This was the Carboniferous age when oxygen levels are thought to have been up to 35% as opposed to 21% now. Most trees probably grew in swampy areas and the wa

This was the Carboniferous age when oxygen levels are thought to have been up to 35% as opposed to 21% now. Most trees probably grew in swampy areas and the water they fell into would have been lower in oxygen. The material would have built up in the water and mud.