An isopach map of the transgressive system of the Mannville Group (Lower and basal Middle Mannville) shows that it occurs irregularly over much of the Western Canada sedimentary basin, in topographic lows on the basal unconformity. These lows were created by one or more of the following: 1) salt solution, 2) erosion of underlying rocks creating drainage basins, and 3) tectonic subsidence in western areas, particularly over the Peace River Arch. Five major erosional drainage basins are delineated on the basal unconformity. The basin through Saskatchewan and northeastern Alberta, created by salt solution, drained northwestward onto the Canadian Shield. The major drainage basin in the central portion of the foreland flowed to the northwest, then turned directly westward, as did two other basins between the Pembina ridge and the overthrust belt. A southern basin drained into the United States. The map shows two sets of linear thickness anomalies (NW-SE and NE-SW) where erosion has cut deeper into the underlying rocks. These linear trends are interpreted to be the result of inheritance from one or more tectonic events. Detailed stratigraphic analysis shows that the Lower Mannville includes several unconformities. In the west, the Cadomin Conglomerate is unconformity-bounded It does not mark the initiation of thrusting and subsidence, but predates this, and was transported into the basin during a period of general uplift. In places, the Cadomin is overlain by a basal sandstone of the Gething Formation (likely equivalent to the Dunlevy and Dalhousie Sandstones of western areas), also interpreted to be unconformity-bounded because of onlap relationships and a channelized upper contact. The mudstone- and coal-dominated typical nonmarine Gething is interrupted by incised valleys at three stratigraphic levels. In central and southern Alberta, the Lower Mannville is also comprised of several unconformity-bounded units within large-scale valleys cut on the basal unconformity. These Aptian deposits (Ellerslie Fm) are easily confused with pre-Mannville valley-fill sands, some as old as Middle Jurassic, and others perhaps equivalent to the Cadomin or basal Gething sandstones in the west. These basal sands may be distinguished from each other by changes in lithology (from pure sandstone to mudstone-dominated fill), petrography (from pure quartzose to lithic sandstones), or on detailed cross-sections which show erosional truncation of units. The Ellerslie Fm is also comprised of a series of unconformity-bounded estuarine to nonmarine sequences, but generally finer-grained than the sands below. In the northeast, a basal Cretaceous sand has been dated as old as Valanginian, and is an erosional remnant of a pre-Mannville unconformity-bounded unit. The overlying McMurray shows three units which are also bounded by unconformities. Tn one of these, the major channel sands with the large-scale lateral accretion surfaces occur in incised valleys. These unconformities in the Lower Mannville play an important role in controlling reservoir distribution in all areas of the basin.