In the Middle Jurassic the seas became shallower and the Middle Jurassic rocks consist of limestones, mudstones, sandstones and ironstones, most of which are quite thin beds, rarely more than a few metres thick.
The Lincolnshire Limestone is much thicker, varying from less than 20m in the north to a maximum of 40m near Grantham, before thinning slightly further south.
The Lincolnshire Limestone was deposited in a clear, shallow subtropical sea that covered much of what is now England. In this sea, small coral reefs developed and small rounded grains of calcium carbonate called ooliths formed. These grains were rounded by wave action so the waters can only have been a couple of metres in depth.
The Lincolnshire Limestone is more resistant to weathering and erosion than the underlying beds, so forms a prominent ridge that gradually gets higher as it is followed south through Greater Lincolnshire. In several places it has been used with the adjacent clays to manufacture cement, but its main fame is as a building stone. Quarries along the length of the ridge provided the stone for Lincoln Cathedral and many of the area’s stately homes, as well as many more humble dwellings in villages along its length.
Near Stamford a series of thin-splitting beds at the base of the limestone are the source of the Collyweston roofing slates that add so much to the character of that town. Elsewhere, particularly in Kesteven, stonewalls around fields indicate the widespread distribution of other thin-bedded parts of the Lincolnshire Limestone.