Rock Basins are a rather cool erosion phenonema that are always intriguing when found. A shallow depression in an explosed granite boulder, perhaps started by the presence of a weeker feldspar phenocryst, is gradually attacked further and deeper by a persitent shallow pool of water. Once the frost and the wind get involved by eddying the water around in the basin scoop then the cycle gets ever more pronounced.
Rock Basins sometimes end up drilling themselves right through the side wall, or even more impressively, through the bottom of a flat boulder, producing an impressively sculpted piece of stone that have been linked to druid ceremonies.
The Rock Basin here was found on the upper outcrop of Hucken Tor SX 551739, although the lower crags also have further examples.
As Dartmoors plutons pulsed and pushed up and through the country rocks of Devon 270 million years ago, some of these metamorphosed sedimentary layers above ,shattered and sank their altered and fractured boulders into the crystal mush of the rising and cooling magma. As they sink they are generally chemically altered, abraded and rounded off by the intense heat and continued pressure of this new super-hot environment. These resultant buried rocks and boulders are permenantly embedded into the granite framework until exposed later within the walls of a Dartmoor granite tor. These bizarre and erratic stones are more commonly known as Xenoliths.
By definition a Xenoliths, meaning ‘foreign stones’.
Dartmoor is one very large pluton of granite however it’s development 270 million years ago has led to some variety to its colours, forms and crystal makeup. Here we have some well developed Feldspar Phenocrysts in a granite mass. This strangley clean surface was found at Calveslake Tor. Each of the white Phenocrysts are approximately 10cm across.
By definition a Phenocryst is; an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock.
Of the long list of 176 Marilyn’s in England, there is only one of its type on Dartmoor. That lonely peak is High Willhays (SX 580892 621m a.s.l.). A Marilyn doesn’t have to be any particular height but it does need to have an elevation of 150m or more of relative height.
High Willhays does have a slightly lesser peak (Yes Tor 619m a.s.l.) and indeed many other linked Dartmoor peaks that do not allow for a fall of over 150m however it exists as a Marilyn due to the fact that it is the highest point on Dartmoor and Dartmoor’s elevation is over 150m above the surrounding countryside.
A Dewey is a hill or mountain in England or Wales over 500 metres (1,640 feet) in elevation, but below 2,000 feet (609.6 metres), with a relative height of at least 30 metres (98 feet) with relation to other hills in it’s locaity, namely you must descend at least 30 metres before you ascend another hill. The Dewey list was compiled by Michael Dewey and published in his book Mountain Tables. There are 13 Deweys on Dartmoor (Beardown Tors is actually 3 tors/ ‘rock stacks’ with the westernmost ‘stack’ being the highest at 513 m).