At the very end of the Triassic the saline lakes progressively merged as the Jurassic sea inundated the area. The boundary between the Triassic and the Jurassic is marked by thin fossil rich limestones, ‘bone beds’ and clays that occur to the east of Gainsborough (The Rhaetic beds).

The early Jurassic sea deposited thin layers of muds with occasional limestones, but in the northern part of Greater Lincolnshire it includes the Frodingham Ironstone, on which the Scunthorpe steel industry was founded. This is an iron rich limestone laid down in a warm, shallow sea. Abundant fossil shells including ammonites, Gryphea (‘Devil’s toe-nails’) and other bivalves suggest that the sea was rich in life. Some of the best fossils in the ironstone are preserved in a characteristic green colour.

The ironstone appears to have been laid down on a sea floor that was sloping downwards to the south. To the north of what is now the Humber there was probably a land area. Further to the south, the water seems to have been too deep for the ironstone to form so muds were deposited. We have thus ended up with a ridge of more resistant rock stretching from Winterton in the north to just south of Scunthorpe, but it stretches for many miles to the east, appearing in boreholes in the Immingham area.

In the central and southern parts of Greater Lincolnshire similar ironstones formed at slightly later times in the muddy sea, when shallow, oxygenated water provided suitable conditions for their formation. The Marlstone Ironstone is a distinctive bed that initially forms a step in the hillsides to the south of Lincoln, before becoming a separate ridge that swings away southwest near Grantham.