Recently acquired marine reflection data provide evidence for a major geological boundary at shallow depth under the southern Bristol Channel, in the hangingwall block of the Bristol Channel thrust. The boundary is recognized to equate with the top surface of a high velocity layer previously identified along refraction/wide-angle reflection seismic lines in the north Devon area. Depth estimates from the reflection data and from a new interpretation of the earlier refraction data are combined to produce a contour map of the shallow boundary in the south-central Bristol Channel and north Devon areas. The boundary has a WSW-ENE regional trend and lies at a depth of less than 2 km immediately north of the Devon coast. It is truncated to the north by the Bristol Channel thrust and deepens southwards across north Devon and west Somerset. The boundary is interpreted as the top surface of a major litho-stratigraphic unit older than the outcropping Devonian sequence and in normal stratigraphic contact with it. However, in the absence of regional geological control the nature and age of the underlying high-velocity layer remain uncertain.