Studies along the southern and southeastern borders of the High Plains have demonstrated the presence of outliers of fossiliferous Ogallala Formation in Borden and Scurry counties and have documented the occurrence of Pliocene deposition as far southeast as Sterling County. The limit of characteristic Ash Hollow seed floras is extended to the southeast. An abandoned Pliocene and Pleistocene valley is described across a prong of Edwards Plateau south of Big Spring, and the drainage of the late Pleistocene Lake Lomax is determined to have occurred in pre-Bradyian Wisconsinan time. A meaningful physiographic boundary cannot be drawn between the southern limits of the High Plains and the Edwards Plateau.