We are at the Stori Brymbo heritage site, which was an iron mine and smelting plant until 1990. Outside it is cool, stormy and wet, but we’re sheltered under a galvanised roof with the wind rattling and the rain tapping. Beneath us, a sandy, crumbling layer of rock formed 300m years ago from a great thickness of mud and silt that settled in a vast river delta system that once covered this part of Wales.
If I squint my eyes, I imagine the vast, stifling, steaming jungle; no grass or flowers, just tall trunks towering above and ferns below, insects flying all around. This was the Carboniferous era in the tropics, long before any dinosaurs or other land animals, and the first “trees” – club mosses, giant horsetails – grew by 10 to 30 metres in a few months, before dying and falling into the swamps to eventually form our coal.
These plants grew so fast and abundantly that they changed the atmosphere, and hastened their own extinction, as we are now. Our guide, Dr Tom Hughes, who grew up locally, wrote his PhD thesis on this, and now he organises volunteers to gently dig and brush away the dust to reveal this ancient forest.
The slow process has its rewards. I find a long trunk of Calamites, as thick as a man’s arm, divided into sections. Some have curving leaves like giant centipedes. After a while, you get your eye in for the marks you’re searching for. I did this so thoroughly on a field trip once as a student that I started seeing them in the wallpaper of the fish-and-chip shop afterwards.
Among our other discoveries were a lovely black fern leaf and a bit of a beetle wing-case. Insects were often too delicate to fossilise, but there are dragonflies found at similar sites with 1m wingspans (I secretly hope I find one). Other patches like dark, shiny snakeskin turn out to be diamond-patterned Lepidodendron – another giant plant.
The dusty hours pass quickly, until our old hips and knees have locked up. We stumble out into the sun for a drink and look at other treasure seekers’ finds.
• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com
