Graptolite fossil: Areas of large mica growth appear white
Scientists have been able to date more precisely the moment when Wales' slate hills were pushed up from the sea floor - thanks to some tiny fossils.
It is thought that the slates, which can also be found in the Lake District of England, were formed in the aftermath of a massive continental collision approximately 400 million years ago.
Scotland and Northern Ireland, then attached to North America, collided with another "microcontinent" containing England and Wales, and southern Ireland.
Mud on the bottom of a sea basin between the two continents was compacted by the force of the collision, turning into slate which was then thrust upwards.
One method of finding the date of this event is to extract naturally radioactive minerals in the slate - these have a fixed rate of radioactive decay which means their age can be worked out with some accuracy.
However, in the case of the Welsh slate, its mineral crystals are just too tiny for scientists to be able to separate out and measure their age.
Slate country: Mount Snowdon, North Wales
Now, though, researchers at Leicester University and the Open University have developed a way around this problem.
They found that when tiny free-floating sea creatures called graptolites were fossilised in the mineral iron pyrite (fool's gold), spaces were created next to the animal remains as they resisted the enormous pressures in the collision event.
These "pressure shadows" allowed the formation of large, pure mica crystals. It is these which have been extracted and dated by measuring the presence of radioactive argon.
The results point to the slates being formed 396.1 million years ago - with this figure accurate to within 1.4 million years either side.
This is by far the most precise date extracted from the slates of the Welsh hills, or indeed probably from slates anywhere in the world.
Dr Jan Zalasiewicz, from Leicester University, told BBC News Online:
"It's so important to find out what created these mountains - to determine the cause and effect.
"However, until now it has been very difficult to date these slates accurately."
Researchers investigating the origins of slates elsewhere in the world could be aided by the new technique - similar fossils have been found in other slate deposits.