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This paper presents initial interpretations of the processes and events responsible for the late Quaternary sequence in the West Mouth of the Great Cave of Niah, in the hot and humid lowland rainforest and swamp forest of Sarawak in Malaysian Borneo. It evaluates the geomorphological context of the site within the known pattern of rapid late Quaternary climate change. Attention is given to the proximity to the sea and the likelihood of humid tropical or cooler drier conditions. The stratigraphic succession is described and four units or lithofacies (2C, 2, 3 and 4) are recognized as being of particular geomorphological and archaeological importance. The key processes operating within the site are the accumulation and subsequent failure and flow of bat and bird guano, hillslope colluviation, and ephemeral stream flow and pond development. Units 2C and 2 contain the critical archaeology, including the Deep Skull from an anatomically modern human, discovered by Tom Harrisson. These were formed by colluviation from a complex cave-mouth rampart and stream flow from within the cave. The stream transported fine-grained sediment to a shallow pond, and both the stream and pond deposits show evidence for prolonged desiccation. Human activity is associated with these surfaces. The human remains and related archaeology are preserved because a mudflow (Unit 3) plowed into and overrode the land surface upon which the humans had lived, resulting in the deformation and burial of the surface and the preservation of the archaeological material. Provisional radiocarbon dates indicate that Units 2C and 2 accumulated from before ca. 45,000 B.P. until ca. 38,000 B.P. Dates bracketing the Deep Skull give this an age of ca. 45,000 B.P. to ca. 43,000 B.P. Overlying the mudflow, Unit 4, a silty diamicton with a relatively high carbonate and organic content, appears to have formed by a mix of natural colluvial and human transport processes, and is associated with human cultural material. Unpublished radiocarbon dates indicate that this deposit formed from before ca. 19,500 B.P. to ca. 8500 B.P. (uncalibrated).This interpretation of the site and its finds has required detailed reconstruction of the changing palaeogeography within and beyond the cave entrance and the nature and rate of geomorphological processes operating within the region, which have been placed within models for rapid Quaternary environmental change. The results suggest that during the earlier period of human presence in the Great Cave of Niah(earlier than ca. 45,000 B.P. until ca. 38,000 B.P.), the climate was episodically wet with much longer periods of relative dryness. During the later period of human occupancy (ca. 19,500 B.P. to ca. 8500 B.P. [uncalibrated]), the evidence is less secure and a slightly moister climate is suggested.