Late Cenozoic stratigraphy and geomorphology, Fort Niobrara, Nebraska
The central Niobrara River valley in northern Nebraska(Fig. 1) has played an important role in the conceptual development of late Tertiary stratigraphy and vertebrate paleontology in the Great Plains. Much of our current understanding of the evolution of such mammals as horses, camels, and proboscideans is based on fossils collected from the Valentine and Ash Hollow Formations exposed in the deep (by Nebraska standards!)canyons of the Niobrara River and its tributaries in Brown, Cherry, and Keya Paha Counties. The Valentine-Ash Hollow sequence in this area represents the northernmost expression of a complex of alluvial deposits collectively known as the Ogallala (variously termed agroup or formation) which blankets much of the Great New Mexico.
The only paved road to traverse reasonably good exposures of the classic Ogallala sequence in the Niobrara valley is Nebraska 12 about 3 mi (4.8 km) northeast of Valentine. Two adjacent road cuts at the northwestern margin of Fort Niobrara National Wildlife Refuge (see. 22, T.34N., R.27W., Cornell Dam Quadrangle, Fig. 2) expose nearly the entire thickness of the Valentine Formation and the Cap Rock Member of the Ash Hollow Formation. Examination of the road cuts, combined with a tour of the Refuge, will provide the visiting geologist with an overview of the later Tertiary mantle of the Great Plains near the boundary between the latter physiographic province and the Central Lowlands Province to the east. Geomorphological relation ships are also exceptionally clear along this segment of the Niobrara River; the very late