The middle Tertiary unconformity, mappable throughout the Townsend, Clarkston, and Three Forks basins (Robinson, 1960), can be mapped also throughout the North Boulder and Jefferson basins. In the North Boulder basin, as in the Three Forks basin, the unconformity is erosional; in the other three basins it may be angular as well. The unconformity separates strata as young as middle Oligocene from strata as old as upper Miocene. The topography in the North Boulder and Jefferson basins at the end of the middle Tertiary erosional episode was apparently similar to the present topography, but drainage was probably to the southwest. Then, as now, one or more broad benches rose gently toward the mountain fronts, and at least part of one bench rose from a scarp bordering the floodplain of the ancestral Jefferson River.