Type Geological formation Other Coal, coquinas Region Montana Primary Siltstone, Sandstone | Unit of Judith River Group Country United States Underlies Bearpaw Formation | |
 | ||
Sub-units Birch Lake Member, Ribstone Creek Member, Brosseau Member, Victoria Member Thickness max 360 meters (1,180 ft) Overlies Claggett Shale, Pakowki Formation | ||
Our fossil ranch in the judith river formation
The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the upper Cretaceous, between 80 and 75 million years ago, corresponding to the "Judithian" land vertebrate age. It was laid down during the same time period as portions of the Two Medicine Formation of Montana and the Oldman Formation of Alberta. It is an historically important formation, explored by early American paleontologists such as Edward Drinker Cope, who named several dinosaurs from scrappy remains found here on his 1876 expedition (such as Monoclonius). Modern work has found nearly complete skeletons of the hadrosaurid Brachylophosaurus.
Contents
Lithology
The Judith River Formation is composed of mudstone, siltstone and sandstone. Coal beds, bentonite and coquinas are also observed.
Relationship with other units
The Judith River Formation conformably overlies the Claggett Formation and Pakowki Formation. It is overlain by the Bearpaw Formation. It is equivalent to the Belly River Formation in the southern Canadian Rockies foothills, the Lea Park Formation in central Alberta and the Wapiti Formation in the northwestern plains.
Sub-divisions
The Birch Lake Member and Ribstone Creek Member are sandstone units recognized inside the Judith River Formation. Other informal subdivisions include the Brosseau Member and Victoria Member, which are considered obsolete due to their inconsistent lateral distribution.
Fauna
Faunal list follows a review published by Ashok Sahni in 1972 unless otherwise noted.
Amphibians
There are three potential species of discoglossid frogs. Hip bones, possibly representing a North American member of the European spadefoot toad family are also known from the formation.
Theropod dinosaurs
An unnamed tyrannosaurine is known from the formation.