How a Tor is formed; Firstly you have a normal hilly landscape; 
Then erosion takes place exposing the harder granite beneath leaving it exposed to be weathered into tors. 
Dartmoor has a varied and vast amount of different geology as can be seen from the following chart. 
Granite The name granite comes from the Latin granum, a grain. Granites are composed of fairly large crystals and have an irregular granular or granitoid texture. Granite is a group name for a family of plutonic or deep-seated acidic igneous rocks. Dartmoor granite is made up mainly of quartz (glassy, grey/white crystals of silicon dioxide), felspar (particularly orthoclase - potassium aluminum silicate: but also plagioclase) and biotite, a type of mica - a complex hydrated silicate of aluminum and potassium with iron, magnesium and fluorine. Dartmoor granite is also characterized by a relatively high proportion of tourmaline, which is blacker than biotite and distinguishable by its finely grooved surface. Some geologists have identified three major types of Dartmoor granite - the contact granite contaminated by minerals from the surrounding rocks; the tor granite which contains the large megacrysts typical of many of the tors, and the finer grained blue granite.
An example of Megacrysts
To log this cache;
Please email me through my profile the answers to the following questions, if i do not receive the answers from you I WILL delete your log.....Simples :-)
1/ Please measure the length & width in Millimeters please, of three of the larger crystals on the NW face of Haytor where the arrow points @ N50 34.820 W003 45.328
A/ Optional photo of the above (not in the log please)
2/ Please explain why you think that these large crystals cooled quicker or slower than the smaller crystals?
3/ Optional photo of yourself with gps at Haytor (can be on the log)
4/ The Dartmoor Granite is connected at depth with the other granite masses of Cornwall, This forms an enormous granite intrusion known as what?
5/ Temperature approaching 1000° C, altered surrounding rocks for several hundred meters beyond the margin, forming a metamorphic rock known as what?
6/ What initially caused the cracks/joints in the tor?
Please email me through my profile the answers to the above questions and await my response BEFORE LOGGING, if i do not receive the answers from you I WILL delete your log.....Simples :-)
My thanks to Dartmoor National Park for permission in setting this cache. References used;
Devon county council web site
www.dartmoor-npa.gov.uk/