The Geologic Setting of the John Day Country: Grant County, Oregon

A regional geologic overview describes the rock units, tectonic setting, and landscape evolution across northeastern Oregon at the junction of the Columbia Plateau and Basin-and-Range provinces. It traces marine deposition in the late Paleozoic, subsequent igneous intrusions of peridotite, gabbro, and later granites, and extensive volcanic activity that produced the Clarno and the ash-rich John Day deposits. These Oligocene beds preserve abundant plant and mammal fossils, while Miocene volcanism and basalt flows sculpted the modern topography. The guide also summarizes mineral occurrences and mining history, current land uses, and includes a road log and map pointing to roadside outcrops and features of geologic interest.