Observations on the geology of the country around Chalfont St Giles, Buckinghamshire

Abstract

This paper presents new field observations from the area around Chalfont St Giles and relates them to existing information from the wider Chiltern Hills region on lithostratigraphy, structural geology, geomorphological and karstic features, and landscape development. In this area the NW-SE aligned valley of the River Misbourne, an internationally rare chalk stream, and flanking interfluve plateaus form a transition zone on the chalk dipslope between the lowest elevations of outcropping chalk around Denham and the characteristic chalk valleys and clay-with-flints capped upland present from Amersham northwards. The area straddles the northernmost extent of both the Paleogene strata, represented by Lambeth Group outliers, and SW–NE aligned pre-Anglian palaeo-Thames River Terrace deposits, and contains an anomalous Lambeth Group, Quarrendon Gap, mid-slope platform. Ground investigations carried out for HS2 have helped substantiate other evidence for significant faulting across the area, with intermittent movement episodes dating from the Mid- to Late Cretaceous onwards, which have affected both the Chalk and Paleogene strata and had a major influence on shaping the valley network and wider landscape. Karstic features are widespread, including contemporary interfluve plateau level streams sinks around the Lambeth Group outliers forming active dolines, together with many concealed chalk dissolution related features. There are numerous, generally small scale, clay, chalk, sand and gravel workings most of which are long abandoned but still provide useful insights into the near surface geology.