GEOLOGIC BACKGROUND

The Judith River Formation yielded the first documented dinosaur remains from North America (Leidy 1856). It occurs throughout much of Montana and southern Canada. Based on a major sedimentological change traceable over a large geographic area in southern Alberta, Saskatchwan, and northern Montana (Eberth and Hamblin 1993), this rock unit has been elevated to group status. However, the change has not yet been identified in south-central Montana. In Montana, much of the stratigraphic and paleontologic work on this rock unit has occurred in the central or north-central part of the state (e.g., Leidy 1856, Hayden 1871, Stanton and Hatcher 1905, Sahni 1972, Case 1978), areas that continue to prove fruitful in their scientific yield (e.g., Rogers 1993, 1994; Montellano 1992; Goodwin and Deino 1989).

In central Wheatland County to eastern Golden Valley County, the Judith River Formation varies in thickness from 70 m to more than 120 m (Fiorillo 1989, 1991, 1997). The vertebrate-bearing interval consists largely of clastic nonmarine sediments (Fiorillo 1991) and nearshore sediments. Underlying the Judith River Formation in south-central Montana is the marine Claggett Formation. Overlying the Judith River is the marine Bearpaw Formation.              

This region has yielded a diverse vertebrate fauna consisting of various fishes, salamanders, lizards, turtles, champsosaurs, crocodiles, pterosaurs, large and small theropods, hypsilophodontids, lambeosaurine and hadrosaurine hadrosaurs, centrosaurine and chasmosaurine ceratopsians, pachycephalosaurs, ankylosaurs, and various mammals (e.g., Dodson 1986, Dodson and Currie 1990, Fiorillo 1987a, 1989, 1991, Fiorillo and Currie 1994, Gao and Fox 1996). Hidden Valley Quarry produced approximately 250 fossil bones and teeth representing 11 vertebrate taxa (Table 1).