A sedimentation model of subaqueous fan deltas dominated by conglomeratic mass flow has been developed from side-scan sonar images of some modern arctic deltas and some fossil examples from southeast Spain. These modern and ancient examples show slide scars in the delta-front sediments and downgoing troughs on the delta slope. The troughs are separated by erosion remnants and (partly) filled with slumped material from the slide scars, from which bed sequences of ancient fan deltas were studied in detail. Some of the relatively small slump deposits (ca. 50,000 m3) show an increase from massive toward stratified bedding away from the slide scar. With respect to the relationship of massive and stratified sediments, a new process called gravity-winnowing has been inferred from experiments and field studies. In this case the stratification occurs by repeated slumping by means of small grain flows and turbulent flows from the just-deposited and unstable head of the debris.