How Geocaching Works
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Prior to the arrival of the granite the area of Dartmoor had consisted of rocks and sediments formed during the Devonian period (400 - 360 million years ago) and the Carboniferous period (360 - 280 million years ago). During the Devonian period the area had been part of a large flood plane in which sediments were laid down which we now see as slates and shales. During the Carboniferous period the area was under a warm sea and the sediments became what are now limestones and sandstones.
The granite of Dartmoor was forced up from below about 290 million years ago towards the end of the Carboniferous period. The granite welled up as molten rock in the southern half of Dartmoor and flowed northwards. It did not break the surface and solidified while still underground. In time the material above the granite was eroded away leaving it exposed exposing the tors. Similar up-wellings occurred along a line extending from Dartmoor to the Scilly Isles, including what are now Bodmin Moor and St Austell Moor.
Granite consists mainly of quartz, mica and feldspar. It also contains some of the native rocks both in solution and as particles and larger lumps. The quality and colour of granite vary with rate of cooling and the amount and type of the absorbed materials.
The feldspar in the granite has decayed in some places. Geologists are still debating how the decomposition of the feldspar took place. The current theory is that hot chemical laden gases rising from below ground soon after the formation of the granite set the decomposition in motion and warm water seeping between the granite blocks did the rest. The process is continuing. The feldspar softens (kaolinises) to form a china clay, kaolin, a hydrated aluminium silicate. This clay is a very fine soft white material. The decomposition does not affect the quartz which remains as an angular sand like material and the mica a pale brownish flaky material. All three remain mingled together in the ground.
In the clay extracting areas around Lee Mill and Wotter the decayed granite is dislodged using water cannons. The quartz and mica are separated from the slurry by gravity and the kaolin is recovered by drying. Using modern methods, the kaolin extracted represents about 12% of the original granite. Although some of the sand is used by the building industry most of it is waste. In the past this waste was discarded in mountainous dumps.
Kaolin was discovered at Red Lake by Richard Hansford Worth, the historian, antiquary and civil engineer, working for Charles Cottier in 1905. Worth also surveyed and supervised the construction of the railway and pipeline to the Red Lake works. Production of clay from Red Lake did not start until the end of 1913 because of the time it took to construct the railway and pipeline. The purpose of the pipeline was to convey the kaolin as a slurry to the drying works at Cantrell near Bittaford. Production ceased in 1932 during the depression. All the equipment, including the railway was sold for scrap and work was never resumed. The buildings were blown up by the army shortly after the Second World War.
The whole of the Red Lake Clay Works are on open access land within the Forest of Dartmoor and we are able to visit them without permission.
From the top of the tip at N50° 29.217 W003° 54.557 you will have a truly splendid view of the surrounding Dartmoor landscape. You will also have a good view of the water filled clay pit. It was over 120 feet deep when work stopped. If possible take a photograph of one of your party at or near the top (not obligatory). Estimate the height and diameter of the tip.
When recording this cache as a find, please:-
1. If you have taken one, post a photograph of yourself or one of your party at or near the top of the tip. (One with the tip in the background would be welcome if the climb to the top is too much)
2. Message us by clicking on "Message this owner" at the top of the page including
a Your estimates of the height and diameter of the tip in metres.
b Your answer to the following questions.
What would expect the main constituent of the tip to be?
How many cubic metres of clay were extracted from the pit? [Assume that the tip is conical, that only 10% of the material extracted was separated as kaolin and that 90% was dumped on the tip.]
(No hints available.)