The Gypsum Spring formation, of Middle Jurassic age, crops out sporadically for a distance of about 150 miles along the western and northern flanks of the Black Hills in northeastern Wyoming and west-central South Dakota. The formation is about 125 feet thick at its northwesternmost exposure 10 miles northeast of Hulett, Wyoming, where it consists of a lower sequence of massive gypsum and red claystone about 75 feet thick and an upper sequence of interbedded light gray limestone and red and gray claystone about 50 feet thick. The formation thins irregularly southward and eastward as a result of truncation by the overlying Sundance formation of Late Jurassic age. The contact of the Gypsum Spring on the underlying Spearfish formation of Triassic age is also unconformable. >

Subdivisions of the Gypsum Spring formation in the Black Hills region correspond closely in lithologic character and thickness with subdivisions of Middle Jurassic rocks recognized by others in central and northern Wyoming and in southern Montana.