The M errions Tuff is the structural-stratigraphic marker of the flysch-like fill of the Hill End Trough, a Siluro-Devonian basin lying in central-western New South Wales and bounded to the east and west by coeval, linear, meridional, shoalwater accumulations of limestones and volcanics. The Merrions Tuff is grossly tabular in form and consists of sheet-like to lobate horizons of dacitic lavas and volcaniclastics. Internal vertical and lateral heterogeneities allow subdivision into members. Although some members are regionally extensive, others are not, so leading to a complex internal stratigraphy. Identification of regionally extensive time planes and analysis of the time-stratigraphy shows that the base of the Merrions Tuff is diachronous. Although mesoscopic sedimentary structures such as sole structures are rare or not exposed, analysis of the parameters (spatial distribution of members, variation of and within members of their shape, thickness, grainsize, sand-to-shale ratios, numbers of sedimentation units, and distribution of vitriclasts), demonstrates that the Merrions Tuff was not derived from a single source and a single eruptive phase, but from several sources which were variously active at different times. Furthermore, through time there appears to have been a shift in the direction of dominant influx of basin fill, from the west through to the south, to the southeast and finally the east. Depositional patterns were controlled by a changing basin-floor topography which was particularly influenced by the emplacement of the very thick member B lava. The effect of this flow was to displace the northwesterly-trending basin axis existing at the beginning of the history of the Merrions Tuff to the east.