A 300-km (186-mi) transect was reconstructed across the 500-m (1640-ft)-thick, 3-m.y. duration Iles clastic wedge in southern Wyoming and northern Colorado using well logs and stratigraphic columns. This wedge developed into the middle late Campanian-aged Western Interior seaway by progradation from the active Sevier fold and thrust belt and adjacent uplifted areas. The wedge thickens basinward because late-stage, widespread uplift of the fold belt caused multiple unconformities across the proximal (former foredeep) reaches of the foreland basin. Transect analysis allows the wedge and its component sequences to be better understood and permits a two-dimensional characterization of the sand and mud distribution. The Iles wedge exhibits 11 low-gradient, regressive-transgressive, high-frequency sequences that were correlated across several hundred kilometers. Sediment partitioning analysis along the transect shows the following. (1) Within the regressive limb of the Iles wedge, the component higher order regressive compartments tend to thicken into the medial reaches of the wedge, whereas transgressive compartments thicken landward. This geometry is driven by preferential erosion in proximal areas during regression, bypassing much sediment to the marine shorelines, and transgressive backfilling into proximal areas previously eroded more deeply. (2) The greatest concentration of sands tends to be located in the proximal fluvial and estuarine facies of the transgressive compartments and within the medial (shoreline) deltaic facies of the regressive compartments. (3) As the high-frequency sequences develop, the effectiveness of basinward sand partitioning reaches a maximum value near the peak regression level of the wedge, apparently reflecting stronger erosion and sediment bypass during these times.