The remainder of the formation was deposited in freshwater fluvial and floodplain environments and is characterized by interbedded sandstone and siltstone, with minor occurrences of carbonaceous shale and coal.

The St. Mary River Formation is part of an eastward-thinning wedge of sediments derived from the erosion of the mountains to the west.

It extends from Glacier County, Montana to as far north as the Little Bow River in Alberta, where it grades into and intertongues with the contemporaneous strata of the Horseshoe Canyon Formation.

The assemblage includes remains of ferns, Ginkgo, conifers,[9] a Trapa-like plant,[10] and at least six types of large monocot leaves in addition to a sabaloid palm.

[18] Edmontonia[20] E. longiceps[20] Montanoceratops[21] M. cerorhynchus[21] Pachyrhinosaurus[18] P. canadensis[18] Regaliceratops[22] R. peterhewsi Albertosaurus[18] [18] Saurornitholestes[23] cf.