Sediments laid down under water are frequently layered, the layers being identified by changes in particle size and/or density. The paper considers an example of such layering examined in a core from the Nares Abyssal Plain using an accurate, non-destructive X-ray technique for measuring density. It also reports a series of laboratory experiments in which a sediment containing sand, silt and clay-sized particles was introduced at a variable deposition rate into settling columns. The development of the bed was monitored using the same X-ray technique for density measurement, and the occurrence of layers, marked by changes in density and particle size distribution, is noted and discussed. It is suggested that a genuinely uniform naturally occurring sediment may well be quite uncommon.