Gravity modelling was carried out along five profiles that traverse the Eastern Cape Province of South Africa in order to determine the depositional surface and isochore thickness of the Ecca sediments. Gravity models of the subsurface rock density reveal that the Ecca Group has a maximum vertical thickness of about 3215 ± 160 m. The maximum depositional surface (elevation) above sea level for the Ecca sediments is about 500 m, whilst the depth below sea level reaches about 10000 m. Correlation of the isochore thickness maps with the depositional surfaces shows that the sediments in the basement highs were subsided, deformed, eroded and deposited in the basement lows. The basement highs served as the source area(s) for the sediments in the basement lows, thus basement highs are characterized with thin sediment cover whilst the lows have thick sediment cover.