The Wabash River, coming out of Tippecanoe County to the east, defines the southeastern border of the county; the terrain here is hilly and wooded areas are common. Fountain County lies across the river. By contrast, the northwest region consists mainly of flat prairie farmland; this continues in Benton County to the north. Along the western side of the county is the border with Vermilion County, Illinois. The small southern border is shared with the north end of the similarly named Indiana county of Vermillion. The state capital of Indianapolis lies about to the southeast.