Sand-flow cross strata, formed by the avalanching of sand down slip faces, dominate the foresets of the shallow-marine tidal sands in the Lower Greensand (early Cretaceous) of southern England. The sand-flow cross strata dip steeply at angles between 20 degrees and 32 degrees , occur in trough and tabular crossbeds ranging between 0.3 and 10.0 m in thickness, and vary in style between tongue-shape and tabular forms, with various intermediate compound types. Narrow, tongue-shaped sand-flow cross strata, hitherto considered as characteristic of eolian crossbeds, are widespread in these marine sands. Similar structures and textures are observed associated with sand flows on small, experimental subaqueous dunes and dry talus slopes. Slow rates of bedform migration and locally increased rates of sediment supply to the slip face are considered prime factors controlling the formation of subaqueous, tongue-shaped sand flows. Swifter bedform migration and more uniform sediment supplies favor tabular sand flows. Discrimination between subaqueous and eolian cross beds cannot be made based on sand-flow characteristics. However, the rare occurrence of grain-fall laminae in subaqueous cross beds suggests that thick grain-fall deposits provide a criterion for distinguishing eolian from subaqueous cross beds.