Magnesian Limestone, which outcrops just to the east of Doncaster, was laid down in a dying sea, the Zechstein, during the Permian period.
Limestones and ironstones of the early Jurassic from the imposing ridge at Grantham.
Chalk forms higher ground around Stevenage. It formed in the Cretaceous period when dinosaurs such as Tyrannosaurus rex roamed on land.
Softer RocksBetween these limestones are softer rocks including desert sandstones south of Doncaster (left), Oxford Clay south of Peterborough and the London Clay from the early Tertiary, laid down in sub-tropical seas and estuaries after the extinction of the dinosaurs.
The valleys of the Thames and the Trent have been carved out from these softer rocks. South of Peterborough fen country has developed on the level ground.