A coast-parallel elongate body of postglacial diatomaceous mud on the inner continental shelf of South West Africa (Namibia) is 740 km long and 76 km at its widest off Walvis Bay. The average thickness of the deposit is 5.1 m, and volumetrically, it totals about 127 km3. Maximum and minimum water depths at the edges of the mud belt are 222 m and 1 m respectively, and the average depth of the deposit is 83 m. The opal content averages 54% (range, trace to 88%) and the density of mud containing maximum opal values is 1.05 (wet) and 0.18 (dry).