Drowned valleys, cliffs, and erosional features of continental shelves require a prolonged period (or periods) of low sea level followed by a period of gradually rising sea level. Because tectonism cannot account for the relatively uniform depth of these features, and because Pleistocene glacioeustatic regressions were too brief, a hypothesis of preglacial low sea level caused by tectonic changes in ocean floors is proposed. Pleistocene glaciations temporarily restored sea level to near its preglacial position. Preglacial and inter-glacial high sea levels were relatively minor incidents in the late Cenozoic evolution of coasts.