A simple physical model of gravitational settling from an unsorted, unflocculated source suspension is presented and an equation derived to describe the grain-size spectra of the resulting bottom sediment. Results of grain-size analyses of sediments from a variety of environments and geographical locations are shown to conform with the postulated model. The characteristic size spectrum, termed ‘one-round’ sediment, identifies a deposit which has settled from suspension with no subsequent reworking resulting in modification of the grain-size distribution. The distribution of settling rates of grains in the suspension may be inferred from an analytical form fit to the bottom sediment grain-size spectrum, along with knowledge of certain physical characteristics of the fluid (e.g. mean velocity profile).