Eastern England from the Tees to the Wash areas - Western Lincolnshire and southwest Humberside

This wedge-shaped area lies between the Western area described above and the Lincolnshire Wolds to the east. The landscape is defined by a series of ridges. Lincoln and Grantham sit on the highest of these ridges; other settlements include Sleaford, Horncastle, Market Rasen and Boston.

Sedimentary bedrock

The Lincolnshire Limestone, quarried near to Sleaford. P213703.

A concentration of boreholes in the north and central part of this area allows the overall form of the sedimentary bedrock geology to be well understood to a depth of at least 1000 m. It is less well understood in the south where there are fewer boreholes. The sedimentary bedrock ranges in thickness from about 500 m in the south to over 4200 m in the north of this area; within 1000 m of the ground surface it mainly includes mudstones, sandstones, limestones, coal beds and layers of mineral salts.

The uppermost layers in this sequence include mudstones, sandstones, limestones and ironstones that were laid down about 200 to 145 million years ago (Jurassic), when this area was largely covered by shallow seas. These oldest layers occur in the west and are tilted gently towards the east so they gradually descend beneath the Wolds. These layers thin dramatically from over 600 m in the south to less than 40 m in the north. As the thickness of these layers changes northwards, many of the individual layers die out.

Mudstone layers of the Oxford Clay in the upper part of this sequence are famous for their fossil remains of marine reptiles, many of which were found when these rocks were quarried for brick clay. One of the thickest limestone layers, the Lincolnshire Limestone (Plate P213703) is an important aquifer. The water in this layer is typically ‘hard’, meaning it is rich in calcium carbonate, which forms limescale in kettles. The water in this aquifer generally flows through fractures within, and between, the individual layers. Several of the ironstone layers have been mined in quarries at the surface and from shallow underground mines.

The Triassic and Permian sedimentary bedrock found at the surface in the Western area, including the Sherwood Sandstone, generally continues beneath these younger layers, reaching a depth of around 500 m in the south-west and more than 1200 m in the east. The underlying package of red-brown mudstones, sandstones, mineral salts and the Magnesian Limestone is generally thicker in the north of this area but die out to the south.

A sequence of still older rocks, including the Coal Measures, extends beneath these layers across much of the area. However their extent is very complicated due to faulting and changes in their thickness. These layers are several kilometres thick in the north but die out in the south. Natural gas is found in some of the sandstone and limestone layers in the central and northern parts of this area.

Basement rocks

The south of this area is the only part of the region where basement rocks are found within 1000 m of the ground surface. These include slates, sandstones, volcanic lavas and once molten granite intrusions that come to within 500 m of the ground surface before descending steeply to the north. These basement rocks are often tilted steeply. These rocks are known from relatively few deep boreholes, so other information including seismic, magnetic and gravity data, are used to interpret their structure.