During the Woodland Period, the Adena culture left large earthworks downriver at South Shore's Biggs Site ( BC to 0). The Hopewell who succeeded them then expanded the complex into the great Portsmouth Earthworks ( to 500). Ashland lies between this group and the related but distinct Armstrong Culture who lived along the Big Sandy. Its Central Park includes six 4- to 5-foot (1.2 to 1.5 m) burial mounds, which have been placed on the National Register of Historic Places. They have not been excavated by archaeologists, but are thought to date to the Adena. The mounds within the city used to be more numerous but were mostly destroyed in its construction. The present mounds are actually restorations, having been rebuilt from smaller heaps; The city recently agreed to fence them.