Tintagel: Lanterdan Pinnacle EarthCache

This is the third of the Tintagel earthcaches, however, this one is away from the village and will require you to take a walk down the coastal path past Tintagel Youth Hostel to Lanterdan Slate Quarry, to view the pinnacle.

Firstly, please don't be put off by the diagrams and detail in the listing. It is there for those who wish to read and understand it in more detail, but the questions are really very straightforward and you are not required to understand more than the basic ideas to be able to answer them.

This earthcache brings you to a viewpoint over the remains of a once thriving slate industry. There is evidence that slate has been quarried here since 1650 and the last operation, the nearby Long Grass Quarry, did not close until 1937.  The slate has been worked in this area in two ways, the traditional digging down from above to form large quarries as was done in Lanterdan (here) and West quarries; and, north of Hole Beach, small quarries working the actual cliff side.  

The reason for bringing you here to Lanterdan quarry is to view the tall pinnacle of rock you will see in front of you. It looks a little like the slate quarrymen were trying to leave a monument behind, but this is not the case – that would not have been profitable. In actual fact, anything up to 95% of quarried slate is often considered inferior – in other words, not good enough to sell, and this is why you often see huge 'spoil' piles, which was basically the slate that the quarrymen considered spoiled. That is the case with this pinnacle – they considered the slate to be no good, so they simply quarried around it. Shorter pinnacles were left in West Quarry for the same reason.

Masonry strongpoints were built in the smaller cliff face quarries to support timber structures which lowered cradles and buckets down the cliff face. The slate itself was exported from Tintagel Haven on boats 'hobbled' in a similar fashion to that used at Boscastle.  A wharf was also constructed at Penhallic Point where the cliff edge was trimmed to form  a  100ft  vertical  face. Ships  would  lie against this face in the natural deep-water berth whilst the slate was lowered by crane down into their holds.

Grain/Cleavage – slate is cut from rock that was formed by the laying down of sediment in ancient seas many millions of years ago. These layers of sediment have since been compressed and heated by geothermic conditions until the layers of sediment lie closely packed against one another. Because the rock still retains these layers, it allows the quarry to split the rock into slates of a consistent thickness. A good slate will split cleaner, be of similar thickness to one another and have minimal coddling (twisting) of the slate being evident.

A second grain running down the face of a slate is also evident in most slate. The very best quality and strongest slates will have a vertical grain that runs straight from top to bottom of the face of the slate. A grain that twists, swirls, or falls horizontally part of the way down the slate indicates that the slate may not be as strong or durable.

There are various types of cleavage within quarried slate, but the type the miners wanted was ‘Slaty Cleavage’. The term used to describe this layering in slate is foliation - repetitive layering. It is caused by the parallel orientation of platy minerals in the rock such as microscopic grains of clay minerals and mica. These parallel mineral grain alignments give the rock an ability to break smoothly along the layers. The quarrymen exploited this property of slate to produce thin sheets of slate that are used in construction projects and manufacturing. 

Inclusions – All slate includes some amount of non-carbonate inclusions - in other words - pieces of other minerals that have become embedded in the slate as it was formed. Some react with water weakening the slate, whilst others are stable and have no damaging effect. The most potentially damaging inclusion in slate is a mineral called 'pyrite' - a shiny yellow crystal of iron disulphide (FeS2) formed from particles of organic material present in the sedimentary muds and crystallised during the rock’s metamorphic processing. This mineral's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold. Another mineral that is sometimes found in slate is Pyrrhotite, which is an iron sulfide mineral, and this can also be a problem.

Significant pyrite inclusions in slate. The inclusions may not always be this large or obvious.

So what is the big problem with having a bit of Fool's Gold in your slate? Well, the issue with the two minerals mentioned above is that they contain iron, and they will rust in water. This rust will run and permanently stain the slate brown. People pay high prices to have a beautiful blue slate roof - they don't want it to rust and turn brown. But the problems get worse. If the mineral inclusion goes right through of the piece of slate, imagine that when the piece of mineral eventually rusts away completely, it will leave a hole in the slate - perhaps a hole in your roof, with water now flooding in. Even if the mineral doesn't go right through, it could greatly weaken the slate when the piece has rusted away causing it to crack and collapse. Another consideration is that if the mineral sticks out as you can see on the photo above, it could also prevent the slate from lying flat.

So, you can understand why the presence of pyrite or pyrrhotite in a slate can be potentially disastrous. To put it more technically for those with an interest in science, when the mineral is exposed to water vapour in the air iron disulphide reacts, decomposing to form powdery iron sulphate and sulphuric acid. How quickly this happens, and its effect on the slate, varies:

• It may just cause a stain around the inclusion
• It may leave an unsightly, rust-coloured ‘run’ on both the slate and those in lower courses
• Its decomposition will leave anything from just a pit in the surface to a hole through the slate
• The slate may delaminate and split, or even totally disintegrate

This earthcache falls within the Tintagel Cliffs SSSI (a Site of Special Scientific Interest), designated for both its maritime heaths and geological features. Many thanks to Natural England for their support in the publication of this earthcache. Please be aware of the protected state of this coastline and don’t interfere with the cliffs or plantlife when you explore. The earthcache is located on the coastal path, you do not need to go down into the main quarry.

Please e-mail me the answer to the questions via my profile. I do read all answers and try to reply to them all; I may not reply immediately so please do not wait for a reply before posting your find. Ideally, please send your answers at the same time you submit your log, or within a few days of your visit. I do check answers have been sent for every log, if you do not send answers within a week your log may be deleted. You are not expected to have any previous geological knowledge, your best attempt at the answers is all that is required. You do not need to enter the quarry.

1. Please estimate the height of the pinnacle. This gives a good indication of how deep the quarrymen worked.

2. Please describe the colour of the slate on the column. Is it the same as the colour of the quarry cliffs? Is there any dis-colouring you can see that might suggest intrusions?

3. Now examine the smoothness of the pillar compared to the cliff edge? Is there a difference? Why do you think this might be?

4. Try and find a piece of slate, any here will have been left as they were spoiled. There will be one somewhere around the area (please do not attempt to go down into the quarry). Examining the rock on the column may also help, but I appreciate you can only do this from a distance. There are two reasons given above for the slate to be considered spoiled (please read for more details). Can you determine which of the two reasons caused the slate here to be left? Please explain you evidence, possible options are below.
i) Can you see any discolouring/indentations/holes suggesting there was a mineral intrusion present?
ii) Do you see any evidence of a problem with the grain meaning the slate couldn't be cut straight, or wouldn't be strong enough?

We would like to see a photograph of you and/or your GPS device with the piece of slate you examined - this provides additional proof of your visit, so please include one with your log if you wish.

Please enjoy your visit to Lanterdan Pinnacle.