A study of rock-samples collected at 26 stations and continuous seismic profiles taken over a distance of 2000 km in the eastern part of the English Channel reveals the existence of a large flat-floored Tertiary syncline which forms an easterly extension of the Hampshire Basin of Southern England. The newly delineated basin, which is here referred to as the Hampshire-Dieppe Basin, forms part of the well-known Palaeogene Anglo-Paris-Belgian depositional basin. The total thickness of Palaeogene strata in the eastern part of the basin is about 380 m and the youngest beds present are probably of Upper Eocene age. Upper Cretaceous strata are exposed around the edges of the basin; the thickness of the Chalk varying from about 250 m in the south and east to 420 m in the northwest. The basin is bounded to the northeast by the Weald-Artois anticline and a strong flexure, the Bembridge-St Valery line, which is a continuation of the Isle of Wight monocline, controls the position of much of its southwestern boundary. To the south of the Hampshire-Dieppe basin lies the Baie de la Seine Tertiary syncline. Here, freshwater limestones of presumed Oligocene age overlie a Middle Eocene marine sequence. Lithologies and microfaunas of the samples are described and related to those of sequences of similar age on nearby land, and some conclusions are drawn about the palaeogeography of the area. The geological structure of the area is elucidated, the paths of faults and fold axes are traced and a geological map is presented.