A set of fluid temperature logs from 54 boreholes in the Chalk aquifer of the western London Basin syncline has been interrogated. Subsurface temperatures in the shallower (<100 m) strata can be shown to be largely controlled by a combination of two factors: (1) depth, with an average gradient of c. 1.8 °C per 100 m; (2) ground surface elevation, with an altitude lapse rate of around 1.0–1.2 °C per 100 m. These factors alone are not sufficient to explain the elevated temperatures observed in the deeper strata at the eastern end of the study area, where the Chalk is overlain by greater thicknesses of Tertiary argillites and where a lower bulk vertical thermal conductivity is thought to be the most likely factor (although this cannot be conclusively proven with the existing dataset).

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