Dartmoor has the largest area of Granite in Britain and is connected far underground to the granite masses in Cornwall. Granite is one of the hardest rocks, which formed in the Carboniferous Period of geological history. There are many geological features on Dartmoor, which also have superstions and folklore attached to them. The Tolmen Stone for example is supposed to prevent/cure rheumatic disorders if you pass through the hole.
So, how does an extemely hard rock like granite get a large hole in it that's not manmade?
There are four main processes of erosion that occur relating to rivers. These are:
1. Hydraulic action
2. Abrasion / corrasion
3. Attrition
4. Corrosion
1. Hydraulic action.
This occurs when the force of the water hits the river banks pushing water into cracks. Air then becomes compressed in the cracks, pressure increases and portions of the riverbank break off and the riverbank collapses.
2. Abrasion.
Here, the sediment carried by a river scours the bed and banks. The fast currents in the river, called kolks, cause pebbles to spin around creating a vortex and eroding the river bed/rock and overhanging ledges. A pot hole or rock basin is created which increases in size over time.
3. Attrition.
Eroded rocks collide and break into smaller fragments that become smoother and more rounded.
4. Corrosion / Solution.
Carbon dioxide dissolves in the river to form a weak acid. This is then able to dissolve carbonate rocks like limestone and chalk.
Logging requirements:-
Message me via the message centre the answers to the following questions BEFORE loffing 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. 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. State how you think the large hole in the Tolmen Stone was formed and give the reasons why.
2. State why you think other methods of erosion did not cause the hole.
3. What is the depth of the hole in the Tolemen stone?
4. How do you think the Tolemen Stone got to it's current position?
5. There are some smaller pot holes on a different stone nearby. What bearing in degrees are they from the Tolemen Stone?
6. A photo of you/your GPS on/by the Toleman Stone would be appreciated, but is not compulsory.