Lower Cambrian–Lower Ordovician passive-margin sediments were deposited across northeastern Washington, northern Idaho, and western Montana. Lower Cambrian strata record the initial transgression onto Laurentia in northeastern Washington. Middle Cambrian–Lower Ordovician units were deposited across a much broader area and record the establishment of a western ooid-algal shoal complex that restricted water circulation in an intrashelf basin that formed between the shoal and craton. Long-standing topographic highs in the region include Montania, which may have controlled the location of the ooid-algal shoal complex, and the Lemhi arch, which served as a western source area for siliciclastic sediment input. The Sauk megasequence sediments were deposited in multiple grand cycles that are regionally correlative. Each grand cycle consists of a shale and sandstone base deposited as sea level transgressed onto the craton. The basal siliciclastic units are gradationally overlain by carbonate, which was deposited as sea level continued to rise and an extensive carbonate platform developed. Some of the boundaries between the grand cycles are unconformities, recording rapid changes in sea level. Meter-scale shallowing-upward cycles are common within grand cycles, but they have not yet been regionally correlated.