During the late Permian Period, around 260 million years ago, a sea flooded a large part of the great continent that Britain was then part of. The sea stretched from Siberia in the east to what is now north-east England, in the west. Leeds lay right at the edge of this huge body of water that had encroached across the desert surface. It was a shallow sea and, in the hot dry climate, its waters quickly evaporated making it very salty. In these conditions limey muds rich in magnesium were deposited which, when compacted, formed the pale yellow Magnesian Limestone. The warm, salty sea was not favourable to many life forms and so the limestone is rather sparse in fossils. Algal reefs and the shells of a few bivalves that could survive the hostile conditions are all that are usually found.

Today the Magnesian Limestone forms a low plateau in the east of the region and is well exposed in cuttings of the M1 between Garforth and Bramham. The attractive pale yellow rock has been used extensively as a building stone and today is still worked as an aggregate.

Magnesian Limestone exposed by the M1 at Garforth