This study demonstrates the validity of using vertical grain size progressions as an environmental indicator for ancient clastic sequences. Previous methods of grain size analysis are shown to be generally unreliable for environmental determinations. Grain size data is interpreted here in terms of idealized sequences for different sedimentary environments. The environments examined in this study include braided river-alluvial fans, meandering rivers and coastal barriers. Braided river systems are characterized by the random variability of grain Meandering size parameters within a vertical section. rivers and coastal barriers are each characterized by a distinct vertical sequence of sub-facies. These stratigraphic sequences are reflected in the vertical grain size progressions that characterize each of these environments. Vertical progressions in ancient sequences correlate well with lateral progressions in modern analagous environments. With improved coring technology, this method of grain size analysis can be used to establish the depositional environment of subsurface sand bodies. Vertical grain size progressions may prove to be an important exploratory tool for the petroleum industry as well as a reliable grain size method for the field geologist.