Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch (or, in chronostratigraphy: the fifth of six stages in the Upper Cretaceous series). The Campanian spans the time from 83.6 ± 0.7 Ma to 72.1 ± 0.6 Ma (million years ago). It is preceded by the Santonian and it is followed by the Maastrichtian.
Two Medicine Formation
The Two Medicine Formation spans from 80 to 74 Ma, nearly the entire length of the Campanian stage. The formation has been dated using Ar/Ar dating at volcanic ash layers located below the top and above the base. The deposition of the formation may be diachronous. The Lower Two Medicine dates to late Santonian to early Campanian times. The Upper Two Medicine dates to middle-late Campanian times.