Poorly exposed Late Eocene strata in the Rock River basin, 115 km northeast of Watson Lake, accumulated in an intermontane valley with a geometry and history controlled by subsidence associated with the Rock River Fault. The sequence, as seen in one outcrop and five borehole sections, is dominated by drab mudrocks with minor sandstones and some thick lenses of coal. The mudrocks accumulated in floodplain marsh and pond settings associated with a low-gradient, possibly anastomosed, fluvial system. River banks were stable owing to the abundance of plant roots in the channel walls. Although channel sandstone and conglomerate were not identified in the core, the abundance of coarsening- and fining-upwards sets of sandstone of splay origin indicates pronounced levee development. Woody coals accumulated in areas well away from the main channel, in a series of elongate forested swamps, which were periodically inundated by flood water.The overall palynological assemblage is typical of the Eocene and Early Oligocene. A Late Eocene age is inferred from the presence of Gothanipollis in combination with the absence of index species for the Early–Middle Eocene and the latest? Eocene and Oligocene. The low miospore diversity indicates a temperate climate. The dominance of the palynological assemblage by Taxodiaceae–Cupressaceae pollen indicates wet–humid conditions.