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 Some of the lithostratigraphical terms used on the 1:10 000-scale maps have been superseded, and a table showing the current terminology is included as Appendix 1.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).