First called Highland, Sullivan County was formed in 1843 and fully organized in 1845, taking its name from Revolutionary War Gen. John Sullivan. Covering 654 square miles of high glacial prairie, it lies in territory ceded in 1824 by the Iowa, Sac and Fox tribes, and the area still contains massive glacier boulders and many Indian mounds. The first settlers were Dr. Jacob Holland and his son, R.W. Holland, in 1836. Milan, the county seat, was laid out in 1845 on land given by A.C. Hill and bought from Hiram Phillips, and an Indian mound there was leveled to form the public square. The last U.S. Land Office established in Missouri was located at Milan from 1849 to 1859. During the Civil War, a Union post was maintained at Milan and guerrilla bands raided the county. Railroad development in the 1870s brought modern growth, and Milan expanded as a shipping center and railroad division point when the C., B., and K.C. later the C. B. and Q. was built east and west between 1871 and 1881, while the C., M., and St. P. was built in the western part of the county in 1886. Sullivan County came to be characterized by rich grain fields and productive dairy, livestock, and poultry farms. Its main watercourses are Medicine, Locust, East Locust, Yellow, and Spring creeks. Early pioneers came from VA., KY., Tenn., Ohio, and Ill. County towns included Green Castle, Newton, Pollock, Boyton, Core, Winigan, Green City, Humphreys, Reger, Osgood, and Harris, founded between 1857 and 1887. Early schools included Green City College, opened in 1885, and Humphreys College and Business Institute in 1884. In the 1880s, Reger was an important railroad tie shipping point. Near Osgood and west of Milan, the widely known Camp Ground and pioneer cemetery stood at the site of an early wagon-trail camping ground, where in the 1850s the Methodist Episcopal Church South and the Cumberland Presbyterian Church organized Camp Meeting Grounds; congregations from across the county met there until the Civil War, when all installations were burned, and the interdenominational church building standing there today was built in 1901.