Water Cycle Study - East Devon District Council

3.3 Geology
Ref EWCS April 2010
Exeter and East Devon Water Cycle Study
3.3.1 The geology of the study area is summarised briefly in the paragraphs below, based upon a review
of British Geological Society maps (Sheets 325, 326, and 339). The geology of the river catchments
and the study area as a whole is already well-documented in the CFMP and CAMS documents
produced by the Environment Agency and in the recently published Green Infrastructure Study for
the New Growth Point [LDA Design 2009]. For the purposes of this Water Cycle Study the
geological classification was determined for each of the identified growth areas – the results are
recorded in the table in Appendix B.
3.3.2 The bedrock geology across Exeter and East Devon becomes progressively younger from west to
east. In the west the relatively permeable Permian strata of the Exeter Group (sandy breccias and
fine sandstones) lie along the east-side of the Exe Estuary. In the hills around and to the north and
northeast of Exeter City these sediments give way to older and less permeable mudstones,
siltstones and sandstones of the Carboniferous Holsworthy Group in the Clyst catchment.
3.3.3 Further to the east, beneath the Otter and Sid catchments, the presence of relatively impermeable
Permo-Triassic mudstones of the Aylesbeare and Mercia Mudstone Groups lead to significant
surface runoff. Beneath the Axe and Lim catchments in the east, younger and variably permeable
Jurassic strata, comprising a sequence of interbedded mudstones and limestones, are capped by
Cretaceous clays, and highly permeable sandstones and Chalk.
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