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The Clay Castles in the Hoarusib River valley are fine-grained deposits in a tributary immediately upstream of a narrow gorge. Sedimentological analysis reveals three lithofacies associations: (1) Channel Facies Association near the mouth of the tributary, (2) Back-Flow Facies Association in the middle section, and (3) Silt and Massive Sand Facies Association in the head of the tributary. Overall sedimentary architecture shows that aggradation of the main Hoarusib channel led to back flooding into the tributary and the deposition of massive fine-grained deposits. Decreased discharge competency, increased sediment load and the presence of narrow gorges within the main valley are suggested as reasons for the aggradation. Inter-fingering of desert-derived sediments channeled down the tributary and backflow sediments from the main river indicate an increase in both upland and desert rainfall during the course of sedimentation. Optically Stimulated Luminescence (OSL) dating of these deposits indicates formation between 44 and 20 ka, with the possibility that deposition occurred during two phases of rapid sedimentation at ca. 42 and 25 ka. (C) 2004 Elsevier B.V. All rights reserved.