The geological framework of the Frome-Piddle Catchment

The purpose of this report is to describe the solid and drift
geology of the catchment area of the rivers Frome and Piddle
and a 5 km-wide buffer zone (together comprising the
study area).
The Frome-Piddle Catchment is centred on Dorchester
in South Dorset and is predominantly a chalkland drainage
basin containing rivers that flow SE into Poole Harbour
(Figure 1). The drainage basin is some 48 km in length and
22 km wide, with the Frome draining an area of 463.7 km2
and the Piddle an area of 187.5 km2.
The catchment has been recently geologically surveyed
at 1:10 000-scale (1985-1997), and most is covered by published
1:50 000-scale maps (Figure 1). These have been
compiled into a map covering the catchment and its 5 km
buffer zone (Map 1). While the map is largely seamless
within the catchment, there is one major misfit in the buffer
zone resulting from the different lithostratigraphical subdivisions
used on the modern Shaftesbury (313) and much
older Yeovil (312) sheet. Some of the lithostratigraphical
terms used on the 1:10 000-scale maps have been superseded,
and a table showing the current terminology is included
as Appendix 1. In addition, borehole, and surface
outcrop data has been utilised to model important geological
surfaces and produce a series of maps (Maps 2-5) and
cross-sections (Sections 1-5).