Category:13. London and the Thames Valley
This account provides a broad perspective of the geology of the London and Thames Valley region of southern England, which extends from Essex westwards through Greater London, Hertfordshire and Bedfordshire and into Oxfordshire and northern Wiltshire. Outside Greater London the main towns and cities include Oxford, Swindon, Newbury, Reading, Bedford, Luton, Chelmsford and Colchester. Figure P902272 provides a geological sketch map of this region showing the rock types occurring in relation to the major towns and cities. Geographically, this region can be divided into three: the London Basin, the Chalk downlands and the Upper Thames Valley. Parts of the region in the west and north lie within other river catchments but their geology is a continuation of that forming the Upper Thames Valley. 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 area.
At the surface the region is formed of rocks laid down in seas that covered the area in the past. The relatively flat landscape is typical of areas underlain by these sandstones, clays and limestones. Even in the Chiltern Hills and other areas of Chalk downland, where the highest land rises to more than 250 m above sea level, the rocks are very rarely exposed at surface. Nevertheless, the geology is well known from quarries and other surface excavations, numerous shallow boreholes especially in urban areas, water wells and changes in soil type. The region has no known economic resources of coal, rock salt or metallic minerals; consequently there has been no deep mining within the region.
At depths greater than about 300 m below the surface, our knowledge depends largely on deep boreholes drilled in the search for water, coal, oil and gas resources. There are more than 140 such deep boreholes in various parts of the region. Some of the deepest boreholes, in the west of the region, reach more than 1 km depth. The results of these investigations are complemented by interpretations of the patterns of variation in the Earth’s gravity and magnetic fields shown by regional geophysical maps and exploration surveys which provide information on the rocks by sending sound waves through the ground (seismic surveys). In the north-eastern parts of the region there are very few deep boreholes and no seismic surveys and, our understanding of the geological structure at depth depends almost entirely on these regional gravity and magnetic surveys.
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