Category:12. East Anglia and adjoining areas: Difference between revisions

This account provides a broad perspective of the geology of East Anglia and adjoining areas, which includes the counties of Norfolk, Suffolk, Cambridgeshire and parts of Lincolnshire and Bedfordshire. Figure 1 provides a geological sketch map of this region showing the rock types occurring in relation to the major towns and cities. East Anglia’s relatively flat and rolling landscape, mainly less than 100 m above sea-level, provides a rich agricultural setting and contains an interesting geological story. This account sketches out the geology to a depth of at least a kilometre and summarises the current and historical use of the geological resources in the region.

The surface geology of the region is known from quarries, coastal cliffs and shallow boreholes. At greater depths, below about 250 m, our direct knowledge comes from about 50 deep boreholes spread fairly evenly across the region. The deepest of these go down over a kilometre, and most were drilled to explore for water or coal. Geophysical surveys, carried out on land or by low flying aircraft, reveal patterns of the Earth’s gravity and magnetic field, and these also give us clues as to the deeper geology of the region. East Anglia is not known to contain oil, gas, coal or metal resources at depth, and there has been no deep mining within the region. As a result detailed information on the deep geology is sparse.

Suffolk and south Norfolk

Our second area of East Anglia comprises Suffolk and southern Norfolk. The main settlements are Ipswich, Bury St Edmunds, Newmarket, Thetford and Lowestoft. This area starts in the west at the ridge of Chalk referred to above and extends from there eastwards to the coast. This area possesses more uniform geology than that described from the Fenland area, and in particular the Chalk either occurs at the surface or beneath a thin cover of young sedimentary rocks throughout the area.

Younger Sedimentary Bedrock

In Suffolk and south Norfolk the younger sedimentary bedrock consists mainly of a sequence of sedimentary layers including the Chalk, dipping gently to the southeast. These rest on the younger sedimentary bedrock layers that occur at the surface farther west in the Fenland, but those layers only extend eastwards beneath the Chalk for a short distance. The younger sedimentary bedrock in this area is up to about 400 m thick and ranges in age from about 150 to 50 million years old. The main elements of the sequence are sandstones and mudstones up to 30 m thick, overlain by the Chalk, which reaches 300m in thickness, and capped near the coast by younger sands and clays up to 50 m thick; so the Chalk is the dominant sedimentary layer of this stack.

Chalk is a fine grained white, or grey rock composed of fragments and microfossils of calcium carbonate; it is a special type of limestone. In its upper parts, black flint nodules (Figure 5) are common. Flint is a very fine grained form of silica and flint nodules were dug from the Chalk in prehistoric times, for example at Grimes Graves near Thetford, and used by early man to fashion stone implements.

The Chalk is a very important aquifer, not just in East Anglia but in adjacent parts of southern and eastern England. Unlike the other aquifers referred to in this account (the Crag and the Sherwood Sandstone) most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along the fractures within it. These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk. The bottom of the Chalk is rich in clay and the thin layer immediately below is a clay layer, the Gault Clay. Water trickling through the Chalk cannot percolate downwards any further when it reaches these clay layers and so flows along the top of the clay until it emerges at the surface forming springs. The water from the Chalk is rich in dissolved calcium carbonate and is referred to as ‘hard’. When this water boils, for example in a kettle, the calcium carbonate is precipitated as “scale”.

Older Sedimentary Bedrock and Basement Rocks

Occurring directly beneath the Gault Clay over most of the area, these rocks occur at depths of about 200m around Cambridge and descending gradually eastwards to depths of about 400 to 500 m below sea level along the coast. These rocks are only known from scattered deep boreholes and most of these penetrated only the very top part of them, there is no geophysical evidence of granites at depth. The Older Sedimentary Bedrock is similar to that of the area to the west being composed of Old Red Sandstone and is mainly confined to the southwestern and northern parts of this area. Its’ thickness is uncertain. In the rest of the area the younger sedimentary bedrock rest directly on basement rocks comprising hard fractured grey mudstones and sandstones that are inclined at steep angles. These rocks originated as sediments on the deep sea floor over 410 million years ago.

North Norfolk

North Norfolk can broadly be equated with an area north of a line drawn from Kings Lynn through Norwich to Great Yarmouth; the other principal towns in this area include Hunstanton, Fakenham, Aylsham, North Walsham and Cromer The coastline here is dominated by low cliffs (Figure 6) with intervening stretches of sand spits, marshes and dunes.

Sedimentary Bedrock

In this area there is a thick sequence of sedimentary bedrock layers which exceeds 1000 m in total between Cromer and Great Yarmouth in northeast Norfolk. As a result, the top of the underlying basement rocks is well below 1 km depth in that part of the area. The same easterly-tilted younger sedimentary bedrock layers described from the rest of Norfolk and Suffolk continue through this area but are even thicker, reaching over 500 m in parts of northeast Norfolk. They comprise interbedded sandstones and mudstones including the Gault Clay overlain by Chalk, which in places in the east is covered by further sands and clays. The lower sandstones and clays are thickest where they are exposed at the surface between Kings Lynn and Hunstanton reaching up to 75m, however they thin out quickly when traced eastwards beneath the Chalk. The Chalk locally exceeds 450 m in thickness with the overlying sand and clay layers reaching 80 m thick but restricted to the fringes of the east Norfolk coast.

The further sedimentary bedrock layers described from the Fenland also continue eastwards beneath the Chalk and are not pinched out as they are to the south. Instead, they form a thickening sequence of layers to the east along the Norfolk coast.

Between Wells next the Sea and Great Yarmouth, Older Sedimentary Bedrock layers are also present at depth. These include sandstones, mudstones, and limestones of the Carboniferous Limestone and the Coal Measures. Elsewhere in Britain some of these layers are encountered nearer the surface and form important aquifers, however little is known about their water content beneath the North Norfolk coast and evidence from similar geological settings elsewhere in the UK suggest that it is likely to be saline.

Basement rocks

The basement rocks in this area are similar to those encountered below the other areas with grey mudstones and sandstones which have been changed by high temperatures and pressures in the long time since they were formed; thin layers of volcanic lavas have also been found in some boreholes. There may be a single granite intrusion in the area north of Fakenham based on the gravity and magnetic data, although other possible interpretations have also been suggested.