The Cretaceous Hell Creek Formation is well exposed in badlands topography in southeastern Montana. Fluvial processes deposited the Hell Creek sediments following the regression of the final Cretaceous Pierre-Fox Hills epicontinental sea. The major depositional environments are the channel-belt and the flood basin. Channel-belts contain channel fill and channel bar deposits of mudstone and cross-stratified and rippled sandstone. Multistoreyed channel-belts resulted from avulsion and lateral migration of channels, occurring repeatedly at the same location. Channel-belts truncate older beds. Flood basin deposits are characterized by rooted beds of carbonaceous shale, mudstone, and rippled sandstone. Overbank flooding caused deposition of flood plains, levees, and backswamps. As a result of avulsion and aggradation, cross sections of lithofacies show mostly isolated channel-belts surrounded by flood plain sediments in approximately equal proportions. Paleogeographic maps of three successively younger multistoreyed channel-belts show the rivers had tributaries and flowed southeasterly. Rivers with single thalwegs of moderate to low sinuosities are indicated. Sandstone composition suggests volcanic with lesser metamorphic and sedimentary sources. The presence of expandable clays and lack of paleosols and coals indicate a seasonably dry climate.
