A Pebble map of Geology

 This is a map of the area around Teesdale in which each pebble is of the correct rock type for the location it marks.

The slates on the left hand edge are the oldest and date from a collision of continents. That will be Scotland and England crashing into each other.

I've known about that for years but this display helped me to realise how far south of the Equator we must have been when that happened because the limestone formed on top of it in coral seas. And the Whin Sill intrusion happened before the red desert sandstone, which I understand was laid down when we had made it as far north as the Sahara is today. So the map turned out to be a bit of a gem that made sense of a set of facts I'd known but never put together.