We all know that Dartmoor is a large granite pluton forming an area of highland in the south west. This rather large igneous pluton exists as part of a much larger batholith that sits deep under the south west counties peninsular, linking areas such as Bodmin Moor , West Penwith and the Isles of Scilly.
Gabbro boulders at White Tor (not granite)
The dominance of this huge granite node Dartmoor can easily draw one’s attention from the other rocks that are associated with Dartmoors scenery. The emplacement of such a large body of granite could not have happened without a few adjustments. These changes to the rock types on Dartmoor can be subtle but they can be spotted with practice.
If you know your rocks then a visit to White Tor SX 542786 and Boulters Tor SX 525780 on the west of Dartmoor will reveal a darker stone than the usual pale grey rounded Dartmoor boulders. The boulders at White Tor are dark grey, mottled and blockier, with sharper fractures than the more rounded edges of weathered granite boulders. Take a closer look and then you’ll see that there is a distinct lack of quartz to the crystal make-up and the minerals are all-together darker (greeny to black). The British Geological Survey suggest a coarse grained Gabbro, as related to Basaltic lava but cooled slowly to make a coarse-grained texture. The mottled appearance that is given the face of some boulders inset. helps reveal one of the differences from that of your more usual granite boulders.
Keep you eyes peeled, not all Dartmoor granite…is granite!
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.