//Ordovician Spotted Dolerite

Ordovician Spotted Dolerite

Ancient rocks

These ‘Bluestone’ rocks are found amongst other exotic stones at Stonehenge. They come from the Preseli Hills, 140 miles away from Sailsbury Plain. So did our ancestors move them 4,500 years ago, or were they carried there by ice sheets 450,000 years ago? The answer is disputed.

Spot the rock

Spotted Dolerites are igneous rocks that formed inside the Earth’s crust. Magma was pushed between layers of mudstones and slates where it cooled slowly and crystalized. The spots are an intergrowth of quartz and white felspar and probably grew as  the the rocks were ‘cooked’, that is metamorphosed, during a period of mountain building.

Healing lichen

22 lichen species have colonised these rough-surfaced rocks. One of these, known as crottle Parmelia saxatilis, is used as a reddish-brown dye for Harris Tweed. But in Medieval times, it was also used to treat epilepsy, but only when it was found growing on bones. It worked best when taken from the skull of a hanged man – apparently.

Where the Ordovician Spotted Dolerite NBG display rocks came from in WalesWhere do the rocks come from?
Eastern end of the Preseli Hills, Pembrokeshire

475 million years old

The Spotted Dolerite display at the NBG

Sunrise over Stonehenge
Sunrise over Stonehenge

Parmelia saxatilis: crottle
Parmelia saxatilis: crottle