Slate Above the Copper Mines

Two rocks, two origins

The Coniston fells are made of the Borrowdale Volcanic Group, a pile of volcanic rock erupted in the Ordovician, around 450 million years ago, when this area was a volcanic arc on the edge of a closing ocean. The rock is mostly tuff and andesite lava: hard, rough, grey-green, and full of fragments.

The slate is younger in origin but formed from the same material. During the Devonian, about 400 million years ago, the whole sequence was buried, heated and squeezed by the continental collision that built most of the Caledonian mountains. That squeeze realigned the platy minerals in the fine-grained volcanic ash into parallel layers, so that the rock now splits along planes that have nothing to do with the original bedding. That property is what makes slate useful, and it is what you can see and feel on the spoil tips here.

What to look for

Walk up from the Coppermines Valley to the quarry levels on the south side of the Old Man. The spoil is everywhere and the distinction is easy once you know what you are looking at.

Slate's cleavage is not the same as bedding. The bedding is the original horizontal layering of ash on the sea floor. The cleavage is the plane the rock now splits along, and it is usually at a steep angle to the bedding. If you can find a block showing both, you have found the whole story of the mountain in one hand.

Logging tasks

Send answers to the four questions below through the message centre. Do not put them in your log. A photograph of you on the spoil with the valley behind is welcome but not required.

1. Find a block of slate on the spoil that shows visible bedding as well as cleavage. Describe the angle between the two planes, using your hand as a rough protractor.
2. Split a piece of slate along its cleavage and describe the surface. Then find a piece of tuff and break it. Describe how the two breaks differ.
3. Explain in two or three sentences why slate splits along a plane that has nothing to do with how the ash was originally laid down.
4. The spoil tips are a hundred years old and still bare. Give one reason why vegetation has been so slow to colonise them, and say what you can see growing on the older tips nearby.

Access and safety

• The quarry levels are on open access land but the tips are loose. Do not climb on the faces, and do not enter any adit or shaft. The mines here are deep and some are unmapped.
• The path from the valley is steep and eroded. It is not a constructed path and it becomes a stream in heavy rain.
• Do not collect samples from in situ rock faces. Loose material on the spoil is a different matter, and a single small piece for your own use is generally tolerated.
• Weather here changes fast. The summit is 803 metres and exposed in every direction.

After the find

The Coppermines Valley below has the remains of a copper and slate industry that ran for four hundred years: water wheels, leats, crushing floors and a remarkable amount of machinery left where it stopped. It is a better industrial archaeology site than most museums.