Paleontology and geology
The undifferentiated rocks indicated on this map form the “red racetrack” around the Black Hills in the western part of the state. They are primarily Triassic in age, with some Jurassic rocks occurring just at the edge of the "racetrack." The Triassic rocks are composed of red silts and clays interbedded with layers of gypsum. Although they are not fossiliferous, they provide evidence of intertidal mudflats and an ancient, humid, coastal plain.