The ancestors of the American Indians of the Eastern Woodlands built Serpent Mound, but archaeologists disagree about when it was built. Some argue that the Adena culture built the mound around 2,300 years ago, the same culture that built the large conical burial mound at the east end of the parking lot. Others think the Fort Ancient culture built the Serpent around 900 years ago; the Fort Ancient culture built the small burial mound near the west end of the parking lot. Serpent imagery became especially important during the Mississippian period, which was contemporary with the Fort Ancient culture in southern Ohio, supporting the idea that Serpent Mound was built by the Fort Ancient culture. Future research may reveal the true age of Serpent Mound, but for now it is not known for sure. A Mississippian stone palette engraved with two serpent-like creatures with rattlesnake tails, overlaid in white, was found in a mound in Mississippi.