The Cardium Formation (Turonian-Coniacian, Upper Cretaceous) of the Western Interior Seaway, Alberta, contains six marine sandier-upward successions bounded by seven basin-wide erosion surfaces. In the subsurface, erosion surface E5 is associated with major oil and gas fields and is penetrated by thousands of wells. A topographic map of E5, based on 6126 well logs and over 1100 measured cores, shows a series of subparallel elongate highs ("steps") and lows which trend northwest-southeast, parallel to tectonic trends and to the actively rising cordillera to the southwest. The steps are about 10 km apart, and the relative relief, step to adjacent low, is up to 25 m. The steps are asymmetrical, with a steeper face to the northeast. The E5 erosion surface cuts into offshore marine hummocky cross-stratified sandstones and interbedded mudstones, and it records a lowering of relative sea level. Our interpretation suggests that erosion was in neither a fully subaqueous nor a fully subaerial environment. Instead, it involved relative sea-level lowering and subaerial erosion, followed by major modification of the landscape during transgressive shoreface erosion. At times of relative stillstand during overall transgression, gravel was supplied to the shoreface and was reworked a short distance alongshore by waves. These conglomerates now form important oil and gas reservoirs. The asymmetry of the steps and lows indicates that the originally flat sea floor was tilted northeastward immediately before erosion, giving a gradient of about 1:1000. During transgression, shorefaces were incised into the tilted surface to form a stepped morphology. Also during transgression, the sea floor was gradually restored to its pre-tilting position. Each of the six marine sandier-upward sequences, along with the erosion surface at the top, occupies an average of 470,000 years, and we estimate that transgression took at least 100,000 to 200,000 years.