St. Mary River Formation, Geological formation in Alberta, Canada and Montana, United States.

The St. Mary River Formation is a geological sequence of sandstone, mudstone, and coal layers found across southwestern Alberta and northwestern Montana. The layers are exposed at the surface in several locations, where erosion has gradually uncovered the rock over time.

The formation dates to the Late Cretaceous, a time when the land alternated between shallow water environments and river systems. George Mercer Dawson described and named the layers scientifically in the 1880s during a government survey of the region.

The formation's name comes from the St. Mary River, where George Mercer Dawson described its outcrops in 1883. Plant fossils collected downstream from the St. Mary Reservoir raised the known leaf flora from 18 species described in 1949 to at least 32, including ferns, Ginkgo, conifers, a Trapa-like plant and a sabaloid palm.

The formation is accessible at several spots in both Alberta and Montana where the rock layers are exposed at the surface and easy to approach on foot. Sturdy footwear is a good idea, as the ground can be uneven and paths are often unmarked.

In one 177-metre section of the formation, more than 100 track-bearing layers were documented, one of the highest densities of such layers reported from any rock succession. A single footprint preserves the first record of skin impressions from the underside of a hadrosaur foot.