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This map of depth to bedrock is based on information from seismic test spreads. The River Denys lowlands, near Melford appear to harbour great thickness of surficial deposits. At other sites the depth to bedrock are less certain.
The digital terrain model above is a close-up of the River Denys area with a compilation of previous data and seismic test spreads, indicating depths to bedrock. Triangles are test spreads, circles are diamond-drill holes, and crosses indicate bedrock outcrop. Larger symbols indicate depths to bedrock of greater than 50 m. Yellow triangles are the locations of seismic test spreads shown below. The sites appear to be on the edge of a fault-bounded basin. Surficial sediment thicknesses exceed 50 m, and subcropping Cretaceous sediments may be expected.
This diagram shows four seismic test spreads measured in River Denys Centre. Site 24 shows an eastward-dipping reflection which represents the gypsum bedrock surface from 48 m to 65 m. A drillhole at the eastern end of the spread confirms this interpretation. The incoherent seismic unit above the major dipping reflector (bedrock) is glacial deposits. Site 25, however shows an unconformity with a westward-dipping unit, beneath the glacial deposit. This is unlikely to be bedrock because the seismic signal cannot penetrate solid rock. This unit may be unconsolidated Cretaceous deposits.
The seismic test spread above is located a few kilometres north of the previous sites (see preceding diagram of digital terrain model). It also shows several distinctive seismic units. Major southward-dipping reflectors suggest a deltaic unit, of Pleistocene age, where the dip is a primary sedimentary feature. These reflections lap onto a flat-lying reflection at a depth of 100 m. This last reflection may represent the bedrock surface. Dipping reflections beneath this, however, may suggest deeper unconsolidated deposits.
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