Judith River Formation, Sedimentary rock formation in Montana and Alberta.
The Judith River Formation is a layered rock unit made of sandstone, mudstone, and siltstone that stretches across Montana and into Canada. The different colored and textured bands visible in the exposed rocks tell the story of ancient river and lake environments from the Late Cretaceous period.
This rock formation developed roughly 75 to 80 million years ago when rivers and lakes deposited sediments across a vast interior seaway in western North America. Over millions of years, geological changes and erosion shaped these layers into the exposure that paleontologists study today.
The formation spans several Montana counties including Big Horn, Blaine, Carbon, and Chouteau, serving as an educational resource for geological studies.
Exposed sections of this formation can be seen at several locations across Montana, particularly along river valleys and cliff faces. Wearing sturdy shoes and taking care while exploring helps protect the site and keeps you safe.
The rocks here contain remains of early mammals and marsupials, some of the oldest known examples from the Late Cretaceous period. These discoveries reveal that mammals were already diverse before dinosaurs disappeared.
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