The Middle Cambrian Arctomys Formation is a mixed dolomite-shale formation that varies from 20 to 160 m thick in the southernmost Rocky Mountains of Alberta and British Columbia. The Arctomys Formation is the lower shaly ＇＇half cycle＇＇ of the Arctomys-Waterfowl ＇＇grand cycle＇＇. It contains no fossils, has a restricted easterly extent, but exhibits mud cracks, salt casts and metre-thick cycles. The Arctomys Formation comprises four facies assemblages: 1) thin-bedded to laminated carbonate mudstones; 2) shale-to-carbonate breccia cycles; 3) carbonate mudstone-to-shale cycles; and 4) interbedded grainstones and shales. The thin-bedded to laminated carbonate mudstone facies assemblage probably records restricted peritidal to shallow lagoonal carbonate sedimentation. However, the metre-thick, upward-desiccating cycles that comprise the bulk of the Arctomys Formation (shale-to-carbonate breccia cycles and carbonate mudstone-to-shale cycles) are most likely the deposits of inland shallow lakes and coastal seepage-fed lagoons that filled and developed dry mud flat soil caps. The most landward outcrops of the Arctomys Formation consist of interbedded grainstones and shales that record sheet-flood deposits on dry mud flats. The best overall modem analog for the Arctomys Formation is the extensive coastal plain of South Australia and our proposed depositional model is based on this modem passive margin setting.