The Fox Hills Formation consists of marginal marine yellow to grey
sandstone with
shale interbeds. It was deposited as a regressive sequence of
barrier islands during the retreat of the
Western Interior Seaway in Late Cretaceous time. In its eastern extent, the formation is underlain by the marine
Pierre Shale in the United States and by the equivalent
Bearpaw Formation in Canada, while in western ranges in Montana and Wyoming it overlies the
Lewis Shale. The Fox Hills is overlain by continental sediments of the
Laramie Formation in Colorado and the
Lance Formation in Wyoming, the latter being the equivalent of the overlying
Hell Creek Formation in Montana. == Fossil content ==