Chiltern Hills - Geology

Geology

The chalk escarpment of the Chiltern Hills overlooks the Vale of Aylesbury, and approximately coincides with the southernmost extent of the ice sheet during the Anglian glacial maximum. The Chilterns are part of a system of chalk downlands throughout eastern and southern England, formed between 65 and 95 million years ago, and comprising rocks of the Chalk Group and which also includes Salisbury Plain, Cranborne Chase, the Isle of Wight and the South Downs, in the south. In the north, the chalk formations continue northeastwards across north Hertfordshire, Norfolk and the Lincolnshire Wolds, finally ending as the Yorkshire Wolds in a prominent escarpment, south of the Vale of Pickering. The beds of the Chalk Group were deposited over the buried northwestern margin of the Anglo-Brabant Massif during the Upper Cretaceous. During this time, sources for siliciclastic sediment had been eliminated due to the exceptionally high sea level. The formation is thinner though the Chiltern Hills than the chalk strata to the north and south and deposition was tectonically controlled, with the Lilley Bottom structure playing a significant role at times. The Chalk Group, like the underlying Gault Clay and Upper Greensand, is diachronous.

During the late stages of the Alpine Orogeny, as the African Plate collided with Eurasian Plate, Mesozoic extensional structures, such as the Weald Basin of southern England underwent structural inversion. This phase of deformation tilted the chalk strata to the southeast in the area of the Chiltern Hills. The gently dipping beds of rock were eroded, forming an escarpment.

The chalk strata are frequently interspersed with layers of flint nodules which apparently replaced chalk and infilled pore spaces early in the diagenetic history. Flint has been mined for millennia from the Chiltern Hills. They were first extracted for fabrication into flint axes in the Neolithic period, then for knapping into flintlocks. Nodules are to be seen everywhere in the older houses as a construction material for walls.