The Hell Creek Formation in Montana overlies the Fox Hills Formation and underlies the Fort Union Formation, and the boundary with the latter occurs near the Cretaceous-Paleogene boundary (K-Pg), which defines the end of the Cretaceous period and has been dated to 66 ± 0.07 Ma old. The thickness of the formation is estimated to have been deposited in about 2 million years. Fauna characteristic of the Hell Creek (Lancian land vertebrate age) are found as high as a few meters below the boundary.