to form and their alignment to change over time. Hint – The granite was once underground and very hot, but now isn’t!
Optional task: Post a photo of yourself or your GPSr with the tor, quarry or St Austell Bay visible in the background.
Feel free to log your find without having to wait for an email from me. However, if your answers are not correct, incomplete, without picture etc., I will contact you by e-mail to verify your find.
Thanks for taking the time to visit this Earthcache and I hope you enjoy your visit.
Additional Hints
(Decrypt)
Tb gb gur yvfgrq pbbeqvangrf naq hfr lbhe rlrf!