//Cambrian Rocks

Cambrian Rocks

World famous Welsh slate

These blocks of Welsh slate were originally thick clouds of muddy water. The turbid clouds flowed down continental slopes and slowly accumulated layers on the ocean floor. You can see the traces of the layers in these rocks.

These muds hardened into mudstones and these and other rocks were squashed to form a chain of mountains 400 million years ago. As the mudstones were squashed the clay minerals within them were forced to re-grow at right-angles to the direction of the pressure, and that’s why slates can be split into very thin, strong slices.

Welsh slates have been used to roof buildings across the world but in our Great Glasshouse, slabs of slate form the floor. It is principally the presence of manganese and iron that gives the lovely purplish colour to these slates.

Have a sniff

Can you smell the slight musky scent of the oakmoss lichen Evernia prunastri? It’s widely used in the perfume industry. Lichens like this, as well as moss, help to break down the slate into acid soils. In Snowdonia, foxglove Digitalis purpurea commonly grows in the thin soils on old slate waste tips.

origin of the Cambrian rocks NBG display: Penryhn Quarry, GwyneddWhere do the rocks come from?
Penryhn Quarry, Gwynedd


510 million years old

Cambrian rocks at the National Botanic Garden of Wales

Slate roof with lichen
Lichen growing on a slate roof

Slate floor of the interior of the National Botanic Garden of Wales Glasshouse
Slate floor of the National Botanic Garden of Wales’ Great Glasshouse

Evernia prunastri
Evernia prunastri

Foxglove line drawing
Foxglove, Digitalis purpurea