Contouring of geophysical and hydrographic data obtained during a regional geological and geophysical reconnaisance programme has resulted in the discovery of an extensive system of narrow, steep-sided, sub-bottom infilled channels. These channels, which occur to the north of the Cherbourg Peninsula, are, to a certain extent, structurally and stratigraphically controlled by the Cretaceous and Jurassic age strata into which they are cut, and appear to be the remnants of earlier river valleys filled with locally derived bedded and unbedded sands, silts, flints, boulders, clays and gravels. The depth reached by this infilling material is variable, ranging up to 200 m below sea-bed. It is suggested that this system may have originated during late Tertiary (Miocene) and that during the Plio-Pleistocene, when sea level was lower, a combination of tidal scour and fluvial erosion entrenched the system into the exposed sea floor. The present tidal regime and the differing physical characteristics of the strata involved suggest that the present bathymetry is a result of tidal scour.