This is an EarthCache, NO box exists, NO log to sign but to claim it as a find you MUST answer some questions and email us the answers, if you do not follow these instructions WE WILL DELETE YOUR LOG!
Dartmoor Granite
To the casual observer, rocks may look grey and uniform, but to the geologist there are many types of rock on Dartmoor, with their own characteristics, texture and mineralogy. Granite is a light-coloured rock with crystals large enough to be visible with the unaided eye. It is formed from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. The type of granite a particular specimen is, depends upon the percentages of minerals that make up the rock, especially quartz (Greyish colour), Alkali(Potassium)-feldspar (White/Reddish), Plagioclase(Sodium)-feldspar (White) and Biotite (Black).
Here are some of the different types to be found on Dartmoor
 |
Tor or Giant Granite
Texture coarse grained, overall colour leucocratic, speckled buff, cream, black. Feldspar phenocrysts present. Essential minerals quartz 30%, feldspars 60%, mica (biotite) 9% Occurrence, widespread upper layers of Dartmoor.
|
 |
Quarry Granite
Texture coarse grained, overall colour leucocratic, speckled buff, cream, black. Phenocrysts very few and small. Essential minerals, quartz 30%, feldspars 60%, mica (biotite) 9% Occurrence in association with Tor granite.
|
 |
Aplite
Texture fine grained form of Granite, colour leucocratic white, pink, buff. Phenocrysts None. Essential Minerals, quartz 30%-40%, feldspar 50%-60%.
|
 |
Quartz Tourmaline
Texture Back(Tourmaline) and White(Quartz) veins.
Usually found as veins within granite.
|
 |
Pink Granite
Texture coarse grained, overall colour leucocratic, speckled pink, grey, cream and black. Phenocrysts occasional. Essential minerals, quartz 30%, feldspars 60% 50:50 (Pink/white), mica (biotite/muscovite) 10%
|
 |
Blue Granite
Similar to Tor Granite, but high level of large Pheoncrysts.
Texture coarse grained, grey, white. Large Phenocrysts present. Essential Minerals, quartz 30%, feldspar 60%, mica (biotite) 9%.
|
Phenocrysts are the large crystals distinct from the groundmass
Leucocratic composed mainly of light-colored minerals
Dartmoor Touchstone
This menhir is probably the first one to be erected on Dartmoor since the Bronze Age. To mark the millennium, John Powls decided it would be a fitting monument to celebrate the new century with and put his idea to the National Park. At the time John Powls was the ‘poet in residence’ with the national park and so he wrote a poem to be inscribed on the stone. The stone was erected on Rundlestone Crest during the summer solstice in 1999. There was then a dedication ceremony held during the winter solstice in December 1999.
The stone for the menhir came from the old prison quarry where Powls was the one-time governor and the poem was carved on it by Kevin Andrews of Polzeath in Cornwall. The touchstone stands at a proud 2 metres from the ground.
Old Prison Quarry
The first prison was constructed from stones obtained by breaking up the boulders lying around the site supplemented by dressed stone from nearby Herne Hole quarry (owned by Mr. Tyrwhitt).
Herne Hole quarry, now disused is on the B3357, just past the Prison Museum on the left hand side, about 300 metre south of the Touchstone.
The Cache
In Order to claim this find we require you to answer a few simple questions. Please send your answers to our email address via our profile or through the message centre on geocaching.com. :
The above co-ordinates take you to the Touchstone about 150 metres off the B3357.
Touchstone
- 1. Describe the colour and texture of the granite used and try to identify any of the minerals that it contains.
- 2. Using the table above suggest what type of granite it is.
- 3. On the back is a carving (about head height), describe it or include a photo with your answers.
Base Stone (on the floor behind the Touchstone)
- 4. Describe the colour and texture of the granite used and try to identify any of the minerals that it contains.
- 5. Using the table above suggest what type of granite it is.
Whilst it is NOT necessary to upload a photograph of your visit in order to log this Earthcache, we would love to see photos of the Blackbrook valley to enjoy the changing seasons.
