Central England area - Eastern area
This area extends southwards from Grantham to Evesham. Leicester, Melton Mowbray and Rugby lie close to the western limit whilst farther east the main settlements are Northampton and Peterborough.
Younger Sedimentary Bedrock
In this eastern part of the region there is a succession of young sedimentary bedrock layers that are tilted gently towards the southeast. This means that as one travels eastwards across the area the rocks at the surface represent higher and younger layers of the sequence. Most of these layers were laid down in ancient seas with the youngest around 160 million years old. The youngest rocks comprise mudstones deposited in deep seas about 150 million years ago, that are preserved to the north and southwest of Peterborough. These rocks are underlain by mainly limestones (Plate P731892) deposited in shallow seas with some layers of mudstone and coarse sandstones and ironstones, deposited when the seas became very shallow. Two layers of ironstone occur, one forming the prominent escarpment of the Vale of Belvoir and the second being present widely in Northamptonshire. They have both been extensively quarried and mined near Melton Mowbray and at Irthlingborough. The oldest layers of the Younger Sedimentary Bedrock are also a mudstone succession called the Lias, similar to that seen in the Cheshire-Shropshire area, which form the broad lowland area from the Vale of Belvoir in the north to the area around Evesham in the south. This entire sequence of sediments reaches 450 m thick along the eastern part of the area.
Older Sedimentary Bedrock
Beneath is the Mercia Mudstone, comprising comparable red mudstones with gypsum as seen in the Central area. The succession is up to 200 m thick and is present to a depth of 550 m beneath the easternmost part of this area. Below the Mercia Mudstone, the Sherwood Sandstone mainly comprises red sandstones comparable to that present in the Central area. These sandstones are about 175 m thick around Grantham, but thin southwards and are absent southeast of Leicester.
Beneath the Sherwood Sandstone these is a 320 to 300 million year old sequence of Coal Measures which were laid down in swampy forests in large river deltas and form part of the older sedimentary bedrock. Although the older sedimentary bedrock does not occur at the surface, the coal reserves are extensive in places, particularly between Nottingham and Melton Mowbray, and have sometimes been mined.
The older sedimentary basement rocks are known to extend down over 1000 m and in places include sandstones, mudstones and limestones up to 350 million years old. This area has yielded oil and has potential as a shale-gas resource.
Basement rocks
Geophysical surveys reveal local anomalies in the magnetic field and the force of gravity in some parts of this area. Such anomalies are common in areas where there are pillar-shaped bodies of granite or similar rocks, known as igneous intrusions. The anomalies mark the solidified remains of bodies of molten rock or magma, similar to those which lie beneath many active volcanoes today at depths of a few kilometres and occur because the granite is lighter than the surrounding basement rocks and so gives a low or negative gravity anomaly. The granite also contains more magnetic minerals than the rest of the basement producing highs or spikes in the magnetic signal. These intrusions of granite have also been proved in boreholes at comparatively shallow depths. They have been drilled northeast of Loughborough at 1130 m depth and south of Melton Mowbray at 450 m depth; these occurrences are thought to be linked at greater depths to form a single mass of granite. Other intrusions occur between Grantham and Stamford at about 850 m depth and also west of Peterborough.