Detailed "Sparker" surveys along a 400-mile segment of the continental shelf of southwestern Africa show a strikingly consistent depth for the truncated inshore edge of an extensive sedimentary rock sequence overlying coastal Precambrian basement. Although depth of truncation varies from 150 to 385 feet below sea level, more than 30% of its extent lies between 340 to 370 feet. The shallower truncations are in the direction of sediment transport from the mouths of the larger rivers suggesting sediment contributed by the rivers tended to bury and shield the sedimentary rocks from the erosive forces of the surf zone. Allowing approximately 30 to 50 feet as the effective depth of surf erosion, indicates one or more sea level regressions to about minus 320 feet.

The former valleys of the Orange River are entrenched in the Precambrian bedrock offshore. The deeper may be traced to at least 300 feet below sea level and the shallower to 250 feet. The depth of entrenchment of the deeper valley suggest(s) a relation to the 320 foot regression level, and the shallower valley implies an accompanying regression to at least 250 feet.

Emergent terraces along the adjacent coast include an older, warped surface and a younger horizontal sequence. Marine gravels containing artifacts assigned to the late Chelles-Achoulian culture are associated with the younger sequence and imply late Pleistocene deposition as does terrace correlation with world-wide eustatic shorelines. The apparently unwarped nature of the inshore edge of the sedimentary sequence also suggests late Pleistocene as the time of truncation.