East Anglia and adjoining areas - Geology
Recent surface deposits
East Anglia has widespread geological deposits of relatively recent origin, formed over the past 2 to 3 million years and spanning the Ice Ages and Interglacial periods. These are known as superficial deposits and include sands, clays and peat deposits which were laid down by former ice-sheets, in rivers, swamps and marshes, or along the margins of the North Sea. At times over the past 3 million years the North Sea has extended inland, covering the eastern part of region, while at other times it was more restricted and the land area extended across much of the present southern North Sea. The superficial deposits are less than 100 m in thickness and are not shown on Figure P902251, examples include the peat deposits of the Fenlands and the Broads, the boulder clays of central East Anglia and the ‘Crag deposits’, sands that are often full of fossil seashells, found in the eastern parts of Suffolk and Norfolk. These Crag deposits (Plate P902286) are a useful shallow source of groundwater; water is present in the pore spaces and flows freely between the individual sand grains, enabling it to be pumped out easily. Deposits like this are known as aquifers and are used to provide drinking water. Most of the superficial deposits are soft and easily eroded, as they have not been deeply buried and consolidated to form strong rocks.
Geology at depth
Below the superficial deposits, or with just a cover of soil where such deposits are absent, are older rocks which geologists broadly split into two distinct types:
- The sedimentary bedrock geology is composed of quite hard rocks formed from tens to a few hundred millions of years ago as layers of sediments which were deposited in shallow seas, deserts and vast river systems in times when Britain lay closer to the Equator and the climate and landscape were very different from those of today.
- The basement geology, which underlies the whole region, is over 410 million years old and mainly comprises harder and denser rocks which have been strongly consolidated, fractured and folded. They include both rocks originally deposited as sediments and others that are products of volcanic activity or formed from the solidification of molten rock below ancient volcanoes.
In the course of the past 550 million years there have been periods when parts of the UK formed a landmass and was being eroded, whilst parts were sinking and new layers of sediment were being deposited. The history of erosion and deposition has not been the same in all parts of the UK. In the East Anglia region the oldest sedimentary bedrock are red sandstones (Old Red Sandstone) overlain by Carboniferous sandstones, mudstones and limestones that are similar to the rocks occurring at the surface in parts of northern England. These rocks are all referred to here as the older sedimentary bedrock.
Subsequently, a younger sequence of sedimentary rocks, including limestones, sandstones and clays, was laid down and is known as the younger sedimentary bedrock. As one travels across the region from west to east the rocks at the surface become younger in age because the layers are slightly tilted down toward the east. These include in the west Jurassic rocks, comparable to those seen along the coast of Dorset and North Yorkshire, they also extend at depth across north Norfolk. They are overlain by Cretaceous rocks, including the widespread Chalk that occurs at the surface over much of the eastern part of the region. These Cretaceous rocks rest on different types of older rock including the basement rocks when traced across the region, this situation is what geologists call an unconformity, it represents a period of uplift and erosion.
Figures P902252 and P902253 provide vertical sections through the geology, referred to as geological cross-sections.