East Anglia and adjoining areas - Suffolk and south Norfolk

From Earthwise
Jump to navigation Jump to search

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[edit]

A massive vertical flint nodule within the Chalk, its length is about one metre. P210913.

In Suffolk and south Norfolk the younger sedimentary bedrock consists mainly of a sequence of sedimentary layers including the Chalk, dipping gently to the south-east. 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 300 m 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 (Plate P210913) 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[edit]

Occurring directly beneath the Gault Clay over most of the area, these rocks occur at depths of about 200 m around Cambridge, 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 south-western and northern parts of this area. Its thickness is uncertain. In the rest of the area the younger sedimentary bedrock rests 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.