Judith River Formation

The Judith River Formation is a fossil-bearing geologic formation spanning Montana, Wyoming and South Dakota in the United States and Alberta and Saskatchewan in Canada, dating to the Late Cretaceous Campanian stage, roughly 79 to 75.3 million years ago. It consists mainly of mudstone and sandstone with secondary siltstone, coal beds, bentonite and coquina, reaching a maximum thickness of about 1,180 feet, and forms part of the Montana Group, overlying the Claggett and Pakowki Formations and underlying the Bearpaw Formation; it was deposited during the same interval as parts of Montana's Two Medicine Formation and Alberta's Oldman Formation. It was explored by early American paleontologists including Edward Drinker Cope, and modern excavations have recovered nearly complete skeletons of the hadrosaurid Brachylophosaurus, making the formation an important record of Late Cretaceous terrestrial life.