The oldest part of the country is the part that lies southeast of a line between about 60°N 5°W and about 62°N 2°W. Rock ages here have been measured to over 400 million years, most of it highly eroded and highly metamorphous basement rock from the Caledonian orogeny, while to the northwest of the line they range from 58 million in the extreme southeast to 52 million in the northwest, mostly basaltic lavas spewed out as the North Atlantic was spreading. This region is rich in minerals, and especially along the border line, very rich ores are found, among them the famous copper mines of antiquity.
Old fossils are found in the southeast, but not in great abundance, though some famous dinosaur finds are displayed in the Southend Museum of Natural History. During its early life, this region lay far inside a vast continent, and life was sparse until extensive forests grew up in the Carboniferous. In the early Permian there are traces of extensive glaciation. Later, forests reappeared, the Permian and Triassic saw wide coniferous forests, and the earlies remains of reptiles are found, as well as many insects. Sauropod fossils are found from the Jurassic and dinosaurs from the Cretaceous. The opening up of the Atlantic lead to a wetter and more coastal climate. The extensive volcanism in the Paleocene and early Eocene made the environment hostile to life, and as the country now was surrounded by the sea, immigration of new species was slow and no mammals are found until the Oligocene. Birds and plants spread more effectively, and from the Oligocene the environment was dominated by grasses and deciduous trees like today.