Within the study area located near Valentine, Nebraska, the Niobrara River is deeply entrenched in Tertiary siltstones and sandstones, and covered with Pleistocene and Holocene eolian deposits of sand and loess. The 100 m depth of the valley reflects the entrenchment. Periods of equilibrium of the river are indicated by the numerous benchlands that lie within and adjacent to the valley. Mass wasting in the forms of landslides and creep significantly alter the form of these terracelands when it is combined with the shifting of the river and saturation by groundwater. Many of the terraces have been correlated with climatic events of the Pleistocene and Holocene epochs. There is some evidence of uplift in the area. The stratigraphy of the low terraces reflects the overall downcutting interspersed with periods of aggradation. Some elements of vegetative community are unique to the grasslands, to Nebraska, and to the Great Plains. Representatives of the Rocky Mountain, eastern deciduous, and northern forests interact with variables of topography, stratigraphy, and microclimate. Active landslides and newly exposed terraces are revegetated in a definite sequence from annuals to hardwood forests or to grasslands in some cases. The parallel zones of woody vegetation on the right bank is disrupted by mass wasting events. The paper birch is a Pleistocene relic and thrives in the valley under stringent habitat requirements such as north facing slopes, springs, and shade of other trees. The results of this study are numerous and diverse. However, in a synthesis, they present a concept of the dynamic interaction of the physical and biological factors in the area. From these individual factors, a grander hypothesis was formulated that describes the Niobrara River as the principal contributor of sand in the formation of the Nebraska Sandhills
