Serpent Mound is located in Adams County, Ohio, at the confluence of Brush Creek and Baker Fork. It is an Effigy Mound, and the most recent radiocarbon dates by a team led by William Romain in 2014, placing construction around 321 BCE and later repaired by the Fort Ancient Culture accounting for the previous radiocarbon dates.
The site consists of a 1,348-foot-long effigy of a serpent, which winds back and forth across the top of a ridge.
The first recorded description of the site comes from the early 19th century when it was observed and documented by pioneers and explorers. In 1846, a survey of the site was conducted by Ephraim Squier and Edwin Davis, who were among the first to recognize the site's significance as a prehistoric earthwork. They published a book, "Ancient Monuments of the Mississippi Valley," in which they described the site and speculated about its origins.
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Ancient Monuments of the Mississippi Valley
Map of Serpent Mound From Ancient Monuments of the Mississippi Valley (See Squier & Davis Book Below)
LIDAR showing the contours of Serpent Mound. Sean Chaney 2019 Using USGS resources.