Delabole Slate Quarry
Geologists are usually very strict when it comes to rules on sedimentary rocks. Sediments are distinguished by particle size into gravel, sand, silt and clay. When a claystone has what is known as fissility or ability to be split in more or less thin layers it becomes shale. Silica cement can make shale harder but usually the rock is fairly soft and weathers easily back to clay. When shale is subject to heat and pressure it eventually turns into the lowest grade of metamorphic or shape-changed rock, slate. The clays have then started to revert to the mica minerals from which they were formed. When transformed from shale, slate grows harder and the cleavage direction gets more pronounced. An easy way to tell slate from shale is to tap it with a hammer. If it rings or “tinks” it is slate. If the temperature and pressure is even higher the slate first turns into phyllite, then schist and finally into gneiss.
Slate comes in different colours and can be grey, black, blue, red, green and even purple. Dark slates usually owe their colour to iron sulfide or carbonaceous material. Red and purple slate usually carries iron oxide (hematite) and green slate owe its hue to clorite, a green clay material. The main minerals in slate are muscovite and biotote, quartz and chlorite. Slate has a dull lustre and a strong cleavage.
Throughout the years slate has had many uses. Examples are building stones, roof tiles, flooring, road paving, pool tables, writing tablets and blackboards. When quarried, slates are split from blocks about 3 inches thick. A chisel in position at the edge of the block is lightly tapped with a mallet. A thin crack opens in the direction of the cleavage and a gentle leverage with the chisel splits the block into two smooth and even pieces. This procedure is repeated over and over again until the original block is split into up to 18 pieces. These are thereafter trimmed to size by hand or by machines. Slate is also crushed to make composition roofing and to be used as a filler.
Delabole slate quarry is said to be the deepest manmade hole in Britain, 425 feet deep and over a mile and a half in circumference. The workings are supposed to have started as early as the 13th century and is definitely known to have been in use since Elizabethan times. In 1841 five separate pits were amalgamated into one single quarry. In those days, hundreds of men were needed for tasks such as blasting, drilling, digging and splitting the slabs. Today only five men are needed to perform these tasks. Modern quarrying techniques and diamond wire saws have transformed the backbreaking ordeals of previous centuries. Up to 120 tonnes of slate blocks is extracted every day.
The coordinates lead to the view point, a ledge teetering on the brink of the gaping mouth of the quarry.
To log your visit you have to email me the answers to these questions:
1. In the car park there is a slate sign saying “COACH PARK and ………..”. Fill in the three words missing.
2. The working face of the quarry, that is the terraced side without rubbish heaps, is facing approximately which compass direction?
3. At the showroom entrance there is a semicircle of wooden pillars. How many are there in the INNER semicircle counting the central pillar as well?
4. A photo of yourself at the pit would be much appreciated but is not necessary to complete the cache.
Send your answers to the cache owner and wait for an OK before you log your visit, please.
The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdoor activity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth. To find out more click HERE.